G
Gordon Clarke
Wayne, you cover so many relevant points I’m having a problem where to begin 
Probably all we won't agree on is the fact that Americans don't know how to spell "gauge" 
In the final analysis it all boils down to reviewing the total cost of manufacturing quality products and ensuring customer satisfaction.
I’ll start off by stating that I’m not advocating for the use of choosing between measurement or gauge use. Making, and continuing to make, a perfect thread is a combination of both – measurement and gauging.
Much of what I’m about to write is obvious but I’ll write it anyway. It’ll also give folks a chance to either think about and agree or dispute what I write.
External thread (screw/bolt)
The OD (d) on an external thread can easily be measured using a caliper or micrometer as the thread just starts off as a cylinder. A snap Go/NoGo fixed caliper gauge (or ring gauges) could be used but I've never seen or heard of anyone doing this. The closer the diameter (d) is to the upper limit the more the cutting tool must remove. The closer the diameter (d) is to the lower limit the less the cutting tool must remove. Carbide thread tips are more prone to breakage at the tip than they are to being worn out. The closer the diameter is to the upper limit the stronger the thread should be – at least in theory. I’ll quickly add that, given a correct thread, the “strength” lies around the pitch diameter.
Internal thread (nut)
The ID (D1) on an internal thread can easily be measured using a caliper as it starts off as a hole. A simple Go/No Go cylindrical gauge can also be used but personally I’d only use this alternative if it was huge mass production. The closer the diameter (D1) is to the lower limit the more the cutting tool must remove. The closer the diameter (D1) is to the upper limit the less the cutting tool must remove. Carbide thread tips are more prone to breakage at the tip than they are to being worn out. The closer the diameter is to the lower limit the stronger the thread should be – at least in theory. I’ll add again that, given a correct thread, the “strength” lies around the pitch diameter.
As manufacturing to the middle of a tolerance is the most certain method of manufacturing consistently good components, then a gauge is far from the ideal method of setting up a machine or process. However, once the machine is set-up and running it is simple to check with a thread gauge. If there is the slightest indication of a too tight or too loose thread then pitch diameter measurement should be carried out immediately to see if adjustment is necessary. It is also common sense to measure the OD (d) and/or the ID (D1) at regular intervals to follow the process.
External tri-roll gauges are excellent but, as the name indicates, they are for external threads. They can also be bought for internal threads but the minimum internal thread for which they can be used is “large” although “large” is relative. Their greatest drawback (in my opinion) is that they are expensive and thus definitely most suited for mass production. When I was QM at one company I bought an external tri-roll gauge plus a setting gauge, to measure a M18x1 external thread that was to be mass produced, only to have design engineering alter the drawing to M17x1 a few days after I had received it. I couldn’t return either the gauge or the setting gauge as it wasn’t my tool supplier's fault, so I was stuck with a piece of “useless” equipment! I was even "chastized" for not having the measuring equpment available for production!
Both types (external and internal) have unfortunately a limited range of scope – especially tri gauges for internal thread measurement. There are much less expensive external and internal pitch diameter measurement methods. The smallest internal thread pitch diameter measurement method for components (not ring gauges) I know of can measure from 6 mm / ¼ inch and up. Choosing the best method is mainly a question of considering costs.
This is just a “general” piece of information, but with a standard UN (ISO Inch) and ISO metric thread the tolerance on the major diameter is approximately 30% greater than that for the pitch diameter and twice that of the pitch diameter tolerance for the minor diameter. Having written that I’ll add that for both these thread types the pitch diameter tolerance for the standard compatible tolerance between external and internal threads is that the internal thread pitch diameter tolerance is approximately 30% greater than that for the external thread. If anyone wants proof compare tolerances with, for example a M20-6g/6H or a 3/4-10 UNC 2A/2B.
Finally Wayne, the answers you gave to the points raised in both Screw thread major and minor diameters – parts vs. gauges and Go does not Go and NoGo does Go are fairly obvious to anyone having worked with threads and experienced some of the “mysterious problems” first hand that can arise. To the vast majority however, these phenomena makes threads even more mysterious and complicated. I remember the first time I tried to screw a thread plug gauge into the matching thread ring gauge and couldn’t understand why it wouldn’t go in. Now I tell people that a thread ring gauge is an inside-out screw and that a thread plug gauge is an inside-out nut. It usually helps as, after a couple of seconds, they nod and smile.
I’ve printed out the “links” you included in your reply and, if it’s OK with you will probably include them wholly or partly in my Thread Information “service”. If I do, I’ll use a link or reference to your company. If I change any wording I’ll run it by you first.
