Screwcutting on the lathe

[Martin ConnellyParticipant @martinconnelly55370]

Sounds like you have the correct setting for 29 degrees. The other issue will be spring in the bar. It is usual to need a number of spring passes with the tool. If you keep all settings the same and do more final passes is the tool still touching the bar or removing material? For long threading jobs a moving steady is useful.

Martin C

[Graham MeekParticipant @grahammeek88282]

Ian,

If you start screwcutting with a tool which has a dead sharp Vee point then you are falling at the first hurdle. Your procedure is correct, but the sharp pointed tool is giving you the wrong starting point. A radius or small flat is required, it is easy to check the radius required with the use of thread gauges or a 10 mm bolt will serve just as well. The radius is not that precise so don't be put off by this requirement.

Even then with the correct radius I would still expect to have to adjust the depth to suit a commercial nut. It is always worth running a plug tap through any commercial nut before hand. I have yet to find one nut that a plug tap does not clean out.

Regards

Gray,

[Martin ConnellyParticipant @martinconnelly55370]

The insert in the picture has a 0.2mm radius which I think gives 0.2mm difference from a sharp point.

Martin C

[old martParticipant @oldmart]

That insert in the picture is for turning not threading, try some threading inserts and a holder.

[Martin ConnellyParticipant @martinconnelly55370]

Old Mart, I have insert threading bits and insert holders and ground HSS bits tools as well as turning inserts and special threading tools. For the job in the picture the 60 degree insert had enough back rake to cut the thread required. I use what works for the job to be done.

Martin C

[HopperParticipant @hopper]

This thread is pointless without pics of your threads and of the lathe set up and tooling you used to make them. Otherwise we are just guessing in the dark.

It's like the 10 blind men describing an elephant from feel. (I'm usually the one at the back saying "An elephant is about the size and shape of a football but warm and steamy.&quot

 

 

 

 

Edited By Hopper on 02/12/2019 00:35:04

[old martParticipant @oldmart]

Why not ditch the 29 degrees and keep the compound parallel to the axis? It is so much less complicated for beginners. The finishing cuts should be only about 0.001" and should not be too much even for a small lathe. Proper threading inserts are sharp and do not cause undue stresses on the machine.                                                                 The other advantage of going in straight is you can use the scales to measure the exact thread depth directly, which is found in decent thread charts.

Edited By old mart on 02/12/2019 14:38:47

[Iain DownsParticipant @iaindowns78295]

So I took some photos.

This is my topslide / cross slide setup and I think it's right.

This is a close up of the cutting tool

Which I think you will agree looks like it has quite a sharp point.

To give you and ideal of scale here it is in a QCTP holder (a 000 type). I think the main body of the tool is 6mm across.

So I expect I need to round the end. How do I know when I've rounded enough?

Thanks

Iain

[not done it yetParticipant @notdoneityet]

One simple advantage of setting the tool to 29ish degrees is that sine of 30 degrees happens to be 0.5, so advancing an amount will be half the depth increase (about 95% actually).

That can provide a good estimate of when to check for the fit of the nut and less depth, per unit advancement of the top slide, for that final fit. In practice, watching the tops of the threads gives one a good idea of nearing the ‘fitting’ stage for larger threads – but not so easy with model engineering threads.

[HopperParticipant @hopper]

Did you use a tailstock centre while cutting your thread? If not, deflection of the job away from the tool is almost certain, which could be the cause of your problem or at least part of it.

Threads in the pic, although a bit blurry, do look about half depth or so.

Is your cross–slide handle one of those that has the graduations that show reduction in the job's DIAMETER rather than the actual distance the tool is moved inwards on the radius? The way you set your thread depth described in an earlier post would result in a half-depth thread if this was the case. Check toolpost movement vs cross-slide collar measurements with a dial indicator to be sure. Or take a cut on a piece of scrap and compare reduction in its diameter with the cross-slide graduations.

Give screwcutting a try using the method NDIY outlines above: Set your tool on the unmachined job surface and set the topslide dial to 0. Then proceed to screwcut using the topslide measurement until you get to .92mm (or whatever your desired depth is) on the topslide graduated collar. As NDIY says, this should be about 95 per cent of the actual "straight in" thread depth and within a few hundredths of a mm of being finished. I use a felt pen to mark the full depth point on the graduated collar so I don't forget and miss it.

