Pretty well everything is ready now to put together Decimus 4 – my latest micromouse. The last part to be done was the wheels. While I had the materials and the tools to make these, I certainly didn’t have the time so I thought I might as well try having them printed, like the motor mounts, by Shapeways.
My experience with the motor mounts was less than totally successful but I understand a little more about the materials and the likely accuracy now so a wheel design was sent off for manufacture.
The design errors so far with Decimus 4 placed some fairly serious constraints on the wheels. I could not make them as big as I wanted so ground clearance was always going to be marginal. the tyres that I thought I had turned out to be a figment of my imagination and I ended up choosing the Kyosho MZW2-20. There will add about 5mm to the overall diameter but they are nice and grippy. There are thinner tyres but I couldn’t find a supplier with the number I needed. That left me with wheels with a 17mm diameter and so the tyres may well need gluing on to be sure they don’t get shed when the mouse it running.
Shapeways managed to deliver in about a week and first impressions were encouraging:
The wheels measured up at exactly the dimensions I set and everything fitted together very well. The hubs are not really big enough because the material is a little more flexible than I would like and so the bearings could do with a little more support. The next design will be an improvement now that I am happy with the accuracy and will allow for normal bearings sitting in a pocket rather than the flanged bearings that I used.
The drive gears are 35 tooth, 0.5 mod Scalextrix gears that are a push fit over the boss on the rear of the wheel. The gears come in packs of five with one each of 34, 35, 36, 37 and 38 teeth. There are matching pinions with 8, 9, 10, 11 and 12 teeth which makes it easy to change ratio without changing the centre-to-centre distance.
Once mounted on the mouse, it is apparent how little clearance is available between the wheels. It should be possible to increase the diameter by nearly 1mm which will help with the ground clearance which is only around 1.2mm. Fine on a good maze but possibly marginal on some.
Because the motor mounts are not really rigid enough, some extra work will be needed before it is all ready. A 1mm spacer placed between the rear faces of the motors will counteract the slight camber that the wheels have and help a little with the ground clearance. Also, solid axles between the mounts will keep the wheels the proper distance apart and counteract the slightly uneven toe-in. While I have always thought a little toe-in to be a good thing, I would rather it was by design. This will also hold the wheels a little further apart to allow a slightly larger diameter.
Otherwise, the setup seems very free running and, remarkably, all four wheels touch the floor together so that is one worry eliminated.
It is far too late to get this working for the UK contest at Birmingham Techfest on June 16th but I do hope to have it running in Taiwan later this year.
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