Practical Screw Thread Information & Tolerances

[stefanhg]

The tolerance of the pitch diameter is ±0.138 mm.

The standard defines tolerance of the crest truncation 0.0034 in = 0,086 mm.

There is a minimum truncation 0,0029 in = 0,074 mm.

Therefore

Minor diameter Tolerance = PD_Tolerance + 2*Trunc. tolerance = 0,448 mm

Min. Minor diameter = Min.PD_E1 - H + 2*Min.trunc. =
= 1.57795-0.07531+2*0.0029 =
= 1.50844 in = 38.314 mm

Max. Minor diameter = 38.762 mm

Your Minor diameter 38,31mm is equal to the Min. Minor diameter and is correct.

 

[Gordon Clarke]

The tolerance of the pitch diameter is ±0.138 mm.

The standard defines tolerance of the crest truncation 0.0034 in = 0,086 mm.

There is a minimum truncation 0,0029 in = 0,074 mm.

Therefore

Minor diameter Tolerance = PD_Tolerance + 2*Trunc. tolerance = 0,448 mm

Min. Minor diameter = Min.PD_E1 - H + 2*Min.trunc. =
= 1.57795-0.07531+2*0.0029 =
= 1.50844 in = 38.314 mm

Max. Minor diameter = 38.762 mm

Your Minor diameter 38,31mm is equal to the Min. Minor diameter and is correct.

Sorry if I seem to belabouring the point, but were the tolerances you gave actually for gauges?

Any comment to my file? I should add that I've written the dimensions and tolerances to 0.01 mm, as the measuring equipment I "recommend" only has a 0.01 mm (0.0004") readout which, for a component, I believe is enough. Giving it to 0.001 mm would make it look more "accurate" than necessary.
In Denmark we have a saying that means something like "shooting sparrows with cannons" when things are given too much detail.

Regardless, thanks for your input - it didn't hurt me to be "forced" to re-check

 

[stefanhg]

Hi,

These are the limit sizes of the minor diameter of the manufactured thread in plane E1.

Min = 38.32 mm; Max = 38.76 mm; or 38.54 ± 0.22 mm

You should correct these sizes with the amount of removed material.

The minor diameter of the working thread ring gauge at E1 is 38.74694 mm; tolerance is + 0.0254 mm.

The minor diameter of the working thread plug gauge is with recess 0.084 mm.

Dimensions for BSPT (ISO Rc) differ from the dimensions in the standard.

See the file.

 

[Gordon Clarke]

Hi,

These are the limit sizes of the minor diameter of the manufactured thread in plane E1.

Min = 38.32 mm; Max = 38.76 mm; or 38.54 ± 0.22 mm

You should correct these sizes with the amount of removed material.

The minor diameter of the working thread ring gauge at E1 is 38.74694 mm; tolerance is + 0.0254 mm.

The minor diameter of the working thread plug gauge is with recess 0.084 mm.

Dimensions for BSPT (ISO Rc) differ from the dimensions in the standard.

See the file.

I've got a feeling we're not looking at this in the same way. I'm not interested in using thread gauges (plug or ring) and gauge tolerances don't interest me either. I leave that to the gauge manufacturers

The problem started with the fact that the company had to manufacture a number of test samples (1 1/4" NPT) in a hurry and didn't have a thread plug gauge. They knew I had something that could measure internal and external tapered threads so they called me. I'm only 10 minutes drive from the company so I could help them immediately. Gauge delivery would several weeks.

The rest of the story is in my earlier submissions.

Your answers, although professionally correct, worries me a bit. The accuracy you give the tolerances to (0.001 mm), taking into consideration the size of the tolerances, frightens people off getting interested in thread measurement as they read thousands of a mil. It's probably the main reason why gauges are so popular as then people don't need to think. Once the machine is set up and running then a gauge is OK - but to set up correctly measurement should be used. "Quality is free" - if you start in the middle of the tolerance.

Surely we can agree that measurement is always better than gauging? Making the "perfect" thread requires both the GO gauge (correct pitch and profile) and measurement (knowing where within the tolerance). Measurement results also drastically reduce the number required in a sample to determine the tolerance spread and machine capability.

Thread measurement is probably one of the areas least advanced over the years mainly because people regard thread tolerances as being much smaller than they actually are. I've often seen people confuse the ISO metric thread tolerances such as 6g and 7H, with axle and hole tolerances, even although the thread tolerances are roughly 10 times as much as the others!

A standard M16 screw has a pitch diameter tolerance of 0.16 mm (0.0063") and a standard M16 nut 0.21 mm (0.0083") - why would anyone want to measure these tolerances in 0.001 mm? The same applies to 5/8" UNC.

I've made a claim a few times and I'll repeat it - I haven't as yet seen a thread that I can't measure the pitch diameter of - even buttress!

