Indexable threading tool tips

Hi All,

I would like to understand the tool tips that are required to cut external threads on the lathe. If want to cut different size and different TPI Whitworth threads do I need to purchase a different tip for different sizes / TPIs ?

Looking at the likes of RDG etc it's not very clear what is required

Not an expert - but have been doing the same research. Looking at APT's offerings, they seem to do 55 & 60 degree tips with "partial profile" which will cut a range of pitches, and a wider range which cut the full profile (presumably including the crests) and for which there is a different cutter for each pitch.

I assume for production work you would choose the full profile tips, but for jobbing and hobby work, the partials make more sense.

Hope this helps.

You can use partial form inserts which will cover a range of sizes, these produce the root and crest to the smallest pitch in their range which will do most of the time provided you adjust depth of cut to suit.

If you want fully formed roots and valleys then you need an insert for each pitch

The 11ER55 that RDG list are partial form 55deg externl inserts

You want something like 11ER $$W for a similar size insert to do a specific Whit external thread where $$ is the tpi

They are known as Laydown Threading Cutters. Whitworth are 55 degree threads so you can use a single tip for a wide range of thread TPI, but of course as the threads become lower TPI there is a limit.

You can buy specific cutters if you wish.

Look at www.apt.co.uk they are specialists

If you already have the smaller RDG holders then APT would be much use as they don't do the smaller inserts in Whit. Try Cutwel

I prefer to use full form inserts, partial form inserts are just a pain. I screwcut a lot of Whitworth threads and it is much easier with full form inserts. The root and crest are properly formed and cleaned up. Partial form inserts leave the crest unformed so it ineeds cleaning up seperately. In addition thread depths are as per theory with full form inserts. Whereas for partial form one has to account for an incorrect root radius. In this picture partial form on the left, full form Whitworth middle and full form metric right:

I buy my threading imserts from Cutwel.

Andrew

As Andrew says the great thing about using full profile inserts is that the thread cut is exactly correct so the depth of cut needed can be taken directly from standard tables. The perfectly formed crest is, to me, merely a minor advantage. I tend to trim the crests slightly on customer jobs as the small flat is more durable when the fitter has "shade tree mechanic Bubba" tendencies. I have almost full sets of Johansson / SKF / Dormer chasers that were commonly used on capstan lathes which produce the full form and can, in home workshop terms, be re- sharpened infinitely.

If you do decide to use partial profile or sharp point inserts you will need to calculate the extra depth of cut needed over book value for the thread you intend to make to accommodate the longer tip. A decent CAD program makes dealing with the geometry much easier. The basic dimensions and geometry of threads can be found in most handbooks but handbook writer union rules seem to demand that the diagrams are too small for easy reading. A decent size one for 60° UNF (American) threads can be found here :-

**LINK**

https://www.gaugestools.com/unified-screw-threads/

By setting out a "nest" of diagrams in a CAD program its fairly easy to see what you need to change. For all practical purposes metric threads are the same.

I strongly advise using the "Zero-to-Zero" thread cutting method if you plan to use partial profile or sharp point inserts because it makes it very easy to reliably set the required changes in thread depth. So much so that its practical to work by trial and error slowly increasing depth of cut over book until a satisfactory fit is obtained using a good quality nut or accurate test piece made using a good quality tap.

Having found the correct in-feed for that thread and that insert you should make a note of it so you can immediately make the requisite changes in set up when doing another thread of that size.

The reliability of the Zero-to-Zero method means that the thread will come out right first time, every time if you have the correct in-feed values.

The Zero-to-Zero method is classically explained as a way of getting the lathe to calculate the correct in-feed when the top slide is set over at, or close to, half the thread angle. This avoids trigonometrical calculations. Generally considered a good thing. However the method works just fine when the top slide is set parallel to the cross side too.

Because all the setting up is done before you start the job goes easier as you only have to keep an eye on the process rather than worrying about feeds, sizes and things. Setting up at the start means what you set is what you get so if the thread doesn't fit you know what you have and have a solid starting point for working out what ...... went wrong (this time!).

In simple terms you start, as usual, with the job cut to the required diameter and tool tip exactly perpendicular to the job.

1) Touch the tool tip to the work then set both top slide and cross slide dials to zero.

2) Pull the tool back a bit using the cross slide, traverse it past the end of the job then feed the cross slide forward past zero by the depth of thread you intend to cut.

If using a sharp point or partial profile insert and intending to find the correct depth by trial and error use the book value for thread depth. If you have already figured out what the actual depth needed is, use that.

3) Re-set the cross slide dial to zero and pull back the top slide to just clear the job.

4) Make the cutting passes with the cross slide at zero, pull it back for clearance when returning ready for the next cut then move it forward to zero for the next pass.

5) Apply cuts as appropriate using the top slide.

6) When all spring cuts have been worked out with both cross and top slide at zero what you cut is what you set.

If using book values with a sharp point or partial profile insert this will not be deep enough.

