You can't set a standard ML7 topslide to 30 degrees from square to the lathe axis. It won't turn that far. When the graduations on the topslide say 30 degrees, it is 30 degrees from parallel to the lathe axis. Which is not where you want it for screwcutting.

With the ML7 you have to use the method common in industry, to save time resetting the topslide. Leave the topslide in the normal 0 degree position, ie parallel to the lathe axis. Then use the topslide to advance the tool by half of the depth of cut every time you put on a cut using the cross slide. So if you feed the cross slide in 10 thou, advance the topslide 5 thou,and so on. This by the miracle of sines and cosines and dumb good luck, moves the tool point in a path that turns out to be a very slight bit under 30 degrees, so perfect for screwcutting. It works for 60 degree and 55 degree threads.

For 29 degree Acme threads, you advance the topslide by a quarter of the infeed. So 10 thou on the cross slide and 2.5 thou on the topslide.

Super 7s and ancient Drummonds and just about every other lathe can pivot the topslide to the correct 30 degress from square, or 60 degrees from the lathe axis, but they forgot to do it on the ML7 for some reason. Cheapness of manufacture I would suspect. You can modify the ML7 with two strips of steel attached to the sides of the cross slide with holes drilled and tapped to take the topslide anchor bolts, putting them at 90 degrees from the original position. Too much faff. Everyone uses the above described method instead. Or just plunges straight in and hopes for the best.

Edited By Hopper on 13/03/2022 12:24:01

Edited By Hopper on 13/03/2022 12:28:21

I don't offset either but took these photos some time ago which illustrate Hoppers point about the angle as it crops up quite often

You want it like this

But I suspect you have it like this

I agree with Dave, Andrew and Jason; offsetting the compound slide seems a complete waste of effort to me.

And why only 0.004" cuts? You might just as well cut with a toothbrush

I never offset the compound, and it could complicate matters when learning. A first skim is useful to check if the pitch has been set correctly. I start with large cuts and reduce the depth after each pass, as the cutter is removing more metal as it gets deeper. The last cut may be 0.001" followed by a spring cut at the same setting.

 It would be a good idea to practice with some aluminium bar, turning off each thread for another go, it is easier on the tooling when learning.

Edited By old mart on 13/03/2022 21:36:19

if you have a retracting topside the set over method is a doddle and still only one knob to twiddle

+1 for a retracting topslide (with carbide)

This is a 3/4" BSF thread screwcut with a carbide insert plunging straight in:

Can't remember at what spindle speed, but not fast, probably 125rpm. Finish looks ok to me. Finish cuts would have been a couple of thou DOC.

Andrew

Er, i know lots of people like the topslide parallel but that's not what it is for. Lots of early lathes didn't have one so it was added to enable tapers etc, You should be able to set it over and leave it there, except for the infernal nuisance of Whitworth and BA being different from unified and metric.

I just reminded myself that the Mach3 threading wizard has an infeed angle parameter you can set. For metric/UN threads this is set to 29 degrees. Obviously there is no topslide involved but the wizard will make a horizontal offset on each cut so the tool effectively advances at that angle.

Here's a thread I cut for an ER16 collet closer using a home ground HSS tool, speed ~215 rpm. Despite what they say Mach3 can thread perfectly well.

Might have changed since the early lathes!

In Jason's picture, where he shows which way round the top-slide is angled for screw-cutting, it can be seen the QTCP hasn't been turned yet to present the cutter at a right-angle to the job. The QTCP is in it's normal position at right angles to the top-slide.

I have a 4-way tool-post rather than a QTCP and it's accurately positioned at a right-angle to the top-slide by a spring-detent. Definitely designed to be used at right-angles, with a positive lock. The tool-post can be clamped at any other angle to the top-slide but the friction grip is reduced and the detent inoperative : not ideal for heavy cutting, but plenty good enough for threading.

An advantage of having the top-slide at an exact right-angle is the tailstock is notched to clear it. The extra space is used to widen the saddle for extra stability.

There doesn't seem to be any advantage in angling the top-slide when screw-cutting with a 'modern' lathe. (i.e the boxy looking machines designed with an eye on carbide a decade or two after WW2.) You can either go straight in to screw-cut, which is what I usually do, or reduce the cutting load by periodically advancing the top-slide slightly on repeated passes.

Not doing a hard-sell on straight-in threading as being the best way to screw-cut on all machines! I think it depends on the condition and design of the lathe, the cutter type, and the material.

Dave