Custom LEGO – A LEGO Holonomic Wheel
This is a work-in-progress, so this post will be updated and changed as the project continues.
Here are the pieces so far:
I want to design and manufacture some custom LEGO pieces because:
- I like LEGO;
- I like designing and making things;
- I want to follow a design project through from start-to-finish, because designing things is quite an involved process, and requires lots of research, sketching, prototyping and mistake-making;
- I have a goal of working for LEGO, so a project like this shows what I’m capable of;
- I’ve seen a gap in the market for some new LEGO elements that would allow the building of an omni-wheel, but would not be a specialised part, and could still be incorporated into the overall LEGO system;
A Holonomic Wheel (or omni-wheel) is a wheel that allows movement forward and backward like a normal wheel, but also allows movement left and right; a holonomic wheel can move forward and rotate at the same time.
It is ideal for manoeuvring in a tight environment (like a forklift in a warehouse) because a vehicle with holonomic wheels can turn on the spot and can change direction without having to ‘turn’.
Here is a previous LEGO holonomic robot I’ve made, based on a Killough Platform, using LEGO Mindstorms 2.0:
Please note: Mindstorms RCX is from 2001 and I had the software running on Windows ME. I’m now on Windows 7 and I can’t program the controller. I’ve got a LEGO remote control that can run the motors individually but it doesn’t work any more so I can only show limited movement. Trust me it works well (see someone else’s video below) and when I can locate a working remote I’ll update with a better video.
Update 1: I found out that it wasn’t a problem with the remote, but the RCX unit needed firmware downloaded. After lots of swearing, when I discovered that it is possible to run Mindstorms software on Windows 7 64 bit, but the infrared tower (that sends the code to the RCX unit) is 32 bit, I got it all to work using an old XP netbook.
Update 2: I got the new LEGO Mindstorms EV3, which apart from PC programming, can be programmed from the brick itself, and also has a great Android app.
A Killough Platform, also known as a ‘Kiwi drive’ (according to Discover Magazine July 1997 http://discovermagazine.com/1997/jul/1997discoverawar1170#.UjJqRNJkPmc) is a ”Omnidirectional Holonomic Platform” developed by Stephen Killough and François Pin at Oak Ridge National Laboratory in 1994:
“Conventional wheeled vehicles can’t move in all directions from a given starting position while simultaneously rotating–a capability arising from a property known as holonomy. The OHP can do this.”
The LEGO Killough Platform is based on the following designs:
How it works – Vectors
In a nutshell, the Killough Platform is based on 3 holonomic pairs of wheels, which by combining rotations, create vectors of movement. Hard to explain (this is a valiant attempt: http://www.technicbricks.com/2008/08/going-to-all-places-in-all-directions_29.html)
Here is my prototypical omni-wheel; one wheel made up of three pairs of chord shaped wheels. The circumference of the main wheel is made up from the outline of the three pairs of wheels:
Each pair of wheels can also rotate along it’s own minor axis as well (blue arrows), perpendicular to the normal rotation of the wheel;
So with three pairs of wheels, two wheels are powered to rotate along their respective major axis of rotation and these determine the vector direction (red arrow), while the third wheel rotates along its minor axis freely, not dragging or braking the vector movement:
It is easier to understand if you watch it in action:
There are a few problems with the above design:
One- it works but it is a really rough ride. Any gaps or breaks in the circumference of the wheels rotation creates a ‘bump’. This problem can be fixed by filling the gaps in the circumference of the wheel rotation by adding more ‘little wheels’, but this adds complexity, more moving parts, more slop, more things to break. Plus it looks a little stupid because you have to use tiny tiny wheels or cogs, turning the wheels more into ‘Mecanum’ style wheels (see below):
The more LEGO parts that are used, the more slop and vibrations are introduced into the motion. If the wheels are used to make a robot, this will reduce turning accuracy and slow the robot down, and greatly increase the chance of breaking down. In order to get LEGO omni-wheels, you end up with large complicated assemblies like these:
Third Party Wheels
Two- there are no official LEGO holonomic wheels. There are lots of third party wheels available but most are small-run custom made. The main criticisms I have of them are:
- Complexity: unavoidable with a holonomic wheel, but they haven’t been designed with LEGOs greatest strength in mind, modularity and compatibility with existing LEGO elements;
- Cost: high, they are small-batch rather than mass-manufactured;
- LEGO semi-compatibility: most can be used with LEGO, but don’t follow the LEGO design language or dimensions. A wheel diameter may be an odd fraction of a standard LEGO brick element high. Also the modularity and compatibility mentioned above, the wheels can not be taken apart and put back together with other LEGO;
The best I’ve found so far. Rotacaster make industrial castors. Very strong and used in lots of robotics projects (see their youtube page).
48mm $21.45 each, 125mm $29.15 each:
3D printing and CAD sharing websites often have LEGO models, which often have creative interpretations of LEGO dimensions. They’ll work in the virtual CAD world but not in the real plastic LEGO world, and with most you can’t tell if they’ve actually been built and tested. $38.67 for a set of 4 on Shapeways is an OK price considering, but the owner has noted “Sadly, the casters only worked when Shapeways offered the rubberized material.”.
$38.67 for a set of 4:
LEGO Education North America online shop sells third party LEGO compatible wheels. The wheels they sell are Pitsco / Tetrix omni-wheels. The small print on their website states: “LEGO Education North America is a joint venture between Pitsco, Inc. and the educational division of the LEGO Group.” so they are sort-of official, They don’t look like an official LEGO element, rather something someone has knocked up at home.
3” (75mm) $24.95 each, 4” (100mm) $29.95 each:
Modern Teaching Aids
Modern Teaching Aids also sell omni-wheels for the “Robocup Junior” robotics competitions. These parts are in a ‘official’ element grey zone. Some robotic competitions allow them, most don’t.
