Screw Threading

It's been a very busy week on the standing press. The base and drawers presented no problems - lots of bits of wood, which always take time, but nonetheless straightforward enough.  The top section was another matter. I still hadn't tried out the  "new"  2.5" wood-screw threading tap, which has been decorating a workshop shelf for over a year, but the time had come:


It likes a little raw linseed oil to ease its passage.

Quite a degree of force is needed as it cuts the full diameter thread:


If there had been time, I'd have had a sleeve welded to the top of the tap to take a heavy T bar. As it is, a section of channel bar and a large crescent wrench will do. I've had to use a half-inch socket extension to get through the deep hole for the thread.

Hard work, but simple enough. Then there's the question of the actual screw itself. This is very much a boot-strapping operation: first use the tap to make a section of female thread in a 2'' piece of wood. Then use this to make the die to cut the thread onto a 2.5" turned billet of suitable wood. The cutting tool is a 60 degree router bit extending into the cut female thread.

Let's just say that it took some ten or twelve hours of trial and error to come up with an acceptable thread. It's just another one of those multiple unknown pentanomial equations (the last disaster in this department was an attempt with Norah to marble paper - was it the humidity, the temperature, the sizing, the ox-gall, the consistency of the size, the age of the size, the type of pigment, or just some arcane aspect of technique that produced those wrinkled smeared sheets of blobby colours?). Threading, by contrast, is much easier. It comes down to the exact positioning and depth of the cutting bit, the precise diameter of the stock, and, most of all, the species and nature of the wood. Yesterday I ran through yew, arbutus and yew again, before finding an odd piece of maple in the wood stack. This worked well enough, and very early this morning I turned the 32" x 2.496" (no, seriously) cylinder and ran it with no great difficulty through the die block:


The cylinder in the block is a sample piece (yew) The huge screw thread lying at the back of the workbench is a section of vacuum hose.

Back to the salt-mine.

(Proof reading later)