Tapped hole position 3

[walkineagle]

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

 

[ajack1]

Walkingeagle are you talking about a fixture?

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?

 

[MadMango]

Or alternately, insert a threaded pin into the 3/8-16 threads, and use the CMM to locate the pin to get your tolerances. We do this on occasion with the Faro Arm we have to check parts.

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[ajack1]

If you go with MadMango's idea I would suggest you look at using shoulder screws as you are then picking up on a machined diameter rather than a thread.

http://www.mjvail.com/holokrome/shoulderscrews.html

 

[strokersix]

I'm not an expert by any means but it sounds to me like special gaging is required meaning various sizes of threaded pins to snugly engage the pitch diameter without contacting the thread root or any other surface.

 

[BillPSU]

walkineagle
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

 

[ajack1]

You can buy go-no go thread gauges but as Bill syas they will not give you a Cpk reading. Most holes on a PCD are a positional tolerance, namely they must lay within a diameter around the mean, so you could check the part by having pins of the minimum metal condition on true centres, but again no Cpk reading.

As Bill says .003 is a very tight tolerance for a threaded hole, hard to see what would justify it.

 

[gbent]

There used to be some devices that used a split threaded member that acted like a spring collet, and expanded against the pitch diameter. They had a projection on them to pick up with your probe. I think they were called Tru-Pos. They worked good, but were extremely expensive.

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?

 

[walkineagle]

This .003" tol. I beleive is required for location for an automated welding process, and is not required for the for the final use of the tapped hole. I will be meeting with the design engineer this week to discuss this
Thanks all

 

[BillPSU]

Since this is a tooling point then I would locate on the minor diameter of the thread to avoid having to screw something in the hole. There may be a problem of damaging the thread though.