Pilot Hole Size and Bit Question

[Eli Gunnell]

Newby here. Is there a rule for the size of pilot hole for a given wood screw size? For example, I want to use a #10 screw. What size bit would be the best for the pilot hole? Looking in the hardware store, most of the smaller bits do not have hexagonal shanks. Will this be a problem in my bit brace?
I am using construction-grade 2x4's, recovered from a teardown; I think it is Doug fir. Thanks for your help!

[Tom Veatch]

Pilot Drill Sizes (http://images.meredith.com/wood/images/pdf/screwchart.pdf)

Hex Shank Drill Bits (http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10051&productId=100053516&langId=-1&catalogId=10053&ci_src=17588969&ci_sku=100053516&cm_mmc=1hd.com2froogle-_-product_feed-_-D25X-_-100053516)

[Eli Gunnell]

Tom,
Thank you, that is perfect!

[David Martino]

Sounds like you're using an old style hand drill/bit brace? Depending on the chuck, some don't work well with hex bits. Round shank or the old kind with tapered 4-sided base are what they used originally.

[Steve Thomas]

Wood Magazine is the best, thats a great easy referance table, I forgot about it and had to go back to find what issue it was in...

Just great. Thanks Tom

[Bill Houghton]

You can also just hold up the possible drill bits to the screw. For the clearance hole, find a bit that's equal to or larger than the body of the screw*. For the "tap" hole, that is, the one that the threads bite into, find a bit equal to the minor diameter** of the screw at the threaded portion.

*If you're using screws, like drywall or deck screws, where the body is smaller than the threads, the "body" of the screw is, in effect, the largest diameter of the threaded portion.
**Minor diameter is the part of the screw that would be left if you ground off all the threads.

Just as important: the clearance hole should always be all the way through the wood that the screw first goes through. Otherwise, it won't pull down properly to the piece of wood to which you're screwing it. On fine work, it's sometimes wise to do a little (very small, just the larger diameter of the threads) countersink on the second piece of wood, after drilling the "tap" hole. Many screws cause the wood fibers to lift up, and the lifted wood fibers can interfere with good seating.

[Eli Gunnell]

Bill,
Thank you for the detailed explanation, I have never read that anywhere before. I'm trying to build a simple bench to use as a saw bench. I did notice last night, what you said about drilling the clearance hole all the way through the top piece. In spite of the pilot hole in the lower piece, I just couldn't get the screw all the way down before shredding the head. That was, until I bored the clearance hole all the way through the top piece.
I hadn't thought of a small countersink in the bottom piece, but that may help with the problems I was having keeping the two pieces from wiggling.

-Eli

[lowell holmes]

I have a set of tapered pilot bits w/counter sink that are sized to wood screw sizes. I bought these about a year ago and had never used the tapered holes before. The wood screws will go in wih light pressure and then engage all the threads at once. The clamping pressure exerted on the wood is incredible. I don't destroy the scre heads any more.

There are several comanies that have these on line. I value my set and will not be without them.

[Jason Roehl]

Absent a table, I generally size my tap hole either the same as the minor diameter (course threads) or ~1/64" smaller for fine threads. I find that this gives better holding power without greatly increasing the driving force.

Here's a trick if your clearance hole is too small: drive the screw home, and the boards will likely be slightly separated. Back the screw out of the "bottom" board and re-drive it. The boards will come together tightly then. Necessity is the mother of invention...

[harry strasil]

on soft woods I don't drill the pilot hole for the screw threads with a drill bit. I use a nail the proper size with the head cut off in a hand drill or archemedian drill and put a bit of beeswax or rub across a candle which makes them screw in with ease.

My theory about using a nail to make the pilot hole is that soft woods are just that and don't have near the holding power that hard woods do. When drilling a hole you remove some of the material which lowers the holding power of the screw threads. When using a nail to make a pilot hole you are compressing the soft wood and leaving behind an open area the size of the internal shank, the wax makes the screw threads slide in more or less and with the compressed wood around the pilot opening you get maximum holding power similar to what you get in harder woods.

Just my 2¢ worth