Screwing: All you need to know

Screws are one of the main metal fasteners used in woodworking. The other is the humble nail. But screws hold better, stronger and longer, they can also be removed for when the piece needs to be dismantled.

 

 

 

But the screw isn't exactly simple. While we go to a hardware store and get away by holding up out fingers with a gap between the thumb and pointer and saying, "teen screw, itna bada"( three screws, this long). This simply doesn't work, if you are half way serious about the wood you are screwing (DAMN, I love inneundo), you would be best served knowing more about "le screw".

The Short Version

"Wood screw, MS, cross head, #8, 1 1/2 inches" 

would mean, a wood screw or half thread or partial thread made of mild steel, with a head that needs a phillips screw driver. The thickness of the shank is 8 on the imperial gauge and the total length is 1 1/2 inches.

The conversion from gauge to mm is in a table below.

Detailed Version

Now screws have a head, a shank, and a thread. Each of these are important for the use you are putting the screw to. 

 

First, there are many many types of metal fasteners with threads, metal screws, machine screws, lag bolts, carriage bolts and more; each created for a purpose. I'm going to focus on the wood screw.

 

A wood screw is a metal fastener, that is used to hold two pieces of wood to each other. The wood screw is characterised by the presence of a naked shaft (shank) below the head followed by the threaded shaft. In use, the top piece of timber takes the unthreaded shank and hence can effectively slide around. The bottom piece which takes the threaded portion of the shaft is drawn tighter and tighter to the top piece with every turn of the screw. If the top piece has a thread portion of the screw, there would be a point where the threads in the top piece would stop the screw from tightening further, even though the bottom piece has not been drawn up properly. There are only 2 possibilities at this point; unscrew and drill a hole in the top piece that is bigger than the shank or try to tighten the screw to the point that the threads in the top piece are destroyed. 

How to buy screws

 Any and all hardware stores will have screws, but some will have a larger variety than others and then you have the specialty stores that only sell screws and bolts.

Now lets take a look back at the screw. It has a head, a shank and a threaded portion.

The Head

The head is the portion you use the screw driver on. There are a few types of heads, the slot head and the cross head (phillips head). The trouble with the slot head is that the screwdriver will pop out at high torque or high speed, making them very difficult to drive. I like the cross heads, these are designed to hold the screwdriver much better. But the problem of jumping out at high torque still continues. There are apparently pozidriv heads which take care of the slip out.

To me the head is only useful to determine the size of the counter sink.

The Shank 

As I pointed out the shank is the only portion in the top piece of timber. Hence drill a clearance hole of the same size in the top pieces; this will save you a whole load of time and effort.

The shank also determines the overall thickness of the screw. Obviously the thickness of the screw determines the strength. Being wood, I have never seen any place where the metal breaks before the wood, but if you use a 3mm dia shank on roofing timbers of 5 and 6 inch thickness then I'm sure we would see a fine example of screw shearing.

So remember, the shank determines the size to drill into the top pieces of wood.

This size is important while purchasing screws. The imperial scale simply has #3, #4, #5 etc. These have some approximate metric conversions

Gauge

Metric diameter (mm)

Pilot Hole size (mm)

Clearance

 

hole (mm)

 

3

2.5

1

3

4

3

1.5

3.5

6

3.5

2

4

8

4

2.5

4.5

10

5

3

5.5

12

5.5

3.5

6

14

6.5

4

7

The Threads 

The threads itself are not very complex. There is a measure of coarse and fine threads, but for wood coarse is best and you never have a variety.

The other thing to note about the threaded portion of a screw is the solid shaft which the threads are attached to, or cut out off. The pilot hole thickness given in the table above generally coincide with the thickness of this shaft. Note that in hard wood, the pilot hole should be a little larger than suggested. The reason being that, hard wood has much less give at the cellular level and it will not make space for the threads. Either the threads will strip off or the wood will split, the latter being the most likely outcome of a pilot hole being too narrow.

The Length

The next parameter is the length. Remember the more threads holding the wood, the better. A 1 inch screw in a 3 inch piece of timber will probably not do hold very well. I generally use screws that are 1/2 an inch less than the total amount of timber. Put another way, I make sure that the threaded portion penetrates to about 2/3rds  to 3/4ths of the wood.

The Material

There are a few materials to choose from. MS (mild steel); strong but will rust. SS (Stainless steel); expensive, will not rust. Brass; expensive, look amazing, soft metal so the threads get stripped easily. If you use these, please buy a MS screw of the exact same measure and pre thread the wood.

Enjoy your next visit to your local hardware. Swagger in and blow him away with your knowledge of screws


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