Thread cutting tools

[Mark BParticipant @markb43031]

I wanted to get some pointers on suitable lathe cutting tools for thread cutting.

I need to cut an internal thread for a custom backplate my Lorch instrument makers lathe. The headstock has a M20 x 2.5mm thread

Having spent some time discovering how to use the quick change gearbox on my Myford ML7R to cut metric threads I'm almost ready to go…

The material I'm cutting is steel (not cast iron).

Can anyone recommend suitable thread cutting tools? I typically grind my own HSS tools, but I wondering if some sort of indexable insert style of tool would be preferable so I get perfect angles of the thread. However having looked the range available is huge!

[Mark BParticipant @markb43031]

[not done it yetParticipant @notdoneityet]

IF you want a perfect thread, I suggest you buy a second tap for twelve quid from tracy tools and use that after first rough-cutting the thread on the lathe. IF there are registration surfaces on the shaft, perfect threads are not an absolute necessity – but the registers would need to be precise.

Edited By not done it yet on 07/05/2020 23:40:07

[Nigel Graham 2Participant @nigelgraham2]

I would concur with that: after all you may as well be fair to the lathe and make the thread as near-enough perfect as you can for modest outlay and little extra work.

Grind a single-point HSS tool as closely as you can to the 60º notch in the setting-gauge; better to be very slightly under than over-angle. Cut to just shy of full depth – take a "spring-cut" at intervals to avoid an increasing extra chip-load especially if plunge rather than angled feeding-in; and one or two spring-cuts on the last increment.

NB: watch out for which step(s) on the thread-indicator to use. I don't know if your lathe's gearbox obviates this need but on conventional change-gear settings on an fractional-inch lead-screw, a metric thread demands the same indicator number every time, or even constant-engagement working.

Finish to size and profile with the tap. If the tap is too large for any tailstock chuck it can be supported and guided with a centre in the tailstock, assuming the tap has a centre-hole as it probably would at that size.

From recent experience, though I was using a bench-drill not the lathe as a tapping-guide, I would think it better to use a revolving-centre if available.

Use an appropriate cutting-compound, especially on the tap, e.g. neat cutting-oil or a cutting-grease such as 'Trefolex'.

'

Also of course, you may wish to make further attachments so you now have the tap all ready and waiting!

'

Indexable tool inserts come in such a bewildering range because they are designed for production work on NC machine-tools, carving all manner of exotic metals at prodigious rates to very high standards of accuracy and finish; so are tailored to suit particular metals as well as a wide range of holders.

The range is less bewildering if you look at what "our" stockists select for our common materials and machining conditions, rather than the tool-manufacturers' catalogues intended for industrial use.

[JasonBModerator @jasonb]

I think you would have a job getting an indexable holder and 2.5 insert down a 17.5mm hole so do a NDIY suggests.

[Mark BParticipant @markb43031]

Thanks for the replies. The approach roughing the thread first with a ground HSS tool and finishing with a tap makes good sense. As you point out perfecting the threads isn't needed as the lathe the has a register.

I do have a question about cutting threads metric threads with my Myford ML7-R with quick change gearbox. To achieve the metric threads I can see I need a metric conversion quadrant and gears as per the chart.

On the Myford, are these metric threads approximations or are they perfectly accurate? Not an issue for this job, but I'm thinking ahead to another tasks where I'm going to need a longer thread cut.

[not done it yetParticipant @notdoneityet]

There are tables on the forum (from the ‘net) showing the errors with differing alternative gear sets for converting to and from either system. I would not worry too much, and only cross that bridge if and when necessary.🙂

[Tony Pratt 1Participant @tonypratt1]

You do not need a metric conversion set to cut decent metric threads on an imperial Myford, using a 33/34 gear you can get good approximations, I can't remember the exact details but there are forum posts about it?

Tony

[Mark BParticipant @markb43031]

Tony,

I'm really pleased you pointed me at the 33/34 tooth gear option. The parts for a metric conversion kit was looking at costing me £250+. A couple of gears will cost me £20 from RDG,

The loss in accuracy in the thread pitch is minimal.

This is the table I found from another forum post

https://www.model-engineer.co.uk/wp-content/uploads/sites/4/documents/gear%20errata%20tables.pdf

So just to double check, this chart talks about the mandrel gear. On my standard setup I believe this is the one arrowed which has 24 teeth.

I so pleased to be able to "almost" make do with my current setup.

[not done it yetParticipant @notdoneityet]

Mandrel = Spindle.

Your arrowed gear is after the tumbler gears.

[Howard LewisParticipant @howardlewis46836]

Brian Wood's book "Gearing of Lathes for Screwcutting" will contain charts that will help.

A lot of mini lathe users use a 63T gear. It is not exact, (you need a 127T for that, but a 20 DP gear is quite big. They are available. )

Another book which may help is No.3 in the Workshop Practice Series. " Screwcutting in the Lathe" by Martin Cleeve.

