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Threading and the tables

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Russ Bulley31/03/2020 12:29:35
12 forum posts
2 photos

I want someone to educate me!

As I have said before, I am not a professional, learning as I go along, and have just finished 8 x 7/8” 9tpi BSW internal threads about 2” long which has taken about three days.

Now my book (British Standards for Workshop Practice - but this seems to apply to all thread forms and tables) says for nuts medium fit, the minor diameter (which I take as h on the diagram) is between 0.7328” and 0.7620” with a tolerance of 0.0292”, depth of thread 0.0711”. So, I bore to 0.740 +/- 0.003” thinking that’s in the tolerance zone, then thread to 0.071” depth, measured with a DTI on the cross slide. But then I test with a tap, and every single thread was really tight and I had a struggle getting the tap down. All are done now, but what am I doing wrong? By the way I am using a BSW insert cutter.

I have to say, even when cutting outside threads, I can never get it right, even though I use a DTI on the cross slide and I have eliminated most wear by adjusting gibs etc. What’s the secret of reading the thread data tables?

Also, if I am using a straight Vee cutter (not a profile insert) the apex of the cutter is actually at H/6 (for BSW) deeper than the bottom of the thread due to the curve. How do you get your depth correct, as you need H/6 + h as a finished height, when you have a tolerance of 0.029”!


Edited By Russ Bulley on 31/03/2020 12:29:54

Edited By Russ Bulley on 31/03/2020 12:30:16

S.D.L.31/03/2020 12:42:12
207 forum posts
9 photos

Can you post a picture of the diagram you are using?


SillyOldDuffer31/03/2020 13:51:44
5631 forum posts
1157 photos

First, I'd work from a tap drill table rather than the theoretical thread form dimensions. I find the tables hard to interpret, especially when tolerances are taken into account. Let someone else do the hard work!

My drill size tap table suggests a 19.4mm drill for 7/8" BSW - that's about 0.768", which - sanity check - is nearly 30 thou bigger than Russ's 0.740" starting point. It's probably why the tap is so tight - the start hole is too small.

Depth of thread is 0.64 ÷ pitch, therefore 0.64/9=0.071" which matches Russ's figure. Hooray!

I'm no expert on threading, but I suggest the lathe is cutting the thread undersize due to the tool shank bending. 2" gives the tool tip plenty of leverage and it tends to cut less deep than the dial suggests. I suppose a professional would measure the actual depth and apply a correction to get to size. I prefer to cut to dial, knowing it's not quite right, and then finish the thread to size by running a tap through. (My goal when threading is to minimise the work the tap/die has to do, not to be more accurate than it. Again, the strategy is to let the tap finish the job so I don't have to do hard sums and tricky measurements.   I only try hard to get threads correct on the lathe when I don't have a tap/die in the right size.)

How important are the tolerances? For most amateur purposes, they're a red-herring. If tolerances really matter, my answer may be misleading!



Edited By SillyOldDuffer on 31/03/2020 13:55:49

Mick B131/03/2020 13:58:23
1552 forum posts
83 photos

When you reached the calculated depth of thread, did you run any spring cuts at the same crossslide setting to take out possible deflection of whatever was carrying the threading insert? Assuming all your calcs and settings were correct, that'd be my first check.

I'm thinking you need to know or calculate the depth of the triangle between the apex of the vee and the crest radius of the standard thread, and add that to your calculated threading depth. That could be the issue.

And +1 on SOD's comment about finishing the thread off by tap too - it's a far simpler and more reliable way to produce a thread for normal fastening purposes. 


Edited By Mick B1 on 31/03/2020 14:08:12

Clive Foster31/03/2020 15:06:20
2154 forum posts
73 photos

Almost certainly your tool tip shape is not quite correct. In my experience almost everyone makes them a little too pointed. I certainly did.

Its almost impossible for home shop guy to grind a single point cutting tip of sufficient accuracy to work to formal tolerances as per the reference books.

Easiest answer is to revert to the alternative flat top form, its in the books as an official alternative, and adopt a threading technique that allows you to make small, predictable, changes in depth.

