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Threading myth .... busted!

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Clive Foster06/09/2021 17:07:43
2882 forum posts
104 photos

Judging by what Grahams Meek says he, like me, primarily uses what I was shown as the Zero-to-Zero / Zero-2-Zero system. Along with the star turner alluded to by Geo.H.Thomas.

The intrinsic forward feed of the top slide is no great issue when using an automatic screw cutting clutch or bed stop with that system. Simply bringing the saddle to the end of the thread, whether automatically with the clutch or by running up to the bed-stop with the slides retracted then bringing the cross-slide to zero and carefully feeding the top-slide forward until it gets to the preset zero produces an entirely adequate finish groove.

Similar results are possible using the "feed forward a little with the top-slide set parallel to the bed every few plunge cuts" are possible if you know how many feed forward movements you intend to do during the process. I know an easy way to set this but that merely adds a smidgin more complication to an over elaborated technique that is totally unsuitable for use by neophytes. There are reasons why things might need to be done that way. But not for any normal thread.

When it comes to Home Shop and generally inexperienced workers the elephant in the room which most pundits ignore is "F**** the thread doesn't fit now what do I do". Usually a bit more feed gets you there but a novice never really knows. Antediluvian penguins like me have the feel to sort it in mid-air but that takes years to gain.

Zero-to-Zero is the only method that allows you to figure out what has gone wrong from first principles.

If you use the cross slide to make the little extra feed when the thread you cut turns out tight it is calibrates the tool in-feed needed to produce the correct thread when using that particular tool. So if the thread depth is supposed to be 1 mm and your first effort turns out tight but, after a few more cuts, an extra 0.1 mm makes it just right you know to set the infeed to 1.1 mm before bringing the cross slide dial to zero. Something you can do with a bit of scrap before risking the real job. Far as I've heard overfeeding so the thread is too deep is almost unheard of with this method. I guess hand ground tooling is usually too pointed.

Forget trying to do that with any other method. Especially if you have the job set up in the chuck with considerable work done needing just a thread to finish.

Just because you have an insert doesn't necessarily mean that "book numbers" will be right. Partial thread ones need clear thinking for best results.

In my experience Johannesson/SKF/Dormer chasers are dead on the money tho'. Harder to find now and not cheap but I'd not use anything else for threads I don't have Coventry dies for. Neither being something a newbie can sensibly afford when every £ scraped off the side of the plate of family finance must do the work of £5 or, better, £10.

It can be hard for experienced folk to remember just how overloaded neophytes and inexperienced workers can be when at the beginning. Lots going on with thread cutting.

What number do I re-engage the half nuts at. Aaargh, they don't want to go in. Oh God its nearly got to the end. Panic, Panic, gotta drop the nut NOW before things go Bang. Phew! Do I need to feed forward a bit this time? How far. Have I gone too much. Eeek finish looks like a ploughed field. Gotta start over. Again! Is this ME business really worth the effort.

Remember.

Much less stressful with an instructor, mentor or mate around to pat you on the shoulder says "Calm down. Take a few spring cuts. She'll come right." Pretty much always does. I did wonder about the attempt to do a thread in one cut tho'.

Clive

Chris Crew06/09/2021 20:44:22
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150 forum posts

There is absolutely no need for anyone to panic, newbie or experienced, when screw-cutting. If you cut the thread at at 1 rpm or 1000 rpm exactly the same amount of material is removed by the tool per pass. If you feel you are going to be fazed by anything when screw-cutting, then cut the thread at the lowest possible speed the lathe is capable of running at. It will make very little, if any, difference to the finish of the thread and will give you plenty of time to think. If you can't even manage that, and I am not suggesting anyone can't, simply stop the lathe, collect your thoughts and start it again.

I actually have an Ainjest rapid threader on one of my lathes which makes screw-cutting as easy as feeding buns to elephants, but even then I don't run the lathe at the highest speed the Ainjest will allow because I like to think that I have just that split second to react if the device fails to trip. It has never let me down yet but I always have one hand hovering over the stop-lever/brake on the Colchester, just in case.

BTW, I always use the set-over top-slide method for both external and internal threads. For internal threads simply mount the tool upside-down in the tool-pool and cut the thread on the back of the hole. Then you only withdraw the cross-slide at the end of the cut as when external screw-cutting. You don't have to think about reversing the cross-slide travel which invites confusion.

Edited By Chris Crew on 06/09/2021 20:47:00

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