# Single Depth of Thread for 26 TPI



## alcostich (Feb 6, 2021)

I attempting to make a draw in nut for a 3C collet. The thread wants to be 26 TPI. Nowhere can I find the single depth of thread for that.
This will be an internal thread. The dimensions on the plan I'm working from call out a minimum Dia of .520 and a max of .635.
Based on the calculation: 1/26 x .866 I come up with .033.
Does that sound right?
Incidentally, my  Machinerys Handbook does not have a listing for this 26 TPI. Or, at least, none that I'v been able to find.

Thanks.

Alan


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## fcheslop (Feb 6, 2021)




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## Scott_M (Feb 6, 2021)

You can also use the formula for obtaining tap drill size and then divide by 2 to get single depth.

outside dia. of thread minus  .01299 x percentage of full thread divided by threads per inch = drilled hole size

if we use 100% and skip the outside diameter we get 

.1299x100= 1.29 dived by 26 =.04961 ( double depth ) /2 = .0248" single thread depth



Scott


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## Tim Wescott (Feb 6, 2021)

Even if I know the exact thread dimensions I'm not a good enough machinist to hit them -- I just get close, then shave & try, shave & try.


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## jkimberln (Feb 6, 2021)

I may not understand this project very well but if you have a min of 0.520" and a max of 0.635" why can't you just use a 5/8"-26 tap instead of trying to cut an internal thread on the lathe?


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## Richard Hed (Feb 7, 2021)

alcostich said:


> I attempting to make a draw in nut for a 3C collet. The thread wants to be 26 TPI. Nowhere can I find the single depth of thread for that.
> This will be an internal thread. The dimensions on the plan I'm working from call out a minimum Dia of .520 and a max of .635.
> Based on the calculation: 1/26 x .866 I come up with .033.
> Does that sound right?
> ...


But your machine has a setting for 26tpi, right?


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## Richard Hed (Feb 7, 2021)

fcheslop said:


> View attachment 122790


HOld on, Alcostich is in USA, our threads are a differnt shape, the 60deg type.


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## DavidBC (Feb 7, 2021)

The thread is a very early English thread. I have attached a jpeg file with the info you are after. It was known as a bicycle thread but is now called British Standard Brass (55 deg). I have a 5/8" x 26 TPI tap that you are after but living in Australia I cannot help you but I hope the attached file does. The column on the right is the tapping drill size. The image was taken of a thread chart of an Australian company called Sutton Tools. 

www.sutton.com.au

Regards
David


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## DavidBC (Feb 7, 2021)

Further to my post if you look at a chart that shows BSF Screw Threads you will find the depth of a 26 TPI thread is .0246".

Regards

David


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## MRA (Feb 7, 2021)

Tim Wescott said:


> Even if I know the exact thread dimensions I'm not a good enough machinist to hit them -- I just get close, then shave & try, shave & try.


Me too!  not forgetting that when I cut threads I sometimes push a burr up at the crowns, which can make things seem tight until I take it off, at which point things are really not tight at all


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## Charles Lamont (Feb 7, 2021)

26 TPI is a pitch that is common in the UK.  1/4" and 9/32" BSF are 26 TPI. "Gas" or "brass gas" thread is 26 TPI in all sizes and is used extensively in UK model engineering designs as the coarsest of the fixed pitch model engineering thread series, the others being 32, and 40 (and sometimes 60). 

All these threads are 55° Whitworth form, as shown above by fcheslop. The thread depth is 0.64 P, so for 26 TPI, the depth of a Whitworth form thread is 0.025". 

Of course Alan's thread may not be Whitworth form, but if it is 26 TPI, I think it might well be. 

(I typed all this before reading DavidBC's posts)


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## fcheslop (Feb 7, 2021)

If you have any 26tpi tap ? .You could use it as a chaser to finish the thread
Like most I suck it and see for the final few cuts.
From memory  British Standard Brass is 55 degrees .British Standard Cycle is 60 degrees. It often causes confusion especially if you want a good fit
The chart works well enough for my simple needs
cheers


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## ajoeiam (Feb 7, 2021)

I would suggest changing that internal thread to something to is more 'common' today. Then you change its mating part and - - - bob's yer uncle.
Why beat yourself about the head and buy very very expensive tools that you are only going to use once?
If you feel you must keep the thread - - - - by all means - - - - I would 'update' the drawing - - - life is too short!

