Thread cutting problem

[Anonymous]

I use the nominal thread diameter. That's true whatever method is used to form the thread – single point screwcutting, Coventry diehead, or split dies by hand and under power on the repetition lathe.

Andrew

[BufferParticipant @buffer]

Andrerw

What is the nominal thread diameter?

Rich

[Ian Johnson 1Participant @ianjohnson1]

Funnily enough I actually made three stainless 10-32 UNF washer faced bolts tonight for my Norton air filter box, but didn't have a die, so I used a 2BA die instead which is near enough.

I always make the diameter a bit smaller when die cutting stainless threads, so ended up at 0.182 not 0.185 as it should be for 2BA, everything worked out great and fits great too. Although when machining brass its nice and easy to thread so I would make the diameter to size.

So don't worry too much about outside diameter if it's under by a few thou, if you get 75% thread contact area that is certainly good enough for model engineering.

IanJ

[BufferParticipant @buffer]

I'm now wondering if the threads are being stripped as I am trying to get the thread started. In other words maybe it is rotating without pulling itself along which is stripping the start of the thread clean.

[Paul LousickParticipant @paullousick59116]

I had a similar problem cutting M2 threads in brass. The die stripped the material instead of cutting a thread when starting. Not sure but die could have been sharp enough and I did not have another.

Problem solved by making the part to be threaded longer and reducing the diameter at the end so the die could start easily (cutting a half height thread). This allowed me to continue the cut on the full diameter. The extra material at the end then removed.

Paul.

[Ian Johnson 1Participant @ianjohnson1]

Try 10 thou undersize and see what happens

IanJ

[Anonymous]

Posted by Buffer on 28/11/2020 21:31:10:

What is the nominal thread diameter?

By nominal I mean the thread size. So in the case of your 3/16" x 40 thread I'd turn to 3/16", without worrying about the 5 tenths, say 0.187".

I don't think having the blank at nominal thread diameter is the problem. I've recently made a batch of wick feed oilers for my traction engines:

The threads on the spigot underneath the body are 1/4" x 40. They were screwcut using a full form insert; the OD measures 0.25" give or take the odd few tenths, some over some under. Likewise the lids bottom left have a 7/16" x 32 screwcut thread. They measure 0.4375" on the OD.

The interesting part is the 5/32" x 40 threads on the copper tubes. The tube OD measures 0.1575". A 2.5mm drill slips neatly inside. That gives a wall thickness of 0.0295". A 40 tpi Whitworth thread has a depth of about 0.016" so that leaves 0.0135" minimum wall thickness after threading. When I made the first batch of these oilers I screwcut the 5/32" x 40 threads. But it was a pain as slightly too much depth of cut and the remaining thin wall gave way. So this time I resorted to a die in a tailstock dieholder. It worked fine and the OD of the resultant thread is 0.1575", so slightly over nominal. But they fit nicely in the mating tapped holes. The threads are properly formed for the full length and without breaking the remaining thin wall. That's why I don't think making the stock undersize is the solution. It's perfectly possible to cut nice threads starting at nominal diameter with a die. Reducing the diameter may appear to help, but it's covering up the problem not solving it. The die I used was part of a present (Triangle brand) some 50 odd years ago and the tailstock dieholder came from Arc, so neither are top notch professional (*), but they did the job fine. A few questions:

Where did the die and tailstock holder come from?

Have the die and/or holder been used satisfactorily in the past?

Was the die opened out to its maximum to check the fit before adjusting?

Have you tried the same setup on something more forgiving like brass?

Andrew

(*) Apologies to Ketan – I'm more than pleased with the tailstock dieholder set from Arc and it does what I expect without fuss, with one minor annoyance.

[BufferParticipant @buffer]

Andrew

The tailstock holder is a homemade one but it works very well normally. The die I'm not sure about but most of my tackle comes from Tracy Tools so should be good. I'm sure I did have it at its maximum opening I wound the centre screw in first then just nipped the other two to stop it falling out. I will try it on some brass and see.

