Saturday, March 1, 2014

Shoot that frame!

Usually one thinks of "shooting" as pointing a gun and pulling the trigger. But this word has a meaning that probably goes back to the Romans. Maybe they made picture frames too, although I doubt that any are left. Most of our extant frames date back to the Renaissance. Those people really did shoot frames. So what do I mean by "shooting a frame?"

When you make a picture frame (think of the last frame you saw, or have hanging on your wall) with miterered corners,  there are three requirements. One of them is that the pieces should be cut at exactly 45 degrees. The second is that the  dimensions of the pieces should be exactly the same. Especially the inside dimensions. The third is that the mating surfaces should be as smooth as you can possibly make them. Otherwise it looks, well, amateur.The big boys -- the pros -- use a guillotine-like affair. This is beyond my means. A miter joint is a really weak joint, you would not like to use it for furniture. For picture frames it works all right. Just.

You could concievably cut miters (Britons read mitres) at any old angle. Say 30 deg. But then the mating part must be cut at 60 deg because the frame is rectangular. The jigs required to get this straight would be quite complex, unless you resort to CNC cut frames. So most of us stick to 45 deg, half the right angle.  I did too.

The crucial step is to build a shooting jig.  Here it is.

It is basically a scrap board clamped in my woodworking vise, which is attached permanently to my dining room table. I have a very small house; everything is multi-purpose, especially in winter. It is really a very simple jig, but it took me all morning to tune it. There is a triangular piece screwed on to the board.  There is a rabbet (Britons read "rebate" which is where our corruption comes from) and the angle between the rabbet and both sides of the triangle is 45 deg as accurately as I can measure it. It took quite a lot of planing (and a very accurate protractor) to get the angles right.  In the rabbet slides my trusty Veritas rabbet plane. Expensive. Worth every cent of it. My frame member is clamped (you can just see the clamp upper left) to the jig. When you plane across, left to right, you shave just a tad -- 0.1mm at most -- of the approximate saw-cut miter. This is called shooting the miter. Adjusting this thing is very difficult. Takes patience. But by George, your miters will be at 45 deg. This is a very old-school technique. Most people use CNC equipment for this nowadays, I suppose.But shooting makes a glass-smooth surface, especially with the Veritas plane which is a low-angle plane.

When I was all through with the shoot, I had a big gap in my frame. Surprise! My dimensions were way off. One piece was a full 6 mm off the other, which is suspiciously close to 1/4". When I laid this out at my daughter's place, I had very primitive layout tools so I suspect I made a mistake because I really cannot deal with RGU. So now I have to take 6mm off one side. I got down to 3mm and then decided to take a break and go back to machining my steady rest. Next post.

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