Guy said > Restoration of the mechanics of my TM200 punch card reader progresses. > There's a writeup here: > http://everist.org/NobLog/20180922_data_in_holes.htm#tm200 Very interesting page, thanks
> Obviously too much 'squish' is undesirable since the roller would get > permanently deformed when left idle in one position. > The 2-part silicone I'll be using for first try at casting rollers has a > cured Shore A durometer rating of 60. Having done some two-part urethane rubber molding at home, I found the results are quite variable and depend on accurate mixing. If you have a vacuum pump and chamber, use it as it's hard to get bubbles out otherwise. Also if the urethane absorbs moisture I found it changed from a translucent honey shade to a solid milky caramel colour, until it dried out thoroughly. But by far the biggest problem I had with home cast urethane is that is has little tolerance for cracks. The slightest split in the material can easily rip right through the cast item. Natural rubber has way better resistance to cracking in this respect. I looked into getting a forklift wheel re-rubbering shop to do the jobafter I gave up on my home effort, they use a much better urethane and cure the job properly before machining, but it wasn't cheap. I have also obtained a good chunk of unvulcanised car tyre rubber for more experimenting, I simply walked into a retread shop and they literally hacked off a strip of the stuff and handed it to me at no cost. I never got around to experimenting with it (ie attempting to vulcanise it under heat and pressure) so it's still in the shed somewhere. It may not be useable anymore after all the long hot summers it's been through. For such small rollers I would suggest just bore out some solid rubber, 60 to 80 duro perhaps, press fit and super glue on to the old boss then machine to final diameter, rather then casting. I think it would last better than home-cast urethane. Steve.