Here is a closer look at the above wheel, it is very ‘engineery’. Like most of these wheels the only LEGO compatible element is the cross axle slot, http://www.holonomicwheel.com/index.jsp:
Made by Okartek http://www.okartek.fi/en/okarroll/ as conveyor belt rollers. Very nice looking, very expensive wheels but they still have the drawbacks of all the other wheels. These appear to allow the individual ‘circumference’ rollers to be removed, but I doubt if they are LEGO compatible. It is designed to be used with the ‘Stingray’ robot kit, so has a specific axle which is purchased separately, and would need to be modified to work with LEGO.
50mm to 120mm, €24,00 each:
The hub insert shows how these wheels are made to be LEGO compatible, a similar system to the Rotacaster wheels, which also allows them to be used with non-LEGO robotics. They have both a 4-roller and 5-roller model.
48mm $15.00 each:
58mm $18.00 each:
Nice range of wheels and they look very well made and at a good price. Most have a square axle slot designed for the VEX square shaft, but they do their own range of what appears to be LEGO-compatible parts ‘VEX IQ’:
200mm travel (approx. 64mm) $9.99 pack of two:
2.75″ (69.85mm)$19.99 pack of two:
4.0″ (101.6mm) $19.99 each:
4.0″ (101.6mm)$24.99 each:
And for comparison, a VEX mecanum wheel, where the added complexity and strength requirements are reflected in their higher cost.
4.0″ (101.6mm) $54.99 each:
Unofficial LEGO Elements
Omni-wheels are used often in robotic competitions, but most LEGO robotic competitions don’t allow non-LEGO pieces, don’t allow modified LEGO pieces, or don’t allow omni-wheels at all. Getting an official LEGO omni-wheel element would be fantastic :
“First LEGO League 2012 FLL Challenge
6 – EQUIPMENT
- Your equipment (robot, attachments, and other accessories) must be made entirely of LEGO elements in original factory condition.”
“Malaysian National Robotics Competition
“Except for special designations in competition rules, materials used by the teams to assemble their robots must be from 9797 LEGO© MINDSTORMSTM Education Base Set, 9695 MINDSTORMSTM Education Resource Set, 9648 Education Resource Set, 9794 LMFS Team Challenge Set, and 9649 Technology Resource Set. The shapes and the colours must be exactly the same.
Modification of any original parts, for example, RCX Intelligent Brick, NXT Intelligent Brick, motors, sensors, etc. is not allowed. Violation of this rule will result in disqualification.”
“3.5.2 For the GEN II League the following applies:
- Robots are to be constructed using strictly LEGO brand pieces, motors and sensors only.
- LEGO pieces can not be modified in any way.
- No other building materials can be used, including glue, tape, screws etc.
- Omni directional wheels are not permitted”
LEGO Design Language and Modularity
To me the 3rd party wheels don’t follow any sort of LEGO dimensions or design language- they work well and appear to be widely used but in my opinion they look out of place. You can only use them for one thing, which isn’t what LEGO is about.*
So I wanted to design some LEGO holonomic wheels that can be used with regular LEGO pieces; they can be taken apart and used for other things. This means using as many standard LEGO pieces as possible, and any new pieces should be able to fit int the existing LEGO system.
I looked at Mecanum wheels (http://en.wikipedia.org/wiki/Mecanum_wheel) but they are complicated and would need to be over-engineered to be stronger than the usual LEGO standard, and possibly not be able to be made out of injection moulded ABS plastic if there was a size limitation, so I’d end up with an expensive LEGO piece that could only do one thing.
Also Mecanum wheels require four wheels to function as a holonomic platform, whereas most other omni-wheels only need three wheels. They look great though:
An Omni wheel (http://en.wikipedia.org/wiki/Omni_wheel) appeared to be a better approach, so I explored that further:
Here are some of the ideas I started sketching with:
I ended up with a triangular assembly of 3 pairs of ’mini-wheels’, whose combined outer surface makes up the circumference of the larger main wheel. The larger wheel works as normal, the 3 pairs of wheels allow the sideways movement.
My design intent was to have the standard LEGO dimensions drive the main wheel size, and so in turn determine the individual ‘mini-wheels’ size. Lots of SolidWorks modelling now to get the geometry right. The ‘mini-wheels’ on the outer of the main wheel allow the sideways movement, so they need to be able to rotate about their own axis, while having a profile that is the circumference of the main wheels rotation:
The part sizes need to balance; a bigger mini-wheel will better cover the circumference of the main wheel, but bigger mini-wheels mean there is less space to attach the wheel on the spoke arm. If the mini-wheel gets too large, it won’t fit in the space allocated:
The early concept for the ‘mini-wheels’ to be on a fork like a bicycle wheel showed it wouldn’t work- not enough space available to keep the whole assembly at a reasonable size, plus the ‘circumference gaps’ got quite large, making for a bumpy ride.
I went for an alternative concept - each ‘mini-wheel’ split by the fork. More of the main wheels circumference was covered by the ‘mini-wheels’ creating a smoother ride. It meant for a new LEGO piece to be designed, a six-armed technic beam:
The only issue I have with this is that LEGO is mostly a ‘square’ orientation, the studs are spaced in rows at right angles to their columns, so a six-armed pattern isn’t compatible with the majority of LEGO bricks (placing a regular 4 by 2 stud brick on top won’t work as the pieces can’t fit together). Some compromises were made so the middle is ‘square’. I figure this is an acceptable feature because LEGO does have lots of pieces that aren’t ‘square’:
LEGO element 32125: “Technic Rotor 3 Blade with 6 Studs”
This is the final design concept:
Three sub-assemblies, each offset so at all times the wheel circumference is in contact with the ground, made up of one six-armed technic beam and 3 pairs of ‘mini-wheels’:
The six arms are required to allow the arms to be stiffened by use of technic pins and liftarms:
Now some prototypes. Prototyping and making is essential to design because it is showing that the ideas work. It’s easy to sketch or CAD something up that looks like it works, and photo-realistic rendering can make anything look real, but an actual real model in your hand tells you so much more about the object. Design is about working out the mistakes before the user does.
3D Prints – Hex Arms version 1
Here are some initial 3D prints of the six-arm technic beam:
The rough surface is due to the 3D printers I am using, a result of the support material that needs to be removed manually.
