That’s the 5 mm punch, where being (at least) half a millimeter off-center matters more than it would in the 32 mm punch.
Unscrewing the painfully awkward screw in the side releases the pilot:
The debris on the back end of the pilot is a harbinger of things to come:
Looks like whoever was on spring-cutting duty nicked the next coil with the cutoff wheel. I have no idea where the steel curl came from, as it arrived loose inside the spring.
Although it doesn’t appear here, I replaced that huge screw with a nice stainless steel grub screw that doesn’t stick out at all.
Chucking the pilot in the lathe suggested it was horribly out of true, but cleaning the burrs off the outside diameter and chamfering the edges with a file improved it mightily. Filing doesn’t remove much material, so apparently the pilot is supposed to have half a millimeter of free play in the handle:
That’s looking down at the handle, without a punch screwed onto the threads surrounding the pilot.
Wrapping a rectangle of 2 mil brass shimstock into a cylinder around the pilot removed the slop:
But chucking the handle in the lathe showed the pilot was still grossly off-center, so I set it up for boring:
The entry of the hole was comfortingly on-axis, but the far end was way off-center. I would expect it to be drilled on a lathe and, with a hole that size, it ought to go right down the middle. I’ve drilled a few drunken holes, though.
Truing the hole enlarged it enough to require a 0.5 mm shimstock wrap, but the pilot is now pretty much dead on:
Those are 5, 6, 8, and 10 mm punches whacked into a plywood scrap; looks well under a quarter millimeter to me and plenty good enough for what I need.
The stiffness of the bike helmet mirror mount suggested a similar clamp would have enough griptivity to immobilize the ball while cutting it in the lathe:
Building the clamp around the lathe’s three-jaw lathe chuck eliminates the need for screws / washers / inserts:
The Ah-ha! moment came when I realized the fixture can expose half of the ball’s diameter for drilling while clamping 87% of its diameter, because 0.5 = sin 30° and 0.87 = cos 30°:
That’s an orthogonal view showing 13% of the ball radius sticking out of the fixture; it’s 6% of the diameter.
Which looks like this in real life:
The socket is offset toward the tailstock end of the clamp (on the right in the picture) to expose half its diameter flush with the surface perpendicular to the lathe axis. The other side necks down into a cylinder of the same diameter to clear the drill bit.
This works nicely until the ball diameter equals the chuck jaw’s 20 mm length, whereupon larger balls protrude into the chuck body’s spindle opening. Although I haven’t yet built one, the 25 mm balls in my Box o’ Bearings should fit, with exceedingly sissy cuts required for large holes.
The fixture doesn’t require support material, because the axial holes eliminate the worst of the overhang. Putting the tailstock side flat on the platform gives it the best-looking surface:
The kerf between the segments ensures the jaws can apply pressure to the ball, whereupon the usual crappy serrated 3D printed surface firmly grabs it.
The fixture is a slip fit on the chuck jaws:
Tightening the jaws shoves them all the way into the fixture’s slots and clamps the ball:
Overtightening the chuck will (probably) compress the ball around the drill, which will (best case) give you slightly oversize holes or (worst case) cause the ball to seize / melt around the drill bit, so sleaze up to the correct hole diameter maybe half a millimeter at a time:
That fixture exposes 9.5 mm = 19/2 of the ball. The drill makes a 6 mm hole to fit the telescoping shaft seen above.
Obviously, you must build a custom fixture for every ball diameter in your inventory, which is no big deal when you have a hands-off manufacturing process. Embossing the diameter into the fixture helps match them, although the scribbled Sharpie isn’t particularly elegant.
Given the angle between the two plates, I didn’t see any way to put a large hole though the center of the ball:
A scrap of wood aligned the two plates somewhat better:
With that as a hint, the Box o’ Brass Cutoffs disgorged a better spacer, although the original screw was just an itsy too short:
Grabbing the modified vise in a machinist’s vise got me most of the way toward the goal:
Polypropylene is grabby, so the drill stuck / rotated the ball inside the vise / made a mess:
A close look at the top picture shows the nasty ring around the hole (on the right side). The vise grips the ball between two holes punched in the metal plates, contacting it only at the right-angle (-ish) edges forming two rings, so there’s really not enough friction against the plastic to hold the ball in position and any slippage results in a gouge. Perhaps pearls / beads / jewelry behave differently?
Fortunately, I had a bag of 100 balls, so a few failures gave me enough of a clue to do what I should have done from the beginning:
That’s silicone tape wrapped around a ball grabbed in the lathe chuck, with a center drill in the tailstock. There’s barely enough traction between the ball and the chuck to get the job done, but it worked out well enough to build a few new mirrors:
There’s obviously a better way, although it took a few weeks to shake out the solid model …
Contemporary vacuum cleaner dust brush heads have bristles in some combination of [long | short] with [flexy | stiff]. The long + flexy combination results in the bristles jamming the inlet and the short + stiff combo seems unsuited for complex surfaces. Shaking the Amazonian dice brought a different combination:
That’s the new one on the bottom and, contrary to what you might think from the picture, it is not identical to the one just above it.
In particular, the black plastic housing came from a different mold (the seam lines are now top-and-bottom) and required a new adapter for the Kenmore Progressive vacuum cleaner’s complicated wand / hose inlet, with a 3/4 inch PVC pipe reinforcement inside.
Early reports indicate it works fine, so I’ll declare a temporary victory in the war on entropy.
The general idea is to hold the wave washer (it’s mashed under the flat washer, honest) above those bumps on the plate holding the mirror and stalk balls. It’s a few millimeters from the end of a ¼ inch brass rod, drilled for the M3 screw, and reduced to 4.5 mm with a parting tool to clear the bumps.
While I was at it, I made two spare mirrors, just to have ’em around: