Perpendicularity to a thread?

[legrand01]

We have a few parts here that have what I feel is a questionable callout of perpendicularity of a surface with respect to a thread. The thread is normally called out as the datum and a counterbore below the thread has the perpendicularity callout. They are about to change the part in question to have a perpendicularity callout of .0005". All diameters are around 0.30"

My problem is that I don't know how this can be measured correctly. The machinists on the floor will usually take a part that has not yet been threaded and use the minor diameter as the datum. I don't feel this is correct, but at the same time I haven't a clue how you would correctly measure this feature. I would think it would be related to the pitch diameter, but how would that be measured (i.e. set up as a datum)? I don't like the idea of using plugs, because that just adds more variation.

Any ideas? Is this callout, in any form, even reasonable?

More info on how this is used: The bottom of the mating part that screws into this part squeezes a few other components together and everything is subjected to high pressure (100k psi) to check for leaks. The design engineers belief is that if the counterbore is not perpendicular to the threads then the assembly will leak. Current parts are leaking and they are reworking them with a plug screwed into the thread and a special reamer through this plug used to clean up that counterbore. Again, with no supporting measurements, only empirical results, backing up this philosophy.

 

[ewh]

Wouldn't controlling the thread relative to the counterbore be more intuitive?

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

 

[legrand01]

ewh, while I agree with this statement, I still don't know how you would control the perpendicularity of the thread to the counterbore. Yes, make the counterbore a Datum, but what to gauge on the threads?

 

[KENAT]

When you say perpendicular you mean the bottom face of the c'bore is perpendicular to the thread right?

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[unclesyd]

How deep is the counter bore?

What are the geometric properties of the assembly?

Is this a one off product?

the reason I ask is we measure some relatively small counterbores with optics, with larger ones being verified using an Air gauge.
Below are links to Air Gauges, Lasers and Vision Systems, and Tool<akers Microscopes.

 

[legrand01]

KNEAT: Yes, bottom of face is perp to thread.

unclesyd: The issue is setting up some way of measuring to the thread, what should be "probed" when doing so?

 

[unclesyd]

First let me correct my last post, another senior moment.

http://www.westerngage.com/index.htm

http://www.keyence.com/usa.php?ad=gaworld0706

http://www.toolmakermicroscope.com/

Are you using the head of the fastener to seal?

Could you come back with a sketch or drawing showing the actual component? A cross section would be neat if you add the thread call out.

The landing area of the fastener has got to be better than 1 1/2 degrees off to have appreciable effect on the fastener integrity. I think the perpendicularity can be ascertained using the following method.
I would get a thread gauge made like the ones on the link below. This approach is used quite often to check taped threads to a surface. Depending on numerous parameters a taped hole Can have slight bell moth so if possible your plug should thread in at least 1 1/2 dia.
This will give you the two planes of reference to use a measuring device as referenced previously. To get another reference plane there are rotary stages that very high precision in all planes if the part is amenable.

http://www.thread-check-tool.com/page3.html

 

[legrand01]

unclesyd: I am familiar with those thread plugs you linked too. We actually have some of them but they are not meant for accurate inspection, but rather for gaging the particular size of a thread and they don't come in the thread designation we need.

I have thought of creating our own for the sizes we need, but:
1) they would still need to be calibrated
2) wouldn't they need to be practically interference fitted in order to work properly for this application?
It's possible I could have them made slightly tapered so they would lock into the holes, but it doesn't give me a warm and fuzzy feeling that they would be accurate.

I've attached a drawing for reference.

 

[unclesyd]

From an unstructured mind.
I haven't deserted you and your problem. You throw ed me a curve ball so I have been doing some calling and checking out a few things. If you area of interest was a little larger there would be an easy way to take out the precision by using a self energized metal seal to get effective sealing.

Looking for ways to pickup the pitch line of the threads in more than one place.

One question, were the bores concentric prior to threading?

What precision?

Do use use a gasket in the area of interest?

If bores are concentric this would enable the use of the bore below the area of interest as a reference to check the perpendicularity of the land to the bore. If the bores are concentric you could also make thread plug with a 1/4" extension that would allow several measurements that would ascertain if the thread pitch line is close to being parallel to the bore. By doing this one might be able to use a reverse indicating gauge to check the land against the lower end of the bore.
If the concentricity of the bores are very good one could use a tight fitting plug on the lower bore and use it a reference plane for a laser measurement of he landing area.
I'm also looking for a precision gauge that works like a Last Word Indicator to check the landing area from the top side.
I'm also looking at using a lead wire to measure how flat, thus the perpendicularity of the landing area using a screw plug with and extension. The area is too small to use pressure indicating film.
Looking at the Co-AX indicator using a shoe to span a couple of threads to pick of the minor diameter of the threads. This would give some indication of where the pitch line is in relation to the bore.

The first two links are or the Co-Ax indicator.

http://www.blakemanufacturing.com/pages/aboutus.html

http://www.cdcotools.com/index.php

Something to file in the windmills is using a high pressure seal based on the Harwood design or better yet the Bridgman Seal.

http://harwoodeng.com/

http://www.rsc.org/chemistryworld/Issues/2010/November/BridgmansSeal.asp

Just remember the impossible just takes a little longer.

 

[unclesyd]

If you haven't resolved your problem take a look at the indicator with a sweep function on the link below. Not any details so you will have to contact them.

http://www.zerogage.com/v4/go.gnf?s...late=h2_i2_w1_f1_001&itemId=5376&product=2240