Making a model using 3D printing can be very seductive. The idea of turning your pretty CAD render into a real thing, exactly as it appears on your screen, just by pressing a button named ‘print’ sounds great. As you can see this doesn’t always happen. This one got a little messed up on one edge, plus you can see the ‘stepping’ of the filament layers and general inconsistencies (like flattened holes) that are a result of the 3D printers I used.
The arms on the white version (ABS plastic on an Up!3D printer) proved too long, so a second version was printed (red PLA plastic on a Makerbot Replicator):
Again you can see the resolution issues. Partly because I built this version upside down as I figured the internal dimensions would be harder to modify post printing- it’s easier to remove material from an external stud , than from an internal tube (the LEGO connection system is referred to as “stud and tube”). The studs look a bit messy:
Good news is that it is more or less the 3D printed parts fit with the LEGO parts. Some filing and trimming has been required. I’m pretty happy with the results, especially with the limitations of the printers I’ve used. It has given me a real appreciation of the quality and tolerances of LEGO.
3D Prints – Mini Wheels version 1
Now for the mini-wheels. All follow the basic shape and dimensions; a cone that is a chord of the circumference of the main wheel:
An ‘internal’ tread (cut into the shape) is better than an ‘external’ tread (protrudes from the shape) as the ride will be better and there is less to wear down, which would affect the circumference dimensions of the main wheel. This is the design I like the best, except the 3D print didn’t create the support material (my error), so you can see where the extruded plastic wasn’t supported and drooped down and made a mess:This tread design is based on this LEGO piece tyre tread (Wheel with Split Axle hole, with Light Gray Tire 30 x 10.5 Offset Tread (3482 / 2346) http://www.bricklink.com/catalogItemPic.asp?P=3482c04):
It should look more like this (a rendered image of the CAD file):
Here is the 3D printed assembly so far, attached to real LEGO. The next version will have the six-armed technic beams the same size as the red version, and a completed wheel will be made up of three assemblies so the whole circumference of the main wheel will be unbroken:
3D Prints – Mini Wheels version 2
Some more wheel re-design. The original wheels (in the photos above) are too large, they butt right up to the reinforcing liftarms, so I made them a little smaller.
The tread on the design I liked was also too deep. I intend to cast the wheels in rubber/ silicon and 2mm deep tread might not work. Plus it didn’t print very well, the deep tread meant the extruded plastic drooped. This is the revised tread pattern that I’ll stick with:
I also realised that the axle slot was too small- the LEGO axles would not fit in. This is a tolerance issue, both with the callipers I’m using to measure the existing LEGO pieces, and how dimensionally accurately the 3D printers can make the parts. I needed to make sure the axle fitted in snugly, but was easy to remove. This is a trial-and error-process. A number of wheels were made where I experimented with the width of the axle slot. The last one was good:
All the remaining wheels needed some Dremel work to file out the internal dimensions of the axle slot. A bit messy but ended up working well
Here is how it works:
The 3D printed Hex arms do not have the tolerances to accurately and repeatedly produce technic holes of 5mm diametre. I worked out an easy and accurate way to clean up the 5mm technic holes- use a 5mm drill bit and a pair of pliers and tap the holes by twisting the drill bit back and forth, worked fantastically:
.RE-INVENTING THE WHEEL – PART 1.
After leaving the project to work on other things, I had a think and came back to it a few days later. One of my aims was to use as many standard LEGO pieces as possible, so maybe I could use some existing LEGO pieces instead of the six-armed technic brick. I experimented with the tri-blades (see above) which would work, but there would be very little lateral strength as the technic bricks couldn’t be reinforced with pins and liftarms, so the wheel would probably fall apart under load:
After a part search I found this set from 2009:
Lego Combine Harvester
Which contained this part, “Technic, Plate Rotor 6 Blade with Clip Ends Connected (Water Wheel)”:
So I bought some from www.bricklink.com (the excellent online LEGO spare part marketplace) and tried to make the the equivalent of my wheel using existing LEGO parts. The good news is that my dimensions on my custom pieces are pretty true to the equivalent LEGO piece, the bad news is the size of the ‘Water Wheel’ isn’t quite big enough for what I want. Same problems as the tri-blades, they have no room for the pins and liftarms, and I’d also need technic bricks that are 1 x 5 long with four holes, which LEGO do not make, so I’ll keep going with the six-arms.
.PROTOTYPES – SHAPEWAYS.
The above prototypes have shown me my design intent works, but I need much better quality prototypes than what I have available, so I have used http://www.shapeways.com/ (online 3D printing bureau) to get higher resolution parts made. You create your part in CAD, upload it to them, choose the material you want it made from, they make it and send it to you.
I’m using their “strong and flexible” material, which is laser sintered then polished post print. Laser sintering (http://en.wikipedia.org/wiki/Selective_laser_sintering) cuts a CAD model into layers and uses a laser to melt the material particles together according to the shape of each layer, so a piece is built up layer by layer. According to the Shapeways MSDS it is “Fine Polyamide PA 2200 (Polylaurinlactam (polyamide 12))” with a grain size of 56 µm, so the part detail will be much improved compared to the 3D printing I’ve been doing. PA 2200 is a Nylon (sometimes called Nylon 12) which is food and medical safe.
The Shapeways prints are of a much higher resolution than the Makerbot and Up! prints, but they are more expensive (not really expensive but not free at the public library) and took about 2 weeks to arrive. So far they seem just a little bit not as strong as the ABS and PLA; instead of having to file the parts to fit with actual LEGO with a Dremel (see below), I managed to push in a LEGO axle by hand. They look fantastic however:
.RE-INVENTING THE WHEEL – PART 2.
