The starter has a machined aluminum flange that allows it to be clocked in four positions. However, even when clocked all of the way up, the starter was hitting the merge collector. D’oh! I unbolted the motor from the bracket, rotated it and confirmed that it could be clocked enough to clear the merge collector if new holes were machined.
Two holes shouldn’t be a big deal, right? Not exactly. The new holes intersect the existing holes requiring the existing ones to be welded closed and machined. In addition, the geometry requires both the shoulder that indexes the bell housing and the mounting holes to be machined on an arc that’s concentric with the motor’s shaft. Unless you can write your name on an Etch A Sketch (if you’re too young to know what one is Google it), the best way to accomplish this on a manual mill is with a rotary table.
6” rotary table mounted to the X/Y table
I had been thinking about buying a rotary table for a while so this was a good excuse to get one. Centering the table under the spindle with an indicator mounted to an arm is a real pain-in-the-ass because the gauge rotates with the spindle which makes it hard to read. After doing a little research I bought a coaxial indicator. You hold the long horizonal arm (or let it bind on something) to keep the gauge from spinning. Mine supports a max of 800 RPMs. Anything over a couple of hundred RPMs provides instantaneous feedback to changes, making it easy to figure out which handwheel to rotate in which direction to get everything centered.
(Left) indicator mounted to an arm (right) coaxial indicator
The next challenge was centering the part on the rotary table. Centering the rotary table was relatively easy because it’s mounted to the X/Y table and the milling machine’s handwheels reliably move the table one thousand of an inch along the X or Y axis. However, adjustments to the bracket need to be done with your hands and even when you get everything centered you can inadvertaly nudge things when clamping the part.
Custom mounting plate and three-jaw, self-centering chuck
Since the starter bracket has a large round opening the solution was to purchase a three-jaw self-centering chuck. To mount the chuck to the rotary table I fabricated a mounting plate from 3/8” aluminum and affixed it with three M10 flathead screws. To center the chuck on the rotary table I did the following:
Mounted a precision-ground 3/4” stainless rod in the spindle with a 3/4” collet. A drill chuck isn’t as accurate.
Positioned the rotary table/chuck assembly under the rod and lined it up by eye.
Lowered the rod into the chuck.
Tightened the chuck on the rod. Since the adapter plate/chuck haven’t been bolted yet this perfectly centered the chuck on the rod.
Bolted the adapter plate to the rotary table.
Zero’d the DROs.
3/4” precision-ground stainless steel rod used to center the chuck on the rotary table
To indicate the hole circle I positioned the X-Y table such that a tight-tolerance drill bit could be plunged, with the machine off, into one of the existing holes via the quill. To ensure that everything was concentric I rotated the table 180 degrees and validated that I could plunge the bit into the opposite hole. I then determined that the new holes should be 31 degrees from the closest existing hole (the one in between had been welded closed).
Now that everything was fixtured and centered, it was time to stop fiddling and to make some chips so I:
Deeply countersunk the two rear holes to allow the weld to fully fill them.
Welded the holes closed from both the front and back.
Removed the excess weld beads:
Faced the back of the flange to ensure that it sits flat against the motor.
Faced the top of the shoulder to ensure that it sits flat against in the recess in the bell housing.
Machined the OD of the shoulder to ensure that it’s concentric with the hole in the bellhousing.
Drilled the new holes from the back.
Countersunk the new holes from the front so that the screw heads cleared the bell housing.
Deburred all of the edges.
The back has been machined and the front is about to be be machined. The weld beads were intentionally large to ensure that the prior holes and countersinks were completely filled.
The new hole has a button head screw in it. The old hole and countersink were between it and the next hole, but you can’t tell they were even there which was the whole point of this exercise.
I wasn’t able to get the bracket on the starter once everything was done. WTF?… After a brief panic I realized that I didn’t machine the relevant surfaces. It was a tight fit before and the welding process must have slightly distorted things. Fortunately I had a flap sanding drum that perfectly fit the hole and with a little tweaking everything fit again.