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Screwcutting on the lathe

Surely, it's not THIS hard?

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not done it yet12/02/2020 23:27:08
4170 forum posts
15 photos

Does anyone make internal threads of 10mm, or less, using the lathe and a single point cutter?? 1/2” square internal threads are small enough for me!

Hopper13/02/2020 01:45:56
4165 forum posts
89 photos
Posted by Hollowpoint on 12/02/2020 20:44:27:


I think we (engineer types) are all guilty of over complicating things at times.

We certainly see instances of the armchair engineers over-complicating things.

But I always bear in mind the words of Nevil Shute, author of "The Trustee from the Toolroom" and other more famous novels (that I read while sitting in my armchair laugh ):

'An engineer is a man who can do for five bob what any bloody fool can do for a quid'.

Shute was a top notch aircraft engineer and modelling enthusiast as well as an author.


Edited By Hopper on 13/02/2020 02:04:08

Iain Downs13/02/2020 17:00:40
562 forum posts
454 photos

Here's the thing. I want to make a steam gland with the insert having an 18mm external diameter screwed into a larger piece (25mm from memory). I've decided to use 1.5mm pitch for the thread.

Apparently this is a spark plug thread so I could actually buy a tap and die set for this. But I don't want to (a - I'm cheap and b) I want to learn how to DO this!).

I'm practicing on 10mm bar because a) I have some and b) I have some (professional) nuts and bolts which fit this.

Once I find I can make an internal thread which matches an external thread, then I will whoop with glee and stick some 18mm bar in the lather and make the actual piece.

5 bob well spent!


Iain Downs15/02/2020 10:28:07
562 forum posts
454 photos

I just realised. I've had lots of good advice on tidying up an external thread (files etc), but how do I tidy up an internal thread?


Andrew Johnston15/02/2020 11:11:52
5201 forum posts
599 photos
Posted by Iain Downs on 15/02/2020 10:28:07:

I just realised. I've had lots of good advice on tidying up an external thread (files etc), but how do I tidy up an internal thread?

That's one reason I use full form inserts; you don't need to faff about cleaning up threads. Why make life difficult? My personal view is that 1.5mm pitch is rather coarse for the application.


Bazyle15/02/2020 11:12:13
5013 forum posts
198 photos

Ian. If you can cut a good external thread you can make a plug tap to finish or completely tap your internal thread. If you make it the same time as you make the insert it should match even if you invent your own thread.

Pete Rimmer15/02/2020 11:44:22
594 forum posts
28 photos

For standard threads you can just put a tapping size drill down quickly to remove the burr. For larger ones you'll have to suffer it or do a clean-up cut with a boring bar.

i never seem to suffer raised burrs so much on internal threads TBH.

Iain Downs23/02/2020 17:08:45
562 forum posts
454 photos

Right. So, the first thing I did was to take Mr JOhnstone's advice and re-think my thread size.

I've got some 10mm x 1mm taps and dies and made a 10x1 nut and bolt as standards.

The lathed bolt was OK and fitted in the tapped nut nicely.

The lathed nut was a bit wobbly so I'd clearly over cut, but I thought it worth having a go with the part I'm trying to make (piston gland).

The nut for the piston gland (external thread) went OK despite having to turn the thread up to a corner.

The gland itself was more daunting as I had to thread it up to an internal corner.

The idea was to cut it up to the rough depth and then try the nut, cut a bit more, try the nut and so on. Not terribly professional, but then I'm not.

I found I was tapping much deeper than I should and the nut still wouldn't go on. I also started to loose track of the depth and at one point the half nut slipped and I had to find my way back into the thread. I also noticed that the QCTP had slipped a bit so that wasn't helping.

In the end I tried a little harder to thread the nut on and - hallelujah! went on. Wobbled a bit, mind you but I thought I was there.

Unfortunately, when I tidied the internal thread up with a light touch from a scraper the nut stopped screwing in and started pushing in.

I think I have to scrap that part and start again. But not tonight - there's only so much I can take!

The lesson seems to be to be a bit firmer about trying the nut - and I might make a non-hardened thread cleaner - a sort of poor man's tap as my try nut. The idea is that it will both try the fit and clean up the threads a bit. Sort of tap profile, but in mild. Or if I go mad I might have a go at making a tap. I have some 20mm silver steel I could cut down. We'll see - all these digressions take away from actually building things!

Thanks for all the help so far. I'll post again when I've had time to try again.


Andrew Johnston23/02/2020 22:18:03
5201 forum posts
599 photos
Posted by Iain Downs on 23/02/2020 17:08:45:

The idea was to cut it up to the rough depth and then try the nut, cut a bit more, try the nut and so on. Not terribly professional, but then I'm not.

That's what I do, at least to get an initial number on the cross slide to aim for. I find a quick brush of the thread helps before trying the mating thread. It's particularly helpful for dusty materials like cast iron. It's surprising how little dust is needed between won't go on and a rattly fit.

Funny thing, but I've just bought M10x1 taps and die. The internal threads on the steam head for my traction engines are shown as 3/8" by 26tpi, aka British Standard Brass (BSB) thread. While taps and dies are still available they're not especially cheap. I'll be making the mating parts with external threads rather than use the commercial parts listed on the drawing, so no need to stick to BSB. The M10x1 thread is pretty close, the taps and dies are less than half price and best of all a spiral flute tap is available. So I only need to buy one tap, not a set of two or even three.


Robin Graham24/02/2020 00:49:49
670 forum posts
152 photos

I've been doing some M22x1.5 external threads to match commercial parts recently, and have experienced some of the OP's frustrations. I ended up turning the blanks to nominal diameter, blueing them up (layout blue), throwing theory out of the window, then going at it with with a carbide insert screwcutting bit until I could no longer see blue on the crests. That seems to me to mean that the major diameter must be right, and the bit should take care of the rest.

After a swipe or three with a file to take off jagged edges and give a bit of clearance this seems to work. For me at least! It's not always easy to judge when the blue has disappeared, but I got an eye for it after doing a few.


Iain Downs06/04/2020 19:09:11
562 forum posts
454 photos

I confess that I still don't think I've 'got it' as far as threading is concerned and my initial go at internal threading was a mess.

So I thought I'd make a tap for the inside.

I had some nice silver steel spare from my Steam Engine crank. This was lathed into a basic shape

tap basic shape.jpg

Then the end squared off

tap squaring.jpg

Then a thread put on

tap cutting.jpg

tap ready for heating.jpg

Next I used a ball nosed mill to cut some slots and prepared for heating

tap on hearth.jpg

Tap here on the hearth, wrapped in wire and gummed up with Boric acid with a bit of meths.

If it's good enough for Chris of Clickspring, it's damn well good enough for me.

Next a heat up and a dip into some sunflower oil.

In a spirit of full disclosure, my first go at this didn't go se well and a file could cut it, so once more into the heat. This time it worked better and I cleaned the boric acid in boiling water and then dipped it in Acetic acid for a day or so to get the scale off. I think I could probably have left it in for longer.

tap in use.jpg

And here we have it cutting a thread. Again in a spirit of openness, I actually cut the bulk of the thread with a lathe tool and mainly used my new tool as a plug tap. It seems to have worked well enough

And here is the piston gland inside and outside! The two parts screw together well, though I fear that the outer part (this bit) got a bit nudged in the process and may not be fully concentric. We'll find out when I put it all together!

piston gland.jpg


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