I use the Zero-2-Zero angular in-feed method which has the great advantage that the depth of cut is set to book values before starting using both cross and top slide dials. When both feed dials read zero what you have cut is what you set. If that's not right its very easy to go past zero until the thread is good.

If you have more than one thread to do the dial zero position can be adjusted to reflect the correct last cut position and all subsequent threads made at that set-up will be correct. The Zero-2-Zero method also automatically sets the correct indeed for the angle at which the topside is set so no maths and the actual set over angle only has to be close. Offset of 25° works fine for me on both Whitworth 55° and Metric / US 60° threads.

I now use Johanneson / SKF / Dormer chasers originally developed for captan and similar production machines. The cutting edge is formed into two threads of the correct profile. It really is pretty easy to work to book tolerances with these.


Edited By Clive Foster on 31/03/2020 15:07:19

Russ Bulley31/03/2020 19:28:18
12 forum posts
2 photos

So thank you for the replies. Yes always do spring cuts. And as I am using Sandvik inserts, think the form will be correct.

I did also say these particular threads were internal.715ff0aa-196b-44df-8f69-8c56b3147e22.jpeg


Michael Gilligan31/03/2020 20:08:14
15478 forum posts
668 photos
Posted by Russ Bulley on 31/03/2020 12:29:35:

[…] But then I test with a tap, and every single thread was really tight and I had a struggle getting the tap down. All are done now, but what am I doing wrong? By the way I am using a BSW insert cutter.


Wild guess, Russ ... the Tap may be over-size [i.e. a looser tolerance than you are trying to achieve]
Working to the standards that you are ; you should probably be using a Go/NoGo gauge


Clive Foster31/03/2020 20:47:27
2154 forum posts
73 photos

Sorry I missed the bit about using inserts in the original post. As you say a full form insert should be dead on the money when it comes to shape.

Are you able to run the inserts at book speeds & depth of cut? I've heard tell that there can be sizing issues if you don't run them in the manner they are designed for.

Obviously not with threading but I have had experience with cuts being a touch shallow when running an insert too slowly. That particular one was bought in error due to not reading the spec properly and missing the Warp Factor 10 cutting speed. Having made the first few parts running at a sane speed I took my courage in both hands, wound the 1024 up to maximum rpm putting the cutting speed at the bottom of the recommended range and got an undersize part. Light push fit went to free rotating but shake free bearing. Which sounds like the sort of size error you were getting.


Russ Bulley31/03/2020 20:58:38
12 forum posts
2 photos

Clive, no I rarely can as I am using suds and at high speed the overspray is terrific. Anyhow charging (9 tpi remember) into a dead end also scares me. So I was using 5 thou increments to try and compensate.

To SOD good point, but why is the tapping drill size way off the table? I still don’t understand this tolerance as all I want is a standard thread medium fit!

I will measure the tap as suggested.

All good stuff guys. So where do you obtain the true machine use data (I acknowledge SODs comments)?


Clive Foster31/03/2020 21:49:13
2154 forum posts
73 photos


The Sandvik data book should tell you what speed and depth of cut its designed to run at. Hopefully its one intended for use on a manual machine so the speeds will be within reason.

Inserts tend to need an adequate cut to behave themselves so 5 thou may not be enough. Not sure about the spring cuts either. Inserts are supposed to be production tools, to greater or lesser degree, so the makers put considerable effort into producing something that doesn't need the faffing around of multiple spring cuts.

Certainly my chasers don't need conventional spring cuts, just a final low feed pass to clean things up to good finish. Which was probably the biggest surprise when I first started using them. I imagine modern inserts will be rather better behaved as being a much more recent development.

Realistically I suspect you will need to accept that you won't get perfect results by simply following the thread reference book and will have to tweak the final cut on test to get things just so. The great thing about the Zero-2-Zero method is that it allows you to make small, measured, adjustments whilst getting things right on the first sample and then reproduce the results for the rest of the job.

Best practice would be to make notes on the actual in-feed needed to get a right size thread off the machine alongside the thread book value. After a while and several threads you may well find that you have a pattern to the results and will know how much to add right from the start.


