Archive for July 22nd, 2019

Mini-Lathe Cut-Off Tool Holder: Improved Clamp Screw

Apparently all mini-lathe cutoff tool holders suffer from the same problem:

Lathe Cutoff Tool - OEM swaged screw
Lathe Cutoff Tool – OEM swaged screw

The clamp tightening screw is made from butter-soft Chinese steel with a swaged hex socket. As you’d expect, the hex wrench eventually (as in, after a few dozen adjustments, tops) rips the guts right out of the socket.

The screw has a M6×1.0 mm threads, but the thread around the hex recess is left-handed. While I could, in principle, print a 127 tooth change gear, rebuild the lathe’s banjo to accommodate it, then single-point a backassward M6 thread, it’s easier to just use a standard socket head cap screw:

Lathe Cutoff Tool - rebuilt screw
Lathe Cutoff Tool – rebuilt screw

The clamp screw passes through the block at an angle:

Lathe Cutoff Tool - blade view
Lathe Cutoff Tool – blade view

Fortunately, the screw is perpendicular to the angled side over on the left, making it easy to clamp in the Sherline’s vise:

Lathe Cutoff Tool - aligning to screw
Lathe Cutoff Tool – aligning to screw

Using the laser aligner seemed like a good idea at the time, but the top of the screw wasn’t particularly well-centered on the hole’s axis. I couldn’t screw the left-hand part (with the socket) in from the bottom and center the block near its surface, because then I couldn’t extract the screw before proceeding.

I used a diamond burr to grind out a flat for the screw head:

Lathe Cutoff Tool - clearing screw recess
Lathe Cutoff Tool – clearing screw recess

The flat came from about twenty manual G2 I-2.5 full-circle passes, stepping down through the hard steel block 0.1 mm per pass, at a too-slow 4000 RPM and a too-fast 30 mm/min feed, with plenty of water squirted from one side into a shop vac snout on the other. The doodle in the background of the first picture shows a first pass at the layout, with the burr centered at X=-2.5; I actually did the grinding from X=+2.5 so most of the passes started in thin air.

The screw head started just shy of 10 mm OD and the burr just over 5.2 mm, so the ensuing 5 mm circles created a flat barely large enough. If the flat were perfectly centered on the screw axis, I wouldn’t have had to grind out another millimeter on the left side (toward the bottom of the tool holder body), but it worked out OK:

Lathe Cutoff Tool - 6 mm SHCS test fit
Lathe Cutoff Tool – 6 mm SHCS test fit

The trial fitting also showed the head stuck out ever so slightly beyond the far side of the block, where it would interfere with the blade, so I turned off 0.4 mm off its OD.

If I had a 50 mm SHCS in hand, I’d have used it. Instead, I extended the threads of a 75 mm screw, then lopped off the end to the proper length. I’ll spare you the ordeal, including the moment when I reached for the cutoff tool to shorten the screw. A bag of such screws will arrive shortly, in preparation for future need.

Now the [deleted] cut-off holder works the way it should have from the beginning.


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