Last week I mentioned my plans to remake the wooden screw for our cider press. The problem is that after the press is tightened so tight, the screw slips in it’s nut…effectively limiting how tight it can be turned. This of course limits how much cider can be pressed. And that’s not a good thing!
I believe the reason the screw/nut slips is the relatively fine thread pitch of the screw, 6 threads per inch (TPI). I say “relatively fine” because 6 TPI would be a coarse thread in metal, but is fine indeed in wood. Here is a picture of those 6 TPI threads:

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I have been doing some reading and researching and have decided to make the following changes in the new screw:
– increase diameter from 1.5 to 2 inches. Otherwise the root diameter of the screw would be made too small by the deeper threads I intend to cut.
– decrease pitch of threads from 6 TPI to 2 TPI. This will make for a “faster” screw and should make for stronger threads as well.
– increase angle of threads from 60 degrees to 90 degrees. Most standard metal screws have 60 degree threads. That works quite well in metal. However, back in the day, most wooden press screws had 90 degree threads. This makes the threads stronger, less prone to breakage.
Below is a sketch of what I have in mind:

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Now comes the rub…the taps and dies to cut a screw and nut like this are not commercially available. Well, not without mortgaging the farm. So, I’m going to cut these using an ancient method. Instead of using a metal tap to cut the internal threads, a wooden tap with a metal tooth and a sheet metal guide is used to SCRAPE the threads in the nut. Then, a corresponding thread is laid out on the screw blank, then hand cut with chisels. Whew! Let’s get started…

The tap:
I started by turning a true 1.5 inch cylinder for the tap. It is turned from a piece of well-seasoned cherry. A section was left at full dimension for the head which will be fitted with a cross bar.

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Next a length of 1/2 inch wide masking tape was wrapped in a spiral around the cylinder, leaving just a saw-kerf width between the spirals. By the way, I could not find any 1/2 inch wide masking tape locally. Or 1/2 inch wide tape of any kind for that matter. So I used my cutting gage – the one I use to lay out dovetails- to cut 1/2 inch wide round and round the roll. Then I was able to pull off strips of the correct width.

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A backsaw was used to cut a spiraling kerf between each wrap of tape. A sheet metal guide will fit in this kerf to pull the tap into the nut to be tapped. A piece of masking tape on the saw marked of the 1/4 inch depth the saw was to cut.

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More to come; thanks for looking !

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