# Converting Imperial and Metric threads to U.S. Threads



## Bogstandard (Aug 9, 2007)

Just to help the newcomers and maybe a few not too new, here are a couple of charts to assist in the choice of converting to US threads if you come across a plan that uses metric or obscure model engineering threads.
Just co-relate by using the BA conversion. eg 2.5mm relates to 7BA. Then 7BA relates to 3-48.














John


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## Tin Falcon (Aug 9, 2007)

Thanks for sharing looks like an excellent resource. 

Here is a link to more thread standards.
http://mdmetric.com/thddata.htm

Tin


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## shred (May 16, 2008)

So I'm poking around the forum looking for a metric-to-closest-imperial thread chart and found this topic.. but the pics have been lost. Anybody got 'em saved somewhere?


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## Bogstandard (May 16, 2008)

Hi Shred,

I can't find the originals, but these might help and you will be able to save and print them out

John


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## mklotz (May 16, 2008)

Here's one I made up some time ago. 

	British Association			Closest American
   	   Threads				  Threads

	BA 	 OD	TPI	PITCH		THREAD	 OD
	Size	(in)		 (mm)			(in)

	16	0.031	134.0	0.19
	15	0.035	121.0	0.21
	14	0.039	110.0	0.23
	13	0.047	102.0	0.25
	12	0.051	90.9	0.28		0-80	0.060
	11	0.059	82.0	0.31
	10	0.066	72.6	0.35		1-72	0.073
	9	0.075	65.1	0.39
	8	0.087	59.1	0.43		2-56	0.086
	7	0.098	52.9	0.48		3-48	0.099
	6	0.110	47.9	0.53		4-48	0.112
	5	0.126	43.0	0.59		5-44	0.125
	4	0.142	38.5	0.66		6-40	0.138
	3	0.161	34.8	0.73		8-32	0.164
	2	0.185	31.4	0.81		10-32	0.190
	1	0.209	28.2	0.90		12-28	0.216
	0	0.236	25.4	1.00		1/4-28	0.250

Notes from ME-05/05/95-529 Post Bag letter by Jim Nicholson
Full series runs from 0 to 22.
Thread angle = 47.5 deg.
Crest and root radii = 0.18p (p=pitch)
Pitches are calculated as p(mm) = 0.9^n (n=number of thread in BA series)
Diameters are calculated as D(mm) = 6p^1.2
Running:

FOR N=22 TO 0 STEP -1 : P=0.9^N : D=6*P^1.2
PRINT USING "## #.### ###.#  #.###";N;D/25.4;25.4/P;P
NEXT

produces:

N   OD  TPI  PITCH
   (in)      (mm)
22 0.015 257.9  0.098
21 0.017 232.1  0.109
20 0.019 208.9  0.122
19 0.021 188.0  0.135
18 0.024 169.2  0.150
17 0.028 152.3  0.167
16 0.031 137.1  0.185
15 0.035 123.4  0.206
14 0.040 111.0  0.229
13 0.046  99.9  0.254
12 0.052  89.9  0.282
11 0.059  80.9  0.314
10 0.067  72.8  0.349
 9 0.076  65.6  0.387
 8 0.086  59.0  0.430
 7 0.097  53.1  0.478
 6 0.111  47.8  0.531
 5 0.126  43.0  0.590
 4 0.142  38.7  0.656
 3 0.162  34.8  0.729
 2 0.183  31.4  0.810
 1 0.208  28.2  0.900
 0 0.236  25.4  1.000


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## shred (May 16, 2008)

Thanks guys.. What I was looking for was a chart like so (much like Marv's first, but with an additional metric column):

Metric..........Nearest BA.......Nearest UNS
----------------------------------------
M2.... 0.078"... 9BA (.075") ... #2 (.086")
M3.... 0.118"... 6BA (.110") ... #4 (.112")

and so on, so when I'm looking at a metric print I can swap in #4-40 instead of a M3 (or 4-48 for M3f)

I finally found one of a sort (I'm still working out what half of it's for, but what I need is on there): http://www.allcap.co.uk/Conversi.htm


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## GailInNM (May 17, 2008)

Here is one that has almost too much information as it has all common threads plus watch threads in order of major diameter.

http://www.watchman.dsl.pipex.com/thread.html

It can be downloaded in Excel format so you can modify it if you like.


