Monday, March 28, 2011

More about screw driving, this time - the screw.

In this segment I talk a bit about the screw itself and what to select for the job. And what to avoid, and why.

Take a look at the picture below and see some of the options:

As you can see there are also a bunch of drills. Many screws need pilot holes to be useful, as the holding power they get is related to the number of and the surface area of the threads in contact with the holding ground. Long screws have more holding power.

Another thing is that pilot hole I mentioned earlier. It does two things in wood. The first is that it prevents the wood from splitting if a fat screw is driven in. It also prevents twisting the head right off the screw trying to drive it into hard wood or knots. A perfectly sized pilot hole is ever so slightly narrower than the shaft of the screw. This means the threads are biting into compressed wood as they force their way in. It makes for a stronger resistance to pulling out.
Lubricate with wax if you are driving a long screw into tough wood. It makes it easier to drive, and to undo but has little effect on the resistance to being pulled apart. I have seen some guys using wood glue as a lubricant. It works on the way in, but plays hob with ever getting that sucker back out.

In the picture above are a number of screws designed to drive into concrete, and each size of screw comes with a recommended drill size. Do not try other sizes, they won't work.

If you look again at the drills you will notice they have little flags (I use masking tape) on them. The flag shows the depth needed to allow the screw to penetrate fully in to the concrete. I leave a little extra depth for the grit and dust produced by inserting the screw, its gotta go somewhere.

Next, look at this picture:

Click on the pictures to get a large size blow up.

These are wood screws. Most wood screws have a smooth shaft closest to the head.The reason is that they are supposed to pull one piece of wood close to another, and if there is thread all the way down, the thread will prevent the top piece from moving snug to the holding piece.

A rule of thumb is that twice the length of screw should be in the holding piece than is in the top piece. So a proper choice for a wood screw is three times the thickness of the piece to be attached.

See the brass screw?

It is also for wood but it is for attaching brass hinges. So there is no need for a smooth bit on the top and the whole length does the holding.

There is also a screw designed to hold wallboard to wooden studs. Two special things about that one. It is designed to not need a pilot hole. See how skinny the black one is? It is also case hardened so it can pierce metal studs if they are being used instead of wood.
The other thing is more subtle. The under side of the head is trumpet shaped, and it has a Phillips (cross) drive slot. The trumpet shape compresses the wallboard slightly as the screw head is sunk a tiny bit below the surface, and this compression hardens the board where it was just weakened by being pierced.

The Phillips head is a sort of self releasing design that adapts well to setting those screws with a ring adapter on the screw gun that stops the screw when it is just below flush with the surface.

But Phillips head slots are not good for high torque applications. That is best done by the Canadian Robertson design square drive. Those square drive are the best for a couple of reasons. The first is that you can put a screw on to the driver and wave it all over the place and it does not fall off.

Great for putting a screw one handed into awkward places whereas a Phillips driver needs a magnet, or curse words to stay on the driver.

Take a look.

I choose Robertson whenever I can for their superior performance. Slots are my last choice, and unfortunately the Phillips head in a number of variations to increase torque but not infringe on the Robertson patent are all too common, and troublesome.

I could go on about the many other types of head from slightly raised, to wide and flat, and specialized screws for holding in particle board, that abomination of un recyclable and mostly un repairable construction stuff made with formaldehyde and sawdust. Ugh.
rave rave rave.

That's all for this one.

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