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Myford M type half nut Thread form?

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Eugene13/02/2018 09:25:41
113 forum posts
5 photos

Lads and Lasses,

 

I'm looking to repair a worn leadscrew half nut on a late model Myford M type, probably following one of the methods published by Geoff Walker.

 

 

This requires making a new half nut in phosphor bronze; although I'm not very experienced in screw cutting I think I've got the basics on how to do this, but there is I think, a problem.

 

 

I'd assumed the 3/4" x 8tpi leadscrew thread to be an Acme type, but looking at it, I don't think it is, there seems to be no flank angle at all, just a plain square thread. That implies no commercially available thread cutting insert, at least I don't see any, so I'll probably be making my own. That doesn't fill me with much confidence, my tool grinding skills are pretty rudimentary, and this cutter needs to be very accurate.

 

 

Guessing, grinding a suitable HSS thread cutter is just a matter of making up something like the business end of a parting off tool of appropriate width (whatever that is) and cutting to the appropriate depth (whatever that is) at the appropriate helix angle (whatever that is).

 

 

So what thread form is it? Anyone have accurate dimensions of the original factory issue? Given that I'd much prefer to buy a thread cutting tool rather than try to bodge one, is anything available? Am I missing something?

 

 

Thanks in advance,

 

Eug

Edited By Eugene on 13/02/2018 09:26:10

Brian Wood13/02/2018 10:25:59
1521 forum posts
35 photos

Hello Eugene,

My late father's ML4 from 1945 had an 8 tpi leadscrew of square form thread and that I suspect was the order of the day at the time. ACME threads were first introduced in the USA before 1900 and clearly were not adopted over here on non production machinery until much later.

Cutting square form threads is actually easier than ACME. The tool should have a width of a thou or so narrower than 1/8 inch with side clearances to avoid flank cutting--all the action is on the cutting face of the tool. You will need to grind it so that it follows the helix angle of the thread form [laid over on it's side as the tool sees it] and cut the thread at 1/8 inch pitch. Thread depth is equal to the pitch, ie 1/8 inch so make the tool to allow some extra depth for clearance

It will make life a great deal easier for you to thread cut much of the form with a standard Vee form tool first to get the meat out of it beforehand, but don't take that deeper than a few thou over 1/8 inch and stop before the vee cut is wider that 1/8 inch otherwise the thread will have a bell mouth.

Cut in small depth increments without making dust until at depth and test it from time to time on the leadscrew to get some confidence. It may well be a bit tight when depth is reached so move the tool a few thou to one side and run another finishing cut until the fit is snug on your leadscrew. I suggest hand power is used as well until you feel brave enough to put on a cut, slowly, under power.

Good luck, you will feel a real sense of achievement at the end

Brian

Edited By Brian Wood on 13/02/2018 10:28:41

Hopper13/02/2018 12:04:46
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2479 forum posts
38 photos

I made a new half nut for my M type a few years back. Yes, it is a square thread. 8tpi. AKA 1/8" pitch. A pitch includes one thread and one 'valley' so your square threading tool bit will be nominal 1/16" wide and the thread will be a nominal 1/16" deep. From memory I ground my toolbit to .063" wide but took a final cut about .067" or so deep, just to provide clearance in the nut over the tips of the leadscrew thread. I ground my toolbit from the piece of 1/8" HSS that clamps into the boring bar. One of those boring bars you can buy from all the usual suppliers. I used the biggest bar that would fit in the hole, which was bored to size of OD minus .125" nominal, but again I think I bored it five thou large to provide clearance on the root of the leadscrew thread.

I would not use phosphor bronze. It will wear out your non-hardened lead screw. I went the other way and just used brass because a) it's easier to machine and b) it is soft and will wear out quicker than the leadscrew, thus minimising wear on the leadscrew. If you feel you must use bronze, I would definitely use leaded bronze, I think in teh UK its commonly called gun metal (?). Here in Oz its known as LG2 standard leaded bronze. It is not as hard as phosphor bronze and is OK to run against non-hardened steel. I think the lead gives some lubricating qualities to it.

Making the tool bit took longer than machining the thread in the "bobbin". I ran the lathe in the normal direction and cut the left-hand thread by starting with the tool inside the job at the left hand end. When the halfnuts were engaged it travelled towards the right. It's a great leap of faith starting the cut where you can't see the tool bit but it works, providing you have the cross slide dial correctly set. Took about five thou cuts and finished with finer cut or two. Bit of squealing from the brass and that springy boring bar but you get that.

