OK, this time I'm attempting to make a screw-box. This device has two blocks and a cutter. The two blocks are the same length and width, but different thicknesses. In his book "The Woodwright's Workbook", Roy suggests making the rearmost block as thick as 6 threads of the screw. With a pitch of 3/8", that's 2 1/4" thick. The front block is about half as thick.
It's best to make them the same length and width for layout purposes. I used the bottom edge as the reference and used a marking gauge to scribe a centerline around both blocks' long axes. Then a square and a knife were used to mark a centerline across the grain. These lines help with alignment of the two pieces later. The blocks were screwed together and marked together for this. The marks also give a center for laying out and drilling the holes.
The front block gets a 2 1/8" hole. This was done with coping saw, incannel gouge, rasps and files due to not having the proper size drill. The rear block gets a 1 3/4" hole (made similarly). Once these holes were made, I used the tap from last week's post to cut the interior threads in the rear block.
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| Turning the tap into the rear block |
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| Testing with the screw that I made last week |
The next step was to shape a cutter. The only steel I had on hand that might be big enough was some 1/4" square O-1 steel. With thread pitch of 3/8" and a 90 degree angle between threads, the math works out that I need just over 1/4" (about 17/64") on each side of a cutter to cut these threads. And it's better to have more than not enough.
Well, I started shaping the 1/4" stuff I had just to see if it might work, but I'm getting bogged down. Here are some shots of the shaping.
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| Creating an angled flat on one corner |
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| Going from corner to opposite corner and back to 1" from the end |
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| Then marked a center line on the resulting flat |
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| Used a hack saw, then square and triangular files to get final shape. Sharpened the edge with slipstone and strop. |
There's another problem when shaping a square piece of steel as I've done. When it is mounted in the rear block, there's a corner of the square rod sitting down into the recess that has been chiseled out. That means I had to chisel the recess with a matching shape. Not sure how well it shows up in the next pic, but the recess is pentagonal - like the roof of a house upside down in the bottom of the recess.
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| The recess for the cutter |
I can tell this isn't working out. I was thinking about ordering some 3/8" square steel, but even then if I orient the steel so there is a flat face at the bottom of the recess, the "wings" of the cutter are just 17/64". Damn! After a quick calculation, it looks like I'll need some 1/2" x 1/4" steel. And I can get it in a 6" length from McMaster-Carr. Let's hope it goes better with that!
