Metric Thread On Imperial Lathe

[RichardShute]

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If the angle is greater than the thread angle then the trailing edge is cut by the tip of the tool, (as the back edge clears the face) and is thus defined by the angle of the slide.
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Bill

Bill,
your first drawing is essentially the same a mine. I showed the saw toothing as a result of each successive cut taken - rather coarse I accept, but it was intended to accentuate the effect. I agree with your other two drawings, I was just too lazy to bother....

Richard

[BillTodd]

your first drawing is essentially the same a mine.

Yes it is, Richard. Sorry I missed that first time through

Of course, all this means is that me and every other Hardinge owner (the ones that follow the manual anyway) have been cutting slightly incorrect threads  (albeit with the greatest of ease).

Bill

[Ned Ludd]

It is regrettable that so few books show that what they mean by "1/2 the thread angle" does not in fact relate to the markings on the cross slide's quadrant. The half angle must be subtracted from 90 and that figure set on the quadrant. This is one of the problems of self taught machinists working in their sheds alone. They have no one to point out elementary errors, whereas those lucky enough to have mentors are blessed.

I must also point out it is a bit of a miss-perception that going straight in (not the set over top slide method) loads the tool on both sides. It does no such thing, once the cut is under way. Cutting a thread is no different to an ordinary sliding cut, just deeper, think about it! For an ordinary RH thread, the tool will only cut on the left side. The only time a threading tool would cut on both sides is when you do a plunge cut. As the old saying goes "unless you know different".
Ned

[Pete.]

Actually now I think of it what you produce using a 30°-off-parallel angle for the compound is a saw-tooth pattern on the right flank of the thread because the angle you're advancing at is shallower than the angle of the cutter. My CAD skillz are lacking so I'll knock-up a quick pencil-sketch.

[Pete.]

Ned I don't agree, if the tool is not loaded both sides what is cutting the right flank of the thread? I see feeding directly in exactly the same as plunge cutting except that you are continuing the plunge cut along the length of the thread, a 'running plunge cut' for want of a better term.

Anyway, here is my crummy sketch depicting the result of feeding in at 60° off perpendicular.

[BillTodd]

Here's a 1" 12tpi thread I cut the other day (@ ~400rpm with a full depth insert in BMS )

You can see the striations caused by the each pass. However, although it is cut at 31° (59° on my top slide), there are no obvious steps on the trailing edge (right-hand side).

[edit] Actually, now that I can see the tips clearly, it looks like I've over done the in-feed (or picked up the wrong insert??) . As it happens I'm going to have to slice off and re-cut the thing as a dual start thread (i.e. 6tpi) as soon as I get the appropriate change gears for the lathe.

Bill

[RichardShute]

Bill,
as you say, it looks fine. A 1 degree error is a near miss, but we were initially talking about a 31deg error and that would make a significant difference.

The saw toothing may be less obvious depending on the tip radius of the tool, but the reduced height of the thread profile would still be the same as each successive cut removes a little more of the crest with the trailing edge of the tool.

Richard

[Bogstandard]

I think you are all missing the major point here.

It really matters not one iota at what sort of angle you are coming in at. Half the thread form angle minus 1/2 degree from the cross feed motion is observed to be the optimum, as at that figure, you are keeping the trailing edge fairly clean cut.

But you should not be worried about what is happening on the trailing edge, purely because you should always finish your threading cut with a minute feed of the cross slide, which then cleans up perfectly both faces of the cut thread, as long as you have set the cutting tip up correctly with the gauge in the first place.

I stated that I use the compound at a shallower angle for cutting very fine threads, which is what I do.

When you set your compound over, you can control the depth of cut much better than by using the cross feed, and by setting it at say 5 degs from parallel to the spindle, you can use the feed of the compound to take parts of a tenth off the diameter of a bar during normal lathe turning operations.

I use that method, but using a slightly larger angle, so that I have much better control of the depth of thread when cutting fine ones.

Maybe not in the books, but it was shown to me many years ago by an old timer as a method to get superior results, and I adopted it. The final tiny cross feed takes care of any 'sharks tooth' effect on the trailing edge cut.

Everyone has their own methods of cutting single point threads, but there is no need to argue over it or shove it down everyone elses' throats because you think their method is incorrect.
Make your methods known by all means, and if someone wants to take it on, then they will do, but for other people who have their own methods of doing it, and if it serves them well, and are quite happy to do it that way, then certainly let them carry on using it.

Nothing is written in stone, in fact for thread cutting, I think there are more conflicting words of text than for any other machining operation, so it has to be a personal decision as to which commandment you follow.


Bogs

[Ned Ludd]

Hi Pete,
As I said "once the cut is under way" how can the tool cut anything on its right hand edge when it is moving sideways to the left. If I am wrong, wouldn't be the first time, the only cut on the right of the tool would be the small incremental in-feed, while the left hand edge cuts full depth, I would not describe this as "crowding" would you?

