Join Date: Mar 2018
Location: Huntington Beach, California
Screws in Hard Wood, 101
OK We use wood screws to hold two pieces of Hard Wood next to one another but NOT to draw two pieces of wood together. Think about that for a moment.
There are two basic styles of screws that we use in Hard Wood. There are wood screws and sheet metal screws. The difference is the shape of the body and threads. A wood screw is tapered and a sheet metal screw is straight without a taper. Unfortunately these days most real wood screws are for slotted screw drivers. Wood screws are usually much more expensive than sheet metal screws. Hence the vast majority of us use the sheet metal variety. The following is intended for the sheet metal variety of screws. Sheet metal screws have threads all the way to the head while wood screws have a shank below the head.
If you are using screws for construction using Hard Wood, there is absolutely no reason for the screw to bite into the top piece of wood. It defeats the purpose of using the screw.
When a screw is driven into Hard Wood, even with a pilot hole, a mushroom is created around the screw as it enters the wood. You need to make allowance for the mushroom in the top or adjoining piece of Hard Wood. A bit of counter sink drilled into the body hole of the top piece solves this problem. The mushroom will prevent the two pieces from drawing up together.
The body clearance hole in the top piece of Hard Wood can be determined as follows. Hold the screw horizontally and at a comfortable viewing distance with a light background. A light colored wall is fine. In the other hand take a drill bit and hold it against and in front of the screw. Choose the smallest drill bit that just barely hides the threads, then select a drill bit that is 1/64 inch (0.4 mm) larger. Use this drill bit to drill the body hole through the top piece.
To determine the pilot drill size use the same technique except hold the drill bit behind the screw. Use the largest drill bit that is obscured by the screw body with the threads plainly visible. Use a drill bit 1/64 inch smaller for hard wood or 1/32 inch smaller for soft wood.
Lubrication is an important part of the process. My preference is the wax sealing ring for a toilet. Go to your favorite M&P hardware store and purchase the sealing ring. Don't get the 'Deluxe' version with the plastic boot. Then stop at the pharmacy and ask for a prescription bottle with the widest mouth available. Remember you are going to put your fingers into the bottle while holding the screw. Use a coffee can in a pot of boiling water to melt the wax and then pour the melted wax into the prescription bottle. Use the wax warm or cold it doesn't matter.
The Actual Joining Process
Prepare the two pieces of wood with the body hole and counter sink in top piece and the pilot hole in the bottom piece. Dip the screw point ¼ inch into the wax. It doesn't matter if the wax is melted or still solid. This should be enough lubrication. The screw, when inserted into the body hole should almost fall through or helped with just a bit of pressure but no twisting required. Align the point of the screw with the pilot and start to tighten. Clamp or otherwise hold the two pieces together and finish driving the screw. If your drill doesn't have a clutch and speed control, drive the screw to almost full depth and then switch to a hand screw driver.
Galvanized screws need more lubrication, maybe a full inch or more of wax.
Brass screws are a completely different animal. After drilling the pilot hole, the hole needs to be threaded. This is tricky. Take a brass screw and at the M&P hardware store find a steel screw that match the thread pitch. Just hold the brass screw against the steel screw. If the threads match up you have a good threading screw. Beware that screws made in the US are going to have a different pitch than those made in a metric country. Just keep the screw diameter the same.
In furniture 1/32" is a Grand Canyon