7) Make extra cuts by feeding the cross slide past zero leaving the top slide set on zero.

When the fit is correct the cross slide dial will read the extra depth of cut needed beyond book values. Add this to the book value and make a note of the result to give the correct depth of cut to set at step 2 when you next have to cut that thread with that insert.

Note that any damage to the tip will upset the calculations and results. Errors will almost invariably be too shallow a cut.

Partial profile inserts from different makers may not have exactly the same shape so if you change insert breeds you may need to alter your personal table of results.

Full profile is lots less faff. But spendy.

Clive

Edited By Clive Foster on 04/11/2022 13:57:34

I can also recommend APT, they sell partial, (which is what you want) and full profile inserts singly. Also, of interest to hobby users is that they also sell certain sizes of turning inserts in smaller quantities than the 10 that you get in a full box. This enables you to try out different grades without breaking the bank on something that ends up being unsuitable for your needs. They also sell individual er milling collets in about three ranges of quality.

Many thanks for all of you replies. I now understand the different tips. APT looks like a very good source for the different tips.

Today for an experiment I cut a 1" x 32tpi External thread with a 55' insert tip. The result was excellent. I then attempted cut the same size thread using the internal tool and tip, The result was OK but the fit to the external thread was slightly "slack" I estimated the tapping size bore for internal thread. Is there calculation to find the correct tapping size bore ?

I keep a bookmark for Motalia, they have some very useful thread charts and sell a lot of goodies for motorcycles.

**LINK**

Peter

You don't say whether the inserts used were full, partial or sharp triangle profile? The type makes a big difference as to how easy it is to get a nice fitting pair of threads.

Once you have a decent understanding of the thread profiles its pretty easy to see whats going on and how to modify the starting bore and cut to get a nice tight pair.

Link in my previous post shows the diagram for 60° threads. There are equivalent ones for 55° Whitworth form. In some ways its easier with Whitworth because the thread crest shortening relative to the theoretical sharp triangle is symmetrical. Assuming full profile inserts simply cutting one or other thread a touch deeper will give any clearance needed. In other ways its harder due to the existence of flat top and truncated versions.

Shortening on 60° threads is asymmetrical which can cause issues.

The proper calculations are not friendly but can be found in decent books and, presumably, on the internet. My reference book, should I ever need such things is "Guide to World Screw Threads" by P.A.Sidders. Machinery's Handbook and other references also have the data but Sidders is easiest to handle.

Never having needed to work to that level of precision with properly controlled allowances I simply cut the external thread dead to size, bore for the nominal crest position of the internal thread and cut the internal thread just enough deeper that it all screws together. Using a test piece if need be to get it right.

Initially you will probably find it helpful to make a short plain land on the outer end of each thread at a diameter corresponding to the bottom of each thread. This will give you something to measure and confirm exactly how deep a cut you have made.

Clive

Edited By Clive Foster on 05/11/2022 22:51:13

Many thanks for all contributions to this 'thread'. I've cut many both Imperial & metric threads over the years - some good, some bad & some indifferent - mainly with HSS tooling.

I have used tipped tools more recently and aprt from being ham fisted and chipping them they seemed OK.

However I really find this discussion useful as I never realised the difference between partial & full form + the hints on usage, so this has been quite invaluable!

All the best Mike

Clive thanks for the reply, the tip I used was a partial form tip and gave a good profile. I will return to the workshop today and carry out further experiments. especially looking at correct bore diameters.

Peter

As it was a partial form tip you clearly went too deep to get the necessary root clearance.

Unfortunately I can't find a good diagram of the Whitworth thread profile explaining the shortening of a real profile relative to a theoretical sharp pointed one. Best I've found is this pretty crappy example :-

**LINK**

https://www.unikgaugesindia.com/bsw-bsf-whits-thread-gauges.html

Basically "H" is the full depth of a theoretically sharp pointed thread

and

"h" is the depth of the real thing cut with rounded crests and valleys instead of sharp points.

"h" is the thread depth given in the data charts.

To get "h" from "H" you divide H by 6 and subtract that from both top and bottom of the theoretical sharp thread. This is called shortening.

The crests of the thread are at the nominal internal or external diameter so effectively a real thread is cut 1/6 th of "H" shallower than the theoretical sharp pointed one.

With a full profile its all worked out for you. Cutting to book depth gives the correct thread with the crests shortened by the correct amount.

With a partial profile you have to go a little deeper to make room for the longer, smaller radius, tip. The shortening now has to be calculated for the finest thread that tool will manage.

Summarising :-

Theoretical sharp thread depth is 5/6 of H where H = pitch multiplied by 0.960419

Shortening for real thread depths is H/6 = pitch multiplied by 0.160082

To get depth of cut calculate 5/6 of H for the pitch you intend to cut and subtract the shortening, H/6, for the finest thread that the partial profile can manage.

Hopefully that helps.

Truncated Whitworth forms have flat rather than radius tips so the shortening is different. Basically the rounded crests are removed reducing the depth by pitch multiplied by 0.073917. So the depth of cut is is reduced by that amount.