Hopper31/03/2020 23:53:33
4404 forum posts
93 photos

Thread tables and British Standard diagrams etc etc cause way too much angst for use in the home workshop. They describe a theoretically perfect thread suitable only for a theoretically perfect world. You should throw them away.

In practice, you are better off to follow SOD's advice and instead calculate your thread depth for BSW or BSF etc as .6403 X pitch.

BUT you need to allow working clearance on real world threads. So for external threads, start with the outside diameter at least five thou smalller than the nominal size. So a 1/2" thread would have an actual diameter of .495". For larger threads, increase this clearance. A 1" diameter thread I would probably make outside diameter .990", so ten thou clearance.

Ditto internal threads. Make the hole up the middle five to ten thou larger than the nominal root diameter. And you find your nominal root diameter by deducting two thread depths (as calculated in the above formula.) from the nominal OD of the external thread.

Final sizing is done by try and fit with the nut or bolt the part is going to mate with. Or by using a tap for the final few thou if available. No shame in that. Quick and effective is always the best way of machining.

Get any of the old books by the old timers who screwcut all their lives without ever using a BS drawing in the workshop and learn the way screwcutting was done on manual lathes for the past 100 years or more. It is quite simple. Recommended: The Amateurs Lathe by LH Sparey for basics and Screwcutting in the Lathe by Martin Cleeve for more than anyone ever really wanted to know on the subject. .

Nigel Graham 201/04/2020 00:13:39
585 forum posts

By corollary I use a die to trim the top of a cut external thread.

It's worth remembering that an insert tool or a chaser will only cut true rounded crests if the thread requires the full depth of the tool.

A tip for depth-gauging. If possible make the part a bit over-length and turn the surplus section to a few thou' above (male thread) or below (female) root diameter. When the threading tool just starts to scratch that, you are almost there. Take a couple of spring cuts then test with the mating part - or if a standard thread, a new screw. Or finish the thread with a tap or die (preferably a die-nut?)

Spring-cuts - I normally take one every couple of passes to ease the growing load on the tool, especially when using straight-in cuts.

Also, I do question this matter of carbide tips "needing" to be run at high speeds. They are indeed made to clear metal at alarming rates, but in industrial production conditions on massively-built CNC machines; and their individual designs are to suit particular materials. I have not had unduly poor finishes using insert tooling at modest rates. If the finish is poor I blame my tool-setting or using a slightly worn or mis-chosen insert, or using "new-to-me" steel of unknown provenance, before the tool per se.

Hopper01/04/2020 00:40:31
4404 forum posts
93 photos
Posted by Nigel Graham 2 on 01/04/2020 00:13:39:

It's worth remembering that an insert tool or a chaser will only cut true rounded crests if the thread requires the full depth of the tool.

That's certainly true. But I never worry about getting the perfect tip radius etc -- that is all part of the angst brought about by looking at BS thread form drawings and the like. As Tubal Cain points out in his ME Handbook, you get something like 95 per cent thread strength with only 65 per cent thread engagement. What you want on the tips and roots of your threads is working clearance. So if the radius has a bit of a flat on the top of it, or is just one big flat, no big deal at all. In fact its good, because its providing working clearance and ensuring the thread is engaging on the flanks as it should do, and not on the tips and roots.

If we were making wing pivot bolts for space shuttles, things might be different. But for general work, not so. Horses for courses.

Howard Lewis01/04/2020 10:27:22
3146 forum posts
2 photos

If you do as Hopper suggests and truncate the thread, (Very slightly Undersize for External, Oversize for Internal) you don't need to worry about getting the correct crest or root radius. It won't be there.

As an Apprentice, we were taught to truncate threads, to prevent interference between root and crest.

There is always a lot of discussion about Tapping size vs % engagement of threads. A reduced engagement, say 65% against 75% will not matter in most of the work that we do. We are not deliberately going to tighten fasteners into yield, for hobby work.

Tolerances become important when making mating parts in volume, so that all have to be capable of being interchangeable with all the others. That's why the BS charts, and drawings, quote tolerances.

