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| Drilling! - small holes on the mill and centre drills |
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| Lew_Merrick_PE:
--- Quote from: John Stevenson on July 18, 2010, 06:03:41 AM ---Centre drills are a throw back to the days of using dead centres in lathes. They have a small pip at the front to aid starting and breaking off in a part that already has 10 hours thrown at it and a 60 degree cone for the centre to be supported. The pip's main purpose was to start the drill, provide clearance for the point of the centre and provide a reservoir for white lead as a lubricant. Unless you are drilling a support hole for a centre forget the centre drills, times have moved on but unfortunately no one has updated any of the books, everything is repeated ad nausuem. Industry today uses spotting drill which are a very stiff, short fluted drill made for accurately spotting holes. Stub drills do virtually the same thing at a quarter of the cost. --- End quote --- 1) Center drills are about 3X as stiff as a spotting drill because less material is removed for the flutes. 2) Here in the U.S. the "cost equation" for center drills versus spotting drills is reversed. A good quality spotting drill costs slightly more than double the cost of a good quality center drill. The main thing is to provide a very accurately placed "start" that places the initial cutting of a drill bit near to the lip (where the grinding is likely to be more accurate) and prevent the center chisel-edge from being used until at least 1 diameter of the drill lip is bearing on the circumference of the hole. |
| John Stevenson:
--- Quote from: Lew_Merrick_PE on July 18, 2010, 12:38:59 PM --- 1) Center drills are about 3X as stiff as a spotting drill because less material is removed for the flutes. --- End quote --- So why don't you see CNC machining centres using them? Less material removed isn't correct, if you take to pip that always breaks off then it's very weak in comparison to the main body. a spotting drill is the larger diameter all the way. --- Quote --- 2) Here in the U.S. the "cost equation" for center drills versus spotting drills is reversed. A good quality spotting drill costs slightly more than double the cost of a good quality center drill. --- End quote --- Same here but now compare a stub drill to a spotting drill, hardly any difference and a fraction of the price of both. This was yesterday afternoons work. Believe it or not there is a 10" chuck under that lot and that is all done from drilling 8,500 holes in a series of 6mm thick steel plate with a 1/8" stub drill, nor pilot or centre drill first. I wouldn't offer advise unless i know it works. John S. |
| Rob.Wilson:
--- Quote from: Lew_Merrick_PE on July 18, 2010, 12:38:59 PM --- 1) Center drills are about 3X as stiff as a spotting drill because less material is removed for the flutes. 2) Here in the U.S. the "cost equation" for center drills versus spotting drills is reversed. A good quality spotting drill costs slightly more than double the cost of a good quality center drill. --- End quote --- I am with John here ,,,,,,,,,, spot drills are stiffer ,,,,,,,,,,,,,,,allot stiffer Yes they may be a tad more expensive,,,,,,,,,,,but they can be re-sharpened :med: ,, ever tried re-sharpening a centre drill ? Rob :D |
| JimM:
John / Rob can you recommend anywhere that sells (small) spotting drills at a sensible price ? Cheers Jim |
| raynerd:
John Stevenson, thanks for the reply, I`m now much clearer. Chris |
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