Can't tell much about the tool from your pic as it seems to have a large booger right on the tip. But probably OK. Usually on a small thread like that, a couple of rubs on a bench oil stone is enough to create sufficient radius. Radius on a small thread like that is tiny and not enough to create the half finished thread you are getting.

If you are not confident to grind your own HSS toolbit you might invest in a carbide insert thread cutting tool, one that will do different pitches of thread, so it does not have the groove in it to machine the crest of a specific pitch. But I don't think this current tool bit is the cause of your problems. I'm rather keen on the cross slide collar graduation theory.

[DiogenesIIParticipant @diogenesii]

"I've moved the cutter forward with the top slide until it just touches the bar. I set the zero on the top slide and advanced it 0.92mm (3 and a bit divisions off a full turn).

Next zero the cross slide and the top slide. Move the topslide back until it just touches the bar again (a bit over one turn)…"

Your tool point looks fine – when you say "move the topslide back", you need to come much further back, and then go forward again to touch, in order to remove any backlash in the feedscrew..

..at present it sounds like you are touching off, turning less than a full turn (to achieve "full depth" zero), then coming back over 1 full turn to touch again.. try coming back, say, 3/4 of a turn, and then go forward again for your final touch..

sincere apologies if I have misinterpreted…

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Iain Downs on 02/12/2019 20:20:11:

[…]

So I expect I need to round the end. How do I know when I've rounded enough?

.

Have a good look at the picture here: **LINK**

https://en.wikipedia.org/wiki/ISO_metric_screw_thread

[strong coffee might help with interpretation]

The tip of the tool should ideally fit the profile shown [twice, in pale grey] for the external thread.

… If it is more pointed, you will need to cut deeper to compensate

… if it is more rounded, the nut might bind

MichaelG.

[pgk pgkParticipant @pgkpgk17461]

I see no-one has mentiond the use of thread wires – fiddly but one of the ways of matching accepted tolerances as opposed to making it fit a matching part.

[Ron LadenParticipant @ronladen17547]

Posted by old mart on 02/12/2019 14:35:40:

Why not ditch the 29 degrees and keep the compound parallel to the axis? It is so much less complicated for beginners. The finishing cuts should be only about 0.001" and should not be too much even for a small lathe. Proper threading inserts are sharp and do not cause undue stresses on the machine. The other advantage of going in straight is you can use the scales to measure the exact thread depth directly, which is found in decent thread charts.

Edited By old mart on 02/12/2019 14:38:47

Thats what I do with the tool square to the job, it seems to work fine for me and I am a relative beginner when it comes to thread cutting.

Ron

Edited By Ron Laden on 03/12/2019 08:29:11

[not done it yetParticipant @notdoneityet]

l just looked at the pic above. How much stick-out from the chuck is that? Looks horrendous. Needs a centre as a minimum requirement! No excuse to say the tool would not fit without fouling the centre – one thinks ahead and leaves a length of blank turned to, or less than, minor thread diameter to avoid any interference and remove it later. Or maybe simply use a half-centre.

[Graham MeekParticipant @grahammeek88282]

I for one can see a centre in the work!

I also suspect the amount protruding from the chuck is to allow time to dis-engage the half nuts before a collision. A wise move if you are doing screwcutting for the first time.

Also having the tool set around at half the thread angle means you do not have to remember what your last cut was, as when the cross-slide is returned to zero, the cut is already set at the last movement.

Regards

Gray.

[Pete RimmerParticipant @peterimmer30576]

Iain I've done a little drawing in cad today and your 0.92mm infeed at 30 degrees comes up slightly short of the minimum depth you'd need to reach the minor diameter for the nut so it would never go on, and it would require a 0.4mm flat to attain the proper flank width at that depth anyway. Obviously you don't want that because your thread is 1. not quite deep enough and 2. has no root clearance.

If your tip width was 0.25mm (10 thou) you could infeed 1.06mm and you would have a usable thread with good root clearance.