The Shapeways parts are much better detailed and more dimensionally accurate, so better show how well my part connects with existing LEGO, and has highlighted some issues:
- the technic brick holes and axle slots need to be a tiny bit bigger, the technic brick holes need to allow the axles to spin freely and the axle slots need to be able to hold the axles while letting them to be put in and removed by hand without too much effort;
- the bottom of the hex arm brick (the “tube”) connects well to a regular LEGO brick, but the top of the hex arm (the “studs”) are too small and do not connect at all to a regular LEGO brick;
I have sent an experimental batch to Shapeways to check what are the best dimensions for overall fit. LEGO has tolerances in the microns (0.001mm) and my callipers only go to 0.1mm, so I’ve had to experiment with a few pieces with different dimensions:
I’ve made 5 different versions of the above technic brick, all with different sizes of axel slot, technic hole and stud width, and I’ve sent these to be printed at Shapeways, so I can test what are the best ‘real world’ dimensions to use, and then go ahead with those. It will also allow me to see what the different material colours look like:
The text on the parts is to remind me what the respective dimensions are. They are 0.8mm high and extend into the piece by 0.2mm, so it will be interesting to see what that resolution looks like on a real part.
Pretty good. Apart from the matt finish and the texture when you pick them up, they almost could pass for actual LEGO.
The pieces did show some of the tolerance issues with this 3D printing. The purple piece had a 0.05mm larger diameter hole than the black piece, but the axle moved more freely in the black piece. That said all the technic holes allowed the axles to move smoothly, there was no ‘notching’ or ‘stepping’ that occurred previiously when trying to get axles to spin in the blue hex arms. None of the text was legible, it was there but so small and couldn’t be made out.
The results for best compatibility with actual LEGO:
- Cross Axle slots 2.00mm wide, overall diameter of 5.00mm;
- Technic Holes 5.00mm diametre;
- Top Studs 4.9mm diametre.
.RE-INVENTING THE WHEEL – PART 3.
The wheel chord size is still bothering me. The current design calls for 3 hex arms, each with 3 pairs of wheels. Each assembly requires 18 wheels, and for a vector-style robot, at least 3 full assemblies are required; in total 9 hex arms and 54 wheels. Also with the wheel chord size as it is, there is only about 3mm clearance between the end of the hex arm and the ground:
Wheel Size and LEGO Units
The wheel diameter is effectively 4 LEGO ‘units’ in length (one LEGO unit being the distance from the middle of one stud to the middle of the adjacent stud). If the wheel diameter is increased to 5 LEGO ‘units’, there will be greater ground clearance and only 2 hex arms would be required per wheel assembly, as a standard technic 4 x 1 brick could be used instead of the middle hex arm. A vector robot of 3 full assemblies would only require 6 hex arms and 36 wheels:
Hex Arm and Beam Length
The 5 LEGO unit wheel version highlighted some more issues. In order to work the liftarms that provide reinforcement would need to be 3 holes long, which at the moment LEGO don’t make:
Making the wheels larger in order to increase ground clearance also creates another clearance issue, as the clearances between the wheels and the liftarms are tiny. I don’t think in the real world with flex and slop and loose fitting parts it would work, as the wheel would rub against the liftarm.
One way to increase clearance between the wheels and the ground and other parts is to increase the size of the hex arms. The hex arm span is 11 LEGO units/ studs. Beams and bricks are usually (but not always) an even number of units long:
Making a 12 unit long Hex arm allows slightly more ground clearance, and allows better integration with the most common LEGO sizes:
.FAILED 3D PRINTS.
The joys of prototyping. 3D printing is still in the future, I’ll know when it’s arrived in the present when the ratio of good prints to bad prints is better than 50:50.
In trying to get a clean base I printed without a support raft, so unfortunately the model moved a few minutes into the print and the whole thing was stuffed:
on the same day a new mini-wheel design failed as well:
and what happens when you don’t initialize the printer:
Two printers took about 3 hours to produce two mistakes and one pretty good print (in the middle). To be fair, it was new filaments, so the extruder’s temperature needed to be tweaked, but both failed prints were cause by not adhering to the base plate, which is pretty rare for the Up! printers, and usually caused by a temperature difference when printing (like a draft):
Moulds for Casting
The mini-wheels are to be made from rubber, and mount onto a standard LEGO rim, in this case part 3482:
So I have to adjust the part so it can take a rim rather than an axle, which is straightforward:
and making a mould is pretty straightforward as well. Enclose the wheel in a box, boolean operation to remove the wheel shape for the box, split the box in half, then add some guiding notches and key holes to align the two halves:
I experimented with the shape to see if it would work, unfortunately the 3D printers I’m using at the library aren’t accurate enough for this sort of model, and removing the support material was a pain:
I’ll need to print a very high quality mould to see if this will work. More expensive set-up costs but casting each piece should be a lot cheaper than individually printing each wheel:
.3D PRINTED WHEEL – MOULD FOR CASTING – 1.
I’ve ordered some updated hex-arms from Shapeways that incorporate the dimension improvements I worked out previously. Recent 3D prints I’ve done myself have worked well, but they do not have the tolerancing and finish of the Shapeways parts, plus removing the support material is a real pain which I can do without.
Shapeways have also introduced ‘Elasto Plastic’ http://www.shapeways.com/materials/elasto-plastic which is an elastomer (rubber-like material) so I’m getting two wheels printed to see what they are like. It will be a good guide to how well the individual wheels work with the standard LEGO rim I’ve chosen:
This is an experiment as there are some tolerancing issues with this material. Shapeways notes:
“The Elasto Plastic models may come out thicker than designed – up to .4mm in the Z direction, and up to .2mm in the X and Y directions. Features on Elasto Plastic models may be offset from their location in the original design file – up to 30% on small models and 10% on big models.”
Plus it is expensive, $19.51 per wheel (12 wheels per omni-wheel, 3 omni-wheels per robot, $702.36).
For this reason I’m going ahead with 3D printing a mould, that I will use with RTV silicon, to make individual wheels, hopefully for a whole lot cheaper:
I found this guide http://whatabout.no/no/home/item/26-the-ultimate-tool/26-the-ultimate-tool where a mould for a silicon water bottle was made using a 3D printer. Great guide which I will follow, when the latest Shapeways prints get delivered.