Probably all we won't agree on is the fact that Americans don't know how to spell "gauge" In the final analysis it all boils down to reviewing the total cost of manufacturing quality products and ensuring customer satisfaction.I’ll start off by stating that I’m not advocating for the use of choosing between measurement or gauge use. Making, and continuing to make, a perfect thread is a combination of both – measurement and gauging.
Much of what I’m about to write is obvious but I’ll write it anyway. It’ll also give folks a chance to either think about and agree or dispute what I write.
External thread (screw/bolt)
The OD (d) on an external thread can easily be measured using a caliper or micrometer as the thread just starts off as a cylinder. A snap Go/NoGo fixed caliper gauge (or ring gauges) could be used but I've never seen or heard of anyone doing this. The closer the diameter (d) is to the upper limit the more the cutting tool must remove. The closer the diameter (d) is to the lower limit the less the cutting tool must remove. Carbide thread tips are more prone to breakage at the tip than they are to being worn out. The closer the diameter is to the upper limit the stronger the thread should be – at least in theory. I’ll quickly add that, given a correct thread, the “strength” lies around the pitch diameter.
Internal thread (nut)
The ID (D1) on an internal thread can easily be measured using a caliper as it starts off as a hole. A simple Go/No Go cylindrical gauge can also be used but personally I’d only use this alternative if it was huge mass production. The closer the diameter (D1) is to the lower limit the more the cutting tool must remove. The closer the diameter (D1) is to the upper limit the less the cutting tool must remove. Carbide thread tips are more prone to breakage at the tip than they are to being worn out. The closer the diameter is to the lower limit the stronger the thread should be – at least in theory. I’ll add again that, given a correct thread, the “strength” lies around the pitch diameter.
As manufacturing to the middle of a tolerance is the most certain method of manufacturing consistently good components, then a gauge is far from the ideal method of setting up a machine or process. However, once the machine is set-up and running it is simple to check with a thread gauge. If there is the slightest indication of a too tight or too loose thread then pitch diameter measurement should be carried out immediately to see if adjustment is necessary. It is also common sense to measure the OD (d) and/or the ID (D1) at regular intervals to follow the process.
External tri-roll gauges are excellent but, as the name indicates, they are for external threads. They can also be bought for internal threads but the minimum internal thread for which they can be used is “large” although “large” is relative. Their greatest drawback (in my opinion) is that they are expensive and thus definitely most suited for mass production. When I was QM at one company I bought an external tri-roll gauge plus a setting gauge, to measure a M18x1 external thread that was to be mass produced, only to have design engineering alter the drawing to M17x1 a few days after I had received it. I couldn’t return either the gauge or the setting gauge as it wasn’t my tool supplier's fault, so I was stuck with a piece of “useless” equipment! I was even "chastized" for not having the measuring equpment available for production!
Both types (external and internal) have unfortunately a limited range of scope – especially tri gauges for internal thread measurement. There are much less expensive external and internal pitch diameter measurement methods. The smallest internal thread pitch diameter measurement method for components (not ring gauges) I know of can measure from 6 mm / ¼ inch and up. Choosing the best method is mainly a question of considering costs.This is just a “general” piece of information, but with a standard UN (ISO Inch) and ISO metric thread the tolerance on the major diameter is approximately 30% greater than that for the pitch diameter and twice that of the pitch diameter tolerance for the minor diameter. Having written that I’ll add that for both these thread types the pitch diameter tolerance for the standard compatible tolerance between external and internal threads is that the internal thread pitch diameter tolerance is approximately 30% greater than that for the external thread. If anyone wants proof compare tolerances with, for example a M20-6g/6H or a 3/4-10 UNC 2A/2B.
Finally Wayne, the answers you gave to the points raised in both Screw thread major and minor diameters – parts vs. gauges and Go does not Go and NoGo does Go are fairly obvious to anyone having worked with threads and experienced some of the “mysterious problems” first hand that can arise. To the vast majority however, these phenomena makes threads even more mysterious and complicated. I remember the first time I tried to screw a thread plug gauge into the matching thread ring gauge and couldn’t understand why it wouldn’t go in. Now I tell people that a thread ring gauge is an inside-out screw and that a thread plug gauge is an inside-out nut. It usually helps as, after a couple of seconds, they nod and smile.
I’ve printed out the “links” you included in your reply and, if it’s OK with you will probably include them wholly or partly in my Thread Information “service”. If I do, I’ll use a link or reference to your company. If I change any wording I’ll run it by you first.
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