Actual help needed to measure actual part features screwed up in an actual machine shop.

Hi, What are the good ways to measure the True Position or I guess concentricity of pitch diameter / major diameter of an internal 1/4 -
28 thread to it's minor diameter? The datum is set to what looks like the major diameter of the ID thread. Is going off the major normal or do most use the pitch diameter? I just scrapped a bunch of parts because I offset a mill/turn lathe's thread mill wrongly. The 1/4-28 was on the face of the sub and I sent the X(center line of the live tool spindle pointing right) *down* 0.010. I meant to send it in the Z 0.010 to scootch in the Min thread call out. And in trying to pull this one out of the fire, we came to realize we have no way of checking the concentricity of PD to to the minor or any other features called out on the center line of the thread. I've got .003 max material positional and the minor is up at the top so it is easily shown - not even argued - the parts are to print. But it aint right and we need to get the checking procedure down. I suggested making the .003 regardless of feature size but that will only make the need to measure stuff just right even more critical. A suggestion was to run the PD to fit those screw-in,ground measury plugs or buy a bag full of graduated measury plugs. But then what to do with tapped holes? We've got the resources to measure pert near any way you folks know of CMM, Laser, String with two knots in it.... Any direction, experiences and help would be much appreciated by me; the guy who just went out in the woods and cut off a long thin willow switch and went back in and handed it to my bosses' boss. And it's not that i don't love you guys the most; but this may show up posted in several other metal head chat groups.
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The first thing iw what class thread are they calling for? The allowances vary as determined by the class of thread.
John
snipped-for-privacy@gmail.com wrote:

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snipped-for-privacy@gmail.com says...

I gather that you want to measure the run out of the pitch diameter OR major diameter with the minor diameter. Is that correct? If so, the simplest & most effective thing I can think of is: -Place the parts in a four jaw chuck -Find a pin that is a tight or slip fit into the minor diameter -Dial the part in on that pin -Remove that pin -Find one of "those screw-in,ground measury plugs" that is a nice fit to the thread you have cut -Remove the handle from the screw-in,ground measury plug -Screw in the screw-in,ground measury plug (as long as you are not TOO far off this will not be affected by or touch the minor diameter.) -Use an indicator to indicate the TIR of the smooth end that is sticking out. -The result is the run out of the pitch diameter to the minor diameter. Indicate in more than one place to find straightness, if that is suspect.
-plh
--
Where are we going and why am I in this HAND BASKET??

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says...

major
most
the
off
out.
Indicate
The best way I kno w of is to cut the part into halves--dont forget to shove both pieces straight up the design engineer's bunghole afterwards
--


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On Sat, 17 Jan 2009 04:53:02 -0800 (PST), " snipped-for-privacy@gmail.com"

Without knowing what your part looks like here are some options;
If your part is round and the tap is supposed Centerline I would use a micrometer the check wall thickness an compare Max. Min. or V-Block and Indicator to check concentricity OD to minor diameter (Hole). Then use MTG gage listed below to check concentricity between minor and pitch diameter.
If not then:
Use a pin gage or probe to check the minor diameter (hole) true position Then use one of these to check Pitch Diameter true position http://www.lavezzi.com/PDFs/Tru-PosPromo%202005.pdf and compare readings.
http://maja.uni-mb.si/files/apem/APEM2-3_127-134.pdf don't know if they have a probe small enough for your 1/4-28
Use probe or pin in hole to check true position of minor diameter then use MTG gage to compare runout between minor and pitch or full thread form. http://www.mtggage.com/i_inspect.html Concentricity between minor and PD. is measured by using fingers "C" and "J~ and rotating the workpiece or gage. Minor diameter size, roundness and taper are checked with a pair of "C" fingers.
http://tinyurl.com/9a25gu
For a quick and dirty, use a tight pin gage to check true position of the hole (minor diameter) and then take an impression of the ID thread and use a comparator and check thread Minor and Major runout, it should give you show if there is a problem.
In your position I would call a couple Thread Ring Gage manufacturers and ask how they inspect their threads. I know they use a standard to set them. I wonder how they check runout and concentricity between pitch, major & minor diameters.
Tom
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On Sun, 18 Jan 2009 19:39:12 -0800, snipped-for-privacy@aol.com wrote:

Correction S/B;
take an impression of the ID thread, use a comparator to check minor and major diameter runout. It should give you an indication if there is a problem.
-- Tom http://tinyurl.com/5okkgz
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