Issue 286 Spot drills

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

I think the article is suggesting that spotting drills are better to use for starting a twist drill than a ctr drill not as a substitute when a 60deg hole for a ctr is needed.

This is the way I use them only getting out a ctr drill when I want to put a ctr into the hole, all other times I use a spot drill. Think I have only seen 60deg ones as insert not solid.

Edited By JasonB on 30/09/2019 17:54:45

I think two thirds of the article relates to lathe work and it does then focus on hole starting in general.

I have done a quick search to find 60deg spotting drills that could be used for lathe centre work but the only ones I found listed as 60deg were actually 120degree!

What I did stumble upon though has shown me that all along I have not been following the correct practice for using spotting drills prior to using a twist drill

Link to spotting drill usage article

Ian P

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?

Maybe the clue is in their mention of carbide drills which could possibly be a bit more fragile than HSS.

I usually just make a small dimple with a 90deg spotting drill which is not really any different to a good old whack on a ctr punch. Could be more of an issue if you are spotting to near or even larger than the drill diameter.

I did find several 60deg point solid spotting drills on the net but the only UK one I found was a bit pricey starting at about £40 for a 3mm dia one. I would also think the more pointed they are the more fragile so there may be less benefit if using to avoid broken ctr drills.

Edited By JasonB on 30/09/2019 18:53:36

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

My question is: Why are centre (Slocombe)u drills that shape - a relatively long narrow (parallel?) start and a 60 degree shoulder?

The only sensible reason I can see is that of providing a lubrication reservoir for when using a dead centre. To provide clearance for the point of any centre would only need to be just a mm (or maybe the tips are longer to allow for resharpening by the user?).

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

I use a 120 degree spot drill then drill with a 118 degree twist drill, this should ensure the central chisel edge of the twist drill contacts the metal first and not the lip as if it did this would probably throw the drill off centre if one of the lips started to bite before the other. I was led to believe this is why a 120 degree spot drill was produced.

Otherwise follow what JasonB does and make a small dimple and not a spot the size of the drill diameter.

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

Ian

I’m not sure if this was intentionally engineered into the design of centre drills but having that small extended hole in the bottom of a centre drilled hole gives a place to start a very small boring bar to true up or re cut a centre .

I think the guy on the Suburban tools youtube channel explained a few reasons for this extended hole in a video he made about lapping centres .

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

Sharp angle punches are 'pricking punches' for picking up scribed lines. You can do this by feel once you have the knack. Followed by a centre punch you get a larger and wider angle hole to start a drill. Usual disclaimer of grandmothers and eggs etc.

regards Martin

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

Didn't Neil address that in the article. ? The 60 degree is so that the hole correctly matches the 60 degree centre for between centres turning and the pilot is to create clearence for the tip of said centre and to hold lubricant. In the past Slocombe drill were also used for creating starting holes for drilling but that is not their design purpose.

regards Martin

Martin,

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

MichaelG.

The extra length also allows resharpening in the vanishingly unlikely scenario of one lasting long enough to become blunt...

Neil

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.

Having read the Editors advice about using Spotting Drills rather than Centre Drills, I searched my collection of catalogues to find what companies sold Spotting Drlls - and drew a blank

So I did an internet search with similar results

Can anybody recommend a supplier of Spotting Drllls that do not require a bank loan to purchase, please?

MC Black

MC, have a look at my second post in this thread for reasonably priced, reasonable quality ones. They are often cheaper than ctr drills of the same quality from these suppliers.