Thanks

[HowiParticipant @howi]

Posted by Buffer on 28/11/2020 21:45:25:

I'm now wondering if the threads are being stripped as I am trying to get the thread started. In other words maybe it is rotating without pulling itself along which is stripping the start of the thread clean.

hi buffer, have you tried reversing the die, NOT all cut(well) in both directions. With regard to nominal size, I have always used the same size i.e 3/16 die, 3/16 material, if machining down to a size then go a few thou under.I usually put a slight taper/bevel at the end of the material to give the die sometrhing to start on.

Howard

[HopperParticipant @hopper]

Posted by Buffer on 28/11/2020 21:45:25:

I'm now wondering if the threads are being stripped as I am trying to get the thread started. In other words maybe it is rotating without pulling itself along which is stripping the start of the thread clean.

 

Are you pushing on the unclamped tailstock with your right hand to force the die onto the job in the direction you want it to travel?

And are you turning the chuck by hand rather than under power?

With a back and forth motion to break the chips as you go?

With suitable tapping compound on the job?

If yes to all of the above, you may have a duff die. Test it on a bit of scrap held in the vice and the die in a regular dieholder by hand.

 

Edited By Hopper on 29/11/2020 09:57:36

Edited By Hopper on 29/11/2020 09:59:02

[Nigel McBurney 1Participant @nigelmcburney1]

The problem could be poor manufacture of the die,I have had a tap from a well known supplier which jammed solid in the hole,it had no clearance ,or possibly negative clearance,replaced without question,and the replacement worked perfectly,i also have a set of set of ME taps and dies,one of the dies just will not cut,others in the set are ok,this was one of those exhibition offers from many years ago again poor manufacture no doubt from the far east,again a well known uk vendor. Again I bought a set of drills at an exhibition, a previous similar .set from the same vendor had been very good. this second larger set was very poor,some drills ground off centre,others with negative clearance. I had lost the receipt and could not be bothered to send them back ,so used them for making counterbores and a lot were resharpened and ground backed off for brass, next year I tackled the chap on the exhibitors stall,he admitted that the had received a poor batch and asked if i had brought them along,and he would replaced them,could not fault that,I just told him that I had used them for other purposes, I did buy a same size set from another source of a known make ,only snag they were 5 times the cost.

[Anonymous]

PM sent – Andrew

[IanTParticipant @iant]

Coming a bit late to this thread – but I used to have this problem too, although maybe not quite so extreme.

I do most of my small brass fittings on my EW lathe and leave it set up normally with a 40tpi screw-cut in mind (but not normally engaged). I use a handle to turn the lathe manually and cut a fairly deep (but not full) thread. If possible, I also allow some extra length on the part being threaded. I have a very thin parting tool that I use to make a slight groove at end of the cut when the part needs to screw up flush.

I then run a die over the part, finishing the thread form. If you have a split die, open it up a little before the first run down the thread – then check that a "nut" tapped with the tap you intend to use for any mating part – will also run down the thread. If not, allow the die to close up a little and run it down again.

The part screw-cut will help the die run on true (I do use tailstock die-holders but on fine threads they still need help). If you have made the part 2-3 threads over length, you can then face the extra length off and slightly chamfer the end. If you leave your "test" nut on, it will clean up your thread-end as you remove it.

Hope this helps.

Regards,

IanT

Edited By IanT on 29/11/2020 12:57:01

[Rod RenshawParticipant @rodrenshaw28584]

Lots of good points made by posters above.

If the diameter and the material of the stock are OK, and the technique and lubricant are OK, then the only thing left is the die which is either faulty or it's being presented to the stock off centre or at an angle?

I understand split dies are intended for use by hand held methods. Good ones will generally be concentric, between cutting edges and outside diameter, but cheap ones may not. I don't think there is any cure for a die which has the central hole and teeth at an angle to the outside faces.

Tailstock dieholders are great but they need to have some slack in them somewhere to allow the die to line itself up with the stock, some are just too rigid, "too well made!." and as has been said, a small die cannot be expected to pull a heavy tailstock.

Rod

[David ColwillParticipant @davidcolwill19261]

Years ago I made up a guided die stock. This is basically a die stock that can take different bushes to suit different threads. In your case the bush could be sized to the larger diameter of the stock. I always find this has a much better feel than when aligned by other methods.

Harold Hall's version

Regards.

David.

