Precision Manufacturing with Mini Mill

[aldaeron]

Hi CD,

Our team has a mini mill with a DRO and were interested in making some simple plates for gearboxes. Since this machine is not as robust as some larger machines I have used in the past, plus it has been used by students for a few years, I am concerned that we cannot repeatably attain the accuracy needed for cutting mounting holes for bearings/CIM and getting the correct spacing for spur gears.

The typical gear spacing formula for 20 DP gears I have seen here many times is: (Total # teeth from both mating gears)/40 + .003.

For a 84T spacing this distance would be 2.103, for 96T it would be 2.403 (Vex demonstrates this here).

Vex bearings are nominally 1.125 (Hex are 1.124)

A CIM mounting boss is 0.750 +/- 0.002

What we want to do is accurately cut a mounting hole for the CIM and a bearing to support a spur gear on some 6061 aluminum bar stock 3/16 or 1/4 thick.

Is the feasible to do this repeatably on this mini mill? What tolerances are acceptable on the mounting hole sizes and between centers? How would you measure these distances (if at all)? For cutting the bearing mounting holes, should the final cut be with a 1-1/8 endmill or should we try to use a 1-1/8 reamer? A stock reamer is way too long to fit in our tiny mill – can we cut the shank down?

Any help is appreciated, especially if you have experience with precision manufacturing on one of these mills. We typically use it for simple operations. We do not want to attempt this if we cannot meet the tolerances and end up spitting out gear teeth. We have been fortunate to get offers from other teams to have parts made on their CNCs, but we are trying to learn to do this in house.

Thanks!

-matto-