.3D PRINTED WHEEL – MOULD FOR CASTING – 2.
Latest Shapeways prints arrived today.
The Elasto Plastic was a lot harder than I expected. Thin walls flex quite nicely but thicker walls are almost the same as ABS or nylon. It has a plastic rather than a rubber feel so it isn’t really what I was expecting. It also came full of un-sintered powder, that spilled everywhere:
The latest version (version 39!) of the hex arms have some slight changes in the underside, to allow long thin LEGO plates to attach better, plus they have incorporated the dimension improvements developed from the previous Shapeways parts:
Most exciting are the moulds. I’m really happy with how they have turned out, really accurate parts, the two halves fit together in a really precise and snug manner. Next step is to cast with RTV silicon:
.3D PRINTED WHEEL – MOULD FOR CASTING – 3.
Here are the first castings of the moulds using Pinkysil:
Pinkysil is a 1:1 skin safe ‘hobby’ RTV (room temperature vulcanisation) silicone used for making moulds. I purchased it from Adelaide Moulding and Casting Supplies http://www.amcsupplies.com.au/ and they were very helpful over the phone and in person, especially with the ‘two pot’ mixing technique. I’ve used Pinkysil as the casting material because my mould has an undercut and the Pinkysil is flexible enough to be pulled out around the undercuts. It gets fantastic detail- pretty much every little flaw in the 3D printed mould is perfectly replicated. It is rated as Shore-A 20 hardness, which is a little softer than I ideally want but more than usable at this stage of the process. It’s also pretty non-toxic according to the MSDS, so it is good for a toy which kids will use.
It sets really fast- 6 minutes mix time and sets in about 20 minutes, so I used a syringe to get it into the mould for speed and also to avoid getting air bubbles in the silicone. This was a pretty good system overall, no mess and accurate. When the silicone cures it doesn’t stick to anything so can be pulled off all the mixing cups and from inside the syringe.
The cast was hard to de-mould at first, but with practice it got easier. The overflow was how I could tell the mould was full, with hopefully no trapped air bubbles. Putting my ear to it I could hear the silicone coming off the mould.
Air bubbles can be seen on the upper surface of the wheel. They look uniform so it looks deliberate. A learning for next time is to design traps for air bubbles- columns of silicone that bubbles can float up into, which can be trimmed post de-moulding.
Some flashing from the de-moulded part. This is so thin it can be pulled off by hand.
Another future learning is to design with the silicone tolerances in mind. According to my measurements this LEGO hub should fit snugly, but it didn’t, and spun in the wheel. Next time I’ll build this into the mould, but to fix my problem now I redesigned a few different sizes of the hub and printed them, the idea being use the best fitting hub. The LEGO hub is 17.0mm in diameter, so I made 17.0mm, 17.5mm, 18.0mm, 18.5mm, 19.0mm, both with and without ribs ribs to prevent spinning. This is what I ended up making, left-to-right smallest to largest, top row with ribs:
Unfortunately the printer couldn’t handle the very small bridging parts, which snapped off as I removed the parts from the support raft. Luckily the 19.0mm with ribs worked perfectly:
This is the next version I CAD’ed up, and it worked great:
The printer couldn’t completely handle the 3mm high “19mm” text, so it came out as “10″:
An axle assembly with two complete wheels and hubs.
I was still having issues with air bubbles. Knocking and tapping the mould sometimes worked well, sometimes didn’t:
I came up with this solution, which worked pretty well. A cheap ($35) random orbital sander, held down without a sanding pad and resting the mould 0n top of the sander. Turning it on and off vibrated the crap out of the mould and I could see bubbles floating to the top of the overflowing silicone. The main problem still appears to be the mould shape itself trapping bubbles. Combining this sander vibration method with a better designed mould looks to be the answer:
Better still was the ‘three cup’ mixing process I worked out. Researching how to remove bubbles from the mixed silicone led me to look at vacuum chambers, but these are really for making moulds out of silicone. You need a lot of mix to make it worthwhile (more than the 30ml per pour I’m using) and while it removes the bubbles very well I can’t justify the $200+ for one. Instead one of the ‘how to’ guides I found mentioned pouring in the silicone into another cup via a long pour- the bubbles are stretched out and pop as they are poured out. I did this a few times and the results were excellent, much less bubbles in the cast wheel:
Half of the latest finished wheel, with Shapeways printed nylon arms, custom ABS hubs and Pinkysil moulded wheels:
.DIMENSIONS AND TOLERANCES.
LEGO dimensions are pretty important to get right, because of the high tolerances of LEGO. LEGO claim a stud has a tolerance of 20 microns/ 0.02mm (http://aboutus.lego.com/en-us/lego-group/company-profile/). I’ve measured LEGO with a set of callipers to get an idea of the dimensions, but there is no way I can get that precision. This is one reason why LEGO is expensive, it is very very well made.
An article on LUGNET (http://www.lugnet.com/~330/FAQ/Build/dimensions) provides a good explanation of estimated LEGO dimensions:
“The actual dimension (rather than the ratios) can be determined by carefully measuring a properly constructed “wall” of bricks. Vertical dimensions are prone to error caused by sloppy stacking, but long horizontal walls constructed from two or more layers of overlapping bricks are not prone to such error. A wall of this type built from Technic beams made in the late 1990’s shows that the stud pitch distance is 7.986 millimeters, plus or minus 2 microns, when the parts are at 25 degrees centigrade (77 degrees Farenheit).
It is also worthwhile to note that LEGO bricks are always made a little smaller than this dimension would suggest. For example, the length of a 2×4 brick is a little less than 4 times 7.986 millimeters. The length is diminished slightly so that there will be a small gap when bricks are stacked next to each other. This gap allows for dust, marred surfaces and other imperfections. If the gap were not there the bricks would tend to push each other apart and the overall construction would be less strong.”
And the dimensions are:
- Brick height 9.582 mm
- Plate height 3.194 mm
- Stud pitch 7.985 mm
.FUNDAMENTAL LEGO UNITS.
