The first decsion to be made is what material to use for the chassis? Mostly I print in the Frosted Ultra Detail (FUD) material from Shapeways as I need the fine detail this material allows for. With the chassis though I'm more interested in making it robust than detailed so I ordered my Mk 1 design in black strong and flexible and black detail as well as FUD. That order represents increasing price and decreasing strength and so if the black strong and flexible material works I'll get the best of both worlds.
Due to the different printing tolerances of the three materials the first job was to check that the chassis actually fitted within the body. Interestingly the FUD print doesn't fit at all. The saddle section where the motor will sit is too wide for the gap in the footplate. I find this very odd given that it should be the most accurate dimension wise. Anyway FUD is the most expensive and weakest material so I'm fine with it ruling itself out of contention. Both the black chassis (ordering the dyed versions seemed sensible to avoid having to get paint near the wheels and motor) fitted perfectly so I moved on to trying to assemble the strong and flexible version.
My preference for using the strong and flexible material is that as well as being the cheapest option it should be the most hard wearing. My hope is that I can get away without bearings and just let the parts run freely in the chassis without it wearing away. I deliberately made the holes in the print the same size as the parts (it's easy to remove material, but impossible to add it back), so I started by reaming out the holes for the axles and layshaft. Once I had what seemed like reasonable clearances I assembled the parts. This was very fiddly and time consuming. Most of the problems came from me not reaming out parts far enough but mainly dealing with the pulling a wheel off each axle and then refitting it. I guess I need a wheel puller and press; any suggestion on where or which model to get would be most appreciated. Anyway after battling away for a little while I ended up with this.
As you can see turning the layshaft by hand turns the wheels nicely. I did make a couple of mistakes though. Firstly I should have opened up the holes a little more, as it ended up requiring quite a bit of force to turn the layshaft. Secondly I managed not to keep the worm gears at the ends of the chassis which means the layshaft moves when changing direction before the wheels start to turn. For a first attempt though I think it proves the general design.
Unfortunately from this point on it was one disaster after another. First I added the main pulley to the layshaft and in an attempt to stop the worm gears moving pushed it up to the chassis. This had the desired affect but made the layshaft even harder to turn. I then realised that given the pulley location I'd need to push the pulley on the motor as far down it's shaft as possible. Doing this I managed to drive the shaft through the plastic back plate of the motor. Ooops! Amazingly the motor still rotates under power. Even then I had to offset the motor from the designed position to make the pulleys match up at which point the combined chassis no longer fits in the body. And to top it all off the motor can't provide enough torque to turn the tight drive shaft.
This was attempt number 1 though so I'm not too disheartened by the outcome. I've proved that the general layout seems to work and that I can use the black strong and flexible plastic which should allow me to avoid using bearings. I also now know that I could do with making a few adjustments to the body as well to give a little extra clearance in a few places. Altogether I'm happy with the progress.
