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Square thread cutting

Square thread cutting

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not done it yet01/04/2020 13:15:26
4514 forum posts
16 photos
Posted by old mart on 31/03/2020 20:35:46:

Since Raglan seem to be uniquely unfortunate in specifying a square thread for your lathe spindle which has no inherent self centring qualities, you need to actually measure the registers and compare the results to the drawings. The rear one is 1.750" on the spindle and the front one is 1.562". Any reduction due to wear will have to be compensated for when the backplate is made.

On the other hand, perhaps they were uniquely fortunate that the registers were the only centring feature and the square threads avoided any interference from any “self-centring” thread form. Cheapness does not always lead to the highest quality resolution for any particular facet of manufacture.

Edited By not done it yet on 01/04/2020 13:15:53

old mart01/04/2020 14:17:42
1564 forum posts
136 photos

The Smart & Brown model A at the museum has a number of backplates to screw onto a conventially threaded spindle (1 3/4 X 8 W), the worst one has at least 0.020" clearance in the register. It repeats just as well as the tightest one which has 0.0005" clearance. With a square thread, the worst one would run +- 0.010" every time it was fitted.

not done it yet01/04/2020 18:01:08
4514 forum posts
16 photos

Your S&B back plates seem very inaccurately/poorly made or very much worn away. My Raglan and back plates are very much OK, thanks. They all repeat very precisely.

But there again, perhaps you are comparing apples with oranges. Does a myford repeat that well with back plates so variable and slack? Further, I do wonder after your reply, why lathe manufacturers bothered with registers.🙂. They clearly didn’t know as much about lathe manufacture as you seem to know. How on earth have they managed to get it so wrong for so long! Good day.

Andrew Tinsley01/04/2020 19:34:06
1074 forum posts

Odd that the late George Thomas was quite a fan of a loose register on the back plate. Following in the footsteps of Commander Barker who used the method for very accurate work.


old mart01/04/2020 20:45:51
1564 forum posts
136 photos

The odd thing about the tight register, 0.0005" backplate, which is one that I made before I saw the light, is that it is a swine to get on and off simply because I made it too good. A clean Myford plate will repeat just as well as any other, the angle of the threads ensure that the plate finds the same position as it comes into contact with the shoulder. Since we have at least ten things that screw onto the museums lathe, I know what I'm talking about.

John Reese01/04/2020 21:09:40
838 forum posts

To answer the original question: A grooving tool us not suitable for cutting square threads because there is insufficient clearance on the leading edge of the tool. The clearance on the leading edge must exceed the helix angle of the thread.

old mart01/04/2020 21:57:27
1564 forum posts
136 photos

The helix angle for 1.75 X 6 is 2.73 degrees, I think that if one of the grooving tools which holds the MGMN02 type insert was used, it would only need a little relieving on one side. If I was doing it, I would use an insert for aluminium, if the backplate was cast iron.

Edited By old mart on 01/04/2020 22:04:32

George Jervis02/04/2020 09:30:04
73 forum posts
42 photos

Hi Old Mart,

Your link you put up is the type of tool holder I was considering but wondered if the cutting tip would stay in place while cutting?

I think I'm going to do as Simon collier suggested and use a suitable bar and drill and tap to hold a cutter. I'll modify a 16mm boring bar shank to suit

Many thanks again for all your help and advise


old mart02/04/2020 15:49:02
1564 forum posts
136 photos

The tips in these grooving cutters are clamped in with a screw. I am unable to get hold of one of mine as the museum is shut for the foreseeable future. The first backplate I made, was using a standard threading bar and cutting towards the chuck. I used a time tested method which is much safer to use, I turned the spindle by hand using a home made handle in the left end. This gives time to think what you are doing and avoids the risk of a crash. Your backplate will have to be made with it turned the other way round relative to the drawing.

Nigel Graham 202/04/2020 21:30:58
590 forum posts

Old Mart -

Would you also recommend turning a run-out groove at the end of the threaded portion first? I prefer to do so on external threads, but I have rarely tried internal thread-cutting.

old mart02/04/2020 22:28:57
1564 forum posts
136 photos

On this particular Raglan lathe, the picture of the backplate shows the equivalent of runout grooves at both ends of the thread, which is helpful. A runout groove can usually work on other designs, I would use one if I could. The Smart & Brown design doesn't require one due to the design, and I don't think Myford would either.

old mart03/04/2020 20:42:35
1564 forum posts
136 photos

Here's a better picture of one of the grooving tools, this one is 1.5mm width and 16mm diameter. It gives an idea of the depth of groove obtainable. I would not buy this particular one as the price is outrageous.



Edited By old mart on 03/04/2020 20:45:07

old mart03/04/2020 21:28:50
1564 forum posts
136 photos

Would you believe it, look what I saw on ebay:


Simon Collier04/04/2020 02:04:16
339 forum posts
55 photos

Since this thread has been running I have been making a mate a reverser screw for his locomotive. It is 5/16 diameter, two start square left hand. Each start was 8 tpi. The tool was about 1/32 wide. When I broke my cross drilled tool on the bms, I ground up a tool from 5/16 HSS tool blank, cutting out a block with the Dremel, and grinding to finish. Minimal side clearance for strength. Note that the toolpost is angled to the helix angles , which is quite a bit at 8 tpi. I switched to free machining steel, sacrificing hex, and put on modest cuts of 1-1/2 thou. Cut 40 thou deep. After the first start, I moved the top slide .0625 and cut the second thread.

The point is, it is very easy to grind up a tool for a job. Some model engineers, not necessarily the o.p., seem to be addicted to insert tooling. I wish they would grab a HSS blank and have a go at grinding a tool or two. They might be surprised how easy it is.




Andrew Johnston04/04/2020 09:18:34
5426 forum posts
631 photos
Posted by Simon Collier on 04/04/2020 02:04:16:

I wish they would grab a HSS blank and have a go at grinding a tool or two. They might be surprised how easy it is.

There's no need to waste time grinding to shape. If a lot of material needs to be removed (like the tool shown) use a mill to shape and then grind to add reliefs.


Simon Collier04/04/2020 10:35:29
339 forum posts
55 photos

I cut the bulk out with the Dremel, in a block. I wouldn't want to grind out all that. I am not thrill seeker enough to try to mill HSS. Would you use carbide cutter and high revs?

Andrew Johnston04/04/2020 10:58:41
5426 forum posts
631 photos
Posted by Simon Collier on 04/04/2020 10:35:29:

Would you use carbide cutter and high revs?

A carbide cutter is a must. General rule of thumb is run at the same speeds as ordinary steel, but with small DOC and high feedrate. Ideally the shear zone needs to be red hot. Just to prove it works here's a toolbit being milled:

embryo cutter.jpg

In this particular case the milling was to achieve an accurate involute profile rather than bulk material removal, but the principle is the same.


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