“The Unofficial LEGO Technic Builder’s Guide“ by Pawel Kmieć refers to ‘F.L.Us’ or ‘Fundamental LEGO Units’, where one F.L.U. is one LEGO stud:
“When the unit of measurement for a LEGO piece is omitted, you can safely assume that it is the stud—for example, a 1×1 brick, a 2×2 tile, and so on. This is how we’ll refer to pieces in this book. You might also see the stud referred to by other names, such as module, dot, or fundamental LEGO unit (FLU). The letter L is used to indicate length in studs. For example, a 6.5L shock absorber is 6.5 studs long. LEGO builders generally measure the height of their creations, however, in terms of the height of a brick or plate. For example, we say that something is one brick tall or one plate tall. Note that one brick tall is equal to 9.6 mm, just a bit more than a stud.”
The LEGO wikipedia page http://en.wikipedia.org/wiki/Lego has a good primer on the LEGO dimension ratios (even though the actual dimensions are the approximate ones):
My larger holonomic wheel is 4.5 FLU in diameter and the smaller wheel is 3.0 FLU diameter. Both are the sweet spot in regards to the hex arm length. Any bigger makes the wheel not fit into the hex arm. I was concerned that 4.5 is LEGO heresy, but I was happy to discover LEGO does have some fractional pieces, there is a 5.5 unit long axle in the new Mindstorms EV3 set for example, so I have no problem in designing a 4.5 unit long element :
.3D PRINTED WHEEL – MOULD FOR CASTING – 4.
The next steps now are to print a whole lot of hex arms, print a lot of my LEGO rims, and cast a lot more wheels. One complete robot will require 3 complete wheel sets, which are made up of:
Per Wheel Per Complete Robot
2 Hex Arms 6 Hex Arms
12 Rims 36 Rims
12 Wheels 36 Wheels
6 LEGO axles (3 long) 18 LEGO axles
6 LEGO technic bricks (4 x 1 long) 18 6 LEGO technic bricks
12 LEGO flat plates (4 x 1 long) 36 LEGO flat plates
6 LEGO lift arms (6 x 1 long) 18 LEGO lift arms
18 LEGO pins 54 LEGO pins
1 LEGO axle (10 long) 3 LEGO axles
.KILLOUGH PLATFORM – VERSION 1.
The finished wheels, rims, hex arms and LEGO RCX 2.0 robot:
This design meant the holonomic wheels flexed a lot along the long technic axel, which meant the wheels quite often slipped off the rims.
.KILLOUGH PLATFORM – VERSION 2.
LEGO Mindstorms EV3, and re-designed wheel ‘cages’, which stiffens up the robot and helps the wheels stay on the rims:
Some more work is required to better attach the new style EV3 motors to the wheel cages, under load they tend to pull away from the chassis, but overall this design works a lot better:
One issue that has come to light with the first moulded wheels is that the space for the LEGO rim to fit into was too big. Even though I measured it multiple times to make sure, the 17.00mm LEGO rim diameter was too small for the wheel, so the LEGO rims spin in the wheel, when the holonomic wheel is turning as a vector the wheels tend to be pulled off the rim- you can see how loose they fit:
I fixed this is the short term by printing my own rims (see previously the 19mm rims above) but I want the wheels to be compatible with existing LEGO. Also printing my own rims add a level of non-LEGO tolerance to the wheels, axles don’t run as smoothly for example.
The only way around this was to test different rim diameters. Instead of making more moulds that may or may not work (time consuming and expensive) I made a mould ‘test rig’, eight different diameter rims to test how they fit. Here is the 3D printed mould test rig (bottom) and the resulting cast (top):
17.00mm is the nominal size, but you can see in the test rig it is too loose (trust me, it’s loose and spins freely):
15.00mm fits too snugly, you can see the bulging and distortion of the silicone:
15.5mm still bulges and distorts as well:
16.0mm seems to be the sweet spot, fits nice and snug without bulging, and doesn’t spin:
.SECOND GENERATION MOULDS.
Version 2 of the wheel moulds arrived:
- Two wheel sizes (previous 35.937mm – 4.5 FLU, plus a new 23.958mm – 3 FLU) so I can determine which wheel size works better;
- Resized rim slot, so official LEGO rims (part number 348224 “Spoked Hug D17mm” a.k.a. 3482 “Wheel with Split Axle Hole”) will fit snugly and to help stop wheels being pulled off the rims due to side loads;
- More top outlets per mould, to assist in removing any air bubbles from the silicone;
- A funnel from the top running down the side of the mould so the injected silicone will fill the mould from the bottom, also to assist in removing bubbles;
- Open-sided bolt holes, so the mould can be opened quicker without having to completely unscrew the wing nuts, and also repositioned so wing-nuts have clearance of the mould features;
Lots of shaking and blowing and tapping out dust and scraping with wire and safety pins and paper clips. I actually couldn’t unblock the larger 35.937mm mould funnel as the ‘pipe’ wound through 180 degrees, so I had to bodge a solution in order to use the mould.
I’m also using a black dye in the Pinksyil, as lovely as the pink wheels are, black looks more suitable and also lets me easily tell the versions apart. 5% by volume so each wheel contains approx. 1-2ml of dye. The dye is pretty messy.
A major problem straight away is the size of the smaller 23.958mm mould. The first casting got stuck and tore when I pulled it out, leaving silicone stuck in the mould. After about an hour of scraping with a pin I still couldn’t get all of it out. This may be caused in part by being the first cast, so it is sticking to the surface of the mould and picking up all the surface nylon dust. What did come out of the mould showed lots of dust stuck in the silicone. Note to self: wash moulds before use.
I fixed the 35.937mm blockage issue by tapping out larger top outlets, using drill bits and a pair of pliers.
The first cast did show a greatly improved lack of air bubbles:
Here is a wheel fitting snugly on the LEGO rim, still some dust but only with about half (approx. 1ml) the amount of black dye.
I think the small mould is beyond repair. The mould shape means the silicone is too thin to be able to cope with being pulled out from the undercut without tearing. A new mould will probably be required.
