Measurements of Major Diameters of plug thread gages

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cklapperich

#1
The major diameter of most of my master plug thread gages seem to about a hundredth of an inch shorter on the ends, though a couple seem consistent all the way through. Why is this? Is this normal? A hundredth of an inch is a lot. :confused: Thanks
 
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cklapperich

#3
I discovered it trying to calibrate these thread gages: our calibration procedures say to measure the major, minor, and pitch diameters. And most of my master thread gages are like this - the last half or the last third or something is suddenly around a 1/100" narrower. I'm using a calibrated micrometer, I zeroed it, I took each measurement 3 times, I know now to use a micrometer. But I must be doing something wrong wrong... right? or its supposed to be like this or something? There's no way there's such a big error on all of them.
 

normzone

Trusted Information Resource
#4
" This seems to me to be a manufacturing defect."

OR

"I must be doing something wrong wrong... right? or its supposed to be like this or something?

I'm going to give the gage manufacturer the benefit of the doubt for knowing their business better than I do. There is likely a valid reason for the gage to be manufactured in that manner, or some error in your technique/calculations.

Or you have detected and publicized a massive coverup in the gage world, and you are now in grave danger :)

Wait a minute - Major diameter? It's to provide a lead in zone for the beginning of thread engagement.

I would contact the gage manufacturer, tell them what I believe I am seeing and ask them to educate me if nobody here provides the answer. Plenty of metrology expertise here though, so I'll be patient.

If you mysteriously disappear at least we know where to begin the investigation;-)
 
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smkeilman

#5
If I assume that these are for setting thread ring gages, the setting plugs can be purchased with a truncated end. The first half of these gages will have a major diameter about 0.010" less than the full form section.

You can find the manufacturing values in the ASME standard for thread gages. (not not the one for manufacturing them)
 
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cklapperich

#6
smkeilman, thank you, and yes you are correct. Which measurement do I take then? The larger measurement only right? Also, would you know the exact name of that ASME document so I can go have a look at it?
 

Jim Wynne

Staff member
Admin
#7
If I assume that these are for setting thread ring gages, the setting plugs can be purchased with a truncated end. The first half of these gages will have a major diameter about 0.010" less than the full form section.

You can find the manufacturing values in the ASME standard for thread gages. (not not the one for manufacturing them)
Just for fun I took three of our plugs and measured them, and I see no significant difference along the length of the threads. Are these old gages? It would take a lot for .01" to wear away, but if it happens it's most likely on the starting thread because it's the one that has th clear the way for the others. If the female threads being checked are consistently tight and the material is hard, that sort of thing can happen. Again, though, .01 is a lot.
 

normzone

Trusted Information Resource
#8
Yeah, even if they were being used as deburring tools (I've seen it done ! ) :( they'd have to be worked all night and day to take them down .010
 
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smkeilman

#9
The ASME std. is B1.2 (GAGES & GAGING FOR UNIFIED INCH SCREW THREADS) and B 1.16M (GAGES & GAGING FOR METRIC M SCREW THREADS

The proper name for what you have is called a truncated setting plug gage. As long as both halves of the members meet the values listed in B1.2 or B1.16M, your member is fine. The pitch diameter should be the same all the way down the member.

The major diameters are supposed to be this way. The reason, I surmise, is to set an adjustable thread ring gage to pitch diameter only using the truncated portion. The rear (called full form) section would then be used to ensure that there is no interference on the ring major diameter. There should be no noticeable difference in drag from truncated to full form on a properly set thread ring.

Also, most of these gages are made to W tolerance, which is roughly a third as wide as the X tolerance for hole thread gages. A micrometer is not really the best tool for checking these.
 
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