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4th axis harmonic drive build

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JayMcClellan:
Thanks PK. The continuous output torque rating of this gearbox is 60 N-m = 8496 oz-in.  Momentary start/stop torque rating is about twice that, and peak torque (shock load) is about 3x. Theoretically at 3A drive current this stepper can deliver 1.9 N-m = 269 oz-in of input torque, so with a gear ratio of 45x  that means 85.5 N-m = 11,400 oz-in of output torque, which is 40% over the gearbox rating. I'm only driving it at 2.8A but it could still over-stress the gearbox by about 30% if the output were stalled at full power. However the gearbox has a 3x safety margin for peak torque so it should not cause any damage if it's only done occasionally (crash!) and not in regular use. In normal operation I should be able to get the full 60 N-m of torque and if it has a typical torque curve the stepper should deliver the required 1.3 N-m of input torque at 2.8A up to roughly 500 RPM which gives an output speed of about 10 RPM. On a 2-inch diameter part that's a cutting feed rate of about 60 inches = 1500 mm per minute, a very usable range for most materials. And that should be WAY more torque than I need to resist cutting forces with the small cutters I'm likely to use so I should be able to push the feed rate to perhaps twice that and still get maybe 30 N-m = 4250 oz-in of output torque. That's 265 lb of force at a 1" radius which is enough to snap most of the cutters I have. The stepper's output drops rapidly above 1000 RPM so I think useful max output speed will top out around 20 RPM. Sorry if that's too much information in answer to your simple question.

PK:
It looks nice and stiff too. Done any heavy cuts?

JayMcClellan:
No, so far the heaviest cuts were on the side of its faceplate, and that wasn't too heavy at about 10 thou. I think I might run into limitations of the 3-jaw chuck before limitations of the gearbox. My plan is to make a secondary faceplate that mates to this one in place of the chuck and has T-slots, which would permit some heavier milling. I might mill the T-slots with it mounted on the 4th axis so they align with the center, haven't decided about that yet. Actually most of the projects I have in mind for it in the near future would be in wood on the CNC router, where it would be unlikely to push the limits of stiffness. I just built it heavily so I can use it on the mill too, but it won't be much more than an indexing head until I get the CNC conversion done on the mill.

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