The older British authors seemed to be solid supporters of using punch marks to start drills. But they would start from scribed lines and then note the intersection with a "prick punch", like a centre punch but with a much steeper point angle. This was capable of very fine marks. Then to guide the drilling a circle was scribed outlining the projected hole. As the hole was started it soon became clear if it was in the correct location as the cut approached the scribed outline. A lack of concentricity could be rectified using a small cold chisel cut on the cut to draw the cut into line. (No kidding, this works to move the hole in the direction of the mark!) All of this antedates the use of DRO's to convince yourself that the hole is going where you think! Bill in Boulder
On 11/5/2015 8:05 PM, mark.kimball2@frontier.com [GrizHFMinimill] wrote:
Hi Ian,
Your comments on why to avoid using punch marks are all good. I try to avoid using that approach if at all possible, but sometimes that is not feasible (read on, and if there is, please elucidate!).
I am in particular referring to the situation where you want to transfer bolt hole locations from one piece to another. Transfer punches are typically used. In the case of blind holes, transfer screws are used.
When I do find it necessary to employ this apparently-necessary evil, after finding the center of the mark as best I can, I start the hole with a center drill so the situation of an uneven lip on the mark won't affect the outcome. I slowly lower the drill to make sure it has created a well-centered divot. This latter action is based on my observation of vibrations that arise and then diminish when the drill is allowed to turn without lowering it any more. The vibrations have a periodicity that is the same as the spindle rotation, so I believe the hole cut by the center drill is NOT immediately symmetrical....and therefore a bad start for the usual spaghetti-like twist drill.
I do have another question regarding the diameter of the center-drilled hole and the subsequent diameter of the "starter" drill bit. Should the drill bit diameter be smaller, so it completely fits inside the hole? I think the drill should be smaller so the ridge on the outer diameter (created by the center drill, no way to avoid) doesn't catch the drill flutes in some random manner and produce a bad start. But on the other hand, I'm not sure if the cutting edges on your average drill bit are exactly symmetrical. If not, that won't necessarily help either. Comments?
-Mark
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