Posted By Engineercub on 02/25/2009 6:16 PM
Does anyone have alot of experience with tapping as I have never done it? Does it involve filling an existing screw hole with a material, letting it harden, and drilling a new sized hole? Any tips, hints, or ideas? Thanks!
-Will
Will,
Tapping takes a measure of easily-learned skills and some few aids: 1) Use a lubricant. I like Kroil, an all-purpose penetrant. W/O going into a lot of detail, cutting threads involves heat,though you'd never guess it, and the lube tends to carry that away from the tap's threads, along with the chips, making a cleaner cut and causing your tap to last longer.
Forget chucking into a drill press--or lathe even, unless you know what you're doing and it's necessary. That's an excellen way to frustrate yourself and break taps. Instead, get a tap wrench--I much prefer the T-handle, collet- type to the 'long-armed' kind that are seductively cheap. The reason is, with a fine tap like we use in this hobby, it's ever so easy to get too much side force on the ends of those handles and break a tap. Point of fact, really cool machinists make a set of knurled 'knobs' that fit each tap. (WTH: it pays the same, and gives exquisite control with fine taps).
You must have a tap drill chart: that tells you for X size tap AND thread, you need to drill Y size tap hole, using what's called, oh my gosh, a TAP Drill. Not a special drill, just the drill sized for cutting threads for that particular tap.
DO NOT USE CHEAP TAPS OR DRILLS MADE IN CHINA. Better to go shoot yourself in the foot and be done with it.
For very precise hole location, use a CENTER drill. These come in sets of common sizes, and are very stiff and short. They're used to precisely 'start' a LOCATED hole. Then you switch to the tap drill.
Last, get a TAP GUIDE, which is a circular piece of steel with various tap clearance holes in it: you put the tap in the T wrench, squirt it with Kroil, stick it in the guide hole, feel for that tap hole in your workpiece, and start in.
GO ABOUT 1/4 of a turn. Then back off. Go perhaps another 1/4 of a turn, being sure you 'feel' the tap engage the first try. Regrettably, there's no way to tell you when too much torque is too much. After you break a few taps, you'll know. I broke my share getting up to speed.
ALWAYS proceed about 1/4 turn (or so) and then BACK OFF. This clears the tap threads of chips. You don't have to back all the way out of the hole, but you'll FEEL the chips clear. Eventually, you'll 'clear through', you back your tap out and BLOW (as in compressed air, or failing, a stiff acid brush with Kroil or whatever, you flush out your brand newly threaded hole. Of which you can be justifiably proud.
Brass is 'sticky'. It likes to 'grab' a drill or tap and 'tear' (phonetically: tare) as in rip chunks of metal, rather than cutting clean. Red brass is better than soft yellow brass, bearing brass is pretty okay, the rest you'll learn as you go.
How do I 'learn as I go?" Ever so glad you asked. Your best friend in metalworking is a common 6" mill ******* file, fine cut. You start stroking every piece of metal that comes your way, and you'll learn a repertory of the hardness of metals. Of course you'll get fooled now and then, especially on steel. For those, go to your grinder and hit it with the metal. Watch the color and spray of the sparks. That's all I'm gonna tell you, except after awhile you'll learn 'the feel' of the metal you have in your hands by how fast it cuts--or doesn't--and the color and type of spray of the sparks.
DO NOT GRIND BRASS OR ALUMINUM ON A STONE WHEEL. People who know better will curse and hurt you, and make you clean the wheel. Which I happen to feel is justice. To grind brass or aluminum, use a belt sander.
REMEMBER: Some patience and observation will teach you far better than any ol' guy like me.
Les
Retired Tool & Die Maker