Tuesday, April 5, 2011

Shop Tips

A few recent simple things...

Drawing the curtain down:
This is an old one, culled from my days at school. Somehow I've been getting by without it, but the current project has enough going on that I really want to keep all of the drawings handy. So, I bought a window shade, and mounted it in a convenient place. The drawings get mounted to the shade, and that way I can pull them down quickly to look at, and give a tug and put them away just as quickly to get them out of the way.

One thing I noticed... this particular one was marked at Home Depot as one that couldn't be cut down in the store. Turns out the reason why is that it's mounted on a mostly metal tube. Go figure.


Tenon cutting on long pieces with the new crosscut sled.

I had to do this today on the side of a bed frame I'm building. Normally when I cut tenons I'm cutting them on smaller pieces, and I can use the table saw fence to gauge the length of the tenon. This piece was almost 7 feet long. I played with the idea of using an end stop at the other end of the board, and decided it really wasn't worth it. I don't like using subtractive cuts* if I can help it. So, referencing small cuts at one end of the board by hooking onto the other end of the board didn't seem like a great idea. So, I set up the stop on the sled, and used a scrap of wood to bump the end I was working on to the proper measurement in relation to the blade... and not the other end of the wood.

*Subtractive versus positive cuts/ measurements: This is a term I came up with recently, I don't know if it's the real term, that was buried in my subconscious, or if it's just one that I made up. Either way, I like it.

A positive measurement is a measurement that's taken directly... in the case of a table saw fence, it's the space between the fence and the blade. The cut that is being made is directly referenced. A subtractive measurement is typically a workaround. For example, cutting a 7 foot piece of plywood is impractical, because the fence on the saw doesn't extend that far. So, I'd set the fence to 11 7/8", to subtract 12 inches off of one end. (11 7/8", + 1/8" kerf width) Another example is cutting a 1/2" thick tenon this way on the saw. I'm not actually cutting a positive 1/2"... I'm subtracting incremental amounts until the tenon is thin enough to fit into the mortise. It's one way to work, but it's not ideal for production work, because any variance in thickness between one board and the next will result in tenons of different thicknesses. Any clumps of dust under one board will cause issues. There are ways to make it work, but the difference here is that to cut a 1/4" tenon in the middle of a 3/4" board, I wouldn't actually be cutting a 1/4" tenon... I'd be cutting two 1/4" rabbets, one on either side of the board. It's not a recipe for reliable accuracy. But it's currently easier than setting up a jig that would positively cut a 1/4" tenon.

In the end, a positive cut is one that is determined in the setup. It's impossible to cut a 6 1/2" wide piece if the fence is set to 6" from the blade. A subtractive cut can be accurately made, but it requires a lot more setup and futzing. That 7' piece of plywood can be cut, but first, you have to make sure that the plywood is actually 96" long, and not 96.5", and that the kerf left by the blade is actually 1/8".

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