While pondering a project requiring a slitting saw, I discovered the clamp on the dial test indicator magnetic mount I’d picked up a while ago didn’t quite fit the 5/32 inch = 4 mm stem on the indicator. The clamp ring is obviously punched from sheet, then formed into its final shape, as the holes are somewhat un-round. Running the proper drill through the holes removed a minute sliver of steel:
And then it fit just fine:
Although it looks like I’m in the process of sawing the ball off the indicator, I’m really measuring the runout, which turned out to be maybe 5 mils = 0.13 mm. The blade is likely too small for what I’m thinking of using it for, so the pondering continues.
The two bigger holes in the clamp fit the equally standard 3/8 inch = 9.5 mm stems just fine, so it’s just another one of those tools where I get to finish the last few percent of their manufacturing.
That’s the V4 R3 version, although I bought it from Makergear rather than fight with all the support required to get a proper bearing opening.
The long M4 screw and spring apply a constant force to the filament against the drive gear, rather than the constant position from the default (and much shorter) stock screw. The lever arm does have some springiness, but not much travel, so IMO the spring works better with the fine teeth in the drive gear.
This drive has a 5 mm hole at the top for the stock PTFE guide tube, which I long ago replaced with ¼ inch OD HDPE tubing to reduce the friction required to get the filament off the spool and into the hot end. The rather hideous hot-melt glue blob holding a ¼ inch ID tube onto the previous drive never failed enough to bother me, but a little lathe action produced a much better adapter:
It’s a chunk of ⅜ inch = 9.5 mm Delrin rod with a 2.4 mm hole through that 5 mm spigot for easy extraction of a gear-mashed 1.75 mm filament. The other end has a 6.5 mm hole drilled 20 mm deep to hold the guide tube.
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 …