Don't Trust Your Tape Measure

Every once in a while on the woodworking forums there's a discussion about the problem of laying out and cutting parts accurately, and these talks typically tally time-tested tips like these:

1. Be careful!
2. Be sure to cut on the waste side of the line.
3. Use a knife instead of a pencil for marking.
4. Set up stop blocks for making repeated cuts.
5. Use relative dimensioning where possible.
6. Don't trust that sliding hook on the end of your tape measure.
7. And so on.

Here's one more tip:  For really accurate work, don't trust your tape measure at all.  Period.

"Really?" you say.  Well, yes.  As it turns out, the machine that makes tape measures uses a rubber belt to print the markings on the tape.  This belt can stretch slightly during the process, and when it does, the marks it puts on the tape will be misplaced by however much the belt stretched.  This means that while a particular tape may be perfectly accurate at, say, the 12-inch mark and the 24-inch mark, it might be off by a noticeable amount halfway in between.

Whether this potential inaccuracy actually matters or not depends on what you're doing.  In any case, though, it's a good thing to be aware of and something to think about if you're having trouble.  For much more detailed information, click here.

A Poor Man's Microadjuster

For repeated crosscuts using a miter saw or a crosscut sled or a radial arm saw, nothing beats a stop of some sort for consistent results.  Same thing for some operations on a drill press with a fence. Problem is, setting the stop can be kind of tedious sometimes.  You make a test cut, bump the stop a smidge, make another test cut, bump the stop half a smidge the other way, and so on.

For the price of a drywall screw, though, you can set up a poor man's microadjuster that will eliminate most of the fussing.  Just drive the screw into your stop and use it to register the workpiece instead of the stop itself.  You can then make fine adjustments to your setup simply by turning the screw. A secondary benefit is that sawdust won't get trapped between the workpiece and the stop like it might if the screw wasn't there.

Drywall screws work especially well for this, for a couple of reasons.  First, they're not tapered, which means that they won't become loose in the hole when you back them partway out.  Second, some of them have exactly eight threads per inch.  That means that one turn will conveniently move the screw 1/8", a half turn will move it 1/16", and so on.

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.

A New Road to Flatness

Sooner or later, after tearing themselves away from the internet and maybe making an end grain cutting board or two, most woodworkers wind up building a workbench. There are a zillion variations on the theme, but one thing that every workbench needs is a top that's flat and likely to stay that way.

The Old Ways

To make flat work surfaces, the traditionalists glue up mondo slabs of wood, grab their hand planes and go to town. Other folks glue up the same ginormous slabs, then flatten them with routers and sleds. Both of these methods involve quite a bit of work, and the router method in particular makes a mess and a half. Worse, because wood is wood, it will move over time and it's a pretty good bet that a thick wooden top that's flat today won't be so in a year or two or five.

A completely different approach is to abandon the thick, heavy slab, and build a torsion box instead. The result is a stiff, lightweight structure that will remain stable over a long period of time. That stability is great if the torsion box comes out flat to begin with. But if something goes wrong during the build and it ends up with a bend or a twist, about the only fix is to start over from the beginning.

A New Way

"If you can't make it perfect, make it adjustable." I don't know who first said that, but it works for a lot of things, including yet another way to make a flat work surface.

Suppose you have a square piece of 3/4" MDF, say one foot on a side. It's going to be flat, and it's going to be stiff and stable enough to remain flat over that small area. But a panel of the same material as big as a workbench top will sag under its own weight, and will not be acceptably flat over its entire area unless it is supported by, um, something flat?

Sounds like a Catch-22. It is, almost, but not quite. Here's the trick. If the panel can sit on a series of regularly-spaced supports, and if those supports can move up and down, then it's possible to adjust the elevation of the panel by a small amount at each support position. That in turn makes it possible to tweak the panel until it's almost perfectly flat. I've done this with both a workbench top and a router table, with very good results.

The Support Mechanism

To make the adjustments easy, I used two 1/4-20 bolts at each position, as shown in the figure. Both bolts are threaded into T-nuts that are embedded into the rigid frame of the supporting cabinet. The bolt on the left is the one that actually supports the top. It bears against a metal disk (a penny, actually) embedded in the top so that the bolt doesn't dig into the MDF. The bolt on the right simply holds the top down and in position against the first bolt.

Both my workbench and router table have pairs of bolts like this arranged in a grid with the pairs spaced about twelve inches apart. Here's what one of the pairs looks like in real life, viewed from the bottom:

Adjusting the Supports

It's easiest to adjust the bolts in two stages. Start by installing all of the bolts that come up through the frame to support the top. Then select a bolt somewhere in the middle as a reference, and use a carpenter's level to adjust all of the other bolts so that they are at the same height as the reference. If you have a good level and do this carefully, the tips of all the bolts will define a surface that is not only flat, but also level.

Next, set the top on the support bolts and install the hold-down bolts. Tighten them until they are snug, but not really tight. Then, using a straightedge for reference, adjust the bolts so that there are no humps or dips in the surface. To raise the surface, loosen a hold-down bolt slightly and then snug up the corresponding support bolt. To lower the surface, loosen the support bolt first, then snug up the hold-down bolt.

This sounds easy enough, but it can be frustrating if done in a haphazard fashion. A methodical approach works much better. Here's one procedure that works well. Assume that your bolt pairs are arranged in three rows of three, as shown in the picture.

Start by placing a straightedge along the diagonal from corner A to corner I. Then adjust the bolts at A, E, and I so that there are no gaps between the straightedge and the top surface. Next, place the straightedge along the diagonal from corner G to corner C. Now, without changing the bolts at E, adjust the bolts at G and/or C so that there are no gaps. Finally, without changing any of the previously set bolts, adjust the bolts at B so there are no gaps when the straightedge is placed along the A-B-C edge, adjust the bolts at D so there are no gaps when the straightedge is placed along the A-D-G edge, and so on for the bolts at and F and H.

The procedure for other arrangements of bolts is similar: first set the diagonals, then without changing any of the previously-set bolts, adjust all the bolts in the intermediate positions between the bolts that have been set.