Went looking for the specs - - - -what a boondoggle!
Doesn't look like there is any way of getting around the issue!!!!!!!!!!
Isn't backward compatibility fun!!!!!!!

Looking at pgs 966-8 (25th ed Machinery's handbook) - - - - well they're ALL oddball threads!!!!


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## danallen (Feb 7, 2021)

Since the pitch is almost exactly 1mm I would use the metric way of calculating tap drill size or thread bore. Subtract the pitch, 1mm or .040, from the nominal OD and start there.


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## JLaning427 (Feb 7, 2021)

I have a South Bend 9" lathe that uses the 3C collet, so this piqued my interest.

I went looking in my "old" Machinery Handbook (11th edition from 1943), since others had already said 25th Ed didn't have useful info.  It lists a bunch of collet sizes, including 3SB / 3C (listed separately but the dimensions are the same).  It lists the thread as 0.645" - 26.

It also has a listing for the United States Standard thread form for 26 tpi.  It lists 26 this as having a 0.0250" depth of thread, and 0.0048" flat width.

I don't have a 26tpi thread gage, but checked my 3C's with 24, 27, and 28 (which I had) and they definitely don't fit.

James


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## JLaning427 (Feb 7, 2021)

Additional follow up.

-USS threads are 60° and are the predecessor of American Standard threads.  I am assuming that they are USS threads, based on finding 26TPI info in the USS section.  And the fact that South Bend, Hardinge, and Cataract (the original designer of the 'C' series of collets) were all US based companies, but it  could be something else.  It could also be British Standard Fine Thread, which shows thread depth at 0.0246 for 26 TPI (I would call that 0.025") and that is a 55° thread, as mentioned above.

-I measured the OD of a couple of my 3C collets, and they were around 0.637" to 0.638" across the threads, vs the 0.645" nominal listed in Mach Handbook.

Good luck cutting those threads!

James


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## MRA (Feb 7, 2021)

fcheslop said:


> If you have any 26tpi tap ? .You could use it as a chaser to finish the thread



A bit pot of mismatched odds and ends which once would have fit a Coventry die head, came my way very cheap (no sets).  I thought I might grind them up for small tools (and make a holder), but I ended up using them for chasers which seems a better use.


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## fcheslop (Feb 7, 2021)

Also if youre cackhanded enough to break a split style die the wee bits left can also be made into chasers. I have some very old hand chasers that often get me out of trouble although I've only hand chased in brass for old scientific instruments.
Keep well


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## Richard Hed (Feb 7, 2021)

MRA said:


> Me too!  not forgetting that when I cut threads I sometimes push a burr up at the crowns, which can make things seem tight until I take it off, at which point things are really not tight at all


Oh, yeas, that happens, keep a file close by and keep the burrs down, however, there is another thing too if you make American style or metrics with 60deg angles:  valleys and peaks on the threads where the peak should be cut down a couple percent.  I find that usually a file over the top is enough if my diameter is what is called out for in the drawing


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## Richard Hed (Feb 7, 2021)

fcheslop said:


> If you have any 26tpi tap ? .You could use it as a chaser to finish the thread
> Like most I suck it and see for the final few cuts.
> From memory  British Standard Brass is 55 degrees .British Standard Cycle is 60 degrees. It often causes confusion especially if you want a good fit
> The chart works well enough for my simple needs
> cheers


This is why I don't bother with Whitworth or the other type of threads, only 60deg


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## MRA (Feb 7, 2021)

fcheslop said:


> Also if youre cackhanded enough to break a split style die the wee bits left can also be made into chasers. I have some very old hand chasers that often get me out of trouble although I've only hand chased in brass for old scientific instruments.
> Keep well



That sounds interesting, old instruments.  I found something called a Henrici harmonic analyser in a bin.  It was in bits and I didn’t know what it was when I found it.  It is kind of related to what was my job (noise and vibration) so that was nice.  If you type that search string into youtube you’ll see a short video of it doing its thing, resolving Fourier components.


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## alcostich (Feb 7, 2021)

fcheslop said:


> View attachment 122790


This looks to be for a Whitworth thread.  I'm looking for a 60 thread for a 3C collet for a South Bend lathe.


