# Thread: Understanding machine screw sizes

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Originally Posted by Doozer
That is because screws are not measured by the nominal diameter.
They are measured to the major diameter.
nominal is also the major diameter; nominal means the reference point or presumed value. If you pick up a 1/4 - 20 screw, its nominally 1/4, .250....but when you measure its say .243". That was Old Marts point, because of the difference between nominal major diameter and actual major diameter it might not be clear to someone what they've got.
Last edited by Mcgyver; 06-23-2019 at 09:01 AM.

2. ken
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Always best to have a machinery's handbook close by. Ken

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The major diameter of a numbered machine screw in the US system is:
The size number X .013" + .060"

Therefore a #4 screw has a Maj. Dia. of .112" maximum.

I do not know why I have remembered this for 30 years, it is inexplicable.

Even more inexplicable is the fact that someone devised a system of screw sizing using this formula 100 years ago.

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Originally Posted by Bented
The major diameter of a numbered machine screw in the US system is:
The size number X .013" + .060"

Therefore a #4 screw has a Maj. Dia. of .112" maximum.
And, of course, using such a stupid nomenclature that isn't open ended, one runs into complexity when the need for screws smaller than #0 (e.g. 0-80) arises. For such screws one has to remember to convert the multiple zeroes to a negative numeral...

00-90 00 => N = -1
000-120 000 => N = -2
0000-160 0000 => N = -3

for the formula

major diameter = N * 0.013 + 0.060

to yield the correct value.

https://www.amesweb.info/Fasteners/M...imensions.aspx

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Originally Posted by Mcgyver
nominal is also the major diameter; nominal means the reference point or presumed value. If you pick up a 1/4 - 20 screw, its nominally 1/4, .250....but when you measure its say .243". That was Old Marts point, because of the difference between nominal major diameter and actual major diameter it might not be clear to someone what they've got.
Nominal size is not the same THING as major diameter. The maximum allowed major diameter is the same as the nominal size for a Class 3A external thread, but is smaller for classes 1A and 2A. And the minimum allowed maximum diameter is less than the nominal sizes for all classes.
Last edited by cameron; 06-23-2019 at 11:02 AM.

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Originally Posted by cameron
Nominal size is not the same THING as major diameter.
fair enough, I was loose in thinking of the OD as the major diameter, but I thought the point clear enough: that the difference between nominal and actual was the source of confusion.

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Originally Posted by mklotz
And, of course, using such a stupid nomenclature that isn't open ended, one runs into complexity when the need for screws smaller than #0 (e.g. 0-80) arises. For such screws one has to remember to convert the multiple zeroes to a negative numeral...

00-90 00 => N = -1
000-120 000 => N = -2
0000-160 0000 => N = -3

for the formula

major diameter = N * 0.013 + 0.060

to yield the correct value.

https://www.amesweb.info/Fasteners/M...imensions.aspx
There are no NC, NF, UNC or UNF threads smaller than #0.

There are many different systems governing the characteristics of smaller threads, but the system for which you have graciously given us the mathematical determinants exists only in your imagination.

I wouldn't want to call your proposed system stupid, but the absurdity of continuing to decrease the size of ever smaller screw sizes by an arithmetical constant seems to have escaped your notice. I can assure you that the absurdity of increasing the size indefinitely by the same constant has not escaped the notice of those who use them, which explains why the sizes larger than #10 have pretty much disappeared from common use.
Last edited by cameron; 06-23-2019 at 11:40 AM.

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somewhat on topic, Ive got some (very old) taps and dies that are machine screw sizes, yet they are *not* using the numbered system -- the thread profile is USS with tiny fractional sizes. They don't cross to anything currently in use. Any guesses as to the age or time period? OK I have a strange fetish about collecting odd threads.

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Originally Posted by cameron
There are no NC, NF, UNC or UNF threads smaller than #0.

There are many different systems governing the characteristics of smaller threads, but the system for which you have graciously given us the mathematical determinants exists only in your imagination.

I wouldn't want to call your proposed system stupid, but the absurdity of continuing to decrease the size of ever smaller screw sizes by an arithmetical constant seems to have escaped your notice. I can assure you that the absurdity of increasing the size indefinitely by the same constant has not escaped the notice of those who use them, which explains why the sizes larger than #10 have pretty much disappeared from common use.
I was wrong. I apologize for posting without checking the facts, and in particular for attributing the absurd extension of the 0.060+0.013N formula below #0 to mklotz.

That extension does exist and includes the three sizes cited by mklotz.

However, those are NS threads, and as far as I know, I was at least literally correct in stating that the NF and NC sizes do not go below #0.
Last edited by cameron; 06-23-2019 at 03:57 PM.

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Originally Posted by old mart
It can be difficult to identify screws as they are usually slightly smaller than the nominal size in the thread charts.
An example are 10-32 UNF and 5mm metric coarse, The 10-32 is 0.19" diameter and the 5mm is 0.1968" diameter. The thread pitch is even closer, the 5mm is 31.8tpi (0.8mm). A thread gauge will look ok at first glance both ways round with only the slightest rock.
I've been tripped up by that a couple of times. easiest solution in my case is to try the screw in a known nut - 10-32 will screw into an M5x0.8 nut but not the other way round. That's pretty much the only one I've had problems with out of the normal screw sizes. Imperial is usually coarser for a given diameter than metric.

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