I attempted to fix the smaller mould by drilling out the access holes, which I thought would allow me to easier pull out the cast.
It sort-of worked, but the issue is that the LEGO rim I’m using is too big for the smaller 23.958mm wheel – there is not enough meat and it doesn’t grab the rim. I’ll have to try something else.
I am still having some trouble with air bubbles. Long pouring and more outlets does reduce the number of bubbles, but I realised that Pinkysil is silicone, and silicone does stick to itself, so I experimented on some wheels that had big popped air bubble holes that I wouldn’t normally be able to use, and with the left over silicone after a mould pour, filled up the holes with it:
It worked really well, apart from having a different surface finish (it is very shiny), so I fixed up a lot of cosmetic holes on other wheels:
### different LEGO rim for smaller wheel###
Here’s my completed army of black holonomic wheels, version 2:
Now with fitted LEGO rims and axel:
Busby Berkeley style, just waiting to be assembled into complete omni-wheels:
and on the robot:
.RE-INVENTING THE WHEEL – PART 3.
…I’ve created an impossible to injection-mould part…
I got in touch with Will Chapman from http://www.brickarms.com/:
I’d found an NPR piece on him and his company- http://www.npr.org/2012/11/26/165574108/an-entrepreneur-expands-the-lego-universe:
and I wanted to ask him what hobby CNC mill (to make prototype moulds for injection moulding) and desktop injection moulder (hand cranked, small shot) he uses for his LEGO compatible weapons. Will very nicely answered my questions (he recommends a mill from http://www.taigtools.com/cmill.html and a http://www.injectionmolder.net/ hobby injection moulder) but pointed out that the hex arm probably couldn’t be successfully injection moulded, as the side pull inserts required to create the ‘technic’ holes would not have enough room to pull out of the mould.
He is absolutely correct.
This is a render of the original hex arm part, showing detail of the ‘technic holes’ closest to the axle slot:
In order to create those holes in an injection mould, an insert the shape of the hole would need to be able to be inserted and removed, in this case from both sides. This is an example (a real mould pin/ insert would look different to this, but you get the idea):
There is not enough room to insert and remove the hole inserts, they would hit each other:
3D printing and CAD has led me up the garden path to think that the hex arm could be injection moulded…shit shit shit…but all is not lost. It would be easier the create a ‘flat plate’ hex arm (similar to part 32125 – three blade Technic Rotor/ Propeller) and use normal LEGO Technic bricks to hold the wheel pairs in place:
Here are the first run of hex thin plate prints:
These are PLA prints from the Makerbot Replicator. The Makerbot has a new extruder and the Makerware software is a great improvement over the previous one, so the quality is sensational and a big improvement. The speed of printing has a great input into the final quality, but also the support material is greatly improved and is really easy to remove, without leaving marks on everything. It has really changed my mind on Makerbots (for the better):Here’s one complete wheel built using flat plate hex arms (excuse the dog hairs picked up off the floor):
One thing I noticed is that the ’12 FLU’ Hex Arm means each arm is 5 studs long, and LEGO do not make 5 stud long technic bricks, so either one vacant space can be seen at the end of each hex arm, or a combination of 2 stud and 3 stud technic bricks have been used, which aren’t as strong. The 12 FLU Hex Arm will probably need to be modified to be 14 FLU, so 6 stud technic bricks can be used.
The orientation of the axle slot needs to be rotated slightly. Part of the benefit of the flat plate hex arm design is that existing LEGO elements (such as 32125: “Technic Rotor 3 Blade with 6 Studs”) can also be used. The orientation of the axle slot of the flat plate hex is slightly out compared the orientation of the axle slot of the Technic rotor blade:
The first finished wheel with flat plate hex arms:
.OFFICIAL LEGO CUSTOM ELEMENTS.
LEGO has their “Fair Play” policy, which is focused more on the trademark and use of the LEGO name and logo:
- “ALWAYS write our trademarks in capital letters.
- ALWAYS use a noun after the trademark, e.g. LEGO toys, LEGO values.
- NEVER add a possessive “s”, plural “s” or hyphen, e.g. LEGO’s design, more LEGOs to play with, LEGO-bricks.
- NEVER change or adjust the graphical design of a trademark, e.g. change the colours or shape of the LEGO logo.”
Fair enough. If you are buying or using LEGO, you want to be sure it’s the actual product and not a knock-off.
Interestingly, LEGO does allow ‘fan’ use, which is great:
“Therefore, we have developed these guidelines to assist Internet users who wish to refer to LEGO products for non-commercial purposes as to the appropriate way to refer to our LEGO trademarks. However, you should be aware that it is impossible to set hard and fast rules about the proper use of our trademarks. This is because our objective is to prevent a use which, when viewed in the overall context, is likely to lead to a blurring or loss of distinctiveness of our trademarks or cause consumers to believe mistakenly that there is an affiliation with or sponsorship by the LEGO Group. Since use in different contexts may produce different impressions (or misimpressions) on the viewer, generalizations do not always apply to a particular case. Nevertheless, we offer these guidelines and ask for your help in order to keep our trademarks strong and distinctive.”
“The LEGO Group owns the copyrights to its building instructions, publications and to the photographs used in our catalogs and on our packages. Copying, scanning and distributing these materials on the Internet would be an infringement of our copyrights. Nevertheless, at the present time the LEGO Group does not object to scanning of limited extracts of these materials in unaltered form for non-commercial purposes of exchange of information or good faith commentary. However, scanned /images/info should not be given such prominence as to indicate sponsorship of the Web site by the LEGO Group. We would ask that the photographs be scanned without distortion or overemphasis of the LEGO logo. A disclaimer and notice must appear indicating that the copyrights are owned by the LEGO Group (e.g. LEGO Group. This is an independent site not authorized or sponsored by the LEGO Group).”
So I’d better put in that disclaimer.