HTH

Howard

[Tony Pratt 1Participant @tonypratt1]

NDIY is correct in that 'Mandrel = Spindle.' i.e. the gear behind the black locking collar top LH, but you are correct in saying the 33/34 gear replaces your arrowed one.

Tony

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Tony Pratt 1 on 09/05/2020 18:09:05:

NDIY is correct in that 'Mandrel = Spindle.' i.e. the gear behind the black locking collar top LH, but you are correct in saying the 33/34 gear replaces your arrowed one.

Tony

.

… because Myford treats the one physically on the spindle as being non-removable except when servicing the lathe.

MichaelG.

.

Edited By Michael Gilligan on 09/05/2020 18:36:30

[DC31kParticipant @dc31k]

Posted by not done it yet on 09/05/2020 17:45:05:

Your arrowed gear is after the tumbler gears.

That may well be so, but your pedantry is not at all helpful to the originator of the question.

Can any of the gears above the one he has highlighted be changed for alternative sizes?

If not, then the term is apt as the mandrel gear is conventionally the first (variable) gear in the chain.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by DC31k on 09/05/2020 18:32:35:

Posted by not done it yet on 09/05/2020 17:45:05:

Your arrowed gear is after the tumbler gears.

That may well be so, but your pedantry is not at all helpful to the originator of the question.

Can any of the gears above the one he has highlighted be changed for alternative sizes?

If not, then the term is apt as the mandrel gear is conventionally the first (variable) gear in the chain.

.

+1 for that assessment.

MichaelG.

[Mark BParticipant @markb43031]

Okay I've completely misunderstood the content of this table then. I thought all I needed to do was change the gear arrowed with 33 or 34 teeth. I got over excited by the simplicity and low cost of what I thought was my solution!

Interestingly I can see that the gear I have arrowed turns 1:1 with the lathe spindle which is why I'd thought this was the one to change.

I've looked at what is involved with changing he gear on the spindle and that isn't so easy.

I'm going to back to basics and work out the gearbox ratios and see what is possible. Suddenly the metric conversion kit is feeling like a simpler solution, but let me work it out. I really want to properly understand how this table was created and how I can use the results it shows.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Mark B on 09/05/2020 18:46:25:

[…]

Interestingly I can see that the gear I have arrowed turns 1:1 with the lathe spindle which is why I'd thought this was the one to change.

I've looked at what is involved with changing he gear on the spindle and that isn't so easy.

[…]

.

That’s the point of the three posts that precede yours, Mark

… Ignore ndiy’s pedantry

MichaelG.

[Mark BParticipant @markb43031]

I ended up replying before I received the latest replies. I think I've done full circle and back to the simple solution of 33/34 teeth where my arrow shows which is a good outcome!

All of this feedback helps me to properly understand and have confidence in a more satisfactory end result.

Thanks all again – I'll post the results of my screw cutting adventure once I've got my extra gears.

[SteviegtrParticipant @steviegtr]

Indeed you do need those gears which I bought some time ago. I also had no idea which ones to change. The nearest answer I got was something about making a new quadrant device. Which I took to be the plate at the rear of the lower gears. With that they went in the cupboard for another time. Still there. I also had a chart somewhere if you do not have it. It lists all the metric threads that can be done. There are some that can't.

Steve.

Edited By Steviegtr on 09/05/2020 23:53:45

[SteviegtrParticipant @steviegtr]

So from the above chart that I have not looked at for some time , it seems you only change one gear for a particular thread pitch. ???

Steve.

[JohnFParticipant @johnf59703]

Mark, I was taught this method as an apprentice in the early 1960's having been asked many time how to I typed this up same years back. You may fined it useful : —

John

Screwcutting Metric / Imperial August 2010 This was passed to me by my apprenticeship mentor in the early 1960’s When screw cutting a metic thread on an imperial lathe you can use the chaser dial and disconnect the half nuts when cutting up to a shoulder by doing the following : –

First make sure you engage the half nuts with the chasing dial on a particular number — number 1 is best

Then make your first cut and at the end of the cut disengage the half nuts and stop the lathe asap now retract your tool exactly as you would if it was an imperial thread — DO NOT move the carriage — this is very important.

Now the thread dial will have passed number 1 so start the lathe in reverse and re-engage the half nuts on number 1 and run it back to the start point of the thread.

No need to disengage here just stop the lathe, apply the second cut and start the lathe forward then repeat the above at the end of the cut.

This allows you to thread up to a shoulder or undercut without fear of a disaster !

Link to an excellent video demonstrating this method from the ME forum March 2019

Some of my free hand ground threading tools

Edited By JohnF on 10/05/2020 07:30:06

[JasonBModerator @jasonb]

Posted by Steviegtr on 09/05/2020 23:55:26:

So from the above chart that I have not looked at for some time , it seems you only change one gear for a particular thread pitch. ???

Steve.

If you want the pitches that are on the list then yes only the one gear in the position arrowed earlier.

If you want other pitches such as 0.5 or 0.8 then you will need to play about with the gear train, not sure if Brian's book includes a suggested set of gears for these pitches.

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