Thursday, May25, 2017 L-36.com

WOOD SCREWS PILOT HOLE SIZE



There are many tables published on the Internet that show pilot hole recommendations for wood screws. The sizes recommended vary quite a lot. The discussion below the table explains why I recommend the one I do.

Screw
Gauge #
Shank Shank
Hole
Root
dec
Root
fract
AWC Pilot
Soft Wood
AWC Pilot
Hard Wood
Recomended
Soft Wood
Recomended
Hard Wood
75% of
Shank
Screw
Gauge #
2 .068 3/32 (+.007) .054 1/16 1/32 3/64 1/16 1/16 1/16 2
4 .112 7/64 (-.003) .075 5/64 3/64 1/16 1/16 5/64 5/64 4
6 .138 9/64 (+.002) .094 3/32 1/16 5/64 5/64 7/64 7/64 6
8 .164 11/64 (-.007) .112 7/64 5/64 3/32 3/32 1/8 1/8 8
10 .190 3/16 (-.002) .130 1/8 3/32 7/64 7/64 9/64 9/64 10
12 .216 7/32 (+.003) .148 9/64 7/64 1/8 1/8 5/32 5/32 12
14 .242 1/4 (+.008) .165 5/32 7/64 9/64 9/64 11/64 3/16 14
16 .268 17/64 (-.002) .184 3/16 9/64 5/32 5/32 3/16 13/64 16
18 .294 19/64 (+.003) .204 13/64 ? ? 11/64 7/32 7/32 18
20 .320 5/16 (+.007) .223 7/32 ? ? 9/32 15/64 15/64 20

I had the occasion to sink some #14 screws in teak using one of the most common recommendations. It was all I could do to sink them and I was using a 3/8" snap-on flat driver socket in a ratchet wrench. No way could you sink this with any screwdriver.

Most of these charts are based on the maximum strength as determined by the American Wood Council (AWC). Their recommendation is for 70% of root diameter for softwood and 90% of root diameter for hardwood. These are in danger of splitting the wood if the hole is anywhere near an edge and way too tight for most work, in my opinion. Another site recommends 85% of root diameter for softwood and 100% of root diameter for hardwood but then publishes a table with larger sizes! Yet another table recommends 100% of root diameter for softwood and slightly larger for hardwood.

The recomendations of the AWC are correct, if you are sinking a 3/8 lag bolt into a 12 foot long 4x12 with an air impact wrench. If you are working on fine woodwork, don't use them! You don't need the last bit of strength and any strength you have will be lost if the screw splits out the side of your work. The AWC specifically does not worry about splitting of the wood or how hard it is to sink the screw, only strength.

My daddy said you should hold the screw and drill up to the light and for softwood the screw should just block the drill and for hardwood, you should just be able to see the drill past the root. In other words, use just under the root diameter for softwood and just over for hardwood.

In the table above, you would find these numbers right at 85% of root diameter for softwood (except for #2) and just the next drill size larger than the root diameter for hardwood. That ranges from 102% to 116% of root diameter. These are the scientific equivalent of my daddy's advice. Perhaps he was taught that by his grandpa, who worked for Thomas Edison. My bench vice came from that shop, my most prized posession.

There is also a rule of thumb that is pretty easy to remember that says use a drill that is 3/4 of the shank diameter. That produces the drill sizes identical or close to my recommendation for hardwood. The softwood sizes are 1/32 under (2 drill sizes)

You decide which column to use for your own application.


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