Cut RMS (royal Microscope Society) threads - Lathe or CNC mill

@nicksilva @TBJK @BobKarnaugh

Hey guys,
This is a tricky one. 36 TPI, 55 degree flank angle, major diameter 0.8000, minor diameter 0.770, and pitch diameter 0.7837…
Yeah little bitty tight threads.

   I’m thinking Al 6061.... HSS cutter 55deg



Nothing to it. Easy peasy

Seriously? Will the lathe be able to get those?

Yep, I confirmed its on the chart.

Thank you.

@TBJK @nicksilva

Will an insert for Whitworth thread angles fit our tools on the lathe? They are 55 degrees instead of 60 degrees.

Any recommendations for local machine shop supplies? I tried CDC and Grainger…

Not any local sources that I can think of. You may be better off if you grind your own out of HSS.


@TBJK @FreddyCalvert @BobKarnaugh @nicksilva

Anyone have experience with the Colchester gearbox…it seems the “W” and the “X” gears are tanked or can’t be engaged…SUCKY…

It’s an orange handle so…

I’d love to help…I’ve never looked at one so I’d need guidance.

Thanks in advance.

Meaning they don’t engage the lead screw or you can’t get the handle in the position?
If the lead screw doesn’t turn then the shear pin is likely toast. If the handle doesn’t want to turn, I’ve had to rotate the chuck to get it to engage/mesh.


Shear pin. I screwed around with rotating the chuck and trying other settings. Y is marginal. Z is good.

Thank you Tim.

I will try setting it this evening. It is not at all unusual to have to play with the power feed output rod while fiddling with the setup levers to get engagement.

Tim ran some threads yesterday. I was going to try to replicate his process, but my day got away from me.

     1) Is it okay to move the leadscrew speed selecting levers while the machine is on?
     2) When you say play with the power feed output you meed the leadscrew itself?
     3) What time this evening do you anticipate being around the space?



I was in yesterday from 2-9pm and I did try setting power feed speeds involving W and X. It can be set, but for one of them I had to wiggle the power feed rod a bit to allow the lever to set into the X position. The power feed rod is the round rod with a notch running its length, not the acme thread rod, which controls threading pitch. Do this with the motor off. Some folks giggle the lever while rocking the chuck back and forth instead of the power feed rod. Do this with the motor off.

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So got them cut…the M34 has some roughness…ideas on how I should’ve done them to get a smoother finish? Besides the obvious get a good machinist to cut them…where is the middle finger emoji when I need it? The part was 6061.

What cutter did you use?

Cutting aluminum? If you didn’t, I recommend WD-40 as the cutting fluid. Try to have everything as stiff as possible, as little overhang as you can arrange. Did you have the compound set at the correct angle, so nearly all the cut per pass was on one side of the thread?

I used my personal cutter. Ti Nitride coated if I’m not mistaken. My machinist said that was a mistake. He said I should always use carbide for Al. He said the Al sticks less to carbide and that the ribbon building up and catching was tearing things up. I used whatever oil was in the dippers. It was good enough for the test piece…

The dippers are color coded. Blue is WD-40, which really is a good cutting lube for aluminum, and the red tops are something else that is good for steel. A lot of people say the best results for AL is tool steel, but yeah, you would likely get better results with other coatings, or uncoated.