The gotcha with truncated Whitworth is that the outer crests are also removed so the starting diameter for an outside thread has to be pitch multiplied by 0.073917 smaller and the bore diameter that much larger.

Calculating depth of cut for partial profile truncated Whitworth is painful!

I avoid such maths like the plague and, if I have to always draw it out on the CAD.

You see why I advise the Zero-to-Zero technique. It makes keeping track of this stuff easier.

Practical folk just make the bore a touch too small, so they know which direction the starting error is in then take fine cuts until things fit!

Clive

PS Don't even think about allowances and tolerances!

Edited By Clive Foster on 06/11/2022 10:48:14

Forgive me. I've read through Clive's Zero-to-Zero technique description several times and I can see that it'd result in a workable thread but isn't there a danger that the minor diameter might be cut too small by over-depthing creating a sharp valley, or have I missed something?

Is that what you meant by the "PS Don't even think about allowances and tolerances!"?

I'm a fan of the alternative approach advocated by Martin Cleeve of setting the top slide parallel to the lathe bed and, after the theoretical depth is reached plus a small allowance, broadening the thread groove until a good fit is achieved using the top slide.

Jon

Edited By Jon Gibbs on 06/11/2022 13:06:02

Jon

The great advantage of Zero-to-Zero is that the thread depth cut is exactly, within the limitations of machine and operator setting accuracy, what you set when feeding the cross-slide past its initial zero position.

If the tool shape is correct and the thread doesn't fit you have set the wrong depth or not fully taken out backlash in the feed screws when first touching off the work before zeroing the dials for the first time.

If the tool point is too sharp then Zero-to-Zero will indeed create an over deep thread with too small a minor diameter.

No way round that.

But if that is a worry the extra feed past zero on the cross slide needed to make the thread fit tells you exactly how much to grind off the end to bring it back to the correct width. Generally with hand ground tooling I would err on the over sharp side. If I needed less than 5 thou extra feed I'd call it good. Now I have the full profile Johanesson / SKF / Dormer chasers things just come out right.

I would only use Martin Cleeves method when opening up an Acme, Trapazoidal or Square thread for minimum backlash after first cutting it over-tight.

I dislike the uncontrolled nature of the parallel movement correction. You have no way of knowing if the error is due to fundamentally too little depth of cut or having an incorrectly shaped tool with too sharp a point. If the depth of cut is correct and the tool too sharp the method, theoretically, works fine. If there is any other error everything is somewhat up in the air an you cannot be sure of exactly what happened or be confident in the accuracy of fit.

None of which stops Martins method from producing a decently functional thread in reasonably skilled hands.

But its not something that should be resorted to if you have access to full profile inserts.

Martins method is probably most appropriate when using the part circle shaped HSS tools ground to either 60° or 55° that fit it Andycraft, Denford and other holders sized for small lathes. These are inevitably too sharp in the point and adjust in for each new thread wasteful. I imagine you could calibrate the side feed needed for different threads.

Clive

Pictures of a Denford version of the part circle threading tool holder :-

The front end is a pretty sharp 60° point. Grind the top to sharpen and rotate to bring it back to centre height.

Zero-to-Zero will always produce an over deep thread with too small a minor diameter when using this style of tool. In practice its not too bad as the tool isn't really made for anything beyond 1/2" or so diameter. Such smaller threads are Coventry die head territory for me!

Clive

Thanks Clive, that's what I thought but I think I was obviously at cross-purposes because I was thinking partial profile inserts whereas you were clearly thinking full-profile.

...and I think your last paragraph, which I think probably also applies to partial profile inserts, means we may be in agreement over the big advantage of Martin's parallel top slide method.

Many thanks

Jon

Hi all it's me again.

Since we last spoke, I have been experimenting with more screw cutting. Today using my Boxford ME10 with screw cutting gear box but no thread indicator. I set up to cut a 5/8" BSW thread.

Set up the gearbox for 11 TPI

Set the top slide at over at 27.5 degrees.

Set the 55' partial threading just touching the work piece and zeroed the cross slide dial.

Closed the clasp nut and left it closed for the duration of the job.

Carried out on scratch cut to prove the gear box set up. All ok.

At the end of the scratch cut run I released the lathe clutch. Wound the top slide back away from the work piece then reversed the saddle back to the starting point. Reset the top slide to zero the proceeded to take 0,005" cuts using the cross slide adjustment.

I took sufficient passes across the work piece the ensure I had approx 0.054" depth of cut.

The thread was cut with no issues other when viewing the threads with a magnifying glass the thread form is not symmetrical. The thread form towards the chuck is different to the thread form facing the tail stock. As the threading tool is only cutting in one direction how is other side of the thread actually formed ?

Could be caused by wear in the machine and/or inaccurate setting of the tool and topslide angle. With threading inserts I don't mess about with settinjg the topslide at an angle. I just plunge straight in so the insert is cutting on both sides.

Andrew