Manual threading on the lathe – problems

[Jim BeagleyParticipant @jimbeagley46363]

Hi all.
Im currently making a Stuart 10V and I’m running into issues with threading rods.
Im using the lathe with a tail stock die holder, and used but quality (Apex/Warrior) dies.
Ive been having issues with some 5BA threads not cutting parallel to the bar stock which for the piston is a problem.

My technique is to turn stock to size (using a collet chuck)
Disengage belt and run die to stock and lock tailstock.
Hand turn chuck whilst holding die against work.

I was considering machine cutting but BA is not that simple to do.

Any tips as to what I could try, or what I could be doing wrong?

Cheers, Jim

[Jim BeagleyParticipant @jimbeagley46363]

[duncan webster 1Participant @duncanwebster1]

Many would recommend a tailstock die holder so you can advance the die whilst keeping it parallel. What I do is put a boring bar in the toolpost, push the die up to the work with the tailstock but keep pushing on the tailstock, don't lock it. The boring bar stops the diestock rotating. For tiddly threads same technique but lock the tailstock to the bed and keep small pressure on with tailstock handwheel

[Nigel Graham 2Participant @nigelgraham2]

How are you holding the die?

Although it is possible to use the tailstock as a makeshift die-holder guide / pressure pad (I have done it), it is not very reliable for very accurate work (I know – I've tried it).

Your second best bet to screw-cutting is to use a tailstock die-holder, and that will probably give you as good a result as you are likely to expect or need.

Is your difficulty compounded by expectation? You seem to mean this is for a piston simply screwed to its rod. Whilst I don't know the specific engine, it is usual to not rely on the thread alone for concentricity. Instead, the assembly should include a register, at its simplest a plain section of the rod fitting a reamed portion of the hole in the piston.

Many builders also leave the piston a tiny bit over-size then skim the last few thou off with it finally assembled to the rod, held between-centres or by other means to ensure concentricity.

'

Can you cut BA threads? Well, they are metric though not obviously so; to a geometrical progression.

5BA is of 0.59mm pitch, or 43 TPI., and 0.014" (0.355mm) deep.

If you screw-cut it about 0.7 thread depth, you can die-cut it to size and profile by die, and on a thread of less than perhaps 8 or 10 turns the pitch error is likely to disappear in the cutting tolerance. Anyway, a slightly tight thread is probably an advantage on a piston-rod.

The awkward bit is grinding the tool accurately without a basic tool-&-cutter grinder.

' '

If your lathe is to Imperial dimensions it is possible, using spread-sheets, to find combinations of its change-wheels that will theoretically generate some close-match mm / BA threads for short, though useful, distances before the accumulating error prevents correct finishing with a guided die.

Edited By Nigel Graham 2 on 13/01/2021 00:42:16

[Ady1Participant @ady1]

The most accurate way is to partially cut the thread on the lathe

then finish off with the die

Then every thread you do is a proper straight 'un

[Paul LousickParticipant @paullousick59116]

Hi Jim,

You have said that you are using a tailstock die holder and used (and quality) dies. But how sharp is the die and is it located co-axially with the work in the holder ?

Screw cutting is an option to start and align the thread but 5BA is small and material can easily flex. An option is to make the threaded end of the shaft longer but with a reduced diameter. The die will start to cut and align itself easier than starting on a full diameter. Then cut off the end to length.

I also used this method on brass when the die did not cut a thread but just reduced the diameter of the shaft.

Paul.

[not done it yetParticipant @notdoneityet]

One very simple alternative would be to ditch the BA thread and select one that can be machine cut on the lathe?

[JasonBModerator @jasonb]

Try the same setup on some mild steel, it's more than likely the stainless steel of the piston rod is what is causing problems.

If that does not work then your Apex die may only have the old quality name, I bought a couple and they were not too good.

