Hole making has come a long way since some unknown caveman drilled a hole in the first wheel. There are numerous methods of making holes in different types of materials. In addition to the use of the standard twist drill, there are inserted drills, core drills, spade drills, gun drills, friction drills and probably a few other types of drills. Holes can also be produced by EDM (wire, electrode and hole poppers), saw cutting, oxy-acetylene torch, plasma torch, circular interpolation, laser cutting, hole saws, treepanning, mechanical punch & die and on and on. All of these processes are in commercial use today and they all have their specific application in industry. I would like to talk about just a few of the problems associated with hole drilling with a standard twist drill bit.
Assuming that we use high grade drill bits, the majority of problems associated with these tools has to do with sharpness of the drill and proper grind geometry. The coolant/lubricant, cutting speed, feed rate and flute design of the drill must also be matched to the material. If all these guidelines and variables are met, the majority of the difficulties related to the operation of these types of tools are reduced or in some cases totally eliminated.
Size, Surface Finish and Roundness: If all of the above criteria have been followed and you are still experiencing any of these three problems, you may have to add another operation to your process.
Size: If your drill is making an oversized hole, try step drilling (some people call this drill reaming). Simply drill the hole undersize before drilling to the final size. NOTE: leaving less material for the final drill operation and using a higher feedrate on the final op. will produce a tighter hole (we will talk about "Getting Reamers to Work" in an upcoming issue).
Surface Finish: Poor surface finish is usually caused by improper chip evacuation, flimsy setup resulting in chatter or built-up edge. Once again, if you have followed the steps laid out in the second paragraph of this writing and are still experiencing problems, try peck drilling on holes that are deeper than 1.5 x diameter. To reduce chatter, try using a shorter drill with a larger shank or move the job to a heavier machine. Try this little trick for built up edge. To remove the unwanted material on a drill bit take the tool out of the machine and place the relief (ground surface of the drill behind the cutting edge) on a flat piece of steel and firmly roll the drill toward the cutting edge. Built up edge will pop right off.
Roundness: How many times have you tried to drill a hole in a piece of sheet metal with a 1" drill that has a 1/2" shank only to end up with a 5-sided hole? The problem is a weak set-up and no support behind the cutting edge. Here are some hints to help solve the problem. Put a piece of back up material beneath the sheet metal to support it. Now take a short piece of fine emery cloth and fold it over once making a square just a little larger than the drill with the abrasive on the inside and the cloth on the outside. Place the folded up square of emery cloth between the drill bit and the material, then drill thru the emery cloth and into the material. Presto - round hole. What you have done is supported the drill behind the cutting edge and effectively created zero relief. This is an over simplification of a common problem caused by lack of rigidity.
Final Note: Almost all of these problems can be solved with final operations on other equipment or with other tools if necessary such as reaming, honing, lapping, I.D. grind or burnishing.
