Cutting Threads, Depth Of Cut

I couldn't post the excel spreadsheet but I can send it to your e-mail. It should be a simple task to convert.

Bob
 
I saw a discrepancy as well. It appears that the Seko chart is intended for use with their inserts which most likely have a specific non zero radius at the tip. In use, you would touch off on the O.D. of the thread and zero your dial there. With a radius, the tool does not need to cut as deep to correctly form the thread. The mystery to me was it appears that the same Seko tool is used for all threads pitches instead of being proportional to the pitch.

It was for that reason that I suggested not plunging blindly into the threading but measuring your cut thread to determine your progress.

The drawing below shows an external 10 tpi thread drawn to specification with some dimensions. The blue line represents the threading tool, the red line is the external thread. As you can see, a conformal threading tool would not have to cut as deep to make the thread. When I grind a threading tool, I usually just slightly break the point instead of putting a specific radius. The thread is not to spec. and is not as strong as a to-spec. thread but it usually works for my purpose.

All the dimensions are proportional so a 24 tpi thread would have a .0365" depth of cut for a sharp pointed tool and a.0295" depth of cut for a conformal tool.

Since the Seko chart show a less deep cut, it is most likely "steel safe" (takes off less material rather than more). I would recommend using their numbers to start.

Bob
View attachment 109044


Bob,

Thank you. Appreciate the work you put into this discussion, very informative to pre-kinder guys in the machining world like me.

I am using the Warner threading tool and bit, my guess they qualify for sharp pointed tool.


nez
 
This was a great post for the people that are confused about cutting threads ( Me Included ). The charts are easy to look at and get you were you need to be. Again a lot of good info on this site. Thanks to all who share their information with others to put into use.
 
Yesterday, I had to thread some 5/16" stainless rod and did not have a chart of thread depths handy. This morning, I decided to make one based on the definition of unified National thread geometry. The external thread geometry consists of a 60o vee shape with the top of the vee truncated to a width of P/8 where P is the thread pitch and the bottom truncated with a radius which is tangent with thread at a distance of P/4 along the thread face from the bottom of the vee.

If using a threading tool ground to a sharp point and advancing the tool using the compound, the distance along the thread face from the major diameter to the bottom of the vee is P -1/8 * P = 7/8 * P = .875 * P. If the compound is set at an angle of 29.5o, the distance is slightly less or .995025 * 7/8 * P = .87065 * P.

If a tool is ground to conform to the specification (i.e., with a radius), the distance traversed by the compound will be less yet and equal to .79167 for a 30o compound angle and .78773 for a 29.5o compound angle.

The chart gives the compound distance for a 29.5o compound angle setting for cutting the standard UNC, UNF, and UNEF threads.

Bob
Note the chart was removed due to an error. A corrected chart is shown in a following post.

View attachment 108742View attachment 108743
Just curious, what percent engagement are you looking for? Machinery's Handbook gives a handy formula easily remembered, to calculate on-the-fly.

MinDiam = MaxDiam - ((1.29904*Engagement)/pitch)

EG.
1/4 20 TPI @ 75% engagement
0.250 - ((1.29904 * 0.750)/20)
0.250-0.048714=~0.201 or a #7 drill ( 13/64 would be 0.002" larger)


doco
 
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