

Hi all:
I did a search but didn't come up with any good links.
I am looking for a thread depth chart. Not the calc of .75 divided by threads/in. I mean the actual depth of the threads or the "minor external diamension" subtracted from the "major external dimension".
I don't always cut with the compound, but just use the cross.
Thanks,
Jake

Do you have a "Machinery's Handbook" ?
Invest in one... even an OLD one...

Divide one by the threads per inch and then multiply by .866
Example: 16 threads per inch
1/16 = .0625
.0625 x .866 = .054 single depth of thread
If you need a chart, work the formula above for all the common threads and make your own chart.

Hi Jake,
I have a thread gage (sometimes called a fishtail)that has the double depths listed for most of the threads. If you don't have one, I suggest getting one. They're handy for grinding and setting up thread tools too.
If that isn't available, I just work from the one that I have memorized; 20 TPI = .065 double depth.
As an example, say that you want to cut 16 TPI.
20/16 = 1.25
1.25 X .065 = .081
16 TPI = .081 double depth.
Works for me anyway.
Mike

Go to my page, download DOT (Depth Of Thread), run it and you'll get an output
that looks like this...

DEPTH OF THREAD CALCULATIONS
Threads angle [60 deg] ?
Threads per inch [20] ? 16
Compound rest angle [29 deg] ?
thread angle = 60.00 deg
threads per inch = 16.0 (pitch = 0.06250 in/thread)
{compound feed at compound angle = 29.0 deg}
(A) dot sharp crest  sharp root = 0.05413 in {0.06189 in}
(B) dot flat crest  flat root = 0.03383 in {0.03868 in}
(C) dot sharp crest  flat root = 0.04059 in {0.04641 in}
(D) dot flat crest  sharp root = 0.04736 in {0.05415 in}
(E) double dot sharp crest  sharp root = 0.10825 in
For American National (60 deg) thread form, subtract 0.0406 in from
major diameter (assumes p/8 flat on crest) to obtain pitch diameter
use any line on threading dial

Regards, Marv
Home Shop Freeware  Tools for People Who Build Things
http://www.myvirtualnetwork.com/mklotz

Wich one is right?.. Is EITHER RIGHT???
Divide one by the threads per inch and then multiply by .866
Example: 16 threads per inch
1/16 = .0625
.0625 x .866 = .054 single depth of thread
OR.........
As an example, say that you want to cut 16 TPI.
20/16 = 1.25
1.25 X .065 = .081
16 TPI = .081 double depth.

Well I'll add another one in there.
I normaly use this little formula. I'm not sure exactly why its this number works. I think it takes care of the normal root and crest sizes. Anyway, simple as can be. Only 1 number to remember, .625
.625 devided by whatever TPI #
.625/16 = aprox .040" depth per side. These numbers are only meant to get you somewhere close, pretty much just like looking at it. When its getting close to a true V shape, you're almost there and should get out the 3wires/ gages, or the mating part.
I also use this formula for undercuts and I add a bit more for clearance. I never had any complaints.

So far NOBODY agrees... this is NOT funny....
but somehow I sorta expect it...
Do any of you make threads to the BOOK ?
Get a "Machinery's Handbook"
Every one of you would be royally Pi$$ed if the spark plugs you bought were made using your methods...

So what does the Machinery Handbook say?
Jake


Wich one is right?.. Is EITHER RIGHT???
quote:

Divide one by the threads per inch and then multiply by .866
Example: 16 threads per inch
1/16 = .0625
.0625 x .866 = .054 single depth of thread

OR.........
quote:

As an example, say that you want to cut 16 TPI.
20/16 = 1.25
1.25 X .065 = .081
16 TPI = .081 double depth.

So far NOBODY agrees... this is NOT funny....
but somehow I sorta expect it...
Do any of you make threads to the BOOK ?
Get a "Machinery's Handbook"
Every one of you would be royally Pi$$ed if the spark plugs you bought were made using your methods...
Why don't you get your BOOK out and see which one is right?