I then soft-soldered the "bobbin" in place on the modified arm and it has been working ever since. The shoulders on the ends of the bobbin take the load so soft solder seems to be all that's needed.

The threads in my old half-nut bobbin (it had been modded before my ownership, many many years ago apparently) were worn down to about .015" thick so it was a miracle they still worked at all. Must have been so work hardened they were tough as blazes and hung in there long enough to make the new nut.

Edited By Hopper on 13/02/2018 12:05:40

Edited By Hopper on 13/02/2018 12:08:45

steamdave13/02/2018 12:08:32
363 forum posts
29 photos

Eugene

Have you thought about about a plastic set of half nuts? There is a good write up in the following link:

http://bbs.homeshopmachinist.net/threads/43645-Making-Acetal-leadscrew-nuts-the-easy-way

Not tried it myself, but may give you food for thought and comments from others.

Dave
The Emerald Isle

Edited By steamdave on 13/02/2018 12:08:46

Brian Wood13/02/2018 14:08:14
1521 forum posts
35 photos

Ooops sorry Eugene,

Some instinct made me look back on this, senior moment here as Terry Wogan used to observe! Of course the tooling is based on dimensions of 1/16 inches so I really do apologize for the misinformation

I'm surprised the others were so polite about it!

Regards

Brian

Eugene13/02/2018 21:51:33
113 forum posts
5 photos

Thanks to all.

I spent six hours driving up and down the M6 today, so I had plenty of "creative thinking time" available. thinking

The thing got a bit more clear in my mind, and you've confirmed what I worked out. Grinding the tool still bothers me a bit, but there is another project in view that requires a straight thread, only an external one this time; I can play around with that first and get a feel for the job.

Dave, I've seen the softened acetal method described before, but didn't fancy it somehow; can't really say why, perhaps it's not "authentic" enough, but thanks for the link anyway.

Brian, the man who never made a mistake never made anything. I find this a great comfort shortly before the hammer hits the thumb.

Eug

Hopper13/02/2018 23:10:04
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2479 forum posts
38 photos

That's the best shot, have a play around with some easier test square threads first. Using a digital caliper to check the tool during grinding helps get the width right and the two sides parallel. You can make them parallel or with a little bit of back clearance so the end is the widest point, just can't make it a tapered wedge shape!

Brian Wood14/02/2018 10:34:40
1521 forum posts
35 photos

Eugene,

Thank you for your kind comment!

I also think it would be a useful exercise for you to play about as Hopper suggested and get a feel for the technique.

One added point is to take out any overhang that you can with the tooling, you will be cutting on the full face width on slender tools and keep it sharp as well

Enjoy the experience

Regards Brian

geoff walker 114/02/2018 13:05:08
184 forum posts
73 photos
Hi eug
I can help you with this but I am currently in Australia so my response is limited via the mobile.
I return home in late March so if you still need help I will contact you then via a p.m.
In the meantime I would suggest that you try to get a copy of the Steve Papworth articles from m.e. magazine in 1996. In particular the issue in which he describes the screw cutting of the half nut bobbin.
All the best for now geoff
Eugene14/02/2018 15:53:35
113 forum posts
5 photos

Hi Geoff,

Thanks for the offer of help, I may well take you up. There are any number of projects in view at the moment, so it doesn't matter if this goes on the back burner.

Before today all I knew of the half nut was that the backlash was around .080" I took the assembly off today and had a closer look; official verdict is a severe case of cream crackeredness, I bet close to half the thread thickness has gone.

As you warned in your article, there is very little meat in the "bobbin", so it looks like a new bracket / assembly of some kind. The leadscrew itself looks to be in fine fettle.

Hopper, thanks for your input. When you say "modified arm" did you attach the bobbin to the existing arm in some way? Geoff went the whole hog and fabricated a new arm that had an adjustable interface for the bobbins' final fit. Looking at it this morning I thought it would save a bit of time to use the existing arm, nor would it be too difficult to build in a bit of adjustment.

Onwards and upwards,

Eugene.

 

Edited By Eugene on 14/02/2018 15:55:30

Hopper15/02/2018 05:36:52
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2479 forum posts
38 photos
Posted by Eugene on 14/02/2018 15:53:35:...

