# Standard & Metric Thread Sizes



## Bernd (Sep 18, 2008)

Another thread has been started on metric unit conversion and how best to convert dimensions from metric to imperial.  In these discussions I haven't seen anything mentioned on hole size or thread conversions. Hole sizes are easy. Say you need to drill a 2.5mm dia., converted to inch would be .098". The closest would be a 3/32 (.09375). Ok not bad so far.

Now how about a tapped hole of M3. ??? What size drill and tap will match that or be close to it. ??? I'll bet Marv's got a program or some fancy way to figure this. :-\ Only other choice I see is a chart. How many have a chart for such a conversion? Ya, I thought so. So anyway back to the M3 tapped hole. The chart say's M3 X 0.5 (that's a 3mm tap and a 0.5mm drill for you newbie's) the equivalent imperial size is 5-40 tap and a #39 drill.

The reason I bring this up is I would like to build a couple of steam engines, but they have metric dimensions. The linear dimensions are fine. I just convert to inch and be done with it. Hole size as I stated above. But the screw sizes got me until I came across a chart. I reconfigured it in Word and uploaded. It's in the General Section under Standard & Metric Thread Sizes Table.

Hope this helps some of you guys out. Download it and print it out to have handy the next time you need to convert from one thread size to the other.

Regards,
Bernd


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## John S (Sep 19, 2008)

Bernd  said:
			
		

> Now how about a tapped hole of M3. ??? What size drill and tap will match that or be close to it. ??? I'll bet Marv's got a program or some fancy way to figure this. :-\ Only other choice I see is a chart. How many have a chart for such a conversion? Ya, I thought so. So anyway back to the M3 tapped hole. The chart say's M3 X 0.5 (that's a 3mm tap and a 0.5mm drill for you newbie's) the equivalent imperial size is 5-40 tap and a #39 drill.
> 
> Regards,
> Bernd



Nope,
M3 x 0.5 means 3mm diameter by 0.5mm pitch.
Tapping size for this and ANY metric thread is diameter minus pitch so in this case it's 2.5mm

Again that can only be defined as one size, 2.5mm, no chart needed, what size is a #39 off the top of your head ?

.


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## Bernd (Sep 19, 2008)

John Stevenson  said:
			
		

> Nope,
> M3 x 0.5 means 3mm diameter by 0.5mm pitch.
> Tapping size for this and ANY metric thread is diameter minus pitch so in this case it's 2.5mm



I stand corrected. Thanks John.



> Again that can only be defined as one size, 2.5mm, no chart needed, what size is a #39 off the top of your head ?
> 
> .


There you go, that's exactly what I mean you need a chart for things like that. 

End of metric disscusion. 8)

Bernd


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## mklotz (Sep 19, 2008)

Bernd,

No, I don't have a program to find the Imperial thread "closest" to a given metric thread.
To do that would require that you define what you mean by "closest". Is it closest match on major diameter or closest match on pitch? Or one could go all analytic and define a function which is some sort of weighted combination of diameter and pitch and find the standard thread that minimized that function but I doubt anyone wants to get that finicky about the problem.

Your list highlights some of these problems. For instance... 

2-56      51        M1.4 x 0.3  1.10 
2-64      50        M1.6 x 0.35 1.25 

A #2 screw has a major diamter of 0.086" = 2.184 mm. So why use a 1.4 mm metric screw?
And why increase to 1.6 mm when the pitch changes and the diameter doesn't?

56 tpi corresponds to a pitch of 0.454 mm and 64 tpi to 0.397 mm. So, why isn't the closest match to 2-56 a M2.2 x 0.45? And why does the metric pitch increase when we go to the finer pitch of 64 tpi?

It would seem that your table has some built-in, undocumented assumptions about what "closest" meant to the table's author.

On the matter of tap drills... my DRILL program will compute tap drills for any Imperial/metric tap, even the non-standard ones and has the advantage that it will allow you to specify the required depth-of-thread to make tapping easier on recalcitrant materials. However, as John points out, if you're happy with 77% DOT, the simple formula:

tapdrill = major diameter - pitch

will work. Note that this also works for Imperial sizes...

tapdrill = 0.25 - 1/20 = 0.2" => #7 drill for a 1/4-20 tap


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## Bernd (Sep 19, 2008)

Marv, my head hurts after reading all you wrote. 8)

Here's where I got my info from http://www.engineershandbook.com/Tables/tapdrill.htm.

As I said, end of metric disscusion. I was just looking for something close to imperial size. Perhaps I should invest in a set of metric drills and taps. Problem is I still can't find metric rough stock to use here in the U.S. of A. 

Bernd


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## CrewCab (Sep 19, 2008)

Bernd  said:
			
		

> I was just looking for something close to imperial size.



Reasonably close :

1/8" = 3mm
3/16" = 4.5mm
1/4" = 6mm 

You can interpolate the rest 

Not exact .. (within 0.5mm) ......... but it's in the ball park 8) ......... just depends how accurate you need want to be 


CC


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## Bernd (Sep 19, 2008)

Hey CC,

I just wanted something close. It didn't need to be right on. 

In other words if the print stated a hole to be M2.5 X 0.45 (4-48 imperial is close) that I won't try to use a 1/4-20 and wonder why it won't work. ???

Bernd


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## mklotz (Sep 19, 2008)

Bernd  said:
			
		

> Marv, my head hurts after reading all you wrote. 8)
> 
> Here's where I got my info from http://www.engineershandbook.com/Tables/tapdrill.htm.



Ok, I'm going to type this really slowly so you can follow along...

After looking at your URL, it seems obvious that it's just a list of the standard size screws available in Imperial and metric. While they're listed side by side, there's no indication on the table that they are the closest matches to each other.

This becomes patently obvious when you get to the larger sizes. Listed next to 1/2-20 is M30 x 3.5. Now do you really think that the closest Imperial size to 30 mm = 1.18" is a 1/2"?

How hard is it to convert Maa x bb according to:

aa * 25.4 = major diameter (in) of nearest Imperial screw

25.4/bb = pitch (tpi) of nearest Imperial screw

The major diameter of numbered Imperial screws can be found from:

major diameter = 0.060 + 0.013*N

where N is the screw number.


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## Bernd (Sep 19, 2008)

OK Marv "UNCLE" 

 :big: :big: :big: :big: :big: :big:


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