Edited By David Colwill on 29/11/2020 13:16:39

[Anonymous]

Posted by Rod Renshaw on 29/11/2020 12:56:32:

I understand split dies are intended for use by hand held methods.

Indeed, but split dies also work fine under power. This is a sample holder and reversing dog clutch that I use on my repetition lathe:

I've just used it to cut some 2-56 UNC threads in brass at 500rpm, as I didn't have suitable chasers for the Coventry diehead.

Andrew

[SillyOldDufferModerator @sillyoldduffer]

I'm confused! What thread is this? I assumed ³⁄₁₆" x 40tpi Whitworth, but my books don't list it. There's ⅛" Whitworth x 40 tpi, but ³⁄₁₆" Whitworth is 24tpi. Likewise UNC 40 tpi dies for ⅛" and 0.112" diameters exist, but the UNC equivalent ³⁄₁₆" size is 24tpi.

Could the problem be that a ⅛" x 40tpi die is being forced on to a ³⁄₁₆" rod?

Dave

[JasonBModerator @jasonb]

SOD its 3/16" x 40 ME, 55deg form but constant pitch over the diameters.

Your maths from the first page was also wrong, generally for 40tpi the tapping size can be 1/32" less than nominal not 1/16"

I've had a few dodgy ME dies and that's what I would put SOD's money on.

 

Edited By JasonB on 29/11/2020 15:12:02

[BufferParticipant @buffer]

thanks again to everyone who has taken the time to reply with advice.

I did what Andrew suggested and cut a thread perfectly on a piece of 3/16th brass. So there is nothing too wrong with the die it would seem.

I managed to cut a thread in steel but only by taking the bar down to about 0.18 but it didn't look very nice. It seems that despite me pushing the die towards the chuck it just wouldn't start pulling itself forward along the bar and it appears to just grind the steel away until it can get going and then it pulls itself along. So I did what I didn't really want to do and set the lathe up to screw cut. In the end I managed to get a very nice test piece cut on the other end of the brass rod so later I will try it on the steel.

I tried to find out at what point I could engage the leadscrew by looking at the workshop practice series book but soon realised it was going to take ages to work it out. So I reversed the lathe after each cut and engaged on the same number each time.

To speed this up does anyone know any easy way to work out when you can engage the leadscrew for a 40tpi thread with a 4tpi leadscrew?

Thanks

[SillyOldDufferModerator @sillyoldduffer]

Posted by JasonB on 29/11/2020 14:58:46:

SOD its 3/16" x 40 ME, 55deg form but constant pitch over the diameters.

Your maths from the first page was also wrong, generally for 40tpi the tapping size can be 1/32" less than nominal not 1/16"

I've had a few dodgy ME dies and that's what I would put SOD's money on.

Ah, all is explained, ta

Unfortunately you're not the first to notice I got the maths wrong again! Sack cloth and ashes for me, and a Dunce's cap for xmas…

Dave

[Anonymous]

Posted by Buffer on 29/11/2020 17:04:44:

…………..when you can engage the leadscrew for a 40tpi thread with a 4tpi leadscrew?

According to the chart for my lathe (Harrison M300, imperial with a 4 tpi leadscrew) any number from 1 to 8. But I've never tried it as I replaced my thread dial indicator with a high speed threading unit which automatically looks after all that.

Andrew

[BufferParticipant @buffer]

I never thought to do that but there it is in the manual a chart with1-6 what a numpty!

[duncan webster 1Participant @duncanwebster1]

4 tpi is 0.25 pitch, 40 tpi is 0.025 pitch, so you can drop it in anywhere. As long as tpi of thread to be cut divided by tpi of leadscrew you don't need to worry

[Rod RenshawParticipant @rodrenshaw28584]

Andrew J, 14.17 pm. I agree.

My point about dies being made for hand held use is that the manufacturer does not always bother much about concentricity, which hardly matters when using a "manual" die-stock.

Even a non- concentric die will work under power as long as there is enough slack in the system to allow the die to line itself up, the power produces the turning motion – and the die wiggles about, to use a technical term, to stay concentric on the part formed thread. With high quality dies this may not be an issue.

Rod

[Anonymous]

There's virtually no "float" in the Britan system. So they must rely on the thread in the die being concentric?

Andrew

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