With a sharp point you need 1.31mm infeed, at 30 degrees to achieve the proper thread width at the pitch line. This is not very desirable because the sharp point makes for a much weaker thread.

Edited By Pete Rimmer on 04/12/2019 11:51:13

[Iain DownsParticipant @iaindowns78295]

Right now I am totally at a loss. And apologies for the gap – a new mill got in the way.

I had another go.

The first thing i did was to compare the cutting tool against an M10 bolt (1.5mm pitch).

Apologies for the quality of the photo – it's one of many things I'm no good at!

by eye and even here, the tool matches the profile of the thread quite well.

Then I set up the lather to screw cut – centre in the headstock and all.

Then went straight in. None of this angled nonsense.

What I found was that at 0.9mm or so a nut would not go on the thread. this is the theoretical depth of the thread. I kept going in 2 though (0.05mm) increments. The first point at which the nut would go on was at a depth of 1.3mm.

Even at that it was a bit of a struggle. And when I ran a M10 die over it there was still swarf coming off.

So. It looks like I've misunderstood the various postings and I need to cut about 1.3mm deep not the 0.9mm I'd understood from reading).

Incidentally, applying O Level maths the depth of a 1.5mm wide 60degree angle triangle works out to be 1.299mm – roughly in accord with what I've found.

The other thing I found was that I was cutting in increments of about 2 thou (0.05mm) all the way up from about 0.8mm or the lathe would stall. Even at that I hard to run a little faster then I was comfortable with (250 rpm) to be sure to get the cutt – which meant more of a struggle to get it to stop at just the right place.

I remain confused about why ALL the books I've read on screwcutting seem to miss out the the depth calculation as it it were easy.

Any advice welcome – as always.

Iain

[Tony Pratt 1Participant @tonypratt1]

Iain,

Just a thought, did you reduce the bar diameter to about 9.8 mm to give yourself a bit of clearance on the bolt crests?

Tony

[SillyOldDufferModerator @sillyoldduffer]

Metric threads are quite easy, at least if a calculator is handy and you have a good memory!

The depth of cut is always 0.87 × pitch, so M10x1.5 is

1.5×0.87 = 1.305mm (approx)

My usual mistake is forgetting my cross-slide is graduated in reduction of diameter, which is half depth of cut.

A confusion may be because properly formed metric threads have flattened tops an valleys which suggests a smaller doc than ×0.87 does. Maybe I'm lazy, but I just go in full depth and then finish off by flattening the tops slightly with a file as it spins. Also helps to give it a quick final polish with a coarse plastic pan scourer,

Dave

[Martin ConnellyParticipant @martinconnelly55370]

If you put a micrometer across the newly cut threads you will probably find the diameter has increased compared to the original stock diameter. There is usually some plastic deformation of material near the crests that pushes it out rather than perfectly cutting it. This is why you can buy full form threading inserts, they clean off this material. The problem is you need one to suit each thread size so are usually not worth the expense for hobbyists. It is also why the earlier advice to run a file over the new thread makes sense. The accurate way to measure threads is the three wire method which ignores problems with crests and roots but is probably over the top for you current needs.

The advantage of threading in the lathe is the thread pitch should be correct, the thread form should be axial to the stock and the thread can be made tight or slack as required. Cutting a thread with a die in a die stock with the material in a vice can result in wonky (technical term) threads. Using a die in a tailstock die holder may result in pitch errors if there is too much resistance to the die following the thread as it is formed. However once the thread has been formed in the lathe it can be polished with a suitable die in a die holder. If you have a split die start with it opened up and see if it removes some material. Try the nut on it and if it is too tight close the die slightly and try again. Repeat until you have a good thread.

Martin C

[HopperParticipant @hopper]

Well done. As you are finding, screwcutting is somewhat of an art and not just science.

There is always a burr, and as Martin points out some deformation, thrown up from the crest of the thread when screwcutting. This needs to be removed by running the corner of a flat file along the thread with the lathe under power. Keep the file moving at all times and run it along the thread several times. Otherwise, yes you will have trouble getting a nut to go on there.

