3 rules for edge-joining with pipe clamps
Nothing can make ill-fitting joints close up tight. Before glueing, dry-fit all the joints. If you see "air," re-machine the edges.
Don't let the stock wander from your clamps' line of pull. That's the imaginary line running through the clamp's drive handle.
Keep the line of pull parallel to the pipe. It seems simple, but there's one pitfall -- using the wrong size of clamps. The largest clamps, called 3/4" (the diameter of pipe they fit), have longer jaws. Use these clamps on stock 3/4" to 1" thick, the smaller on thinner stock. Otherwise, you'll have overpull or underpull, as shown below.
To use pipe clamps larger than the wood thickness insert spacers, as shown below. This brings the center of the wood up to the line of pull.
Another technique is to "roll over" the larger clamps on the edge of the wood to compensate for their size, as shown below left.
To use smaller clamps on thick stock "double-clamp," as shown below right. Place one of the smaller clamps so that its line of pull centers in the stock just as a larger clamp would.
Alternate clamps from side to side first on top, then bottom, working from the center toward both ends to even up the pressure.
Overtightening clamps will either bow or cup the workpiece. Or, the pipe itself will bend slightly.
Illustrations: Jim Stevenson
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