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# How to Measure the Minor Diameter of Internal Threads

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Post Number #1
24th January 2006, 03:18 PM
 leetimothyj Total Posts: 15
How to Measure the Minor Diameter of Internal Threads

We are purchasing slotted hex nuts (9/16"-18, class 2B) for a load supporting application for overhead material handling equipment. We have been measuring the nuts with both a thread pitch (go / no-go) gage and a cylindrical plug gage to measure the maximum minor diameter. According to ANSI/ASME B1.1, the minor diameter for this internal thread can be a maximum of 0.515". The nuts conform to the thread pitch gage, but we are rejecting them based on an oversize condition on the minor diameter, e.g. 0.520" - 0.525". Our supplier says that this is not a valid measurement, that you only use the pitch diameter. I am not that well versed in fasteners, but my thought is that the less material (i.e., the larger the minor diameter), the less support and greater potential for the nut to strip out. If this dimension is not 'valid' then what determines the minimum amount of thread surface? Thanks in advance for your input and any direction to an 'expert' source (not that you aren't experts!) I did once hear the definition of an expert as "ex" = has been + "spurt" = a drip under pressure. I am an ex-spurt in many fields!

Post Number #2
24th January 2006, 03:39 PM
 Jim Wynne Total Posts: 14,160
Quote:
 In Reply to Parent Post by leetimothyj We are purchasing slotted hex nuts (9/16"-18, class 2B) for a load supporting application for overhead material handling equipment. We have been measuring the nuts with both a thread pitch (go / no-go) gage and a cylindrical plug gage to measure the maximum minor diameter. According to ANSI/ASME B1.1, the minor diameter for this internal thread can be a maximum of 0.515". The nuts conform to the thread pitch gage, but we are rejecting them based on an oversize condition on the minor diameter, e.g. 0.520" - 0.525". Our supplier says that this is not a valid measurement, that you only use the pitch diameter. I am not that well versed in fasteners, but my thought is that the less material (i.e., the larger the minor diameter), the less support and greater potential for the nut to strip out. If this dimension is not 'valid' then what determines the minimum amount of thread surface? Thanks in advance for your input and any direction to an 'expert' source (not that you aren't experts!) I did once hear the definition of an expert as "ex" = has been + "spurt" = a drip under pressure. I am an ex-spurt in many fields!
Your purchase order should specify the standard you expect the nuts to meet. If the supplier accepts the order and the nuts don't meet the standard, you're within your rights to reject them, regardless of the particular reason. You shouldn't specify "Class 2B" without also specifying "UNC" or "UNF" (e.g.), and the national standard. Whether or not the supplier has a case depends on what you ordered from them.
Post Number #3
24th January 2006, 03:59 PM
 leetimothyj Total Posts: 15
Clarification; The nut was specified as "9/16"-18, UNF, 2B" and stated standard as ANSI B18.2.2. Thanks, Tim
Post Number #4
24th January 2006, 04:17 PM
 Jim Wynne Total Posts: 14,160
Quote:
 In Reply to Parent Post by leetimothyj Clarification; The nut was specified as "9/16"-18, UNF, 2B" and stated standard as ANSI B18.2.2. Thanks, Tim
You're welcome. If you specified the standard, you're on steady ground.
Post Number #5
23rd February 2006, 10:40 PM
 Wayne Total Posts: 268

Quote:
 In Reply to Parent Post by leetimothyj We are purchasing slotted hex nuts (9/16"-18 UNF-2B).... We have been measuring the nuts with both a thread pitch (go/nogo) gage and a cylindrical plug gage to measure the maximum minor diameter.... Our supplier says that this is not a valid measurement, that you only use the pitch diameter....
I sell thread plug gages for a living. Your supplier is wrong. The thread plug gage does not fully check the thread. The GO gage checks the thread maximum material condition of the pitch diameter and the major diameter but does no measurement of the minor diameter. The NOGO gage only checks to see if the pitch diameter has not been machined too large. It does not measure either the major or minor diameters. To fully gauge an internal thread, a Minor Diameter GO/NOGO Plain Pin Gage must be used.

BTW: For an external thread, the same is true of the major diameter. It must be measured by a tool other than the thread ring gage.
 Thank You to Wayne for your informative Post and/or Attachment!
Post Number #6
21st August 2008, 03:44 AM
 Gordon Clarke Total Posts: 282

This isn't exactly a "quick" reply as I've only recently found this forum. Still, better late than never
I agree with Wayne 100% in that gauges don't "measure" - they inspect for right or wrong. By measuring you always know where you are with relation to the tolerance.

I have invented and patented (and sell) a method for (among other things) measuring thread pitch diameter and can in fact also measure internal threads from M6 - 1/4".
I have on two occasions supplied thread measurement inserts for measuring diameter D on an internal thread. It is true that the tolerance for this is the only diameter (major diameter on an internal thread) that doesn't have a specified tolerance and is only given as "min." This applies to both Unified Inch Screw Threads (UN) and Metric Screw Threads. Of course this doesn't mean that it can't or musn't be measured.

My personal opinion on making the "perfect" thread is to use a combination of solid thread plug and7or ring gauges and pitch diameter measurement. A good thread is in the middle of the pitch diameter tolerance. Measurement also reveals how much of the tolerance is being used and whether or not the process is being controlled. A gauge doesn't reveal this. Inspection by solid gauges usually requires a much larger sample size than measurement. Another advantage is that when setting a machine up, if the thread cutting tool is set for the middle of the tolerance process control can be reduced and focus moved to viual inpection of the cutting tool. Keeping away from the tolerance limits also means less wear on the solid gauge.

Another Rule of Thumb for UN and Metric threads is that, if the pitch diameter tolerance for an external screw thread = 1, then the pitch diameter tolerance for an internal thread is 1.3
The pitch diameter tolerance for threads is invariably the smallest tolerance on the three diameters. If the cutting tool isn't damaged or worn then measuring the pitch diameter is usually enough to ensure that the overall profile is OK and the thread within tolerance.

if tolerance 2A is known then all other tolerances (1A, 3A, !B, 2B and 3B) follow this one.

If anyone has questions then I'd be happy to answer - assuming I can

Gordon

Last edited by Stijloor; 21st August 2008 at 07:55 AM. Reason: reference to website removed.
Post Number #7
21st October 2008, 04:00 PM
 MACIEK Total Posts: 8
Re: Measuring minor diameter of internal threads - How To?

Hello.
Does any one have an idea where i can obtain threaded ring gauge minor diameter tolerances?
Post Number #8
21st October 2008, 04:49 PM
 Gordon Clarke Total Posts: 282
Re: Measuring minor diameter of internal threads - How To?

Quote:
 In Reply to Parent Post by MACIEK Hello. Does any one have an idea where i can obtain threaded ring gauge minor diameter tolerances?
It will depend on the type of thread you want this information for. There isn't a universal answer for "thread ring gauges" I notice by your spelling of gauge that you're probably not from the USA If it's a metric thread then ISO 1502 will probably be your best bet.
If you give more information i'm certain you can get the correct answer. Wayne (gageguy) is certain to know the answer if he reads this I can probably help too, but only if I get all the information you have on the thread type and size.

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