 

[stefanhg]

Hi Gordon,

Some points about measurement system analysis for suitability and analysis of the measurement uncertainty, for M16, PD-tolerance = 0,16mm acc. to VDA 5, MSA 3, FORD, BOSCH,

1. Resolution <= 5% of work piece tolerance = 0,008 mm, recommended resolution 0,005 mm
2. Suitability of the measuring instrument - uncertainty sources: reference standard, bias, linearity, repeatability,…
Umi <=0.2*0.33T = 0,011
3. Suitability of the measurement system - uncertainty sources: Umi, temperature, operator, method,…
Ums <=0.2*T = 0.08 mm

 

[Gordon Clarke]

Hi Gordon,

Did you make analysis of you measurement system for Suitability, or analysis of the measurement uncertainty?

I think NO.

Some points for M16, PD-tolerance = 0,16mm acc. to VDA 5, MSA 3, FORD, BOSCH,

1. Resolution <= 5% of work piece tolerance = 0,008 mm, recommended resolution 0,005 mm, your is 0.01 mm
2. Suitability of the measuring instrument - uncertainty sources: reference standard, bias, linearity, repeatability,…
Umi <=0.2*0.33T = 0,011, expected min. Umi for your instrument is 0,04mm
3. Suitability of the measurement system - uncertainty sources: Umi, temperature, operator, method,…
Ums <=0.2*T = 0.08 mm, expected min. Ums for your system is 0.12 mm

You must make the analysis.

Hi Stefan,
I've just logged in to apologize if you felt what I wrote was criticism. It wasn't intended as such - it was written to emphasize that we were looking at the "problem" from different angles. The information Wes wrote was the type of thing I was looking for i.e. practical information for a machinist. Your information is for the calibration guy sitting in his measuring environment

Re your remarks on uncertainty, reliability etc. then one of the secrets of obtaining certainty and reliability is to measure a component with a known pitch diameter before starting. Personally I have a calibration piece with a known external and internal pitch diameter with me when I measure thread pitch diameter. It's only really necessary when measuring internal pitch diameter as, like virtually all internal measuring instrument, they should be checked (or calibrated) before use.

The second, and main, secret is that I always use a pressure device to ensure uniform measurement pressure as that is the main reason for digital caliper innacuracy - too much or too little pressure! I'd feel more comfortable if you looked at the website again - and especially the Test Report from an authorized lab. I've measure thousands of times and never had a deviation greater than 0.02 mm.

Look at (broken link removed)

Remember the product is intended for setting up a machine and checking at intervals to see movement and deviation. The GO thread gauge need only be used to verify full thread profile - due to possible cutting tool wear or breakage.

Again, the measuring inserts are made for the machinist - not the gauge calibration guy.

I also have a digital caliper with a 0.005 mm display (and twice as accurate as a standard digital caliper) plus I also have a digital caliper with a 0.001 mm display and guaranteed as accurate as a micrometer.

Finally, I've personally measured the same calibrated thread gauges with 3-wires, micrometer thread inserts and my thread inserts - I got the same value within 0.01 mm on each! It would be suicide or madness for me to claim this accuracy if I wasn't certain.

If our paths ever cross I'd love to demonstrate

I can't resist this - measuring thread pitch diameter with 3-wires is regarded as the most accurate. What if the flank angle isn't exactly as specified???? Even thread gauge flank angles have a tolerance!

Now I'd better be careful or I'll invoke the wrath of Gageguy (Wayne)

 

[stefanhg]

Hi Gorden,

The information Wes wrote was the type of thing I was looking for i.e. practical information for a machinist. Your information is for the calibration guy sitting in his measuring environment

The references VDA 5, MSA 3 and criteria are for the measurements in the manufacturing, not for the lab.

 

[Wayne]

...I opened ANSI/ASME B1.1 and expected to be able to just read off the [internal thread] diameter. No such luck! I had to calculate backwards from the actual pitch diameter at the correct plane depth. ... Is it me that is blind ... I couldn't find the information I needed. Of course it can be calculated but why should that be necessary?

I raised your question to one of the members of the B1 Committee and his response is:

The pipe standards do not list minor diameters or major diameters for taper pipe. There is a very good reason for this ( the same reason you have to use 6 step gages to inspect pipe). The amount of crest, and root truncation is small compared to the diametrical tolerance as derived by standoff (+/- 1 turn). A given diameter with tolerance would not maintain the profile.ie….If you were at the maximum functional pitch diameter (L1 plug goes in 1 turn) and the minimum minor that could calculate at minimum material, the crest would be too sharp (or too wide at the opposite scenario). It’s all about profile! To that end, pitch diameters, lengths, and truncation (/ flat widths) only are given. The only help is B1.20.1 Page 23 in the appendix.

 

[Gordon Clarke]

I raised your question to one of the members of the B1 Committee and his response is:

" It’s all about profile! To that end, pitch diameters, lengths, and truncation (/ flat widths) only are given. The only help is B1.20.1 Page 23 in the appendix".

Hi Wayne,
Long time no hear
I can follow the reasoning but I'll bet most thread standards were written long, long before CNC machines were about the only thing used in machining industry. Progress (with most things) moves rapidly.
I suppose the thing I am most interested in is if all can follow my thoughts on machining as much as possible before cutting the actual thread. In the "good old days" before CNC, machining a taper before cutting the thread would have been too time consuming.
Maybe one day the machinist handbooks will contain this type of information?

 

[goodguyj]

great advice
thanx