Has the look of a double start to me.

Grind up a tool like a parting tool but with clearance so it does not rub on the helix of the thread. First thread I ever screwcut was a 1/4" x 16 sq

To cut a 5tpi thread on your Super 7 with gearbox, you basically set the gearbox to cut 10tpi then alter the change gear train between mandrel and gearbox to double the revolutions of the gearbox input gear. Brian Wood's excellent book "Gearing of lathes for Screwcutting" has a chart showing that for the ML7 and variants with gearbox, 5tpi can be cut by setting gearbox to 10 tpi and fitting a 48T gear to the mandrel gear position. Brian posts on here regularly so I am sure he can confirm/deny the S7 would be the same. Presumably it would be.

As said already you would grind a square ended tool bit with extra clearance on the leading flank to accommodate teh helix angle of the thread. 5tpi thread has a pitch of .200" so tool needs to be .100" wide and thread depth is .100" Tool is plunged straight in, no angled topslide.

If your piece of bronze turns out to be a bit tough to machine, as it sometimes can be in certain grades, an old dodge is to rough the thread out as a V thread, taking care not to exceed the width or depth specs of the finished thread, then finish it off with the square tool.

You might cut a few practice square threads in some scrap material before tackling the main job.

Edited By Hopper on 23/05/2019 13:46:48

Edited By Hopper on 23/05/2019 13:48:03

Here's one I made earlier:

It's 8 tpi and about 1" diameter, cut as an exercise before I made the proper brake screw shafts.

The (HSS) cutting tool was simply a square parting off shape with the leading edge relieved to account for the helix angle. I made my tool a couple of thou less than the nominal 1/16" width. The thread is square, so you need to plunge straight in as the flanks are perpendicular to the shaft. Leave the top slide parallel to the lathe axis.

A square thread can bind on the crest, on the root and on the flanks. These are independent; fixing one has no effect on the others. Assuming you already have the matching internal thread the crest and root interference can be sorted by calculation, ie, select the correct OD for the blank and cut deep enough to provide some clearance at the root. That just leaves the flanks. That's why I made my tool slightly narrower. Having cut the thread you can advance the tool a known distance (a few thou) with the top slide and shave that amount off one flank. Try the nut for fit and repeat as required.

I can't help with the lathe settings (my lathe will cut 5 tpi) but you might be able to select 10 tpi and fudge the drive train by a factor of 2?

Andrew

I was informed during my apprenticeship that the crest clearance on all pitches and any tpi of square threads is a constant ten thou (0.010 ins), Jasons comment could be correct that does look like a two start square thread,which would be difficult and likely to damage the Myford gears,thats why lathes srewcutting charts start at a pitch that the lathe gearing can cope with.One way round this is to apply the drive to the leadscrew rather than the Headstock spindle. A similar approach is taken when milling threads on a universal mill, a short lead attachment was used.Again during my early days there was a regularly need to mill acme threaded shafts,so a temporary arrangement was fitted to the mill table,consisting of a slow speed motor driving via a vee belt to a pulley on the worm handle on a dividing head,the index plunger was removed. the cutter was about 5 inches dia teeth ground to acme angle and mounted on an arbour in a universal head, thread cut at one depth setting,quicker and more constant than screwcutting on a lathe.One way to make a screwcutting tool is use a round toolbit,grind the bit to the required thread angle then mount the tool in the tool post so that the tool can be rotated to the correct clearance by trial and error,then mark the tool so that the top cutting face can be ground at the correct angle.

I'm in the single start thread camp.

Bookbinding presses are intended to apply, and hold, high pressures. A multistart thread would mitigate against that.

Andrew

So, (at risk of teaching my grandmother) to bottom out the one start/two start debate:

With a Sharpie follow the crest of the thread around the undamaged bit for one full turn. As you come back round it will be completely obvious whether you are one thread or two threads (or more!) along the work.

HTH Simon

That's not very nice.

But I wouldn't use the CNC mill, I'd screwcut it on the lathe – it'd be quicker.

Andrew

The 2.5 tpi setting requires me to transpose a couple of gears in the drive train. I did a quick experiment with the correct gearbox settings but not transposing the gears, which equates to 10 tpi. At 40 rpm it seemed pretty sloooow. So 40 rpm and 2.5tpi would be fine using the conventional half nuts. With the Ainjest I'd probably be up at 85rpm or more. The problem isn't stopping but starting, as you need to get the drive fully engaged before the carriage starts to move significantly. I suspect one may well get away without needing a travelling steady.

Andrew