Spotting Drills vs centre drills
In MEW 212 Chris Smith from Taunton chastises Harold Hall for using centre drills for starting a hole and recomends using a spotting drill or stub drill instead.
This is something that I also do so I thought I’d investigate a little further, starting with a quick explaination of terminology, what are these different bits?
A stub drill is a short drill where the flutes only run for a short distance along it’s length. For this reason it’s stiffer than a regular twist drill or “jobber”. The stub drill can’t be used for deep drilling. It has a 60° tip angle which is the same as a regular twist drill.
The spotting drill takes this to an extreme and only has flutes for one turn. This is used for creating a small spot which can then be used to start the hole drilling with a twist drill. The hole angle of 90° that is larger than the twist drill’s angle so that the tip does not come into contact until it reaches the bottom of the spotted hole. I don’t have any of these in my workshop but I do have a pilot drill in my holesaw which is very similar is appearance to the spotting bit.
Note: “Spotting through” is the process of using one piece of work to line up the holes in a mating component. “Spotting through” is useful when you don’t know the position of the holes and hence can’t mark them out on the mating part. A spotting drill would probably be the best bit for this job too.
A centre drill is there to produce a hole in the workpiece for the purposes of supporting work in a lathe or turning between centres. It has a thinner pilot hole followed by the 60° countersink.
Although not a drill it is also worth mentioning the centre punch which is used with a hammer to mark the place on the metal to locate the hole.
The next question is why don’t we just start drilling with a drill bit? The reason for this is that the drills can be flexible and any inaccurisies in the material or drill will cause it to wander and effectively drill the hole in a different place to intended. This effect is worse if your drill is blunt or has been incorrectly sharpened.
The three stiffer drills mentioned above have less flex and hence you are more likely to have a good start. Once you’ve produced a spot following on with a twist bit should produce a good result.
Looking online the centre drills are very common and there seem to be a few suppliers for stub drills but very few for spotting drill bits. I think this might explain why people use the centre drill instead. The disadvantage of using this is that the angle of the centre drill is the same as for the twist drill and hence it is possible that there may be more contact which in turn could lead to wandering. However if you pair you centre size correctly with your twist bit this can be lessened. The other disadvantage of the centre is that the tiny pilot is brittle and can be snapped of. The way to avoid this is to ensure that the work is correctly clamped and only use it in a pillar drill or lathe. Using a centre drill in a hand held drill is a way to quickly break it (I know this from experience).
So Chris is right in that there is a right tool for the job and the spotting drill is the one for this job. However given their hard to find nature of the spotting drill I think that I’ll likely stick with my centre drills and centre punch. I am however tempted to make some stub drills by grinding down some of my longer drills. This could be particularly useful for the smaller sizes which do have a tendency to flex and often I don’t need to drill deeper holes. However smaller bits can be challenging to sharpen.
See also CncCookBook – When to use a spot drill
Cromwell Industrial is the only supplier I’ve found that sells spotting drills, these are about £10 although I’ve never used them as supplier.
Can i use a spotting drill bit in a hand held drill? Say I am drilling on a large item (ie. boat, lawnmower) where a drill press is not feasible.
I am thinking the steps should be: prick punch, center punch, spotting drill in a drill guide, then a twist bit. Overkill?
I’m always trying to learn best practices.
Hi Jason, a spotting drill would be better than a centre drill in that case as the small tip would snap easily. There’s a couple of reasons to go through a sequence, 1) the position is very important or 2) the size is very important. So if I was drilling a 10mm hole in a lawn mower and was not bothered about the precision then I’d probably use, centre punch (to stop skidding), 4mm, 8mm, 10mm. It’s best not to jump straight to the large size as it puts a lot of load on the drill and hence makes wandering more likely.
HI, nice post. One question please, not quite relevant. How you sharpen your drill bits? do you like to use drill doctor or use other ways?
Hi Erik, thanks for the question. I don’t have a drill doctor I sharpen some using a jig and the bench grinder but I’ve not have much success.
Standard jobber drills are 118°. I thought spotting drills were 120° included angle so the drill tip centres. A 90° spotting drill would start the jobber on a larger diameter, where if one cutting lip is longer, the centreline would push over leading to wandering.
I haven’t found a cheap source of spotting drills either.
Hi William, both 90° and 120° spotting drills seem common. You are correct in thinking that the 120 would cause the tip of the jobber to cut first. If that gives a better result depends on the quality of your drill bits. I did a test of drill bits a few years back. Since writing this, I have seen spotting drills that are just a few £ and those should last a long time so are probably a good investment.