As Bogs very wisely says, nothing is set in stone about thread cutting. What matters is that you can produce a decent thread, first and foremost. With more experience you can vary your methods to suit your different needs or tooling. There are Pros and Cons for all the various different methods of single point thread cutting. I have never seen a fully comprehensive treatise on single point threading, but I am sure it would run to hundreds of pages and even then some chap working quietly by himself in a shed, would have yet another different way of doing things. One of the best books for the home machinist is the third in the Workshop Practice Series, "screw cutting in the lathe" but it is now sadly a little outdated, even so it still makes a good primer for those new to the subject. If it were to be up-dated it would have to include some modern developments like Bogs' own type of self lifting tooling.

What is important is that instructional books, or other teaching methods, are not open to misinterpretation by an averagely mechanically minded person.
Ned

[andyf]

I think that I was misled by the video. The camera angle was such that I mistakenly thought the topslide was set at a much greater angle than 29.5 deg to the cross-slide axis.

Andy

[RichardShute]

I think that I was misled by the video. The camera angle was such that I mistakenly thought the topslide was set at a much greater angle than 29.5 deg to the cross-slide axis.

Andy

That is the nub of this whole discussion.

In the video John gave a clear description of the well documented rotated-compound-slide method and says the cut is taken on the left of the tool with the right of the tool just scuffing the rear face to keep it clean. BUT... when the top slide was rotated in the video, it was indeed rotated by 29.5 deg from the spindle axis as John agreed in a later part of the discussion.
The inevitable consequence of this is that the top of the thread crest is gradually removed with each successive cut as the thread is formed. In the end, although the root diameter is correct, the crest of the thread is undersize by roughly half the thread height as shown in my diagram and Pete's and implied by the last of Bill's.

It does, surely, matter a lot what the approach angle is, if the result is a significantly truncated thread. Using a low angle to get a fine feed is a well established technique, but in plain turning there is no concern about preserving a particular tool path, in thread cutting, by definition, there is - it's the thread.

Ned, I was wrong to say the crest is removed by the heel of the tool, that is not the case, the top of the thread is taken away by the tip on successive passes. The tool could only cut on the right as well as the left if the compound angle were less than half a thread angle from the cross slide, but not at the 30deg from spindle axis we are discussing. Sorry for mis-leading you.

doubleboost, I would be interested to know what is the OD of your finished thread. If you started with 40.00 and cut a 1.5 pitch thread, with the compound set as shown in the video I would expect you to have lost about 0.6mm per side so the finished OD would be about 38.8, depending on the tool tip radius.

Richard

[Pete.]

Hi Pete,
As I said "once the cut is under way" how can the tool cut anything on its right hand edge when it is moving sideways to the left. If I am wrong, wouldn't be the first time, the only cut on the right of the tool would be the small incremental in-feed, while the left hand edge cuts full depth, I would not describe this as "crowding" would you?

Yes I would.

Let me start by saying that I am a novice machinist and as such have been and am happy to be corrected if something I say is not right.

When you directly in-feed, you take the first pass and the tool cuts both sides. The leadscrew moves the tool along it's path continually cutting both sides up to the end of the thread. When you set up for the next pass, the leadscrew moves the tool along exactly the same path as controlled by the leadscrew, but the tool is fed in extra depth, so the tool is still cutting both sides but deeper.
Look at it another way, imagine you were cutting a dead-square thread with a full-width square tool. After the first pass, you return and feed the tool directly in using the cross-slide and take another pass. Will the tool now cut the thread deeper by cutting on the end of the tool, or wider by cutting on the side of the direction it's travelling?

As Bogs very wisely says, nothing is set in stone about thread cutting. What matters is that you can produce a decent thread, first and foremost. With more experience you can vary your methods to suit your different needs or tooling. There are Pros and Cons for all the various different methods of single point thread cutting. I have never seen a fully comprehensive treatise on single point threading, but I am sure it would run to hundreds of pages and even then some chap working quietly by himself in a shed, would have yet another different way of doing things. One of the best books for the home machinist is the third in the Workshop Practice Series, "screw cutting in the lathe" but it is now sadly a little outdated, even so it still makes a good primer for those new to the subject. If it were to be up-dated it would have to include some modern developments like Bogs' own type of self lifting tooling.

What is important is that instructional books, or other teaching methods, are not open to misinterpretation by an averagely mechanically minded person.
Ned

Wise words indeed but there should always be room for someone to disagree with anything that's said.

