[ZZII 527]

Update on the A2 tool steel carrier plates:

After cutting them on the waterjet, finishing the pin holes with a 5/32" drill bit, and filing the double-D to be a close fit, but not a press fit, I tempered the steel today. Here was my process:

1. Heat at 950C (1742F) for 20 min.
2. Air cool (A2 is air-hardening tool steel) until warm enough to touch.
3. Temper immediately at 650C (1202F) for 2 hours. Air cool. Repeat temper for two hours.

The high tempering temperature brought the hardness down to a C40 on one plate and a C39 on the other. (Tensile strength around 182 ksi.) I am hoping that this won't be too brittle.

A2 holds tolerance well in heat treatment, so the press fit pin holes worked fine. The double-D was a bit tight, but after a bit of sanding they seemed to go on easy. One gearbox assembly went smoothly. Putting the two stages in first and then fitting in the shaft later is much easier than messing around with a press fit shaft and carrier. It ran smoothly with the motor, drawing about 2.5A. (Is this normal? Good? Bad?)

The second gearbox is still having alignment issues, binding slightly in particular positions. I think the carrier plate may need some more filing to get it on straight. Maybe a fresh look at it tomorrow morning will do the trick.

So it is indeed possible to make a working HRC40 carrier plate out of A2 tool steel. It's a nice steel, because it can cool slowly, not deforming much, and still harden. McMaster also carries 5/32" (4mm) stock, saving one machining step. But it is somewhat expensive and needs to be heat-treated to get to the right hardness, so it probably isn't the ultimate solution.

The most important thing I learned so far is that a press fit makes assembly (disassembly) and alignment VERY difficult, and so I wouldn't recommend it. If I cut more, I'll cut them exactly 9mmx12mm. If the plate can self-align in the motor, it will run much more smoothly. But your results may vary, of course.

For now, I think I'm done. As long as the second gearbox goes together okay and nothing breaks this weekend when we drive the heck out of it, I think I'll be happy. We are using 12:1, 1 CIM per side, so I don't think I need to make a sqare-hole version in anticipation of needing the harder shafts. But I might cut some just in case..