Monday, February 28, 2011

Beyond Relative Dimensioning

Once upon a time, I thought you could draw up a plan for a woodworking project, make all the parts according to the plan, and then have them magically fit together.  After all, that's how they make cars and washing machines and ink jet printers.  The result was, of course, that things almost fit, but not always, and not always very well.

Relative Dimensioning

Eventually I learned the trick of relative dimensioning.  When you're making a cabinet, for example, you should make the carcass according to the plan, then toss the plan aside and make the doors and drawers to fit.  In fact, you should toss your tape measure aside, too, and where possible transfer dimensions directly from the already-built parts to the not-yet-built parts.

The same idea applies to all kinds of projects where the parts have to fit together.  Here's a really simple example.  This is a drill press table.  Minus the T-rack, it's just a piece of melamine with some solid wood trim around the edges.

Just for grins, let's say the melamine itself is 18" x 24", and the trim is 2" wide.  In theory, then, the shorter trim pieces on the ends should be 18" long, and the longer trim pieces at the front and rear should be 28" long.  But rather than try to cut out all five pieces ahead of time and then assemble them, it's much better to do it like this:
  1. Cut the melamine to size and rip all the trim stock to width.
  2. For the end trim pieces, transfer the actual width of the melamine to two pieces of the trim stock and cut them to length.
  3. Attach the end pieces of trim to the melamine.
  4. For the front and rear pieces, transfer the actual length of the resulting assembly to two more pieces of the trim stock and cut them to length.
  5. Attach the front and rear pieces to the melamine.
If your cuts are accurate, this should give good results.  But they still might not be perfect, for any of a number of reasons.  Such as:
  1. Your weren't able to cut the trim pieces to exactly the right length.
  2. One or more of your cuts was not perfectly square.
  3. When attaching the trim pieces to the melamine, you weren't able get everything lined up perfectly flush.
  4. Etc.
So what to do?  In many situations, the only answer is to try to work more accurately.  And that of course requires practice, diligence, patience, care, and all manner of similar good stuff that is not always in abundant supply.  In other situations (and the drill press table just happens to be one of them), a different approach can give better results with less skill.

Cut It Big and Trim to Fit

The trick is to cut your parts a little bit larger than you really want them, and them trim them to their exact size. What I'm getting at here is different than "sneaking up on the fit" by carefully removing small amounts of material until the fit is correct.

Here's how it would work with the drill press table. Start by cutting the melamine piece to the desired length, but a little bit wider than you want. Then attach the end trim pieces as shown at A. Make them a little bit long, and also a little bit wide. Now trim the assembly to its final width as shown by the dashed lines. The result, at B, will have the end trim pieces exactly flush with the melamine.

Now add the front and back trim pieces and trim again as shown by the dashed lines at C to get the perfect result shown at D.

As I mentioned, this trick doesn't apply always, but when it does, it will save you some time and a few headaches.  Maybe you can find ways to use it in your projects.

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