Wednesday, August 5, 2015

Making screws

Just back from a three week holliday in Italy. I'll post some pictures of Verona later, they had some fascinating medieval stuff down there. Later this month I'll be on another trip, so I don't have much time to continue with the carving yet.

I was looking at the hardware for the cabinet. The hinges will be attached to the inside of the door, and the outside of the cabinet sidewall. Usually in the 17th century hinges would be attached with long nails, going all the way through the wood and then being clinched on the inside. this is how I will attach the hinge to the cabinet sides. But for the door it wouldn't look so hot to have a nail protruding through the front. When looking at some antiques I think they wouldn't have cared much anyway, but I decided to use some short screws and leave the face of the door unmarred.

Screws have been in use in furniture since the 15th century, but very sparingly. Screw cutting lathes are a late 18th century invention. Before that time they were made by hand. The nail and screw manufacturing was often done as a kind of home business. A geographic area would specialise in this kind of trade and many families would do this kind of work all day at home. The screw was made from a length of wrought iron, hammered to a round circumference and then swaged out in a die to form the head. Then the threads would be filed. This leaves a rather irregular shape of course.

My local shop quit selling slotted head screws, so this is a perfect opportunity to try my hand at some screw making too. I use low carbon steel in the round, so I don't need a real blacksmith shop. I made a split die first, which is just a chunk of steel cut through the length. Then a hole is drilled and countersunk to create a shape for the screwhead. The steel rod protrudes a bit above the die, and is then hammered down into the countersink with the fin of a heavy hammer to really drive the steel into the recess.

That forms the basic shape of the screw. The slot is cut with a hacksaw, not too precisely in the middle, because I suspect many of the old time screwmakers must have been pretty drunk. You don't see many perfectly centred examples.

To lay out the threads I don't measure anything. I take a regular woodscrew along side as an example and nick the steel at regular intervals with a triangular needle file. I try to remember the angle of the file so the slope of the thread ends up reasonably close to what it should be.

From these nicks I file each one progressively further around the steel blank. As soon as I get to the other side I will have to wiggle around a bit to make the two ends line up. This doesn't matter, there is still ample time to correct the basic spiral. And it doesn't matter if the threads don't end up perfectly.

Then I get a saw file and then deepen the grooves until the edges come to a sharp(ish) edge. At last I use a thin flat needle file with the corner to make a bit more room down in the grooves. Here are the four in the rough.

 And this is after shortening and how they look when used to attach the hinge.

It was a fun little job. After some experimenting, it took me about two hours to make these four screws. It's a pitty most of it will be burried in the wood and not be visible, but the heads have the nice irregular shape of a real antique hand cut screw. A look you just don't get from a hardware store screw.

No comments:

Post a Comment