But, at the extremes of tolerance, you can have parts within drawing tolerance, but producing fits that vary between tight and loose. If the tolerance on both parts are + / - 0.001, with a nominal 0.003 clearance, you could finish up with only 0.001 clearance or 0.005 if the mating parts are at the extremes of the tolerance band.


Hopper01/04/2020 10:45:51
4404 forum posts
93 photos
Posted by Howard Lewis on 01/04/2020 10:27:22:


As an Apprentice, we were taught to truncate threads, to prevent interference between root and crest.

Exactly where I learned it. I vaguely remember seeing a Standard thread form drawing at tech college but they were certainly never referred to on the machine shop floor. The only crest radius in sight might have been from running a file down the thread to knock the burrs off.

I think the angst about radiuses and perfect forms has come about with the availabiliity of readymade full-form carbide inserts designed for use in production CNC work which is a whole different ball game. We just ground our own HSS tools and rubbed the tip on an oilstone a few times to round the end to stop edge chipping. Those were simpler times I guess.

SillyOldDuffer01/04/2020 14:32:41
5631 forum posts
1157 photos
Posted by Russ Bulley on 31/03/2020 20:58:38:


To SOD good point, but why is the tapping drill size way off the table? I still don’t understand this tolerance as all I want is a standard thread medium fit!


All good stuff guys. So where do you obtain the true machine use data ...



I have a number of reference books. They vary a bit depending on the target audience, for example a tool-maker or draughtsman might need all the gory details, a professional might need a few fits, while the amateur wants to keep it simple.

The Model Engineer's Handbook (Tubal Cain) is a good basic reference. He gives one figure for 7/8" BSW tap - 19.5mm.

I have a Newnes Engineer's Reference Book which is a bit more grown up. For 7/8" BSW tap drills it suggests:

  • 47/64" for a Class A (Close Fit),
  • 3/4" for a Class B (Medium Fit), and
  • 49/64" for a Class C (Free Fit)

Tubal Cain recommends 19.5mm for Model Engineering. That's 0.768", rather looser than a Newnes Class C Free Fit ( 0.7656" )

Another good book (second-hand) is Machinery's Handbook from the USA. Mine is the 20th Edition, which does US, British and Metric. 2480 pages of light reading! It recommends a 19.25mm tap drill for 7/8" BSW ( 0.7579" ), which lies between Newnes Class B and Class C fits. Machinery's is lightweight on BSW, obsolescent in 1975, but it does detail on Metric and Unified Screws.

So variations in the Engineering literature! Where did your table come from? It's not unknown for mistakes to creep in.

As others have noted, a BSW thread made fully to specification would have rounded valleys and peaks. However, it's common for lathe cut BSW threads to ignore the rounded top, and for the operator to gently flatten the tips off with a file. In the USA they decided Whitworth's rounded top didn't add much value and simplified NS by specifying a flat top. Metric did the same.

What this boils down to is how important the fit is; what's it for? A safety critical thread will be made to a high-standard but for many ordinary purposes threads are made slack. Loose threads are preferred when ease of assembly matters more than maximising strength. In practice, most fasteners are on the slack side.

The tolerance thing is confusing. They're specified for inspection purposes. When I make a batch of threads it rarely matters if some are tight and others loose. As long as they fit together I'm happy. Not so in manufacturing, where interchangeability matters. Gauges are used to confirm threads are made within tolerance, ie not too big or too small for some defined purpose. Machines are set to make threads within a tolerance band, and readjusted as the tools wear, and again when the tool has to be changed. Although the resulting threads can be a long way off perfect, the variation is under control and acceptable within limits. Few of us need to work that way.







Edited By SillyOldDuffer on 01/04/2020 14:34:38

Russ Bulley01/04/2020 19:06:04
12 forum posts
2 photos

I want to say thank you to everyone for the contributions above. Very helpful!


S.D.L.01/04/2020 20:05:08
207 forum posts
9 photos

From memory when we used to use sandvik internal boring bars the shim under the insert has to be different angles for different TPI inserts, there were at least 3 angles maybe more. I suspect for the feed in to be to theory dimension the angle needs to be correct.


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