46 - Utility Workbench, страница 15
Routing Outside Threads
If you're making a lot of threaded parts, you can cut the outside threads quickly and accurately "with a router and a simple jig, see photo. Note: We used a jig manufactured by the Beall Tool Company, see Sources on page 31.
TAR Here again, the jig comes with a tap used to cut inside threads, see inset The tapered "nose" of this tap is removable, so you can cut threads to the bottom of a stopped hole. (You'll need to buy a separate "bottoming" tap to do this with the threadbox kits.)
SETUP. After tapping the hole, setting up the jig only takes a few minutes. The base of the router is attached to a metal mounting plate that's screwed to a hardwood support block, see drawing below.
This support block holds a plastic insert that's threaded on the inside to guide the dowel. As the dowel is twisted through the insert, a special V-groove bit cuts the grooves that form the threads, see margin.
As before, the grooves "follow" the threads in the insert. But here, the first thread is centered on a hole in the top of the insert. So to ensure accurate results, the tip of the bit must be centered on this hole.
A A solid carbide V-groove bit used to cut the threads won't need sharpening nearly as often as the cutter in the threadbox.
A To rout the outside threads in a dowel, twist it through the plastic insert in this jig like you're making sausage. A hand-held tap cuts the inside threads, see inset photo.
SLEEVE. The solution is a metal sleeve that fits into the hole in the insert, see detail V To center the bit, you just mount it in the router and lower the bit into the sleeve.
MOUNT ROUTER Before removing the sleeve, secure the router to the mounting plate with a pair of
brackets. Be sure the brackets are tight. You don't want the router to shift. Also, don't forget to remove the sleeve. (You'll have to lift the motor housing out of the base to do this.)
DEPTH OF CUT. After reinstalling the motor housing, lower the bit until the tip sticks down into the insert about V32". This is a good starting point But you'll want to cut some test threads to make sure they fit.
ROUT THREADS. To do this, flip the router on and stick the end of the dowel into the insert until it "bottoms out." Now slowly rotate the dowel in a clockwise direction.
It's helpful to hold the dowel close to the jig. If you hold it at the end, there's too much leverage. And even a small amount of movement up or down will tip the opposite end into the bit and chip the threads.
THREAD SHAPE. After threading part of the dowel, take it out and look at the threads. They're fairly flat on top, see detail 'a.' So these threads won't be as likely to chip as the sharp threads cut by the threadbox. The goal is the widest "flat" possible that turns smoothly through the tapped hole. If you need to "fine tune" the fit, just adjust the depth of cut.
Bracket secures router to mounting plate
Mounting Plate anchors^ router to support
Support clamps to bench and houses insert ___
Insert - is threaded to guide dowel