Also, you should take a cut over the outside diameter of the blank before screwcutting so the thread OD is both concentric and slightly under the nominal size for clearance's sake.

As for your query about books and thread depth calculations, it's in all the old books I have. (Sparey, Duplex, GHT etc) Interestingly they usually quote a thread depth of about .64 x pitch for a 60 degree UNC thread. Which is way off from the modern recommendation of .87 x pitch for 60 degree metric. Bit of a mystery but I think the latter refers to a pointed tool bit with the job OD at the nominal maximum. In line with a pure right angle triangle calculation The old books I think assumed a radius on the tool tip and the blank OD starting out undersize for clearance. Hence less thread depth.

But once you have worked out what works for you and your lathe, stick with it and go for it!

Whichever way you go, the ultimate check is try and fit to the nut. Theory and measurements are one thing. Getting the darned things to screw together can often be something again.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Hopper on 10/02/2020 01:00:04:

…

As for your query about books and thread depth calculations, it's in all the old books I have. (Sparey, Duplex, GHT etc) Interestingly they usually quote a thread depth of about .64 x pitch for a 60 degree UNC thread. Which is way off from the modern recommendation of .87 x pitch for 60 degree metric. Bit of a mystery but I think the latter refers to a pointed tool bit with the job OD at the nominal maximum. In line with a pure right angle triangle calculation …

But once you have worked out what works for you and your lathe, stick with it and go for it!

…

 

 

Pretty sure Hopper is right.

An amateur thread cut with a 60° sharp V should be 0.89 x Pitch deep. But properly cut UNC/UNF/Metric threads aren't sharp V's. See footnote #

For practical reasons correctly formed UNC/UNF/Metric threads are flattened slightly top and bottom. Not allowing a sharp V at the base makes the thread stronger by removing a stress raiser. Slicing off the sharp top makes the thread easier to insert, less likely to jamb, and harder to damage accidentally. All good stuff, but tweaking the shape of the thread to meet the exact specification alters the dimensions slightly. The specification says the depth of cut from the top of the 'V' to the bottom flat is 3/4 of Pitch, which about 0.65 as per Sparey and friends. That means the cutter cannot be a sharp V, without special manuipulation. The depth of cut is measured from the outer diameter, which is later removed. Quite confusing!

Then choices can be made:

• For rough work, where the thread doesn't have to be a strong as possible, it's reasonable to cut a sharp 0.87 deep V thread. It will fit. But this isn't a 'proper' thread.
• If the thread specification matters, better to cut 0.65 deep with a V cutter. The resulting thread won't fit, but it can be finished to size and shape with a die. The partial thread cut by the lathe is done to minimise wear and tear on the die rather than to produce a fully finished thread.
• It is possible to produce flat bottomed threads with a V cutter, see the angled tool-post technique rather than plunging straight in.
• If a properly shaped carbide or HSS thread cutter is available, cut to 0.65.

Also bear in mind Martin's point about plastic deformation. Machining often deviates somewhat from theory because tools wear, materials squidge, everything bends, and the operator makes small mistakes. Most of the time in a hobby workshop, I feel it's not necessary to meet industrial standards, for example, a sharp V is 'good enough'. Exactly how a job is done is often a judgement call, all part of the learning process. We'd get bored if it was easy!

Dave

# The thread form of all three is identical – no difference in shape, they differ only in pitch, so the same sums apply in this case to both Imperial and Metric.

 

Edited By SillyOldDuffer on 10/02/2020 10:18:39

[David Standing 1Participant @davidstanding1]

Posted by SillyOldDuffer on 10/02/2020 10:13:39:

 

 

 

materials squidge

 

Don't they just. What a great description

 

 

 

Edited By David Standing 1 on 10/02/2020 10:34:28

[SteviegtrParticipant @steviegtr]

A friend of mine who restores Classic bikes, mainly BSA. Has had a lathe for over 35 years. A Colchester I think. Huge monster. His attitude is why would I ever want to screwcut when I have all the taps & dies.

But I can see the satisfaction of having a go at it. Something on my list for the future. Have a chimp to help me now so what could go wrong.

Steve.

[Author]

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