Moved thread

Rather than start a new thread but since this one is morphing away from Andrew's specific article I thought I would make some comments on the current MEW 'Editors Bench' in praise of spotting drills.

I am a fan of spotting drills but I have not yet found (but I'm sure they must exist?) 60 degree versions that can be used (with a truncated centre or drilled clearance) with standard 60 degree centres. Neil mentions 90 degree ones which are fine for spotting but as the article starts with discussing centre drills it give the impression that they could be used to put centre holes in for lathe work.

Ian P

Edited By JasonB on 30/09/2019 17:56:35

Interesting article Ian. However, I'm struggling slightly to see why the small angle spot drill is bad because it must have a similar effect to a centre drill or pilot hole in that the first contact will not be the centre of the drill?

Fragile to the point of not making it worthwhile to use one (might be a pun there)

My thinking used to be that a conical spot-drilled hole would be good so the drill (smaller diameter than that hole) must start concentrically within it, now I am not so sure.

If the following drill does not have symmetrical cutting edges then the drill will (bend) be pushed over sideways (even if the job has not moved off the same location), and as the drill moves downwards to the point where the tip make full contact and actually starts drilling it may then start off centre. Obviously can only happen with a badly ground drill but whilst that same drill might drill oversize in a centrepopped or shallow angle spotdrilled recess, at least it will stay on the same centre.

Ian P

Partly to provide a lubrication reservoir, but almost all because it's impossible to grind a drill with a pointed end, even a spotting drill has a very small flat on the end.

Of course, one could dub the end of the centre so it wouldn't bottom on a non-pointer hole.

The big advantage gained with spotting drills is that if the face is not completely normal to the drill or has any irregularity, a jobber or longer drill can walk around before cutting and then tart off centre. The spotting drill ensures that it won't do that.

Edited By Mark Rand on 30/09/2019 21:37:58

If you read the article carefully, you'll realise that Neil just wants a free drink!

Ian

Is the theory of larger angles for spotting drills matched by something similar for centre punches? I have bought centre punches in the past that have been supplied with quite a blunt point ground on them. It may be that they are intended for drill positioning purposes where sharply pointed ones are not or that they are for larger drills with a wider chisel point.

Martin C

I can answer that one. It’s so the tip breaks off the smaller sizes so you need to buy new ones!

Martin,

I think ndiy’s query was based on his perception that the pilot portion appears unnecessarily long for its intended purpose.

MichaelG.

A spotting drill with a 60 degree angle would not be suitable for use with centres as you could not be sure that the tip of the centre was not touching before the flanks. A standard centre drill ensures the flanks support the work rather than the tip.

I only have 90 and 120 degree spotting drills, but would like 60 degree ones simply because it is easier to see where the point is before drilling. The 120 degree one masks the marking on the workpiece, and would work best if used after the initial spotting had been done.

Ideal candidates for four-facet point, I would have thought.

Neil

Tracy Tools have got spot drills in stock, but not on their website. Give them a call (and tell them it was via MEW!) or there's a link here

Neil

Thanks to Pete Duckett for letting me know.