46 - Utility Workbench, страница 13
I begin a threading project by cutting the inside threads with the tap, see photo. Later, this makes it easy to test the fit of the outside threads on the dowel and make any necessary adjustments to the threadbox.
PILOT HOLE. The first step is to drill a pilot hole for the tap. As a rule, this hole should be W smaller than the tap. (For example, drill a l3/8" hole for a lV2n tap.) This way, there's plenty of material to form the threads.
LUBRICATION. The tap will fit quite snug in the hole. So to make it easy to turn, it's best to apply a generous amount of oil. (I use linseed oil.)
CUT THREADS. After letting the oil soak in, it's time to cut the threads. The bottom end of the tap has a slight taper that helps center it in the hole, see Step 1 in drawing below.
Even so, the important thing is to make sure the tap goes straight into the hole. (It will still cut if it goes in at an angle, but the dowel that threads into the hole will wobble.)
To accomplish this, twist the tap slowly and evenly in a clockwise direction, applying a small amount of downward pressure, see Step 2. At first, the tap should turn freely, as if you're opening a can with a can opener. But
after a couple of turns, you'll start to feel resistance. At that point, back the tap out about a quarter of a turn to clear the chips, see Step 3.
Now simply repeat this process until the tap cuts all the way through the hole, see Step 4.
The secret to ending up with crisp, clean threads is selecting the right dowels.
TYPE OF WOOD. For starters, stick with close-grained hardwoods like maple, cherry, or birch. (I've also had good results with walnut.) Open-grained woods (like oak) have a frustrating tendency to chip out
STRAIGHT. You'll also want to check that the dowel is straight. It's difficult to cut consistent threads if it's bowed. Also, make sure it's free of knots or defects.
SHAPE. The shape of the dowel is also important. As the wood dries, the dowel may go out of round. (You'll know by the egg-shaped ends.) These dowels feed
crookedly through the threadbox which causes the threads to chip.
SIZE. One final consideration is the size (diameter) of the dowel. Many dowels are slightly smaller or larger than their stated size. So you may want to take the threadbox with you to the lumberyard and check the fit.
What you're looking for is a smooth, sliding fit. If it's too tight, the dowel will bind as you cut the threads. (Some dowels won't even fit in the threadbox.) If the dowel is too loose, you'll end up with slop in the threads. (For sources of "true" dowels, see page 31.)
A There's more to selecting a dowel than simply choosing the type of wood. Especially if you plan on cutting threads in the dowel.
TURN TAP CLOCKWISE TWO OR THREE TURNS
DRILL HOLE Ve" SMALLER THAN TAP SIZE
REPEAT STEPS 2 & 3 UNTIL TAP CUTS THROUGH HOLE
INSERT TAP AND CHECK FOR SQUARE