Gary E,
By the time you have the thread data found in the handbook, The thread could already be cut! It just isn't all that technical.
I stand by my "formula". I use that because I happen to be able to remember it.
Another way to do it:
Pitch X 1.3 = double depth (Handbook worshippers will probably want to use 1.299 in place of 1.3)
These numbers are not for a pointed tool, but rather for a tool that has a proper flat at the tip. I think that this is where the descrepency comes from. The single depth of a thread with a sharp root is indeed .866 X pitch. These threads are weaker though.
Incidently, all this was derived from the "Greenfield Screw Thread Manual" which I believe is still in print.
Mike

Well, I must say I have calculated each of your formulas including the DOT program and none of you are the same.
Since I don't have a handbook, what does it say?
Jake

It says it doesn't matter how you want to calculate it as long as the end result is good.
Going with only a calculated depth without checking with 3wires or a ring gage is NOT acceptable in normal production where many parts have to interchange.

Here you go...
http://homepages.tesco.net/~A10bsa/uncpro.htm
Regards, Marv
Home Shop Freeware  Tools for People Who Build Things
http://www.myvirtualnetwork.com/mklotz

Here you go...
http://homepages.tesco.net/~A10bsa/uncpro.htm
Addendum: Note that, from the above page,
hn = 0.54127/16 = 0.033829
which agrees exactly with what my program calculates for
dotflat crest to flat root.
Regards, Marv
Home Shop Freeware  Tools for People Who Build Things
http://www.myvirtualnetwork.com/mklotz

SND,
Good point. A formula is only good for getting you in the ballpark. Always stop feeding in a few thou shy of the calculated position and start gaging for the last bit.

.130 / number of threads per inch
example .130 / 12 = .01083333333
For 75% thread I believe

JS:
That is sounding good. My fishscale says .93 for 14tpi.
so 13/14 = .92857
.92857/2 = cross slide depth
Correct?
Jake

Jake 
The Theoretical Single Depth of Thread for either the Unified or ISO Metric threadform  which are both 60degree threads with a 1/8 Pitch flat at the Major Diameter and a 1/4 Pitch flat at the Minor Diameter  can be calculated easily from the Thread Pitch:
Theoretical Single Depth = (5/8) x Pitch x Cos (30 degrees)
Working a numerical example for a Unified threadform of 1/16 inch Pitch:
Theoretical Single Depth = (5/8) x 1/16 inch x Cos (30 degrees)
= 0.625 x 0.0625 inch x 0.866
= 0.0338 inch
Working a numerical example for an ISO Metric threadform of 1.25 millimeter Pitch:
Theoretical Single Depth = (5/8) x 1.25 millimeter x Cos (30 degrees)
= 0.625 x 1.25 millimeter x 0.866
= 0.677 millimeter
The Unified and ISO Metric threadforms are modified (truncated) V screwthreads with a 60degree included angle between the flanks. In a 60degree SharpV screwthread, the measuredalongtheflank length would be exactly equal to the Pitch. The flats at the Major and Minor Diameters on the Unified and ISO Metric threadforms result in a reduction of the measuredalongflank length to 5/8 that of a samepitch SharpV screwthread. [This means that the Theoretical AlongFlank Infeed of a compound / top slide set at 30 degrees to feed along the flank can be calculated as (5/8) x Pitch, which for a Unified threadform is exactly equal to (5/8) / Number of Threads per Inch.]
The Single Depth of the screwthread is the measuredalongflank length of the screwthread times the Cosine of onehalf of the V angle.
Ok, the arithmetic says you can combine the 5/8 and Cos (30 degree) factors to create a "magic number", but doing so is a BAD idea. It would be far better to learn the fundamental geometry of the Unified / ISO Metric threadform well enough that you can rederive the equations whenever you need them.
That geometry really is pretty straightforward, and once your understand it, the terms 5/8 and Cosine (30 degree) will come to mind far more readily and reliably than will magic numbers.
John

I'm the only right one here. I hardly ever calculate thread data. I cut threads until they look like a thread then gage them with the nut or male thread, mike them over wires, or thread mike them if they're in range of the thread mikes I have.
I also cut threads with the standard root flat P/4 to P/6) on the tool tip. Never cut threads with a pointy tool.
All the thread data and the NIST formulae it takes to derive it from scratch are in Machinery's Handbook.
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