Perpendicularity to a pitch diameter 1

[Tizbass]

I need some info on how to measure this properly. The feature is a spot-face with a M14 x 1.5 -6H Rolled thread (oil drain plug) The feature control frame Perp. 0.15 to Datum D is on the spot face. It is also a critical characteristic requiring SPC. D is the pitch diameter of the thread. Do I need an optical comparator to measure this or is there a way to do it on our CMM.

Thanks, Randy

 

[ewh]

Just a comment...
It seems backward to me; I would have located the pitch diameter relative to the spotface, but what do I know?
Sorry this doesn't really help with your problem.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[CoryPad]

The perpendicularity requirement is for fluid sealing on the spot face surface. The internal and external threads, the spot face, and the bolt/screw/plug contact surface are the relevant features, and it is common to use the screw axis as the datum. The pitch, major and minor diameters all are used to some degree in industry.

I am not familiar with all CMM methods, but I imagine there is a way for the CMM probe to calculate a pitch diameter with the equivalence of an "over wires" measurement. You could use a thread gage inserted into the hole and measure against it. I would contact thread gaging specialists for this:

http://www.johnsongage.com/

http://www.greensladeandcompany.com/

 

[flash3780]

Interesting question as this is a common callout. I always assumed that a functional gage that was threaded into the hole would be used to check perpendicularity/position of the pitch diameter. Of course, doing so would project your tolerance over the length of the gage.

 

[ewh]

My reasoning was that the spotface usually exists before the thread, so would locating the thread relative to the spotface be acceptable (since relevant features are still being referenced), or would it cause functional problems?

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[dgallup]

It should be relatively easy to have an indicator holder that threads into the screw thread and lets the indicator sweep around the spot face to measure the perpendicularity. Trying to do it the other way around would be more difficult.

 

[fsincox]

I am sorry; I believe I must question this. All of my time in engineering I have seen surfaces called out to threads. SAE standard J514(Hydraulic Tube Fittings) Sept 2004, for example, clearly shows fitting features called to the thread pitch. I have seen it in Parker catalogs, ISO standards, bearing locknut threads, etc. Almost every time a question like this is asked of GD&T people they hem and haw, say well maybe these people don't understand proper applications or something like that. I have seen it too long to believe it any more, plus it think it passes "do it seem reasonable to you", test.
What I consider to be a large disconnect in GD&T is caused, in part, I believe, by this kind of wink wink well maybe “they just don't know what they are doing” if it is not politically popular with whomever you are speaking with. It seems reasonable, to me, that these applications want what is specified, whether it is popular or easy to check.
My trainer used to say if you measure a football field does it matter which end you start with? If you want to turn it around go ahead but don’t say he doesn’t want what he wants
Do you know how hard it is to find these final numbers, or how lucky we are to finally have this kind of information?
Do we really care which comes first, in the end, is that what you believe down deep?
I know lots of people don’t know what they are doing, to so quickly undercut the ones that seem reasonable undercuts the progress we have made.
Frank