Screwcutting on the lathe

[SillyOldDufferModerator @sillyoldduffer]

Posted by Steviegtr on 10/02/2020 20:06:49:

…

His attitude is why would I ever want to screwcut when I have all the taps & dies.

…

Your mission is to educate the poor deluded bodger! He's wasted a Colchester for 35 years, be gentle with him.

Even owning a complete set of taps & dies there's advantage in screw-cutting with a lathe. I already mentioned the value of reducing wear and tear on dies by using a lathe to cut a partial thread as a guide. Another important reason is that lathe cut threads are accurately aligned; they don't wander as taps & dies are prone to do.

There's more – lathes can make any thread pitch from the gears it has available, and thread non-standard diameters, and make taps and dies. After being used to true up the face, etc, etc.

On second thoughts, don't be gentle. The chap has an attitude. Off with his half-nuts, he'll never miss 'em!

Dave

[SteviegtrParticipant @steviegtr]

Ha ha good one.

Steve.

[Andy CarlsonParticipant @andycarlson18141]

I've enjoyed reading this thread having been taught single point threading by an apprentice instructor in the 1980s and then come back to lathe work in the past couple of years screwcutting is definitely more tricky than you plan for… for all of the reasons stated.

Regarding getting the feed to stop in the correct place without crashing, yes running in reverse might be an option but only if your workholding device doesn't attach to the spindle with a screw thread. Mine does.

One option I have used myself and will chip in here is to use a handle to turn the spindle manually for the last few threads. I've used this myself and it makes it dead easy to cut a thread up to an exact stopping point – with the handle you can feel when you reach the end with no problem at all.

My 'handle' is nowhere near as sophisticated as those I've seen elsewhere. I bought some hex steel bar from an oddments bin, bored and threaded this with the 32 TPI thread to fit the back end of my spindle. The 'handle' is just a bog standard hex socket and a ratchet handle from my socket set. Works a treat.

[HollowpointParticipant @hollowpoint]

I must be the odd one out, I don't find screw cutting difficult at all. I picked it up quite quickly and I actually really enjoy the process. I'm no expert though and its probably because I don't bother getting bogged down with all the technical details. I've always cut threads with the cutter at 90 degrees to the work and just go slow and steady, I use the handwheel graduations as nothing more than a guide. Trying to make accurate measurements with them is ridiculous in my opinion, especially when you have backlash, deflection and machine flex to consider. Just keep checking the thread with a thread guage or by screwing a nut on until it feel right. Very few of us need perfectly formed nasa grade threads! Keep it simple.

[SteviegtrParticipant @steviegtr]

I want to chip in here. Get it chip , no ok. The strong point I want to make is something dad said to me when I was younger. A lot younger. Was because I was doing so bad at school he asked me about the history lesson's & said do you enjoy them, I said no. Do you like maths, no, do you like anything I said yes. What is it. Metalwork, I love it. Then that is something you will be good at, because if you don't like it you will never be any good at it. If you love it you will excel. MMMM if I could just find the on off sw for the lathe. I have some 33 34 change gears for the myford & have just got to have a go at metric on an imperial. I am sure it will all end in tears but after reading this thread I must at some point have a go.

Steve.

[not done it yetParticipant @notdoneityet]

Hollowpoint,

You are a true hobbyist – not bothered about time and simply enjoy getting to the end product.

(a thread in this case).

Percy Verence was good at what he did, for that very reason. He always managed to get the job done eventually.

Lots of things are akin to building a house with a pack of cards. If you are missing any of the layers, the top ones will collapse – or not be supported properly, thereby making it difficult to finish the job.

If you miss out on the basics, the harder parts can become overwhelmingly difficult. That is the way I look at maths, for instance. If Percy had cried off at the first hurdle he would not have achieved as much as he did.

[Bob Unitt 1Participant @bobunitt1]

Today I was making a part for the later version of the Worden tool & cutter grinder (mine is over 15 years old). The part called for an accurate 1/4" x 40 TPI thread 1" long, so i screwcut it to a smidgen oversize and finished it off with a die. This gave me a thread which was truly concentric with the part, had an accurate thread–form, and was a slightly tight shake-free fit in a tap-threaded hole – which was exactly what I was trying to achieve.