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## alcostich (Feb 7, 2021)

Well, I don't have a 5/8 x 26 tap. And, yes, I can set the lathe to cut a 26 TPI.  It was also suggested to use something more common. 
But, then I would have to dump a drawer full of 3C collets.


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## Richard Hed (Feb 7, 2021)

alcostich said:


> Well, I don't have a 5/8 x 26 tap. And, yes, I can set the lathe to cut a 26 TPI.  It was also suggested to use something more common.
> But, then I would have to dump a drawer full of 3C collets.


My suggestion is, if you have a capable lathe, make the threads on it.  Try a practice piece first if you have not done a lot of threading, particularly if it inside threads.  Also, take a look at Joe Pie's (Pizinscki?  sp.) method in Texas.  It's really a superior way to do it.


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## Hairball (Feb 8, 2021)

You really never need a chart for any 60 deg thread. You just need a simple formula. Here it is with an example.  Subtract 1 divided by threads per inch from major diameter. Here is a common one in the US  SAE  1/4-20.   1/20tpi equals .050.  Subtract  .050 from major diameter .250 equals .200. That is your tap drill size. The chart calls for a #7 drill. #7 is .199  That is the closest drill to .200. Even easier for Metric.  Example  6mm x 1mm. Subtract 1mm from 6mm equal 5mm for tap drill size. For the 3C collet measure the OD of the threaded part of the collet and subtract 1/26 (.038). That result will be your bore size for the nut. I think the total depth of cut is gonna be about .024 but creep up on it using the collet as a try bolt, lots of variables thread on the bore. Bar flex, spring passes etc


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## fcheslop (Feb 8, 2021)

The depth for sharp 60 degree V thread form is calculated by

D=pitch x cos30 = .866 pitch

= .866 /number of threads per inch

The depth for American National form is calculated by:

.6495 x pitch

Depth for Unified is:

.6134 x pitch


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## Richard Hed (Feb 8, 2021)

fcheslop said:


> The depth for sharp 60 degree V thread form is calculated by
> 
> D=pitch x cos30 = .866 pitch
> 
> ...


You are absolutely correct as I chekt it out.  For those of you who know a bit of trigonometry, what follows you will understand.  Those of you who are not familiar with trig, read no further as I don't wish to confuse anyone.  Altho' using the above method with "cos(x)" yeilds the correct solution, the conceptual idea is incorrect.  One should use the "sin(x)" method which is sin(60)=.866  (the same as cos(30).  It's just a point about how we are taught to use trig.  We are taught the wrong things about trig, that's why it is so difficult for people to use.  

Trig was invented because of map-making.  Map making uses circles and triangles together, but we are taught trig using triangles alone, this makes the conceptual frame work extremely difficult and nearly impossible to remember as it is TOO ABSTRACT.  Add the circles and it becomes a "visual" frame work which is easy for humans.  (Dogs would use hearing or smell so it would be extremely difficult for them to auditorize or olfactorize trigonometry.  )  Notice that one could also use the Pythagorean Theorem for this but who wants to muddy the wine?


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## fcheslop (Feb 8, 2021)

Ive had the attached chart for sometime and it is handy.
Just dont tell our lass Im right she will never believe it
cheers


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## MRA (Feb 9, 2021)

Wow - Zeus book eat your heart out


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## terryd (Feb 9, 2021)

MRA said:


> Wow - Zeus book eat your heart out



I must now throw out both my Zeus and my Machinists handbook as well as the laminated chart taped in the lid of my taps and dies fitted box, damn

TD


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## Ken I (Feb 9, 2021)

I have on a few occasions been required to cut a thread where the mating part is too big or attached to something too big to test the fit - so you have to get it right first time.
Then I use a sharp point (so you know exactly where your point is on scoring the bore or O.D.) and the 0.866 calculation (for 60° forms) thus far it has worked every time - its a most gratifying feeling when it fits.
Regards, Ken


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## DavidLloyd2 (Feb 10, 2021)

fcheslop said:


> Ive had the attached chart for sometime and it is handy.
> Just dont tell our lass Im right she will never believe it
> cheers


Thank You


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## fcheslop (Feb 10, 2021)

Happy it is of some use.
Keep well
cheers


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## mcostello (Feb 10, 2021)

I cannot seem to down load the thread file. What am I doing wrong?