[David George 1Participant @davidgeorge1]

Hi Jim when using my tailstock die holder I start with the die opened to maximum diamiter, using the center pointed screw, to make the thread as large as possible first. I find that taking a smaller first cut and then closing down subsequent cuts till you get correct thread size works best. Make sure that the center screw has a point that opens up the die when tightened and to loosen the two outer screws to allow the die to expand first then nipping them when adjusted. I bought a cheap steel die from Tracy Tools recently, i only wanted it for one item,and the first time I used it I expanded it and it broke into two when I expanded it. I rang them and they said no problem and in the post next day there was a new HSS die free supplied.

David

[Martin KyteParticipant @martinkyte99762]

Whilst not a solution to your threading question, when making the piston and rod start with an oversized piston. Drill and tap for the rod. Make the rod and attach to the piston. Hold the rod in a collet and machine the outside of the piston. That way the piston and the rod have to be concentric.

regards Martin

[SillyOldDufferModerator @sillyoldduffer]

The method is fine. Suspect the die! ' USED' rather than 'QUALITY.'

'Quality' counts for nothing if the tool is worn, blunt or damaged.

Dave

[Jim BeagleyParticipant @jimbeagley46363]

Thanks for all the thoughts and suggestions.

I have at least 2 dies so I can try again with a different one.
I like the idea of the tapered lead-in for alignment – I’ll give that a go too.
Its an imperial lathe but I do have some gears for it so I may be able to get to 43TPI somehow so that’s another option but as said, I’ll need the tool and 5BA is pretty small!

At the least I can swap the BA for something I can easily machine cut and then finishing the piston diameter on the rod is obviously a great idea.
Only issue I have now is I’ve run out of suitable stainless – bah.

Cheers all.
Jim

[larry phelan 1Participant @larryphelan1]

As Jason said What,s in a name ??

[Ron LadenParticipant @ronladen17547]

Hi Jim,

Just a thought but you say your dies are quality but have you checked their fit in the die holder, the tailstock die holder that I have is ok but not the die holders when the grubs are secured they dont hold the die square. I get around it but its a job on the list to turn up some new holders.

Ron

[Jim BeagleyParticipant @jimbeagley46363]

My dies are Apex and Warrior. They’re not new but donated to me by a friend who was in turn gifted them by a model-maker. They are genuine, and although old are relatively un-used. I’m sure they’re better than a new chinesium one.

[Clive Brown 1Participant @clivebrown1]

However good / bad the thread on the piston rod, I would always machine the piston OD "in situ" on the rod, as Martin Kyte suggests.

[Howard LewisParticipant @howardlewis46836]

I cut threads with a Tailstock Die Holder, but one that can float, so that the Die self aligns on the workpiece.

Ditto for Taps. (Made an ER 25 holder to grip the shank of the Tap. Slips if there is any hang up, rather than breaking the Tap )

The grubscrews that adjust the Die can push it off centre, so that a "drunken" thread results.

+1 for assisting the Die onto the work, and having an arm on the Die Holder that can rest against the Toolpost to prevent rotation (Safer than trying to hold by hand )

I failed to cut ME 40 tpi (0.016" / 0.406 mm deep ) threads on brass, until I adopted these methods.

Howard

[old martParticipant @oldmart]

I only recommend split dies used in at least 2 stages. Having the diestock resting against the front of a drill chuck in the tailstock and advancing the tailstock quill gently helps to keep the die straight. Some steel types do not like being threaded however good the dies are. With a tailstock die holder, you could try not locking the tailstock, just push it gently as you turn the chuck.

Edited By old mart on 13/01/2021 16:12:26

[noel shelleyParticipant @noelshelley55608]

Hi Jim, are you using any lubricant ? Though not cheap Rocol is very good especially on hard to cut material. You only need a tiny amount and it's pointed bottle makes doing this easy. Noel

[Stuart BridgerParticipant @stuartbridger82290]

I don't recall having any issues with my 10V piston rod.
Floating Tailstock die holder and decent quality sharp dies are the answer as other have said.
I hadn't discovered Trefolex when i built my 10V but that has been a revelation when threading since.

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