One of the reasons for this is that the patent has expired on the LEGO brick (see http://en.wikipedia.org/wiki/The_Lego_Group#Trademark_and_patents) hence the flood of cheap copies (see: http://cobi.pl/en/, http://www.hasbro.com/kre-o/en_US/, http://www.megabrands.com/, http://www.sluban.co.uk/ et. al.), but this also allows people (like me) being able to design and manufacture custom pieces that are LEGO compatible, without being sued. I think LEGO needs to continue to embrace this culture, but so long as it follows the LEGO ‘design language’ and quality.
Ultimately my goal is get these pieces made and sold, preferably as official LEGO elements. Apart from working for them, LEGO do have some options for members of the public to get custom LEGO sets and parts made.
In the past LEGO have brought fan sets to production. “LEGO Factory” and “LEGO Design By Me” allowed customers to design their own sets using the LEGO Digital Designer http://ldd.lego.com/en-us/download/ and some of these were converted into production sets:
These services are no longer available however, the current options are:
LEGO Cuusoo is a fan voting site for new LEGO sets. If a submitted idea gets 10,000 votes, it then is reviewed by LEGO and if successful it gets put into production, with the submitter earning a royalty of 1% of the total net sales of the product.
They have a few items in production as a result of the process:
and a LEGO ‘Back to the Future’ themed set (which will be the next to be produced):
The LEGO Cuusoo project guidelines (http://lego.cuusoo.com/guidelines) states three types of ideas:
“You will receive credit and compensation for your original ideas. We recognize three types of ideas, as follows:
- Original Model Ideas. You will receive 1% of net sales if your LEGO CUUSOO Project is chosen and the LEGO Group commercializes it as a LEGO set.
- Original Part Ideas. You will receive a one-time flat fee as remuneration for a part idea that is produced, to be determined by the LEGO Group.
- Original Model Ideas based on Licensed IP. You will receive 1% of net sales if the model you submitted is based on third-party intellectual property (e.g. Star Wars™) and is your original work.”
The LEGO Cuusoo ‘cheat sheet’ http://blog.lego.cuusoo.com/2012/05/29/cheat-sheet-how-to-pass-the-lego-review-with/ does not recommend new custom LEGO elements:
“Models should not depend on new LEGO element molds.
While we love considering new elements, if a model depends on a new element, there are more potential factors on which it can fail review. All new LEGO elements go through a very strict design and review process before they are accepted into the LEGO system. An existing element in a new color is OK, but a new element in one set isn’t likely.”
The BTTF set did require some custom parts however (http://blog.lego.cuusoo.com/2013/02/22/interview-with-team-bttf/):
“Customized figurine configuration. First, I sketched some ideas for clothing and hairstyle while watching the movie, and tried to reproduce them for a LEGO Minifigure. The hairstyle in particular is entirely handmade and cannot be reproduced. I used a magnifying glass while creating the parts so that they would appear just like those in the film.”
So LEGO Cuusoo is a future option if I want to take this project further, and here is the official LEGO Back to the Future set:
LEGO Rebrick http://rebrick.lego.com/ is similar to LEGO Cuusoo but not focused on production, rather sharing models that LEGO fans have made:
“ReBrick is a social bookmarking platform where adult users can share, organize and discuss user-created LEGO content.The content on ReBrick is not LEGO sets you can buy in a shop, but creations made by teenage and adult builders, who use their creativity to build their own models called MOCs (My Own Creation).”
This does also mean some brick customisation, mostly done virtually:
And of course some people have taken the Kickstarter path with their own custom pieces:
“The MinuteBot Base is a thick, robust base plate that can be used for construction of robotics based on LEGO Mindstorms. MinuteBot Base extends a LEGO beam into a rectangular plate with 21 x 30 holes. The dimensions and holes are a perfect fit with LEGO products.”
The interesting thing about this project is the disclaimer:
“Legal stuff: The MinuteBot Base and other MinuteBot products are not endorsed by or officially connected to The LEGO Company. LEGO and Mindstorms are trademarks of The LEGO Company.
That said: We have checked with the LEGO people – the company is based in our home country of Denmark – and they have no problems with us creating this project. They are really cool people.”
Quite a lot of third-party custom LEGO element products are available, all deliberately compatible with LEGO:
All the custom builders have disclaimer regarding not being official LEGO pieces. AltBricks states:
““This is NOT manufactured by LEGO. It is a custom element made for altBricks from ABS with the consistency of regular brick plastic. Fantastic clutch power.”
“Big Ben Bricks designs and manufactures custom toy bricks that are compatible with LEGO.”
“LifeLites products are made exclusively by LifeLites, LLC which has no affiliation with or endorsement from the LEGO® Group. Some LifeLites products contain repackaged LEGO elements. The LEGO Group is not liable for any loss, injury or damage arising from the use or misuse of LifeLites products which contain these repackaged elements.”
And lots and lots of custom minfigs and accessories manufacturers, the majority have military themes:
“The custom minifigures are created with new and used (in excellent condition) Lego parts and quality custom Lego accessories from a variety of suppliers (see Links page). We use printed Lego parts, custom Pad Printed parts, and decals to complete the design and look of each custom Lego character and model set.”
“Non-BrickForge items used to demonstrate compatibility have been credited and are the property of their respective creators…BrickForge makes no guarantee that our products are compatible with other manufacturers. We strive to insure proper dimension and fitment but have no control over variations that may occur in items produced by other manufacturers.”
“BrickArms LLC is a small toy company specializing in original, custom designed LEGO® compatible weapons and custom minifigs. All toys are original, and we produce high quality, low cost toys that fit perfectly within the LEGO® world.”
NPR have an article about custom manufacturers such as Brick Arms and the niche they fill:
“Now, you might think Lego would try to sic its lawyers on companies like BrickArms — but it doesn’t. And while Lego doesn’t publicly endorse them, it acknowledges they offer something some of its fans want.”
*the more specialised a LEGO part is, the less LEGO-like it is. If it can only do one thing, it is hard to adapt it into something else. Almost as bad as mixing some knock-off bricks or worse with the real stuff, and nothing fits right and the plastic looks cheap and you cut yourself on the poor quality flashing and you are history’s greatest monster.
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