Reamer Regrinding

In the near future I'll need to make a bunch of things with tight-tolerance 22 mm bores, so instead of buying a 22 mm reamer, or just boring the holes like a normal person, I tried out re-grinding a 7/8" reamer (22.225 mm) reamer down to 20 mm by setting up my toolpost spindle as a toolpost grinder.

Here's the setup:

The reamer is set up between centers, and the toolpost spindle has a 4" grinding wheel on an arbor.  There's a mist-coolant dispenser to keep everything cool and lubricated.  The center on the chuck-side was turned in place out of some steel scrap, to ensure its concentricity with the spindle.

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All together it turned out pretty well.  I turned down the shank and ground it as well, so it will fit in the lathe's drill chuck.

I overshot my target 22 mm by 5 microns, but that's not too shabby, considering the ticks on the lathe's cross slide are 25.4 microns each.  It seems to cut a tiny bit oversized, so it works out in the end to give me a very tight slip fit onto a 20mm ground shaft.

This method leaves no relief behind the cutting edges, so as you might expect, they drag a little bit.  The surface finish inside the bore is okay, but definitely not as nice as I'm used to getting with reamers.  Over all, this was definitely way more time and effort than just boring the holes like I normally would, but it was a fun exercise, and good to get some experience using a toolpost grinder.

Toolpost Spindle, Part 2

I've mostly finished the toolpost spindle.

I found the perfect spindle motor on All Electronics, of all places, for $17.  It's a very cute brushless inrunner.  I dyno-ed it at 500 watts peak output at 48V, running of a 17A sensorless e-bike motor controller.  I forgot to take a picture of the inside, but the rotor has a thin steel sleeve around it, so the magnets won't fly off the rotor at high speed.  So I'm not worried about running it at 48V, even though it's nominally a 24V or so motor.

Unfortunately, none of my motor controllers are quite the right for this thing.  I wanted ~48 V, 10-20 amps, with hall sensor position feedback, and closed-loop speed control.  Fortunately, Charles and Bayley recently acquired a massive pile of mostly-not-working hoverboards.  The motor controllers in hoverboards pretty much exactly fit the bill.  They even do closed loop speed control with hall feedback!  I did a little investigation, and it seems like they run ST's motor library, doing dual-FOC on an STM32F103.  At low, speeds, they block-commutate, and at high speeds the phase currents become sinusoidal once they can interpolate between hall edges.

Phase current at low speed, scoped with the LEM-Stick:

Phase current at higher speed:

Fortunately, other people have figured out the serial protocol the hoverboard controllers use, so it was fairly straightforward to get it spinning the motor.

Unfotunately, the hall effect sensors in the All Electronics motor had very advanced timing, in the wrong direction for my application.  I opened up the motor, broke the glue holding down the hall sensor PCB, and reversed the timing:

I CNC milled an aluminum connector housing, for a DB25 connector.  Each phase has 6 parallel pins, plus 5 pins for hall sensors:  The steel front plate was made with a Bridgeport and hand files to match the motor curvature.  The 2-stepped HTD pulley was done on the CNC mill.

I made a height adjuster out of a chunk of steel.  I still need to make a nice thumbwheel for it.

I shoved a Nucleo and the hoverboard controller into the shell of an old G5 Mac Mini.  The power button on the left also came from a hoverboard, and I found an appropriately labeled knob to stick on the speed potentiometer:

Here it is mounted on the toolpost.  The spacing between the two pulleys is fixed, and I was able to choose the two sets of pulleys to have almost exactly the same center distances and belt lengths, so there's no need for a belt tensioner:

I've used it for a few jobs so far., including in-place drilling a bolt patterns into a big pulley and hub I turned, and also for deburring some internal ring gears:

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.