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## terryd (Feb 11, 2021)

Hi,
I came across this thread specification for the 3C collet which apparently came from an actual collet drawing.  it looks like a British Standas Cycl;e thread which has a 60° thread profile

.640-.004  - 26-NS-RH
P.D. .615-.003



DavidBC said:


> The thread is a very early English thread. I have attached a jpeg file with the info you are after. It was known as a bicycle thread but is now called British Standard Brass (55 deg). I have a 5/8" x 26 TPI tap that you are after but living in Australia I cannot help you but I hope the attached file does. The column on the right is the tapping drill size. The image was taken of a thread chart of an Australian company called Sutton Tools.
> 
> www.sutton.com.au
> 
> ...



Hi David,

I beg to differ.  BSB (British Standard Brass thread) is not the same as BSC (British Standard Cycle thread)  BSB (not actually a British Standard thread) is standard Whitworth form i.e. 55° but BSC (aka CEI - 'Cycle Engineering Institute')  as originally used on bycycles and motorbikes is in fact a 60° thread form so that is the most likely thread form for the collets.  All BC threads above 1/4" are 26 tpi although some particular sizes can also be 20tpi.

Taps of that size are still available in the UK from The Tap and Die Company at approx $20 + delivery.

Stay safe and healthy,

TerryD


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## alcostich (Feb 26, 2021)

After two unsuccessful attempts, finally got one to work. Seems either I didn't understand the dimensions
 on the drawing or those dimensions were wrong. I bored the nut to .0609 which is what my SB collet draw-in 
tube is. Then, although the formula for single depth of thread calls out .033 I turned to .025 and that worked.

Alan


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## 10thumbs (Feb 27, 2021)

.6495 divided by tpi = single depth of cut.


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## alcostich (Feb 28, 2021)

The formula I used, and can't now remember where I got it from is:

1/26 is .0384
.0384 x .866 is .033

However, .025 worked just right.

Alan


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## Richard Hed (Feb 28, 2021)

alcostich said:


> The formula I used, and can't now remember where I got it from is:
> 
> 1/26 is .0384
> .0384 x .866 is .033
> ...


There are two methods one can use:  the trig method and the Pythagorean theorem.  Ultimately, they are exactly the same thing which can be proven mathematically but the Pythagorean theorem is easier to use.  
Knowing that an isoceles triangle has all three sides being equal makes this very easy to use for the American style and metric styles which both use 60deg angles on all three sides.  for the British 55 deg angles, this is a bit harder but ultimately the same formulation.  

Construct an isoceles triangel with the base at the bottom, labelled A, the left side label B and the right side label C.  Then cut the triangle from the tip to the center of A.  Call this H for height.  From the point where H meets A, which is A/2 and meets at a right angle, to either point to the left or right meeting either B or C could get a special name like "a" or maybe just continue to use "A/2" which is what I will do.  Then useing the Pythagorean theorem:  H^2 + (A/2)^2 = B^2 = C^2.  We know the length of A (and thus A/2) and B which are 1/26 in inches which is .0385 and A/2 = .0192.

then using the formulation, H^2 + (A/2)^2 = B^2 we solve for H^2:
H^2 = B^2 - (A/2)^2 and pluggin in the numbers above we get:
H^2 = .0385^2 - .0192^2 = .00148225 - .0003705 = .00111169

simply take the square roots of each side:  H = .03334
This is the mathematically correct solution, that is, it is as close enough "approximation" that we need in machining this.  However, there are other factors as we machinists know, for instance, machining is never as clean as a mathematical formula.  There are always factors of machinability that math does not address.  If both the inside and outside diameters are "perfect" the items are actually not likely to fit, as we all know a bit of clearance is needed,  Usually a couple thous minimum and the tops of the sharp threads reduced a small percentage.  for instance, say one has an OD of 1" and 1/8th" threads (1-8tpi).  It might be easiest to cut the 1" down to begin with by a few thou before you even begin your threads.  so with IDs.

Am I wrong?  Let me know.  Also, using trig the very same number will come up for the above calculations.  I hope this was not too long winded and I hope even more so that it is clear and easily understandable.


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## mu38&Bg# (Feb 28, 2021)

alcostich said:


> I bored the nut to .0609 which is what my SB collet draw-in
> tube is. Then, although the formula for single depth of thread calls out .033 I turned to .025 and that worked.



