cutting threads

[markbackshall]

Hi,
I am new so I hope I am in the right place and get it right.
I am cutting external thread 7/16 x 32 TPI in Myford .
The thread depth worksout to be 0.020" for this thread.
I get a good cut and it looks fine using a 55 deg tool.
When I measure the OD of the thread from peak of thread to peak, it is less than the started 7/16, by about 7 thou.
No matter how many ways I try to cut the thread it always work sout like this.
I have tried cuting straight in, angling the top slide over by half of 55 deg, straight in and advancing the top slide a few thou every couple of cuts.
Even if I cut a thou each cut I still get the same undersize over the finished thread.
Is a 7/16 thread as finished really 7/16 over ther crests ? or is the only the stating size.
I have tried cutting a relief of 20 thou at the end of the thread prior to starting to make sure that the depth of cut is right , by watching that the last cut just scratches this.
It seems like the tool is shaving a bit off the thread peaks.
The tool is 1/4 inch and is commercial cut .
I use a Moore & Wright gauge to make sure its suare to the job
THANKS
Mark

[SteveW]

Mark,

It occurs to ask if the tool you're using has a sharp point or is rounded. If your tool is too rounded the shoulders will be be wider and therefore cause the crests to join up below nominal diameter. Maybe starting with a sharper point will get you to the core diameter before loosing the crest.

There is a nice little picture of a thread with all the numbers on page 4.1 of "The MODEL ENGINEER'S HANDBOOK". The book also confirms your 20thou thread depth.

If you have a male thread to follow try measuring it with a micrometer across two small (small enough the sit inside the thread valley but big enough to be proud of the major diameter) drill shanks.

Try: www.boltscience.com/pages/screw4.htm

p is thread pitch

Thread 'sharp' point to 'sharp' valley is 0.96p

Thread 'rounded' point to 'rounded' valley is 0.64p

The crest/valley radius is shown as r = 0.137p (max tool point)

Effective diameter = D = 0.64p
Minor core diameter = D - 1.28p

Hope the above numbers helps