March 30, 2017

Lathe Toolpost Spindle

I've been thinking for a while it would be nice to have live tooling for the MITERSlathe, for things like grinding and indexed drilling parts like hubs.

So far I've made everything out of random bits and pieces found at MITERS.  For the spindle, I started out with this old R8 to ER16 adapter:

I meticulously turned the shank down to 20mm.  I used sandpaper and Scotchbrite to bring the shaft to within a couple tenths down the whole length.

This was an incredibly satisfying measurement.

The bearings lightly press on the shaft - it's a tight enough fit that you just barely need an arbor press, but you can still easily un-press the bearings without damaging them.

Here's some of the housing machining.  I started out with this huge brick of mystery-steel I found on the floor of MITERS.  I got really lucky - it happened to be perfectly sized for machining a tool-holder,  and machined wonderfully.  It fly-cuts especially nicely, producing an incredibly shiny surface:

I squared up the block on the mill, cut the dovetail, and spot-drilled the spindle bore.

I did all the spindle boring on the lathe.  I started out by placing the housing on the toolpost, centering it vertically, and indicating the back face to be parallel to the travel of the lathe.  I put drills in the lathe's collet chuck, and use the lathe kind of like a horizontal  boring mill, to guarantee that the spindle bore was true to the  the lathe's travel:

To machine the bearing bores, I put the housing in the 4-jaw chuck and indicated in the drilled holes, as well as the faces, to make sure the housing was both square and centered.  I used this big indexable drill bit as a boring bar, because it has wonderfully sharp positive-rake inserts which produces a much better surface finish with low cutting pressure than the usual negative-rake turning inserts do.

I used a different bearing arrangement than I did with the tiny lathe, and I'm very satisfied with how it turned out.  I only had deep-groove ball  bearings to work with, rather than angular contact bearings, and I used a 3-bearing arrangement.  There's a pair of bearings at the front of the spindle, with a 0.002" shim between their inner races.  When the spindle housing is assembled, the outer races get squeezed together, so the shim provides a fixed preload.  This puts the bearings in a face-to-face arrangement, making the spindle somewhat tolerant to misalignment of the back bearing, if there is any.  The back bearing is a very close slip fit into the housing, and the outer race is lightly preloaded outwards by a wavy washer, to keep the balls in contact with both races.  Finally, there's a spacer between the inner races of the front and back bearings.  When the bolt on the back of the spindle is tightened, it squeezes down on the inner races and spacer, locking everything together.

Over all, I'm much happier with how this arrangement worked out compared to the lathe spindle.  Since preload is fixed, it takes no fussing to get the spindle to spin smoothly without slop, assuming the shim is sized correctly.  Also, since the front bearings are so close together, the preload will hardly change as the spindle heats up and expands.  For the lathe, I left the spindle on for a while, and pre-loaded the bearings with the spindle at normal operating temperature.

A plate on the front of the housing presses the outer races of the bearings together, to preload the bearings:

To test it out, I threw a Lovejoy coupling on the back, and a mating one on the output shaft of an old orbital sander motor, which was roughly appropriate speed.  Conclusion:  the spindle part works great, and, unsurprisingly, the orbital sander motor was terrible.  Needs more brushless servo.

Here are the two mounting configurations.  I tested it out with a 3/8" endmill, and the spindle was rock solid, producing an excellent surface finish without any signs of chattering.

Still to do:
- Find a better spindle motor, and make a motor attachment and height stop.
- Make an indexing system for the lathe spindle, so you can precisely lock the spindle to certain angles.

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