Edited By Bob Unitt 1 on 12/02/2020 19:22:23

[HollowpointParticipant @hollowpoint]

Posted by not done it yet on 12/02/2020 08:25:13:

Hollowpoint,

You are a true hobbyist – not bothered about time and simply enjoy getting to the end product.

(a thread in this case).

Percy Verence was good at what he did, for that very reason. He always managed to get the job done eventually.

Lots of things are akin to building a house with a pack of cards. If you are missing any of the layers, the top ones will collapse – or not be supported properly, thereby making it difficult to finish the job.

If you miss out on the basics, the harder parts can become overwhelmingly difficult. That is the way I look at maths, for instance. If Percy had cried off at the first hurdle he would not have achieved as much as he did.

I'll take that as a compliment 🙂

I think we (engineer types) are all guilty of over complicating things at times. I know I do. This is when we get frustrated. Having a go and making mistakes is the best way to learn anything!

[not done it yetParticipant @notdoneityet]

Having a go and making mistakes is the best way to learn anything!

Would agree for most things, but there are a few things I might want to get right first time! Bomb making and parachuting might be on that list – not that In would want to do either of those at my age!

[Iain DownsParticipant @iaindowns78295]

Thanks for all the comments guys.

It looks like the weekend before I get back in the shed, but I will have another go at a 10mm bar and then work on an internal thread to match (shudder). If that all goes smoothly, I will thread the piece that I actually want to make!

Iain

[Pete RimmerParticipant @peterimmer30576]

Iain you're making it hard for yourself turning a thread on such a small diameter for a practice piece. A larger diameter thread would be a lot easier.

[not done it yetParticipant @notdoneityet]

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!

[HopperParticipant @hopper]

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 ):

'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 DownsParticipant @iaindowns78295]

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

[Iain DownsParticipant @iaindowns78295]

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?

Iian

[Anonymous]

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.

Andrew

[BazyleParticipant @bazyle]

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 RimmerParticipant @peterimmer30576]

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 DownsParticipant @iaindowns78295]

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 10×1 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.

Iain

[Anonymous]

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.

Andrew

[Robin GrahamParticipant @robingraham42208]

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.

Robin.

[Iain DownsParticipant @iaindowns78295]

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

Then the end squared off

Then a thread put on

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

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.

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!

Iain

[Mark ReaParticipant @markrea67084]

Have read this and no one has mentioned tool height. It is important especialy when screwcutting

[peak4Participant @peak4]

It wouldn't be the first time that I've used one flute of a smaller tap as a chaser to tidy up an internal thread.
Obviously it has to have the correct tpi, and it's all abit rough and ready.
Last night I needed to clean up a very tight threaded collar for a spindle, I'm not sure if there was damage to the thread, or whether some swarf had embedded itself.

I certainly didn't have a 5/8" x 22tpi tap, but one flute of a 5/16" BSF tap seemed to do the job OK.
Yes I appreciate the helix angle wasn't correct, but at least the collar fitted when I'd finished.

Bill

[Martin ConnellyParticipant @martinconnelly55370]

Thread milling cutters can be used for multiple diameter threads of the same form because a small cutter in a large diameter does not need to match the helix angle. The smaller radius on the tool means it is not going to interfere with the formed thread already cut. If you buy a cutter there is usually a minimum size for acceptable threads but no maximum. If memory serves correctly I think the general rule is that they are ok to use if the cutter diameter is less than 60% of the thread diameter being cut. I would expect a similar rule can be applied to chasing threads with a suitable small tap. An exercise in 3D cad would show if there is any excess material removed by your BSF tap.

I have reduced a 1" tap to a single point threading tool to cut lathe backplates with no issues (I had a damaged tap to mangle so no destruction of a perfectly good tool). The helix angle did not have to be taken into account.

Martin C

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