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## mklotz (May 18, 2008)

When one wants a specialized chart such as this, it's easier to generate it than to try to find a source for a readymade one.

Ten minutes of programming yielded a program that produced the chart below.

Column A is the screw OD in mm.
Column B is the closest BSA screw number (BSA numbers only go to a maximum of zero (typical idiotic inverted, closed end scale) so a "-1" indicates that there is no equivalent BSA number.
Column C is the nearest numbered screw size (another idiot closed end scale). -1 refers to size "00" and -2 refers to size "000". For sizes greater than #12 (have you ever seen a #14 tap?), the decimal equivalent of the millimeter size in column A is given. Thus, for example, the closest screw size to 6.25 mm is 0.246" and one would presumably use a 1/4-20 or 1/4-28 thread.

If you want the table with a larger range or smaller step size in millimeters, let me know. It's a simple matter of changing a number or two in the code and recompiling and rerunning the program.




A......B.......C
-----------------------
0.500..20.000..-2.000
0.750..16.000..-2.000
1.000..14.000..-2.000
1.250..12.000..-1.000
1.500..11.000..0.000
1.750..10.000..1.000
2.000..9.000..1.000
2.250..8.000..2.000
2.500..7.000..3.000
2.750..6.000..4.000
3.000..5.000..4.000
3.250..5.000..5.000
3.500..4.000..6.000
3.750..4.000..7.000
4.000..3.000..7.000
4.250..3.000..8.000
4.500..2.000..9.000
4.750..2.000..10.000
5.000..1.000..11.000
5.250..1.000..11.000
5.500..1.000..12.000
5.750..0.000..12.000
6.000..-1.000..12.000
6.250..-1.000..0.246
6.500..-1.000..0.256
6.750..-1.000..0.266
7.000..-1.000..0.276
7.250..-1.000..0.285
7.500..-1.000..0.295
7.750..-1.000..0.305
8.000..-1.000..0.315
8.250..-1.000..0.325
8.500..-1.000..0.335
8.750..-1.000..0.344
9.000..-1.000..0.354
9.250..-1.000..0.364
9.500..-1.000..0.374
9.750..-1.000..0.384
10.000..-1.000..0.394


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## shred (May 18, 2008)

Thanks Marv. I figured somebody was trying to be clever making the #14 the same as 1/4", but it seems to not quite be the case. 

Anyway, for this particular project all I need is thru about M5 and I have those.

IIRC not every metric step is 'legal' for a thread (much like you don't ever see #7 or #9 in the UNS system), so it might be possible to cut down the chart. The information-design part of my brain thinks there should be a nifty X-Y layout wherin the Y axis goes from say ~0mm to ~25mm on the left side and 0-1" on the right and across the X in between are the various common model-maker thread systems and drill schemes with entries in the correct position for each of the typical sizes in that system. Then you could just slap a ruler across the size you wanted to convert from and at a glance, get the nearest up and down in the other systems as well as metric and imperial conversions and applicable drills. With some extra mojo, you could even have a column for tapping drill sizes.


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## mklotz (May 19, 2008)

Shred,

The program is just a tool to automate the process of developing the equivalencies between the three systems. Extract the information you want and arrange it however suits you. Given the small number of screw sizes used in model making, making a personal chart shouldn't require much time.

I know that not every multiple of 0.25 mm is a standard metric thread but, from a programming point of view, it's easier to just step through all the values and then let the user discard whatever he doesn't want.


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