Screw cutting problem

[Michael Cooper 5Participant @michaelcooper5]

Posted by Hopper on 18/03/2022 08:20:44:

Posted by Michael Cooper 5 on 17/03/2022 22:29:56:

When cutting threads the basic set up should go something like set the compound slide to half inclusive angle.Set dial on compound to zero.If possible turn a root diameter at either the start or exit of the thread using cross slide.When the root diameter is achieved set cross slide to zero.Clear tool from work by backing off just enough using compound slide.Select RPM and touch on work using only the compound slide.Wind down the bed till your of your work.Set your first cut on compound and and chose a number on your chaser dial and engage.At end of each pass(remember to disengage) clear work using cross slide and wind down bed past the work to the beginning.Wind in cross slide to zero , put cut on compound slide and engage on same number.Repeat until compound reads zero.I would set up like jasonB as it cleans up trailing face nice.

Well, that's one way of doing it, out of many as discussed above. I think maybe there are as many ways as there are machinists!

Unfortunately the OP's lathe, an ML7, does not allow you to rotate the topslide to half the thread angle. It is limited at 45 degrees from the lathe axis so can't make it to the required 61 degrees or so. So he is limited to either straight plunge cutting or the common method used in industry of advancing the non-angled top slide by half the amount of infeed.

Oh, my apologies,I didn’t know the Myford had this limitation on its compound slide.Thanks for the Lesson 👍

[Anonymous]

Posted by Hopper on 18/03/2022 08:27:57:

Yes I have almost always found screwcutting to be a "fit and try" effort the final five thou or so…

Exactly, I use thread depth as a guide, but use a mating part as a gauge for final fit. I aim to achieve a shake free fit, and agree that a couple of spring passes can be the difference between go and no go. For brass and cast iron a brush to remove swarf dust also helps. I rarely use dies, many external threads are screwcut, or done using Coventry dieheads.

I screwcut a lot of 32 and 40 tpi ME threads, where thread depths are small, A few thou can be the difference between a shake free fit and a rattling poor fit. My ME taps and dies are old and carbon steel, so it's not clear that they are particularly accurate. Consequently trial fits are essential.

My original comment in response to Clive's comment was partly tongue in cheek. My lathe is ex-industrial and I have an Ainjest unit fitted. The unit was bought on a whim, just to experiment, but I've found It makes screwcutting, especially blind internal, much easier, and faster. So I don't see it as cheating. I use techniques that are appropriate to the machines that I have. I distrust people who are dogmatic about the right, or wrong, way to machine something. I recall a forum member telling me that CNC milling wasn't really true modelling, until they got a CNC mill that was.

Andrew

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