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Lathe screwcutting

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Plasma05/03/2019 16:31:19
326 forum posts
41 photos

Hi all,

I'm trying my hand at screwcutting in the lathe.

I've managed to set the metric pitch right and using an inverted tool in the front tool post with the lathe running in reverse I can cut a right hand thread away from the headstock.

I just seem to get quite ragged results, torn thread from or what look like multiple starts.

Any advice gratefully received. I've watched loads of videos but they don't seem to help.

I've tried infeed with the cross slide and with the compound slide set at 30 degrees but no big difference.

I need to cut some larger threads than available with taps and dies so i have to crack this.

Regards

John Rudd05/03/2019 16:45:18
1365 forum posts
58 photos

I'm no expert at screwcutting, but I would leave the tool as normal rather than invert it, run the lathe in the conventional manner and cut to a relief at the end of the thread......back to basics as they say...Set the tool at right angles to the work, with the compound slideoffset to 30 deg if its metric, adjust DoC with compound and back tool out using crossfeed..

So,

Rather than blame your technique, what material are you practising on?

Are you threading using an imperial or metric lathe, is the leadscrew engaged all the time or are you disengaging it? Using a thread dial indicator? ( I cant remember if the leadscrew remains engaged for m or imp....) Tool on centre height?

 

Edited By John Rudd on 05/03/2019 16:49:10

john fletcher 105/03/2019 17:09:14
515 forum posts

I'm no expert here. When I'm cutting a metric thread I put on some feed on the top slide, zero the dial, cut the thread,. stop the lathe after cutting the thread. Next wind out the top slide and reverse the lathe manually LEAVE the lead screw engaged. .Apply some more feed and do the same again, several times. John

JasonB05/03/2019 17:31:10
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Moderator
15743 forum posts
1646 photos
1 articles

two thoughts

1. If it's an imperial lathe and you are cutting metric threads don't disengage the nuts.

2. Make sure you have the topslide at 30 deg (or 29.5) to the face of the lathe not the central axis

Edited By JasonB on 05/03/2019 17:34:46

John Reese05/03/2019 17:56:33
768 forum posts

Jason B is correct about not disengaging the half nuts when cutting metric threads with an Imperial leadscrew. The same applies to cutting Imperial threads with a metric leadscrew.

When cutting threads with the spindle in reverse the topslide needs to be angled opposite to what is used when cutting with the spindle going forward. The topslide should be parallel with the trailing flank of the tool, less 1/2* or so.

If cutting metric threads with a metric leadscrew or Imperial threads with an Imperial leadscrew it is possible to disengage the half nuts. There are rules for which line or number on the threading dial where the half nuts can be engaged. Instead of looking them up or remembering them I choose to always engage the half nuts at the same number on the threading dial.

John

SillyOldDuffer05/03/2019 18:26:58
4527 forum posts
971 photos

Posted by Plasma on 05/03/2019 16:31:19:

...

I just seem to get quite ... what look like multiple starts.

...

Sounds like the thread dial is being misread, or it needs a different pitch wheel to be fitted. (Not all lathes support this.)

Thread dials work best on imperial lathes doing turns per inch. Metric threads are based on pitch, which isn't quite so user friendly.

On an imperial lathe doing metric the thread dial is meaningless, so always maintain thread/cutter alignment by keeping the leadscrew engaged at all times.

On a metric lathe, the thread-dial instructions need careful study and the dial may not cope with all pitches. Missing pitches are made by leaving the leadscrew permanently engaged. This is the general method that always works.

On a metric lathe the pitch wheel under the thread-dial engages with the leadscrew to show alignment as on an imperial lathe. Unfortunately some pitches need a different dial to leadscrew ratio, which may not be available. The thread dial on my metric WM280 has only one pitch wheel, and its thread dial can't be used to make some common threads, boo hoo. From memory my metric mini-lathe had 3 pitch wheels and did more pitches. Provided you remembered to set it up right...

Usually I'll tell anyone not already dead from boredom that metric is immeasurably better than imperial. Cutting metric pitch threads on a metric lathe is so awkward it must be the kludge that proves the rule.

Dave

Barrie Lever05/03/2019 18:54:41
316 forum posts
56 photos

Plasma

Being as how you are practising for the bigger project, I would strongly recommend that you setup a little more conventionally than your description, so up right tool, top slide at 30 degrees as per Jasons illustration, wind the cut on from the top slide as it puts less force back into the tool, if your lathe is big this is not so important.

The really nice finishes on threads come when you finish the thread with a chaser, I think this might be important on your project as the threads are a fairly big diameter and thus very visible and probably more likely to bind up if something is wrong.

Be sure to retract the tool for the return travel if the leadscrew is staying engaged, always do the retraction of the tool on the cross slide and go back to the known position before starting the next cut.

Finally if in any doubt don't disengage the lead screw, with it permanantly engaged you cannot go wrong, if I was thrown into workshop in the middle of nowhere (these type of things happen to me!) and I was told to get cutting a thread on some old clunker lathe, I would do all of the above described.

