Precision Manufacturing with Mini Mill

[RoboChair]

Originally Posted by aldaeron

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.

If you have a DRO most of the hard locating work is done for you. You just need to make sure the gibs are adjusted well to keep it square and lock it in place during critical hole operations. If you need to make something larger than your bad travel you can use locating holes to re-zero or locating pins on the bed to shift it a known amount.

Your mounting holes should be drilled to no tighter than a close fit for your bolts, but can be as much as a free fit if you are having tolerance stack up issues.

For your 1.125" holes you have a few quick and dirty options that your mill can accommodate. Quickest would be to use a step drill such as this or this, they produce reasonable holes while requiring much less torque. You can then use Retaining Compound to help hold your bearings in place and fill any gaps in your bearing bores.
Another option is a fly cutter or boring head, both can easily make a hole close in size with little work and the boring head can be dialed in to exact size but takes time and skill. You can then finish up with a 1.125" or 1.124" reamer with a cut down and reduced shank.
This is the important thing about reamers, drill bits, and the like; almost none of them have hardened shanks, only the important bits get hardened. Just take a good file to the shank and if it is able to scratch and bite into it then you can machine it however you like. Somewhere down the shank it will stop being able to bite in and just skate across the metal and that marks the spot where you can no longer work the metal normally.
It is also important to note that reaming a hole does not require precise locating of the bit, it will center into your hole just fine and can be easily done as a second operation with a drill press, after all the precise machining is done. If your reamer is rigid and stiff when you go to ream a rigidly mounted part, any runout in your reamer will be transferred to your hole and it will end up larger than the reamer is supposed to make it. For this reason you will probably not want to ream your holes with your part rigidly mounted.