Single point tool to cut an internal 5/8-10 LH ACME thread

I'm trying to source a single point threading tool (or insert) to cut an internal 5/8-10 LH ACME (it may even be stub ACME,) thread. No amount of googling is yielding anything other than size 16 carbide inserts which I think will be too big for a 5/8" thread.

Any recommendations?

I don't yet have the ability to grind my own HSS tools. The irony being that this is to make a new nut for the overhaul a (new to me) Clarkson grinder.

RDG sell a tap but its a second and I suspect I'll need an almost finished thread before I stand any chance of being able to turn it/cut with it? I'd also like to be able to fine tune the size to match the existing screw which is no doubt worn.

Could you use a 'boring bar' type toolholder with silver steel to make the toolbit - then you can file to exactly the profile and relief you need and harden it..

I doubt you will find a size 11 insert for 10tpi let alone with the clearance for left hand.as by the time the large tooth has been formed on the insert it will remove too much of the carbide and the holder will start rubbing too.

You could also think about turning a long tap with a long taper from silver steel and use that to finish a roughed out screw cut thread.

Edited By JasonB on 10/11/2021 07:15:17

Hi Mark I made a 3/8 square thread LH bronze nut for an M type lathe crossslide. I ground the tool from a solid piece of HSS by hand by relieving thd stem first then grinding the cutter to give bottom clearance. The width of the tool was then ground to shape and the clearances were ground on last. I had made a tap from silver steel which was hardened just to finnish the thread. Although the thread I cut was a square thread I don't see why you couldn't cut an Acme thread just the same as you can make the tool slightly under size and using the compound slide machining the flanks til the thread Fitts.

David

Carmex do a miniature 10 tip ACME insert, I had a quick look but couldn’t spot a specification stating how small a thread you could cut but 5/8” is not so small. I use Carmex miniature and sub miniature threading tools for normal threads and they are pretty tiny.

Ian

As you can get a tap and your skills have have yet to reach the stage of accurately grinding a suitable single point tool the simplest approach would be to grind a close, but undersize, tool and finish off with the tap. Something like Davids made a bit narrower than the tip of the thread should do fine. But a bit of taper on the sides wouldn't hurt.

With most of the material pre-removed the tap won't have to work too hard and so should go through fine. I'd arrange to end load it in some way so it doesn't have to do all the work of pulling itself through as well as cut the thread. Maybe get get creative with the drill press to help push.

Adjusting fit by turning a few thou out of an undersize nut is a tricky job. You can't see what's going on and any slight error in the angle of the tool sides, whether in the grind or not quite dead nuts parallel set up, makes the whole thing a bit of a lottery. On a small machine there is a tendency to cut the first turn or so slightly oversize due to the tool initially only working on one side as it enters the job. Large machines generally have enough mass not to suffer but small ones tend to ease back a thou or two under the initial load. Net result is that the first thread is still a touch tight when the rest of the nut is fine. Probably best to make the nut overlong and, when down to the final few cuts, initially just do the first turn, withdraw the tool and check the whole nut before a full pass. Odds are the first turn won't end up all that wonderful so if the nut is overlong it can easily be cut off.

Personally I'd just make the nut to size with the tap and make a new screw to fit. With external threads you can see what's going on so its all much easier. Screws invariably wear unevenly so making a nut to fit a whole screw with minimal backlash isn't really possible.

Hafta say that when I put my Clarkson into service I said "Should really make a new feed screw." as this one is sloppy. 20 years on and multiple coats of looking at the old screw is still working well enough to be going on with. Its more likely to get a DRO than a new screw and nut pair.

Clive

Edited By Clive Foster on 10/11/2021 10:19:07

Would an alternative approach be to mount the work on the lathe saddle and tap from the headstock with the carriage set to the appropriate pitch?

Can't see any reason why not. Might even be able to go straight through with the tap if you have a suitable collet. It will spin in a three or four jaw if it has any significant amount of material to remove. Hard steel shank on hard steel jaws with limited contact area isn't exceptionally secure.

Not a method I'd use unless I were sure it would work. Alignment is less than trivial and once started the job has to keep going. Due to the simultaneous turn and feed anything interrupting the phase lock between them will have disastrous results. Quite probably breaking the expensive tap.

Keeping a Coventry die head settled in the sprung region is quite enough for me.

Clive

Clive:

I have only tried this method once but I think your cautionary comments are spot on.

I suppose that alignment could be assured by boring/drilling the tapping hole from the headstock and it would be very helpful to have a proper tapping chuck that grips both shank and flat (I used a collet chuck).

In similar vein, I believe Harold Hall showed a tapping stand where the tap was positively driven at the desired pitch.

I think a second tap will have full thread for part of its length. Since this is not for a blind hole there will be no problem pushing a second tap all the way through to cut the thread to full depth. Undersize threads can occur with ring series tap sets for tapping hard material if you don't use all taps in the set but I think one described as a second tap would not be a ring series tap. You could ask the suppliers to confirm this is the case.