Good luck.

Barrie

Andrew Johnston05/03/2019 19:51:40
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4710 forum posts
532 photos

Torn threads and multiple starts are probably caused by different issues. First we need to sort out the multiple starts. More information is needed:

What is the OD and pitch of the metric thread to be cut?

What lathe is being used, and is it imperial or metric?

Can it cut the required pitch?

If it can, be aware that higher spec lathes have multiple gears on the thread dial indicator - you need to use the correct gear for the required pitch

Note to SoD - hooray for imperial and tpi smile

For the torn threads, again more information is needed:

Is the tool HSS or carbide threading insert?

If an insert where did it come from? Hint: bang good is the wrong answer.

What is the material, and where did it come from, ie, known specification or gawdonlynose scrap?

A picture of the setup would be helpful.

Andrew

Plasma06/03/2019 11:01:34
326 forum posts
41 photos

I knew my post lacked enough detail to cover all the bases as regards screw cutting.

I'm trying to thread free cutting steel but have tried brass with better results.

The reasoning behind the inverted tool is to feed away from the headstock which is safer than trying to stop before a crash.

I've tried an insert tool of doubtful heritage and a hand ground HSS tool which seemed to give better results.

Perhaps it's just a case of practice makes perfect.

Thanks to all for their input.

Martin Kyte06/03/2019 11:11:34
1463 forum posts
24 photos

Are you using any cutting oil?

Martin

larry phelan 106/03/2019 11:33:38
458 forum posts
11 photos

Screwcutting is one of the jobs I really like doing,and although I am no expert [far from it ! ],I get fairly good results and great satisfaction from it.

May I suggest that you pick up a copy of Sparey,s book,where you will find ALL the info you need,explained in very simple terms [even I could understand it ! ] Everything is explained and he mentions that even at that time,leaving the half nuts engaged for all threads was becoming common in industry. I never use the thread dial for any threads.

I have cut threads up to 2 1/4 " 8 tpi without any trouble,left and right hand. Sounds like you are missing your start point each time,also,no need to invert the tool,works OK right way up. Tool was nothing more than HSS,ground by eye to form.

Give it a try,it will come to you.

Andy Carruthers06/03/2019 11:43:57
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249 forum posts
23 photos

One item not mentioned here - I am no expert and you probably know this - is depth of cut which for Metric is 0.6134 * pitch with 60 degree tool

Chris Trice06/03/2019 11:47:20
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1362 forum posts
9 photos

Larry, I think the point of the inverted tool was because Plasma was cutting away from the chuck rather than towards it.

AdrianR06/03/2019 13:53:57
265 forum posts
20 photos

One draw back of the inverted tool is that the cutting force is upwards. This would be lifting the saddle instead of pushing it down. You would need a robust machine and well adjusted gibs.

That could explain the poor cutting as your tool bit will be jumping up and down.

peak406/03/2019 13:57:42
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786 forum posts
67 photos
Posted by Chris Trice on 06/03/2019 11:47:20:

Larry, I think the point of the inverted tool was because Plasma was cutting away from the chuck rather than towards it.

Indeed so, and it's probably also worth mentioning, that if Plasma wants to continue with that method, offsetting the top slide needs to be at 29.5° the other side of the cross slide handwheel.

i.e. the cut is put on the leading, rather than the trailing edge of the tool.

I've actually done similar jobs with the lathe in reverse, but using an internal screwcutting tool on the rear side of the work, so you can see the cut in progress.

Bill

Jon06/03/2019 14:40:12
988 forum posts
46 photos

Think most are trying to baffle him, start logically keep compound 90 degrees on and do it the way the pros do.
Will then iron out and or eliminate that way of doings things.

Rest as Andrew said yesterday.

John Reese06/03/2019 19:36:42
768 forum posts

Two things I do to help prevent tearing:

Use a sulfur based cutting oil. The dark cutting oil used for pipe threading is excellent.

Grind some top rake on your tool. This is especially helpful on gummy materials. On-edge carbide threading inserts are available with positive rake.

jason udall06/03/2019 19:46:02
2010 forum posts
41 photos

Err just checking....when you inverse tool.....did you check tool height.... don't ask

Phil Whitley06/03/2019 19:54:54
851 forum posts
115 photos

I think AdrianR has got the problem, my Colchester has no facility to adjust the saddle fit to the bed well enough to cope with an upward cutting force There is play and you can lift the saddle very slightly, so it certainly would not cope with this, are the half nuts loose or worn?, could be many things, what sort of lathe is it? Give us a clue!

JasonB06/03/2019 20:15:27
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Moderator
15743 forum posts
1646 photos
1 articles

Strange how everyone and their dog seems to suggest inverted parting tools when the subject comes up but then says it won't work for threading?

I' quite often turn large flywheels with an inverted boring bar where load is far greater and get perfectly good results so don't think that's an issue unless the lathe is really worn

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