Metric screws and their inferial counterparts

[mklotz]

The most important feature of this chart is the "easily confused with" column. When identifying unknown screws, these are the ones to watch. Alternatively, if converting project plans from metric to inferial or vv., these identify the closest matches in the opposing system.



Standard metric screws and their inferial counterparts

dm = diameter (mm)
pm = pitch (mm)
di = diameter (in)
pi = pitch (tpi)
tdm = tap drill size (mm)
tdi = tap drill size (in)

 dm  x  pm   (  di  x  pi )    tdm  ( tdi )  easily confused with

 1.6 x 0.35  (0.063 x 72.6)   1.25  (0.049)  [0-80 = 0.060 x 80]
 1.8 x 0.35  (0.071 x 72.6)   1.45  (0.057)  [1-72 = 0.073 x 72]
 2.0 x 0.40  (0.079 x 63.5)   1.60  (0.063)  [2-64 = 0.085 x 64]
 2.2 x 0.45  (0.087 x 56.4)   1.75  (0.069)  [2-56 = 0.085 x 56]
 2.5 x 0.45  (0.098 x 56.4)   2.05  (0.081)  [3-56 = 0.099 x 56]
 3.0 x 0.50  (0.118 x 50.8)   2.50  (0.098)  [4-48 = 0.112 x 48]
 3.5 x 0.60  (0.138 x 42.3)   2.90  (0.114)  [6-40 = 0.138 x 40]
 4.0 x 0.70  (0.157 x 36.3)   3.30  (0.130)  [8-36 = 0.164 x 36]
 4.5 x 0.75  (0.177 x 33.9)   3.75  (0.148)
 5.0 x 0.80  (0.197 x 31.8)   4.20  (0.165)  [10-32 = 0.190 x 32]
 6.0 x 1.00  (0.236 x 25.4)   5.00  (0.197)  [1/4-28 = 0.25 x 28]
 7.0 x 1.00  (0.276 x 25.4)   6.00  (0.236)
 8.0 x 1.25  (0.315 x 20.3)   6.75  (0.266)
 8.0 x 1.00  (0.315 x 25.4)   7.00  (0.276)  [5/16-24 = 0.3125 x 28]
10.0 x 1.50  (0.394 x 16.9)   8.50  (0.335)
10.0 x 1.25  (0.394 x 20.3)   8.75  (0.344)
12.0 x 1.75  (0.472 x 14.5)  10.25  (0.404)
12.0 x 1.25  (0.472 x 20.3)  10.75  (0.423)
14.0 x 2.00  (0.551 x 12.7)  12.00  (0.472)
14.0 x 1.50  (0.551 x 16.9)  12.50  (0.492)
16.0 x 2.00  (0.630 x 12.7)  14.00  (0.551)
16.0 x 1.50  (0.630 x 16.9)  14.50  (0.571)
18.0 x 2.50  (0.709 x 10.2)  15.50  (0.610)
18.0 x 1.50  (0.709 x 16.9)  16.50  (0.650)
20.0 x 2.50  (0.787 x 10.2)  17.50  (0.689)
20.0 x 1.50  (0.787 x 16.9)  18.50  (0.728)
22.0 x 2.50  (0.866 x 10.2)  19.50  (0.768)
22.0 x 1.50  (0.866 x 16.9)  20.50  (0.807)
24.0 x 3.00  (0.945 x  8.5)  21.00  (0.827)
24.0 x 2.00  (0.945 x 12.7)  22.00  (0.866)
27.0 x 3.00  (1.063 x  8.5)  24.00  (0.945)
27.0 x 2.00  (1.063 x 12.7)  25.00  (0.984)



[kngtek]

Marv,

Very useful table!

An alternate approach is to physically check machine screws using 'Screw Thread Chek'r' plates - manufactured by Ruelle Industries (Ruelle Industries, Inc. - Home of the Screw Chek'''r') and sold on amazon.com (see attached pictures). These plates feature both drilled holes and threaded holes - Corresponding to the 2mm through 14mm portion of your table.

Most hobby machinists probably already have drill hole checkers and screw thread gauges, but these checking plates are different, combining the following features:
- drilled clearance holes for checking the screw diameter,
- accurate threaded holes for checking the diameter/pitch of a threaded screw,
- markings for the required clearance drill size for each unthreaded screw hole, and
- markings for the required tap drill size for each threaded hole (based on 65% to 75% of full thread).
Ruelle also appears to manufacture rebadged Chk'rs - e.g. by General.

Two 'Metric Chek'rs' (Nos.1 and 2) cover respectively the ranges from 2 to 7mm and from 8 to 14mm, and the 'Inch Chek'r' covers the range from #1 through 5/16" - Including all the coarse and fine thread series for both metric and inch. When used in combination to check an unknown machine screw these are, in effect, a double-double set of go-no go gauges.

These Screw Chek'rs are clearly intended for use by N.A. machinists since all of the drill size markings on the three Chek'rs are numbered and fractional inch drill sizes - A metric machinist would use metric drills instead (and obviously metric taps).

I have found these to be extremely useful. They're reasonably priced (at us$ 17 each) and very durable (1/8" thick steel). I've attached photos of two of the three checking plates - These two plus Metric Chek'r No.2 are the most useful of the various Chek'rs offered for sale IMO.


Regards,

Gary (kngtek)
Calgary, AB