# Thread Depth



## rake60 (Oct 4, 2007)

When it come to cutting a perfectly fitting thread there is no perfect 
answer to thread depth.  There are just too many variables involved.
ie: tool tip radius, tool spring, cutting finish, ect.
However there are some good guide lines.  If you own a thread gauge the
standard "Double Depth" of threads are stamped on the back side of the
gauge.





Divide the number given on the chart by 2 and that is the approximate 
depth per side of the thread.
Today I was setting up a job on my machine that called for a 33 X 3.5MM
thread.  My cheat sheets don't cover metric threads. Our head programmer
told me to use the _*.649519*_ constant to check the CNC's opinion of the 
depth.  I've only been a machinist for 20 years so naturally this was new 
to me.    

You determine the actual pitch of the thread.  We'll use a 12TPI for an example.
*1 / 12 = .8333333333"
.8333333333 X .649519 = .05379"*
Convert that to a double depth *(.05379 X 2 = .10825")*
Huh, same answer as the number *.108"* given on the thread gauge...
I will admit I'm getting old, but never too old to learn.

Again it is not a perfect solution, but it gives you an idea of where your
headed to.  I know there will be people offering the perfect answer
solutions. Those solutions will work 100% of the time on paper.  In real life
when the mating part screws on it's right.  Until then you take
another cut.  If you can throw it on from across the room, call it a practice
piece and try it again.  :wink: 

Rick


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## mklotz (Oct 5, 2007)

Your magic number (0.649519) comes from geometry.  If you look at a thread diagram, you'll see that the depth of thread (sharp crest to flat root), is given geometrically by:

D = [[3/8]/tan(30)] / n = K / n

where:

n = thread pitch in tpi

and the thread angle is assumed to be 60 degrees (as it is for American and metric threads).

Doing the math , K evaluates to 0.649519

K appears in another important thread-related calculation.  The equation for the tap drill diameter as a function of percent depth of thread is:

Dt = Dm - 2*K*dot/n

where:

Dt = tap drill diameter
Dm = major diameter of thread
dot = percentage depth of thread expressed as a decimal fraction (e.g., 75% is expressed as 0.75).

Thus, for the common 1/4-20 with a 75% depth of thread, we would have:

Dt = 0.25 - 1.299 * 0.75 / 20 = 0.25 - 0.0487 = 0.2013

or a #7 drill.


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