tailstock threading

[paul raynerParticipant @paulrayner36054]

I came across this last night, I found it quite interesting and very useful. As I have had similar problems.

[Nigel Graham 2Participant @nigelgraham2]

Thankyou for that, Paul.

Once past Gardeners’ Question Time it is very useful – and news to me!

That’s the first time I have heard of this problem. I’d always assumed my loose threads due entirely to me, but I’d never realised the subtle flaw in the “obvious” method.

I often use a conventional die-holder as shown, but not to gain that floating effect as I had not realised its significance. Instead it is because my tailstock die holder, a commercially-made item with tanged shank, tends to slip in the tailstock (on a Myford 7), and is awkward to use anyway. In fact, using its key pin against a rigid surface could even push the whole thing slightly off-centre.

Using a die holder as in the video does hold one trap: it is rather easy to start the thread slightly angled. If that happens it will cut a wavy or eccentric thread.

 

Sometimes it is possible to use the face of the tailstock barrel as the pusher, but a Myford 7 is a rather cramped machine at times, so I use the face of the drill-chuck.

Reversing a die or tap off under power, I start it back manually half a turn or so.

Whether die-cutting, screw-cutting, or using the die to profile a single-point cut thread, sometimes I turn a little core-diameter stub on the work end. It acts as a depth indicator for screw-cutting, and helps a die’s alignment. It can be faced off afterwards if wished, but studs are sometimes left with such a spigot as a thread-protector and assembly aid.

I still cut a little thread-start chamfer; if screw-cutting, using the thread tool itself.

[JAParticipant @ja]

I had a quick look at the video.

Although he took great care I would not dream of cutting a thread with a die under power. As for reversing to get the die off I am surprised that all the Chinese lathe owners on the forum have not pointed out that the chuck is likely to come off the shaft.

There is a firm in Rutland, Arrand, that made very good tooling for Myford lathes. They ceased about 20 years ago but not before I bought a tail stock die holder from them. It is floating and a go tool. At a recent club auction that included workshop tools, quite few Arrand items from a deceased member’s estate came under the hammer. Unfortunately another member also recognised them. The bidding did liven up the proceedings.

JA

[JasonBModerator @jasonb]

Unless it is very coarse/large I tend to do all my threading under power with a die in the tailstock holder. Most of what I do does not need a tommy bar or key, just a good grip of the die holder which also acts as an effective clutch when you are threading to a shoulder.

The other option is to use solid (unsplit) dies which seem to be the norm in industry these days – just look at a Dormer catalogue and combine that with a close fitting hole in the holder then your die won’t get pushed off to one side by the screws. I have been going over to this method as you don’t spend time faffing about adjusting a die every time you come to use it.

One I made earlier

[bernard towersParticipant @bernardtowers37738]

Is it the fact that the die holder has a larger internal dia than the tailstock holder?

[JasonBModerator @jasonb]

Die holder both tailstock and hand held has to be larger than the die if you are going to be able to spread it. Assuming the cutting part of the die is concentric to it’s OD then it will be pushed to one side by the screws. Bit of shim will sort that out if you are bothered.

Probably more noticable on his new Myford than the usual old ones that will have some play in the barrel which will allow a small amount of float. Obviously zero clearance in his as he says it is spot on!

[old martParticipant @oldmart]

The slight eccentricity is more of a problem with smaller thread sizes. With small sizes I start with a split die opened up in a holder simply sitting against the end of the tailstock quill to keep it square. I never cut such threads under power. Bigger threads usually get singlepointed unless the quality is not important.

[peak4Participant @peak4]

I think the chap on the video commented that he couldn’t measure whether the die was concentric.

Surely, he could check that the pre threaded stub is concentric, then thread the die onto it, just the die, without holder, and clock the outside.
He might need to check both the tops of the threads, and use a bit of wire to clock thread depth.
First run the die onto the stub, whilst still in a hand die holder to spread it, then loosen the screws to let the die pinch down and remove the holder.

Next add the tailstock die holder, only just engage it on the tailstock mandrel to support the weight, rather than constrain the headstock end, and then clock the outside of that whilst turning the chuck.

Assuming that the inner and outer diameters of the holder are concentric, that would show any run-out for the mounted die.

Also, just barely engaging the mandrel whilst threading, would pretty much simulate a hand die holder; does it then cut to size?

I may have missed it, and I’m not replaying the full video for a third time, but did he compare the bore diameter of the tailstock and hand die holders?

Further to that, did he measure how far the die opened out in each holder.
OK the OD would be tricky, but you could measure the outside spacing of the opposing pair of holes (east/west if you like with the split at north). Probably easier than trying to measure the thread cutting tips.

As well as hex and fixed round dies, I also have at least one round die where the dimples on the outer are at east/west, rather than north-east/north-west as in a conventional split die.
This one has a grub screw across the gap to set the size that way.

Bill

[Nigel Graham 2Participant @nigelgraham2]

JA –

Unscrewing the chuck…

Not only on Chinese lathes. That can happen on the Myford and many other older lathes with screwed-on chucks. Even my Harrison L5 is like that, with a rather absurdly small nose for such a hefty machine.

In practice the torque imparted by the die running back off the thread is far less than that holding the chuck on the mandrel. Anyway, you do not, or should not, run the machine fast to reverse the die off.

In fact the chuck seems to tighten itself so much it can be a battle to remove it without harm to oneself, or the lathe. Whichever is worse.

Usually I find trying to pull the chuck round against the die to cut the thread, is what unscrews it. (I have a mandrel handle but find it not very satisfactory.)

[JasonBModerator @jasonb]

The other thing that can make any eccentricity worse is people putting metric OD dies into holders intended for imperial dies. Easy enough to see that a 25mm OD die will sit off ctr in a holder intended for a 1″ die

[Nigel Graham 2Participant @nigelgraham2]

There seems a curious irony here: if the lathe is elderly and worn it would seem more likely to give accurate results using a die holder, than would a new one!

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