Martin C

Ever thought of making the nut out of plastic, my DW miller stripped the table feed nut also 5/8 x 10 ACME and after getting a tap from Tracy tools I had some polyacetal plastic in stock and it tapped a 40mm long thread with ease, its been on the mill for 10 yrs now and done loads of jobs , it saved making a fiddly screwcuttling tool up

I carried out the same task on a Clarkson just after I completed my apprenticeship over 55 years ago,now in those days you were expected to grind a hss tool to cut the threads,no carbide inserts in those days,and the nut was bronze.on that job the wear in the screw was done by carefully setting the screw in the Boxford lathe and just skimming the sides of both thread flanks,then screwcutting the nut to suit the modified thread. Now as it is LH the clearance on the internal screwcutting tool is opposite to a RH hand tool. This year I had the job to make some jack screws for a hundred year old showmans caravan they were 3/4 by 4 tpi square ,I advised the owner to use 3/4 by 5 tpi acme as a tap was available and to copy the original exactly would have been too expensive, steel screw (en8) threading was ok it was the nut that proved difficult,5tpi really needs some power to tap, so some brass hex was aquired for the nut bored out and screwcut with a v tool then the tap was held in a cross slide bolt on attachment with an ER collet which was really tight, the hex bar could not slip in the 3 jaw.and in very slow speed with a lot of pressure on the saddle hand wheel the tap was pushed into the brass bore,and it tapped ok,if the pressure on the handwheel is relaxed the flanks on tap will cut like a boring tool, I did not fancy using leadscrew set to 5 tpi,to control the pitch, its easier to spoil the job rather than risk the lathe on commercial work.to finish the job I turned the hex to fit its housing if I had the Clarkson job now I would buy a tap ,rough out with a v tool, shape is not critical, hold the work in a 4 jaw chuck for extra grip and hold the tap in the toolpost,I do not advise holding the tap in the tailstock, it can spoil the lathes accuracy. then make the screw to suit the nut,use free cutting mild steel as it is easy to machine with a very good finish and from personal experience mild steel with a very good finish will last a long time.I have a Clarkson,bought at a sale,its a good machine though its feed screw and nut can all too easily pick up the grinding dust as it rather exposed,though where I worked (15 employees)the Clarkson covered all the cutter grinding requirements for over 10 years before it needed attention.

A second cut tap merely has a shorter taper than a first, so running the tap right through should produce a full depth thread.

It just means drilling the workpiece to the tappinbg size for then bthread.

Tubal Cains Model Engineers Handbook quotes the depth of an Acme thread as being 0.500 Pitch.

So for a 10 t[pi thread, the depth would be 0.050" to give a tapping size for 10 tpi as 0.525"

The nearest would be a 13.3 or 13.4 mm. The nearest Imperial drill would b 17/32, which at 0.531 is 0..6" oversize.

The slightly decreases engagement is unlikely to matter.

A 1/2" drill cutting slightly oversize; as it well might, would allow you to force the Tap into the brass without too many problems. Brutal it works!

Howard

This is the sort of job where the wise first timer, most especially relatively inexperienced folk seeking to expand their range of skills, invests in some delrin or equally machining friendly material to practice with. Such practice not only gets the method dialed in but also allows you to try different ways and select which one works best for you and your equipment.

Speaking of delrin I wonder if there is space to adapt the moulded delrin technique successfully used by many folk to produce "zero backlash" half nuts. Basic idea is to squeeze two bored but unthreaded half nut sections together around a heated screw. The delrin flows around the screw and fills the thread so no backlash. Various write-ups on the internet. Seems to be no intrinsic reason why the two half nuts couldn't be trapped in a suitable metal sleeve with the required attachment points to hold it to the machine. Which assumes there is sufficient space to accommodate an assembly inevitably bulkier then the standard metal nut with integral thread.

On a lightly loaded application like this a single half-nut would suffice to drive things, lots of Colchester lathes only have half a half nut for screwcutting duties so the idea is clearly sound. If the single half nut were on the other side of the screw to the fixings the space needed should be usefully less than accommodating a pair.

Clive

An alternative to the hot delrin idea might be to make a mold and pour white metal nut. Simple block mold and then machine or file to suit final shape. Black the screw before hand and it should unscrew ! Just a thought and might be worth a try. Tracy do many special taps so they might be worth a call. Noel.

If you google long enough you might find a ready made nut you can modify.

I made a feed screw nut by the ‘heat ‘n’ squeeze method. Never used it as I had a spare and then changed my lathe.

Most certainly no backlash!. It needed free-ing off a tad for use (but may have been OK on the most used part of the screw).

I don’t know how long it might last in use - nobody seems to have provided their later experiences. I made it longer than the brass original, inserted in the bored-out old nut and secured with small screws and a washer.

Edited to add that a denford user reports a lot of use from his nuts made this way.  Obviously some reported successes on the net now.

Edited By not done it yet on 10/11/2021 20:24:19

Thanks folks. I may well try turning a 60 degree thread on the lathe and then run the acme tap through it. If it is too sloppy I may follow the suggestion of turning a new screw.

Need to recut the dovetails first, certainly for the X axis, both male and female, and probably the female for the Y as well ! All good fun.

Edited By Mark Davison 1 on 10/11/2021 21:11:00

Mark, My advice to you would be to make a simple grinding jig to grind the requisite tool to cut your thread, both inside and out. Even on a cheap bench grinder it really isn't that difficult with an enlarged rest and a little care, even down to about 3/8" internal.

I always cut internal threads on the rear face of the bore with the tool inverted because you can see what is happening to a certain extent and you withdraw the cross-slide at the end of the cut as if cutting an external thread but this is not so important with a left-hander as the tool will be moving outwards anyway.

Go slow, the tool only removes the same amount of material if you are going at 1 RPM or 1000 RPM so the slower the better in my book. Also, cut the external thread first to use as a gauge for the internal. You may find that you may have relieve the flanks of the internal thread a little by advancing the top-slide a couple of thou. to get a nice fit.

If you cut it with the topslide parallel to the lathe you can advance the tool with the topslide a number of times once at depth, this will give you a cut far more like an ACME but with 30deg flanks so even less to clean up with the tap.