Tapped hole position
Tapped hole position
(OP)
Hi Everyone,
I've been challenged to inspect a 3/8-16 tapped hole with a cylindrical position tolerance of 0.003" on the pitch diameter. This is a high volume process using a CMM. Is there an attachment or gadget that can help me achieve a degree of certainty of my inspection findings. I need to achieve a Cpk 1.67 wich so far has been difficult.
Thanks all
I've been challenged to inspect a 3/8-16 tapped hole with a cylindrical position tolerance of 0.003" on the pitch diameter. This is a high volume process using a CMM. Is there an attachment or gadget that can help me achieve a degree of certainty of my inspection findings. I need to achieve a Cpk 1.67 wich so far has been difficult.
Thanks all





RE: Tapped hole position
If so I would suggest you CNC or jig bore a plate with all the holes oversized and have shouldered pins in them with a taper and then a smaller parallel diameter on top, put a second plate underneath and spring the pins up. This way the taper sits on the true centre of each hole and you can simply pick up on the top diameter with the CMM once the part is clamped in place.
Is this what you are asking?
RE: Tapped hole position
"I think there is a world market for maybe five computers."
Thomas Watson, chairman of IBM, 1943.
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?
RE: Tapped hole position
h
RE: Tapped hole position
RE: Tapped hole position
A thread is a composite feature. The minor diameter of the thread, the pitch diameter, the major diameter, and you are dealing with the angularity from the axis for all features of the threaded hole. Assumptions must be made first that the pitch diameter is the critical feature you want to measure. Maximum material condition is allowed, there isn't an expandable thread gage available. The suface around the threaded hole is perpendicular to the axis of the thread.
Make a inspection plug at low limit for the pitch diameter with a contact shoulder to tighten the gage to the surface centering the gage into the threaded hole. The gage will then have a gaging diameter protruding above the threaded hole. If there is inaccuracy of the thread being perpendicular to the axis or the contact surface is not perpendicular the measurements will not be accurate.
A functional gage would be a better choice but a go-nogo gage does not produce Cpk data.
I want to question the .003" tolerance. Is it a diameter locational tolerance, a linear tolerance or a radial tolerance. I also question the need for a .003 locational tolerance on a threaded hole. I do not think tapping a hole will produce that accuracy of a locational tolerance. Thread milling will probably have to be used to produce that tight of tolerance.
My thoughts
Bill
RE: Tapped hole position
As Bill says .003 is a very tight tolerance for a threaded hole, hard to see what would justify it.
RE: Tapped hole position
As for shoulder screws, they will have to be selected carefully. The tolerance between the pitch diameter and the body diameter is not tightly controlled.
I also agree with Bill about the tolerance. Is .003 someones bad solution to a tolerance stackup problem, or is this part so expensive only ground thread fasteners will be used?
RE: Tapped hole position
Thanks all
RE: Tapped hole position