With the very greatest of respect, I actually don't agree with Bogs that it's acceptable for the compound angle to be greater than half the included thread angle especially when it's double because I think it will produce the saw-tooth form as shown in my sketch. I don't see how a few thou cleanup cut is going to be enough to turn a 90-degree included angle thread into a 60-degree one especially on the OP's 12TPI thread, but as I said I'm a novice machinist with no more than a handful of single-point threads to my name so there might be something I'm missing there. I certainly won't stamp my feet and get all silly if I'm proved wrong, but I'd like to BE proved wrong rather than be told not to argue when I think I'm not.

EDIT: It was Bill Todd who cut a 12TPI thread, the OP cut a 1.5mm pitch thread for his collet chuck.

[Ned Ludd]

Hi Pete,
Argue away, without discussion how is anyone going to get out of the rut of narrow thought?

I, too, have to be convinced about cutting threads at shallow angles, as I wrote in an earlier post I would have thought that the last third of the cut would have to be in-feed, to correct the thread form. OOPS, I just checked back to see exactly what I posted, no post. I wrote a post, I know I did,  but it seems it did not get through and I did not check, silly me. 
Ned

[Ned Ludd]

Hi Guys,
My apologies, I must have saved my post and not sent it. The following was written as a reply to Bogs and should have appeared on page one.


Hi Bogs,
I do have a little experience in thread cutting, and the only way I can see the method in the video working would be if the last third of the cutting was of the "straight-in" method. By doing it that way, after the bulk has been taken away, the correct thread form could be produced by the insert. Though having said that, as he is using "proper" inserts, why not just go straight in and use the insert as it is designed to be used?

While on the subject of threading, I have been experimenting with the swing up tool. I think yours is a stiffer and superior  design than the German chap's. To be fair to him, he was only cutting very fine threads not 1/2" Whit!  I have tried JohnS's with die head chasers, but on some metals the loading is, I find, too much. I am currently trying one using triangular carbide threading inserts, which shows promise. The next experiment will be with the Oz design of tangential threading tool in your style swing up holder.
So far I have found the best method of getting a clean cut thread is to use the Oz cutting tool shape in my own design holder, using the offset top slide method but with a cross slide stop for quick action.
Ned

[doubleboost]

Hi
I made another chuck (for the rotary table)
I used the same insert but went straight in this time.
The diamiter before machining the threads on both pices was39.8mm.
The finished thread measures 39.7 on both jobs and the threads look identical.
I proberly lost the 0.1 as i polished the finished thread with emery tape.
Regards
John

[BillTodd]

Well, I braved the freezing temperature in my garage to cut a thread (20tpi with a sharp V tool) into a piece of scrap HRS at 30° (to parallel) and again at 61° (I'd normally use 59°)

The result was a failure as 30°, partly because of the Hardinge's fast retract top slide tore up the thread as it was pulled out, so requiring me to use the cross-slide to retract (thus ballzing it up). As expected the tool was cutting on both flanks, which doesn't make for a nice looking chip. You may be able cut a thread at this angle, but frankly it doesn't seem or feel 'right' and you'd get a better result feeding straight in with the cross-slide

At 61° the tool was again cutting on both edges which creates a squiggling tearing chip that does not compare to the nice curl at 59° that I'm used to. I will try it again (when it's warmer) with a full-depth insert to see if it make any difference.

Bill

[doubleboost]

Hi
Bill
One of the benifits of using the compound slide is that the tool is wound back on the cross slide (boxford do a threading stop for this very reason).
Like you say it is a bit cold to play in the shop  bad enough at work
John

[BillTodd]

Like you say it is a bit cold to play in the shop 

I'm only half Geordie, so I'm not as well adapted to the cold; -2° is all I can cope with 

[doubleboost]

off topic but it is christmas

[BillTodd]

geet . Aa'd not seen tha before

(Translated by http://www.geordie.org.uk/)

Bill

[RichardShute]

Hi
I made another chuck (for the rotary table)
I used the same insert but went straight in this time.
The diamiter before machining the threads on both pices was 39.8mm.
The finished thread measures 39.7 on both jobs and the threads look identical.
I proberly lost the 0.1 as i polished the finished thread with emery tape.
Regards
John

Thanks for taking the trouble John, I'm very curious to know how this comes about. One aspect I had not included in the sketch I did before was the shape of a real thread which has a clipped top and a radiused root, both of which reduce the effect, but I still can not see how your result comes about.
Here is a notional 60 deg thread with no flat top and no radius:


This is a single groove made with a 60 deg tool fed in at 30 deg from the spindle:


and this is two adjacent grooves on a 1.5mm pitch, showing the clipping at the top of the form:


That makes no allowance for the flat top or root radius and is the form I was using previously. Not strictly accurate I know, but it domonstrates the point of discussion.

This is a true thread form with a flat to a depth of H/8 and a root radius at the full quoted depth of 0.92 (both figures taken from a Zeus book):


This shows the top again clipped, but by a reduced amount of 0.33 because part of the top is already removed by the flat and the root radius also reduces the amount removed.

