Spotting Drill or Centre drill.

I made a small turned part some years back and the hole ended up off centre when I used a centre drill to start the hole. I switched to using a spotting drill on the replacement part and I’ve used them ever since. I wouldn’t be without one or two now although I still use centre drills when needed. I’ve settled on using an 8mm on most jobs for some reason. Unlike centre drills you can’t snap the tip off so I’ve not needed to replace any yet.

Centre drills in the lathe, spotting drills on the mill if required (sometimes spot a layout to finish on the mill drill or drill press.

My centre drills have always self centred perfectly on the lathe, I can't imagine a way to stuff that up

Assuming there's no 'pip' left on the piece in the chuck and you are using a centre drill which either doesn't centre correctly or the tip breaks, there are several causes. One is a worn tailstock barrel. Another, peculiar to Myfords (only?), is that the hole in the back of the tailstock barrel into which that small, round dowel with its key fits, has become distorted. This can cause the barrel to "twitch" when you start drilling. Geo Thomas published a remedy in the M.E. some years ago. [I think it's also included in his Model Engineers Workshop Manual Vol 1].
When done on my original & well-worn Super 7, it cured the breaking of centre drill points.

If you suffer from the problem of the loose / wobbly tailstock, centre drills are a risky proposition, so a spotting drill would be a good solution. The reason there is a small (brittle) drill on a centre drill is to give clearance for the point of the centre. One simple solution would be to use a spotting drill and grind off the very end of the centre. Pragmatic.

Murray

Both. Whichever I get my hands on first.

Hi Everyone,

Looks like I got my sums wrong regarding spotting drills/stub drills. I,ve just been looking them up and they are a different beast altogether,so well they might be,when you see the price of them !

No doubt it depends on your level of expertise,but I suspect that not many could justify the cost of them. This might explain why I have never seen them offered by any of the usual suppliers we use,I dont think they would exactly fly off the shelves.

I never heard of them until now [like a few others ],never saw one even,so I think I,ll stick to my center drills,for now.

One thing I love about this Forum is the amount of information to be had,to say nothing of the advice,and all for free !

Keep it up,and I,ll check before I make another blunder.

By the time the average beginner has snapped the pilot off a couple of ctr drills they work out no more expensive and as I said earlier £2.50 is not that expensive. Also depends on what you are paying for ctr drills, a BS0  (1/8" dia shank) Dormer has a list price of £20!

They will also save you having to buy a small dia CSK bit as they will do that task too

Edited By JasonB on 10/02/2018 09:35:48

I gave up using centre drills to start drill holes 20 years ago. Spotting drills are they way to go and once purchased are cheap because they last for years and are easily regrindable. Look out for offers from MSC but avoid the solid carbide ones they chip easily.

Well I’m pleasantly surprised. I had thought I may be in the minority but it seems the majority of you use spotting drills.

I now wonder how many of you use it’s bigger brother, the Drill Mill? I only have one in carbide.

Is the spotting drill something that was developed for CNC machining that has found another use in general manual machining? There seems to have been strides made in cutter technology possibly driven by CNC, but as NC and CNC have been around for 65 years or more it may be coincidence that cutter technology and CNC have walked together.

Mike

Spotting drills and chamfer mills seem to be almost the same thing. As far as I can tell, the main difference is whether or not they have a sharp end ie can drill directly into stock.

Not sure what a "drill mill" is?

Murray

Hi all, spotting drills are indeed used comprehensively in industry especially in CNC machining, a centre drill is for centring a component that is to be held between centres or to be supported by a revolving lathe centre, it is not however the preferred weapon for starting a twist drill. On a CNC mill or machining centre when all hell is being unleashed particularly when drilling large matrixes of holes it is the easiest thing in the world to blink & miss the centre drill breaking on the 2nd hole, unusually it would be your backside that the bootprint of blame ends up on, in these circumstances a spotting drill solves the problem with the added bonus of allowing the chamfering of said holes before they are drilled, bargain!

Now I realise we ain't all got CNC machines in our private caves (apart from the lucky few) however the same logic applies, whether you use manual, what used to be called "automatic", or full blown computer controlled lathes the same logic applies, for many years I campaigned two 2" capacity bar machines producing 960 various components the large majority of which had holes drilled with HSS twist drills, once I had settled in I invariably spot drilled to chamfer size then drilled to size, in 14 years I never had a spot drill break, I rest my case.

Bob H

I suppose it must be easier to re-sharpen a spotting drill than a centre drill.

I use stub drills in the lathe and mill and have no problems.

Regards.

David.

I seem to remember that JS advocated shortening the fragile tip of a centre drill to preempt it breaking. It serves no purpose when long apart from allowing the possibility of a regrind. From Bob's insight it would seem the spot drill is the cutter of choice for good reasons. If the spot drill was not an option when the text books were written the the centre drill being short and ridged would be best thing to start a drill even though it was probably designed to make centres and not start drills, it just found another use. A carbide spot drill has all the desirable features to provide a start for a drill with the bonus of finishing the hole entry as Bob illustrates.

Mike

This is a Drill Mill.

**LINK**

Handy for cutting small V grooves without resorting to rotating the work or the head of your mill.

I think it gives clearance for the point, for when you actually want to use it for accommodating a lathe dead centre.

Michael W

And possibly a little reservoir for lubricant for the old fixed centres.

It is usually much longer than needed just for point clearance so can stand a bit of shortening, a fair point Hopper about lube reservoir I do remember ramming as much tallow as I could get in the hole.

Mike

When I started my toolmaking apprenticeship one of the first thing taught was shortening a centre drill point.