Single Depth of Thread for 26 TPI

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MRA

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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.
 

alcostich

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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.
 

Richard Hed

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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.
 

Hairball

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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
 

fcheslop

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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
 

Richard Hed

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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
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?
 

Ken I

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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
 

terryd

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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

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

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
 

alcostich

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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
3C Collet Block.JPG
 

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alcostich

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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
 

Richard Hed

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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
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.
 

mu38&Bg#

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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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