Yes, this is expected. I thought I posted a response with these figures here, but maybe I had a reboot before I hit post. The formula for modern 60° threads is universal and understanding basic diameter, pitch diameter, and allowances/tolerances allows you to make any thread. It's all in the Machinery's Handbook or ASME standards.

The trig is simple, but applying it to threads take a bit more effort. .033" is based on full thread depth, but this is never the case!! Depth of a sharp thread for internal thread is 3/4 total height of .033 or .025 from the minimum minor diameter of .602". You need to add some for clearance according to the tables. And you would adjust thread depth according to the starting minor diameter (because you're touching off the minor for your measurement) as pitch diameter and major/minor are independent.

When the mating part is available, one can get by without being very particular about the math. But you run the risk of other mating parts not fitting if you make pitch dimater too tight. I have one such 5C collet block which is so tight, some collets won't thread in.


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## Thread man (Jul 5, 2022)

Maybe the easiest way to find the thread depth on any Whitworth profile thread (55º) is simply to multiply the pitch (in decimals) by 0.640327 (0.64 should be accurate enough for most ☺) The 0.64 applies to threads that have the correct radii.

The correct radius is 0.137329 times pitch.  Personally I'd use 0.14.

The thread depth for H is 0.96 times pitch.


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## Thread man (Jul 5, 2022)

Probably most Americans don't think about it but the most commonly used thread profile (in the world) for pipe threads is the 55º Whitworth.

BSPP is now "G" and BSPT can be R, Rc or Rp depending on whether it's external or internal. The "new" designations are ISO.


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## Thread man (Jul 7, 2022)

I'm wondering if anyone wants similar information re 60º threads? Metric and ISO Inch (UNC etc.) have identical profiles except for a very small difference in the radii.

While I'm at it ☺ US pipe threads (NPT & NPS)) only have 5 different pitches. Whitworth profile pipe threads (taper and straight) only have 4 different pitches. Typisk (?) but the only pitch the two types have in common is 14 TPI.


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## clockworkcheval (Jul 8, 2022)

A small side remark that is probably obvious to many: when cutting a thread with a pitch that can divide the pitch of the lead-screw in a whole number you can disengage and engage the halfnut at will and still stay in pitch. For example with a 4 mm pitch leadscrew you can do this with treads of the pitches 4 mm, 2 mm, 1 mm, 0,8 mm and 0,4 mm. The benefit is that you can use the stops to dis-engage and thus cut at a much higher cutting speed resulting in a nicer finish.


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## Ken I (Aug 1, 2022)

clockworkcheval said:


> A small side remark that is probably obvious to many: when cutting a thread with a pitch that can divide the pitch of the lead-screw in a whole number you can disengage and engage the halfnut at will and still stay in pitch. For example with a 4 mm pitch leadscrew you can do this with treads of the pitches 4 mm, 2 mm, 1 mm, 0,8 mm and 0,4 mm. The benefit is that you can use the stops to dis-engage and thus cut at a much higher cutting speed resulting in a nicer finish.


I gave up on using that idea or the indicator dial years ago simply because I've had the half nuts "ride" improperly closed and therefore out of pitch - it only has to happen once and your job is destroyed. I have had it happen on a number of different lathes including a wonderful Graziano but much more prevalent on hobbyist machinery.

My method is slightly more time consuming but is guaranteed.

I screw in reverse with the tool upside-down - that way I can cut at whatever speed I like without any danger of crashing into the bottom of a blind hole or a shoulder. No accuracy is required for where you stop. You never disengage the half-nuts until the job is complete.

This is a particularly useful method for screwcutting blind holes.

I then retract the tool and go forward slowly - turning the last bit to the shoulder or bore bottom (undercut) by hand - hand turn the chuck backwards to remove lash - set tool to next depth cut - (check rotation is set correctly) - hit start and go like the clappers away from the work.

Almost foolproof - unless you hit start with the incorrect rotation - which as you can imagine, I have actually done.

Regards, Ken


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## clockworkcheval (Aug 1, 2022)

Hello Ken, I hear and feel the crunch! With my Schaublin 102 VM off about 70 years old when I set the endstop on the halfnut relief the carriage will come to a standstill after 2,5 mm. So with a blind hole I turn a relief groove at the inner shoulder of 2 mm wide and then very carefully set the stop for the tool also 2 mm before the start of the groove. After simulating outside the bore I pray and engage. So far all went well. But maybe I should switch to your method before the hideous crunch gets me!


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