Friday, November 24, 2006
Odd bits of know-how picked up along the way...
I finally got around to mounting those guide tabs after mounting the GM8 gearmotor. They smoothed out the movement of the x-axis rather dramatically. Feeling ambitious I decided to see if I could get a thrust collar built for the z-axis positioning stage as well. The threaded drive rod for the z-axis is quite short. As a result I decided to see if I could get by without the sliding joint that have proved necessary in the y and x-axes.
While I had a coupling bolt I thought it might be nice to see if I could make a thrust collar out of something else. I first thought to simply make one out of a blob of CAPA in the manner of Vik. After a while, however, I realised that I had a drill and tap for 3/8-24 studding, so I decided to see if I could make one out of poplar.
In fact, I could. The poplar thrust collar was quite a nice fit and had no backlash whatsoever. I then began working out the excess friction out of the collar with the GM8.
While the GM8 had sufficient torque for the task the torque loading of the rather small drive shaft on the coupling between the GM8 and the threaded drive rod generated a considerable amount of shear stress in the CAPA liner of the coupling. The stepping action of the GM8 induced what amounted to hysteresis in the CAPA which resulted in heating of the liner.
Eventually the liner melted. Given that the GM8 only generates about 3200 gramme-centimeters of torque, that this could happen is a worrying prospect for the use of CAPA to make parts for drive trains such as couplings and gears. The GM8's torque output is about the same as one encounters with a bipolar NEMA 17 running at slow stepping rates.
It would seem that we can look forward to using extremely conservative design parameters in gear trains made of CAPA.
If I understand correctly, this failure is due to the classic soft material under repeated strain between two stiffer materials. Usually it is solved be placing a more rigid material at the critical contact point, and then reverting back to the softer/cheaper stuff around it. In the case of the steel coupler and CAPA, it was the exact oposite!
I wonder if this failure would also happen if the entire coupling nut was an oversized chunk of CAPA? No concentrated compression points...?
I think good reprap gears and couplers might be made if something stiffer was set in the center of the part where all the stress converges. Maybe low temp alloy? Good epoxy (maybe not the 5 minute stuff, but the commonly available slow cure stuff is much tougher usually) might work. Good epoxy would also cure the present problem, it's too rigid to slack off and heat up like the CAPA just did. A simple positive mold could be used while it sets to prevent glueing the motor shaft in it for all eternity. :)
I like the simple poplar thrust collar, very good ultra simplification for a repstrap machine. Amazing how it is possible to cut complexity down sometimes!
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