Screwcutting in the lathe

Appreciate some advice. I am almost at the end of building a Worden Cutter Grinder (Hemingway), and am now making the lead screw traverse kit. The whole thing is based around a piece of 20mm dia galvanized steel tube, with the ends turned down to 3/4". This then has a 26TPI 25 thou deep thread cut on each end. I decided to make a dummy end to try everything out, as there is very little left on the tube once the thread is cut. My problem is that the thread which I have cut with a carbide tool is absolutely rubbish. I tried two different tools - both carbide tips - so where do I go from here? The external thread I can clean up a bit, but there are "nuts" to go on each end which worry me. I cannot "see" to clean up the internal thread. I am concerned about tool tip height, and cutting speed. I have a speed controlled motor so it can be slowed right down to a "walking pace". Any advice gratefully accepted.

I'd suggest that your problem is very likely to be down to the material. A piece of galvanised steel tube doesn't sound good for readily producing a fine finish. Can you use, say, En1a solid bar instead? It will be much better to machine.

If you're really stuck with the tube, are you using cutting oil? That might help.

Edited By Clive Brown 1 on 20/07/2020 14:27:49

I suppose the 3/4" 26 TPI nuts could be tapped or finished with a tap.

Agreed that galvanised steel tube is unlikely to be novice friendly when it comes to screwcutting. As Clive Brown suggests a nice piece of known quality steel would be far better. I'd go for a free cutting leaded steel. Ordering by EN nmbers can be a bit problematical these days. I've had some very unexpected stuff.

if you are stuck with crap material a thread cleaning up file can work wonders.

Has to be said that on typical home shop machines sharp HSS threading tools are much better behaved than "the carbide tip wot I could afford". Good carbide is generally industrial tooling optimised for specific jobs, mostly on high speed CNC machines. For home shop folks its usually a matter of finding a breed and specification that also works well on our slower, lighter machines. Correctly selected carbide works very well indeed but the sharp edges are inevitably more brittle and more vulnerable than HSS. Frequently very little manual mishandling is sufficient to take the sharpness of the edge off a threading tool by enough to stop it giving a good finish. Such damage may only be visible under a microscope.

For internal threads its important that you have sufficient heel clearance on the tool to ensure that only the cutting edge touches the nascent thread. I've wrecked a job or two by forgetting to verify that I still had clearance when the tool reached full thread depth!

Clive

I am not familiar with the kit, Brian ... but I infer from this description that you might be making things unnecessarily hard for yourself:

https://modelengineeringwebsite.com/Worden_13.html

MichaelG.

Thanks everyone. Very interesting build article by Vince Cutajar. (Via Michael Gilligan.) To Clive, the piece I have done so far is on EN1A, as far as I know, not the tube. The build article suggests 19mm by 0.5mm pitch, and his photos show a tap cutting through the locknuts. I think that may be the way to go.

Thanks again, everyone.

Should be no issues screwcutting proper EN1A as its supposed to be free cutting steel. Supposed being the operative word. My experience is that there is a significant amount of cheap crap steel sold as EN1A. Better to ask for the more modern designation 230M07.

I'd avoid any supplier just listing EN1A without a more modern designation. It really has very limited industrial level uses these days as its not really up for structural parts taking any sort of loads and welding is less than ideal.

Clive

Brian as Clive suggested dump the carbide and grind your own HSS tool, stone or hone it to ensure a fine finished edge then use plenty of lube, try different lubricants, I find a mix of cutting oil and Rocol works well on this type of material or many other steels. The material you have chosen will not be easy to attain a high grade finish but it can be done.

Good luck

John

I have recently cut 26tpi threads on 1/2" tube for a friend who was making some light fittings [26tpi is the thread in lampholders ] It seems that it is also the thread used in old ballcock fittings but in that case the od of the tube was 9/16"

I cut the threads using HSS without any problem. I do have an old 26tpi tap from way back, could it be cycle thread ?

26TPI was Cycle Thread, but also BS Brass thread, which was quite possibly used in light fittings.

I think I would chose anything other than galvanised tube for this. You can buy 20mm od x 16mm id stainless tube quite cheaply for example.

Martin C

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The link that I posted above shows plastic tube [supplied in the kit] being used

... Unless I have misunderstood, this component is a cover for the traverse leadscrew.

MichaelG.

I think the adjustable stops on the cover will chew up plastic over time and make a mess of the galvanised tube as well.

Martin C

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I would agree completely, Martin ... if the adjustable stops were on the cover

Scroll down to the photo titled ‘Here is the completed unit.’

MichaelG.

Edited By Michael Gilligan on 21/07/2020 11:03:22