Can anyone shed light on why this appears not to be reflected in the thread actually cut.

Richard

[andyf]

I’ve been puzzling over the remark which Bogs made before his data got lost, and have come up with these approaches to “going in at an angle”.

Method 1. The diagram below shows how I normally set up for threading, with the topslide set at 60º to the work and at 30º to the cross-slide (actually 60.5º or 29.5º, but I’ve rounded them off). Depth of cut is applied using the topslide.



Method 2. The diagram below shows the set up for threading when applying depth of cut with the cross-slide, but also shifting the topslide (and thus the tool) towards the headstock a little between each pass to produce the angular feed effect. The rule of thumb seems to be that the topslide is shifted left half the amount by which the toplide has been fed in.


Method 3. The final diagram below shows the situation which seems to have caused controversy.  The topslide is at 30º to the work and at 60º to the cross-slide. This set-up could be used to simplify the “cross-slide parallel to the work” method. Trigonometry shows that advancing the topslide when it is set at this angle will advance the tool into the work by half the amount indicated by the topslide dial. So in this case the cross-slide and topslide can be advanced by the same amount as shown by their dials to produce the same result as Method 2 with its parallel topslide. This avoids the bother of remembering by how much the cross-slide has just been advanced and then advancing the topslide dial by half that amount. Each dial is advanced by the same amount in Method 3.



Am I on the right track. or have I got myself completely lost?


Andy

[Ned Ludd]

Hi Andy,
What an interesting idea, and  yet another method. 

Going back to the original post, the following You-Tube was pointed out on the ME site today. Alright, it is yet another video on threading and perhaps it is a touch long, there are five videos for the one thread, but I always like to watch others working. Oh, and it does seem a bit like an advert for tips and oil.

Ned

[Bogstandard]

I think you are going into too much theory, and I would suggest you read my very first statement on it.

if doing super fine threads

If you are going a lot deeper, then you will run into all sorts of problems, as shown above.

There are reasons for cutting on the leading face, and that boils down to removing metal without leaving behind the raggedy a**sed threads you usually get when using the straight in technique. What I actually do for very fine threads is to rough it out, without going too deep (hence the shallower angle, which gives greater control of the depth) then use a final straight in approach to clean the back edge up, with maybe one or two tenths infeed. I hope all that reads correctly, I've only just got up.

I find it works for me, but in theory, it shouldn't work. But as with a lot of things, practical doesn't always match theoretical.
That is why I hate it when people start throwing formulae around, it is just too confusing for the laymen who read such posts as ours, it makes them feel so inferior, that they never even try to do something out of the ordinary.

As I stated before, that was what was shown to me, where that old chappie got it from, I have no idea, but somewhere in the past, a machinist must have tried it out and found that it worked for him as well.

But I will say one thing, my tiny threads are always as smooth as silk.

I think where a lot of people go wrong is that they are only looking at standard sizes when threading, say 1/4" diam x 40 TPI. But in fact, you can just as easily have 3ft diam x 40TPI, exactly the same thread profiles, but on a much larger diameter. The critical part is getting that very fine thread to work with it's female counterpart.

Anyone who has visited my shop will see that I can get up to all sorts of weird things, but unfortunately, at this time I cannot show you anything, maybe next year after the patents are completed, but that also includes some strange  but very fine threads, all single point cut.

I hope Peter (HS93) doesn't mind, this is a job I am attempting to finish off for him, but here is the sort of thing I am on about. Making a few new blades for a model steam boat variable pitch propellor.

The hub is less than 1" diameter and is full of bevel gears, and the threads have to be a spot on match for the ones already cut in there. Because if they aren't, because of the very tight tolerances on the unit, the whole lot would sieze up. I have made five stubs, just in case I bugger up on the machining to follow.





So as I was saying in a previous post, if you find a method that works for you, use it. If you like what you see, try it, but don't always expect it to be right for you.


Bogs




 

[andyf]

Bogs: I think you are going into too much theory.....

You're right, of course. But it's so cold in the shop that sitting indoors and theorising is all I feel like doing just now. Of course, what really should be doing is sitting indoors finishing off my tax return, but that's an even less attractive proposition than a sub-zero garage!

My thanks to Doubleboost for his video. I wish I had a 127T gear to get exact metric threads out of my imperial leadscrew, but there isn't room for one in Mod 1, so I would need to use a smaller Mod for the 127T and whatever it meshed with. After a lot of calculations involving my standard gears with two rather than one sets of compounded gears in the train, I can get get down to 0.01% error or less on the common metric and BA pitches. That's 1mm out over 10 metres of thread, or 0.0001" over 1", and my Chinese leadscrew may be no more accurate than that in the first place.

Andy