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Re: Re: Re: Re: Re: Re: Re: Drill Bits
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Posted by Fred OH on July 24, 2000 at 08:20:04 from (208.32.116.49):
In Reply to: Re: Re: Re: Re: Re: Re: Drill Bits posted by F14 on July 23, 2000 at 11:46:46:
Hi F14 from Fred,
I guess as a life long machinist, I can fill in some of the blanks I've seen so you'll be well informed. The term "bits" refers to the wood shop. The term "twist drills" refers to metal cutting. My smart a**ed instructor in Navy Machinist School nailed me on that one! The speed of a grinding wheel is around 5000 surface feet per minute. If you laid the wheel down and rolled it, it would roll 5000 feet in a minute. Almost a mile a minute or around 55 MPH.Some bench grinders deviate from this a little but it is good practice. The faster a wheel runs, the harder it acts and hotter. The white wheels refered to (and gray) are Aluminum Oxide and are used to grind steel. Green wheels or Silicon Carbide are used to grind tungsten carbide and non ferrous metals. 36 grit for roughing and 60 grit for finishing on steel is the general accepted practice. One item not mentioned above is to always keep the cutting edge horizontal when grinding twist drills and never grind toward the cutting edge, always away from it. Also never quench the drill in water, but we all do. Just don't stay on it long enough for it to turn purple, thats too hot and will take some of the heat treat out of the twist drill. Also never purchase 135 degree split point twist drills, their too hard to sharpen by hand and you have to thin the web every time. (They're for production and tough materials). That little chisel point has to push its way through the metal (no clearance on it). Never buy cobalt twist drills if you're gonna use them in a drill motor (electric drill), they're too brittle. Look for HSS (High Speed Steel) on them. Generally thats M2 steel and has been the industry standard for years. High Carbon steel twist drills are good only for wood, plastics, and aluminum. Slowed down they will drill mild steel and with coolant will drill tougher material. You don't need the titanium dioxide coating, thats for production cutting. The word from the machinists that have tried the China twist drills is they are not quite as good as the American brands, just need sharpened a little sooner. (More reason for some of us to learn to sharpen them).I've used them and they are fine for maintenance work. The heel clearance that was refered to can run 8 to 12 degrees (thats the area behind the cutting lip) It takes practice to get it right. The standard angle on general purpose twist drills is 118 degrees. Although the harder the material, the less the angle. Ever drill through something soft only to have the drill grab at the other side and maybe even break? Try grinding or stoning a small flat on the face of the cutting edge to eliminate the rake angle. Won't grab then, but requires more pressure to cut and more heat build up (but does the job). Don't have a gage to sharpen with? Hold it up (toward daylight) and cup your hand around it and rotate it. You can tell if the flutes are equal length that way. Need coolant? Diesel or soapy water for aluminum, motor oil for steel is better than nothing. (even used) but harder to wash out of your clothes. All mechanics should learn to sharpen twist drills. Once you learn the basics, you can sharpen a 1/16" or a 3" if need be. My motto: "You got to get smarter than the metal". This is a good place to do that. L8R----Fred
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