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Archive for January 30th, 2017

Screw Cutting Fixture: Not Quite Right

A stack of PCB proto boards arrived from halfway around the planet and, in a rare preemptive strike, I made some holders before I need them:

PCB Protoboard - in holder

PCB Protoboard – in holder

That called for trimming eight 4-40 screws, so I also tried a first pass at a screw cutting fixture:

Screw cutting fixture - 4-40 insert

Screw cutting fixture – 4-40 insert

That’s an ordinary 4-40 brass insert epoxied inside a drilled hole, with a snug 4-40 clearance hole on the other side.

If I needed slightly longer screws, they’d get a jam nut on the inside, but in this case I just tightened it firmly, ran the lathe in reverse, and cut against the far side to keep the screw from working loose:

Screw cutting fixture - in use

Screw cutting fixture – in use

The insert is on the left side of the fixture, just under the screw head.

Unfortunately, my faith in epoxy bonding led me astray: there’s just not enough griptivity to lock the insert inside that drilled aluminum hole. Despite my taking sissy cuts, the cutting forces pushed the insert out of its hole. I completed the mission by cutting the last four screws by hand.

The original idea for the fixture would have me turning the fixture from steel and tapping the screw hole. That puts a lot of labor into something that may get chewed up fairly quickly, so I wondered if a brass insert would suffice.

Not in that orientation, it doesn’t. Putting the insert on the other side of the fixture (to the right, away from the chuck) would have the cutting forces pushing it into the fixture, which should have been obvious from the start. So it goes.

When I must cut a larger screw, I’ll redrill that fixture, put the insert on the other side, and see how that plays.

If I turned the fixture from steel with similar drilled similar holes, I could braze / silver solder the insert into the hole: that would prevent it from turning, even if the jam nut wasn’t quite up to the task.

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