46 - Utility Workbench, страница 14

TECHNIQUE

Outside Threads

cut by sliding the cutter in or out.

CUT THREADS. Once you're satisfied with the fit, clamp the "real" workpiece vertically in a vise and lower the threadbox onto the end of the dowel. You'll feel a "thunk" as the cutter contacts the end of the dowel.

Now grip the threadbox by the handles and slowly rotate it in a clockwise direction. At the same time, apply steady, even pressure downward. There's not much resistance — about as much as using a corkscrew.

If the threadbox gets harder to turn, it's probably because chips have clogged the throat opening. To clear the chips, simply back off the threadbox about a quarter of a turn.

As you continue to turn the threadbox, it will work its way down the dowel. This exposes the newly cut threads on the end of the dowel.

THREAD SHAPE. The ridges on these threads are fairly sharp, see detail 'a.' Because of this, they're a bit fragile, and they may chip with use. But that won't weaken the thread. The thick root of the thread that fits in the groove provides the strength.

UNSCREW THREADBOX. Once the dowel is threaded to the desired length, there's just one thing left to do. That's to "unscrew" the threadbox by backing it all the way off the dowel.

Once the threads are cut inside the hole, you're halfway done. Now it's time to pick up the threadbox and cut the outside threads in the dowel.

TWO PARTS. The threadbox consists of two wood blocks that house a V-shaped cutter, see drawing below. The cutter fits into a notch in a threaded post Tightening a nut on the end of the post locks the cutter in a "pocket" in the upper block.

Just a word of caution. The post is made of brass, so it's fairly soft. So to avoid bending (or breaking) it, be careful not to overtighten the nut.

METAL INSERT. The cutter is positioned right next to a metal insert in the upper block. This insert is threaded to match the threads cut by the tap. The reason is simple.

When you rotate the threadbox around the dowel, the cutter makes a groove that immediately "catches" the threads in the insert. These threads then pull the cutter around the dowel. This creates a spiral groove that fits the threads in the tapped hole.

CHAMFER & OIL. To get the first thread started (and keep the fragile edge from chipping), I sand a chamfer on the end of the dowel. Here again, applying oil softens the wood fibers

and makes it easier to cut the threads.

TEST CUT. The threadbox I used was already adjusted by the manufacturer. But it's still a good idea to cut threads in a scrap piece and check the fit of the dowel in the tapped hole.

Don't expect wood threads to fit as tightly as metal threads. (A good fit will seem a bit loose.) This prevents the threaded parts from binding when the wood expands or contracts with changes in humidity. Note: To improve the fit, adjust the depth of

Upper Block houses insert and cutter

Nut tightens on post to secure cutter

Slot for chip ejection

V-shaped cutter fits into notch in post

Lower Block guides and supports unthreaded

SHARP THREAP

DOWEL

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ShopNotes

No. 46