Centre Drill Leaves a “Pip” - Sometimes

All, this evening I had to centre drill for some 1.4mm dia. holes in flat steel. I used the SX2P Mill, and a small centre drill. When subsequently drilling with the tapping size, I noticed the drill was moving in a circle around the centre hole. Closer examination revealed a small pip or dome within the centre hole. The centre holes were very shallow - just used the straight part, not the angled part of them.

I changed the centre drill thinking it was faulty, and the next two holes were fine. The one after that had a dome again.

So what causes it? The spindle doesn’t seem have any play, the check seems fine, and the drills worked ok in the lathe ever since I got them. Don’t ever remember having this problem before.

The centre drill on the right is my go to for when i need to drill a hole. The left one for drilling for a centre on the lathe.

Steve.

Either the spindle or the chuck is allowing the centre drill to move in a circle rather than rotate around it's axis.

However, I never use centre drills as a precursor to drilling holes on the vertical mill. As Steve says I only use centre drills for drilling centre holes in work for use on the lathe or cylindrical grinder. For drilling on the vertical mill i use 4-facet drills so there is usually no need, and no point (pun intended), to use a centre drill first. For accurate placement (better than a thou or two) i will spot drill before using the normal twist drill.

Andrew

I've taken to using 6mm spot drills when I'm planning on using a twist drill, rather than a lathe centre.
They are a bit less likely to break as well.

Bill

If you take a close look at a lathe centre drill the cutting face is too broad to be used for spot drilling.

Steve.

As Steve says the size of your ctr drill can leave a larger "flat" than the end of the drill you follow up with. For a 1.4mm drill you would want a ctr with a 1.0mm or 1.2mm end. (what size did you use) These are quite delicate and like Andrew and Bill I also use spot drills which if you get good ones have a thinned web so the cone they produce has a very small "flat" at the bottom. If it was an actual pip and not the flat then something is not right with chuck, spindle or a bent/blunt drill.

For something like that I would have spotted with a 90deg spot drill to leave approx 2mm dia mark as that saves having to deburr the hole and in softer material reduces any bulging around the hole. Then drilled and tapped. If I had a 1.4mm split point stub drill then on that job I would have gone straight in with that but as I don't use BA much these days don't have that tapping size in stub length.

Edited By JasonB on 23/07/2021 07:15:00

I do wish the bad idea of using centre drills for spotting holes would die. It's taking far too long...

Too many YouTube videos of people using centre drills as spotting drills. That's why people think it is the right tool for the job. There was a recent thread on the merits of using a spotting drill instead of a centre drill.

Martin C

Edited By Martin Connelly on 23/07/2021 07:44:30

...and before YouTube, there was Model Engineer, other mags, books and teachers of school kids and apprentices, all advocating the practice... It's become a tradition.

.

Well-spotted !!

[yes, the pun is intentional] it’s all the fault of the English language

Mark a centre-line, then use a centre-punch, and then …

MichaelG.

What no dot punch first

At least now one of a regular suppliers is stocking spot drills and stub length drills though I'd have liked to have seen a smaller spot drill on the list.

Edited By JasonB on 23/07/2021 08:10:24

Hi, I don't have any spotting drills and have always used a centre drill and have not had any problems, however I do have a centre drill / chuck combination that can't be used, as the centre drill is too long for the chuck jaws to grip it without the back end of one of the jaws going into the spiral of the other end of the centre drill, which pushes the using centre, slightly off centre. I first notice this when using it once, when I couldn't understand why I got a wobble on the chuck when starting to make a centre, my first thoughts were a bit of swarf in the taper bore, but not matter what I did, still got a wobble. Using a different centre drill or a different chuck for the offending centre drill, eliminated the wobble.

Regards Nick.

Nick you could always grind the end off the long centre drill

Roy

Hi Roy, I could, but it's OK in other chucks and I do have smaller ones that are OK for the said chuck.

Regards Nick.

It's worked OK for me for 45+ years, and I'm not going to maintain stock of yet another tool type because of a bee in somebody else's bonnet.

Centre drills cut on the periphery as well as the lip to give good rotational CL concentricity, and have the additional advantage of starting the hole with a very small pilot dimple, allowing good check of position under magnification.

In the OP's case, it seems to me likely that these pips (and I've never seen any such) have more to do with spindle play, tool eccentricity or workpiece mounting than the type of tool itself. I wouldn't think that switching to spot drills would have much chance of fixing it.

The tip of the centre drill I was using was about 1.6 mm.

I’ve never had this issue before. The drill seems to run true, and there’s no play in the spindle that I can detect.

I sometimes use a very short drill for centering, but not for small sizes like this.

I guess I’ll get some spotting drills then.

Thanks all.

The centre drill on the left has a wide flat at the tip which is not ideal for starting a very small drill. When it is used for its intended purpose, the 60 degree flanks provide the seating for a centre, and the tiny extended nose makes sure that the tip of the centre is not touching the workpiece. A proper spotting drill has very little length of flute for stiffness and a much sharper point which leaves a better starting hole for any size of drill. The drill on the right looks like a sheet metal drill rather than a centre drill.

In the past, most people would only have had access to jobbers' drills and centre drills.

It made sense to use a small centre drill to create a depression into which a larger jobbers' drill would fit.

Collectively, the hobby got into the habit of using the tip of a centre drill to spot for small drill sizes, because it usually works but the risk of snapping the tip is high.

Spotting drills cast no more than centre drills, so the first time you avoid snapping a centre drill the cost is saved.

Alternatively they can be ground from broken longer drills, just use a slightly more acute tip angle.

Neil

Just a thought - could initial pressure when starting to drill have anything to do with it? A tentative start might leave more scope for the tip of the drill to catch on any irregularities and be forced (very slightly) off line, leading to a steadily worsening orbit of the point around the true centre. Which would, if drilling continued, either end with the pip being sheared off (most likely) or the point of the centre drill breaking off. Maybe you're inadvertantly stopping at the point before either of these happen? Which might explain the random nature of the problem

Possibly a more aggressive "feed rate" (can't think of a better way to describe it) would cure the issue?

Is there any mill scale or other irregularities on your flat plate?

Rob

.

… plus the traditional [and effective for spotting] spade drill.

MichaelG.