Hopper, thanks for your input. When you say "modified arm" did you attach the bobbin to the existing arm in some way? Geoff went the whole hog and fabricated a new arm that had an adjustable interface for the bobbins' final fit. Looking at it this morning I thought it would save a bit of time to use the existing arm, nor would it be too difficult to build in a bit of adjustment.

Onwards and upwards,

Eugene.

No, I used the original arm. Some previous owner had done the modification and installed a bobbin. All I did was make a new bobbin, unsolder the old one and solder in the new one I made. I think Papworths article gives details on doing this. I'll have a dig through my Drummond file and see what I can find.

Basically , the mod consists of setting the arm up on the cross slide table and using a between centres boring bar, or a boring head held in the headstock spindle etc, to bore out the original thread plus a little bit more to make room for the bobbing. I don't remember my bobbin's exact dimensions but I made it from 1" brass bar so the large flanges on the ends were 1" and from memory the smaller diameter of the bobbin was only maybe 15/16" or a tad less.

You say your half nut is down to about half thread thickness. No need to panic. Mine was down to 10 to 15 thou and still worked well enough to make the bobbin!

Edited By Hopper on 15/02/2018 05:52:39

Hopper15/02/2018 05:56:03
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2479 forum posts
38 photos

PS, I just looked in my Drummond file and found the dimensions fo rthe bobbin I made.

OD was nominal 1" on the ends, turned down to .945" diameter in the middle section. Ovearall length was 1.600" and the length of the reduced diameter section was 1.475".

Hole up the middle was bored to .625" and thread cut a few thou deeper than standard to bring it out to .752", just for a tad of clearance on the thread tips. I used a 7/16" diameter boring bar with a piece of 1/8" HSS in the end ground down to the 1/16" wide square profile, protruding just over the .062" from the bar to provide a tad of chip clearance.

I turned it and screwcut it in one setting as one piece then cut it in half with a hacksaw and filed it smooth. Kept the spare bit for next time it's needed. It might be a few thou short of the full half circle but won't matter. The last bobbin lasted over 50 years that I know of so I don't think it will be me doing tghe next change.

Edited By Hopper on 15/02/2018 05:57:46

Edited By Hopper on 15/02/2018 05:58:12

Edited By Hopper on 15/02/2018 06:00:03

Eugene15/02/2018 09:38:29
113 forum posts
5 photos

That's really helpful, thank you.

Eug

Hopper15/02/2018 12:59:57
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2479 forum posts
38 photos

As far as machining the arm to take the bobbin goes, I have not done that myself because a previous owner beat me to it. But I imagine you could get a piece of 5/8" BMS bar and centre drill both ends and then put it between centres on the lathe. Then temporarily clamp the halfnut arm to the BMS bar so the halfnut thread bears on the bar. Then you would have to pack and clamp the arm to hold it in that position. Then take away the temporary clamp and remove the BMS bar. The halfnut arm is then ready to be "bored" out using a between centres boring bar or maybe a headstock spindle mounted boring head, until the old thread is removed and a half-round "hole" is created the right size to hold the bobbin.

Eugene16/02/2018 20:35:07
113 forum posts
5 photos

Hopper,

That's pretty much the technique Geoff outlined in his article, as developed by Steve Papworth.

Looking at my bobbin, if I bored out the old threads, there would be precious little left, if anything. I'll have to do some carful measuring, but the Mk.1 eyeball says it won't do. For that reason Geoff made a complete new arm.

Probably the pre war Drummond machines had a lot more meat in the sandwich than post war 1946 Myfords have.

Eug

Hopper17/02/2018 04:12:14
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2479 forum posts
38 photos

Ah, yes. Once Myford got involved, the rot set in . You are probably quite right. There seemed to be plenty of "meat" on my 1937 model. That makes it quite a bit more of a job then. I think the files section on the Yahoo Group Drummond site has a drawing for making a whole new piece to add on to the existing arm, after machining off the old nut section.

I don't know that the bobbin concept was developed by Papworth. It had been done to my lathe sometime before my father bought it in 1952 in the UK. Same (presumeably) previous owner had also bored out the headstock bearings and poured in white metal to reline them, scraped the bed, graduated the leadscrew handwheel, made their own screwcutting gauge and indexable cross slide and top slide collars and a few other bits. So he was a busy lad, whoever he was. The old lathe must have done some work to require that kind of attention by 1952 - it was only 15 years old, almost new! Apparently it had been used to make aircraft parts during the war. Whether in a factory or by a home shop contractor I don't know.

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