Archive for February 17th, 2017

Screw Cutting Fixture vs. Lathe Ways

A length of aluminum hex bar became a nice 10-32 screw trimmer:

Screw cutting fixture - 10-32 - first cut

Screw cutting fixture – 10-32 – first cut

The hex neatly fits a 5/8 inch wrench, so I can tighten the jam nuts enough to run the lathe forward, part off the screw, and clean up the end just fine.

Unfortunately, the second test cut didn’t work nearly so well:

Screw cutting fixture - 10-32 - wrecked

Screw cutting fixture – 10-32 – wrecked

With the cross-slide gib adjusted to the snug side of easy, the cut put enough pressure on the parting tool to lift the way on the tailstock side about 4 mil = 0.1 mm. The parting tool submarined under the cut, dislodged the fixture, and didn’t quite stall the motor while the chuck jaws ate into the aluminum.

Well, that was a learning experience.

After tightening the cross-slide gib to the far side of hard-to-turn:

  • Put a longer screw in the fixture
  • Grab it in the tailstock drill chuck
  • Crunch the hex end of the fixture in the spindle chuck
  • Remove the screw through the spindle (*)
  • Put a slight taper on the end of the fixture threads with a center drill
  • Deploy the live center to support the fixture

Like this:

Screw cutting fixture - 10-32 - rechucked

Screw cutting fixture – 10-32 – rechucked

Turns out that angling the bit by 10° dramatically reduces chatter. If I had BR and BL turning tools, I’d be using them with the QCTP set to 0°, but they weren’t included in the set that came with the lathe.

It’s a good thing I’m not fussy about the diameter of that cylindrical section:

Screw cutting fixture - 10-32 - reshaped

Screw cutting fixture – 10-32 – reshaped

I knew the craptastic lathe ways needed, mmmm, improvement and it’s about time to do something.

(*) By concatenating all my ¼ inch socket extension bars into an absurd noodle capped with square-to-hex adapter holding a Philips bit.

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