You reach into your box of assorted screws, pull one out that looks about right for the job at hand. Now, you need to know what size it is so you can select the right pilot drill to use. You could measure its diameter and then work backwards through the formula:
Diameter = 0.060 + 0.013*(screw number)
to find its number so you can use one of the tables below to determine the pilot hole size. You say you're not fond of inverting equations in your head? Well then, you need to make one of these handy little screw sizing gages...
Any piece of sheet steel will work; mine is 2" OD. My gage has the following layout...
N = screw number
D = screw diameter according to the formula above
X = drill used to drill hole (err slightly on high side to provide clearance)
Code:
N D X
4 0.112 #33
5 0.125 1/8
6 0.138 #28
7 0.151 #24
8 0.164 #19
9 0.177 #15
10 0.190 #11
12 0.216 7/32
Once drilled, stamp the screw number next to the drill.
Use is trivial. Simply drop the screw into the smallest hole it will fit and read off the screw number.
You may want to gang drill several gages; the workbench and each tool bag should have one.
[I believe there is such a thing as a #14 screw but I've never seen one. If you use that size in what you do, include a hole for it too.]
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WOOD SCREW DATA
N = NOMINAL SIZE
T = THREADS PER INCH
D = DIAMETER OF UNTHREADED ROOT (in)
B = BODY DRILL SIZE (in)
S = PILOT DRILL SIZE IN SOFT WOOD (in)
H = PILOT DRILL SIZE IN HARD WOOD (in)
Code:
N T D B S H B S H
-------------------------------------------------------------------------------
0 32 .060 52 75 66
1 28 .073 47 71 57
2 26 .086 42 65 54
3 24 .099 37 58 53
4 22 .112 32 55 51 1/8 1/16 5/64
5 20 .125 30 53 47 9/64 5/64 3/32
6 18 .138 27 52 44 9/64 5/64 3/32
7 16 .151 22 51 39 5/32 3/32 7/64
8 15 .164 18 48 35 11/64 3/32 7/64
9 14 .177 14 45 33 3/16 7/64 1/8
10 13 .190 10 43 31 3/16 7/64 1/8
11 .203 4 40 29
12 11 .216 2 38 25 7/32 1/8 5/32
14 10 .242 D 32 14
16 9 .268 I 29 10
18 8 .294 N 26 6
20 8 .320 P 19 3
24 7 .372 V 15 D
Recommended Drill Sizes for Self-Tapping Screws (Sheet Metal)
N = Number
T = Threads per inch
OD = Outside Diameter
A = Drill for heavy metals
B = Drill for light metals
Code:
N T OD A B
2 32 .086 49 49
4 24 .112 41 41
5 20 .125 36 36
6 20 .138 32 32
7 19 .151 30 30
8 18 .164 28 29
10 16 .190 20 21
12 14 .216 13 14
1/4 14 .250 3 4
5/16 12 .313 I H
3/8 12 .375 R Q
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