Centre Drill Leaves a “Pip” – Sometimes

[Dr_GMJNParticipant @dr_gmjn]

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.

[Dr_GMJNParticipant @dr_gmjn]

[SteviegtrParticipant @steviegtr]

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.

[Anonymous]

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

[peak4Participant @peak4]

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

[SteviegtrParticipant @steviegtr]

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

Steve.

[JasonBModerator @jasonb]

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

[Kiwi BlokeParticipant @kiwibloke62605]

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

[Martin ConnellyParticipant @martinconnelly55370]

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

[Kiwi BlokeParticipant @kiwibloke62605]

Posted by Martin Connelly on 23/07/2021 07:43:30:

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.

…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.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Kiwi Bloke on 23/07/2021 07:54:13:

Posted by Martin Connelly on 23/07/2021 07:43:30:

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.

…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.

[JasonBModerator @jasonb]

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

[Nicholas FarrParticipant @nicholasfarr14254]

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.

[roy entwistleParticipant @royentwistle24699]

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

Roy

[Nicholas FarrParticipant @nicholasfarr14254]

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.

[Mick B1Participant @mickb1]

Posted by Kiwi Bloke on 23/07/2021 07:37:38:

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

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.

[Dr_GMJNParticipant @dr_gmjn]

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.

[old martParticipant @oldmart]

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.

[Neil WyattModerator @neilwyatt]

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

[Grindstone CowboyParticipant @grindstonecowboy]

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

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 23/07/2021 13:45:10:

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

[…]

.

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

MichaelG.

[Anonymous]

For many holes you simply don't need to centre, or spot, drill first. The holes in this brass fixture –

– were drilled No.70 (0.71mm) to fit 0.64mm pins soldered to a small PCB prior to press fitting into a plastic housing. Driling was done on a Bridgeport and no centre drill or spot drill was used. Even the smallest of both leaves too large an impression.

Of course, I'm lazy and I'm not going to waste time doing operations that are not necessary.

Andrew

Edited By Andrew Johnston on 23/07/2021 14:47:34

[Clive FosterParticipant @clivefoster55965]

Centre drills are not only short and stiff but also accurately ground with the tip properly concentric.

When, as was and frequently still is the case, drills are sharpened by hand the chisel point is likely to be a touch out so true starting in a centre pop cannot be guaranteed. Hence the use of centre drills to provide a start both in the home workshop and industry, especially the non production side. Spot drills have only really become a generalised thing since the CNC takeover. Certainly when I started messing around with machines back in the 1970's spot drills were very much on the special tooling list and I was instructed to use a centre drill if areally accurate hole position were needed.

However starting out with a 1.6 mm point centre drill is brave and asking a lot of typical home shop machines to provide the essential stability should the job be less than perfectly flat. Far safer to use a larger centre drill to provide the initial start cone and swop for the smaller one if needing a hole to provide chisel point clearance for easy drilling.

Going deep enough to get at least some of the centre cone formed is said to give a more reliable start when using a drill that is not factory fresh. Because the chisel point of a standard drill doesn't actually cut it takes a fair battering and deteriorates faster than the cutting lips. The angle mismatch between a centre and a drill point allows the cut to start as a line somewhere up the, probably sharper, lips. By the time the angle mismatch betwixt centre and drill lips has been worked out there is reasonable support for the drill so its more likely to go straight.

One day I shall go through my collection of de-tipped centre drills and convert them to four facet grind spotting drills. Like Geo. H Thomas I save them "because the good quality HSS will be handy for something" and pretty much never use them up because the right type of something never arrives!

Clive

[EmgeeParticipant @emgee]

I have spot drills with different angles, most used is the 120 deg type as it allows 118 deg drill centre points to start at the centre provided not just the lips on the OD of the pre-drilled point.

Emgee

[Nigel McBurney 1Participant @nigelmcburney1]

Common practice in my early days 1950/60s was was to mark out , then centre punch with a small punch (prick punch) using eyeglass to position punch by sliding point of punch along one scribed line until it met the other marking line.then with centre drill in a drill chuck on a drilling machine position the centre drill over punch mark and drill the hole.Never saw or heard of a spoting drill and never used a Bridgeport type mill with quill as the company did not have one.most of the material drilled was thick bright mild,brass and ali castingstypical instrument making where accuracy was important and no complaints about the centre drills. Next job was making prototypes for making printers and converting typewriters to electronic operation mid 60s. There the company approach was different,it was all thin steel prototype pressings, so holes were again accurately marked out but then drilled with a small twist drill,and for bigger holes a larger twist drill used to follow up.no problem with hole positions and quicker, In the 1970s I was a procurement engineer on early hard drives,lots of small holes at really tight positional tolerances mainly in gravity die cast aluminium ,some in machined surfaces other holes in as cast surfaces and a lot of them tapped,with a very resolute company policy problems with hole positioning became nightmare, by then two axis punched paper tap was the main control systems , and various spotting drills were tried to stop drill wander particularly on cast surfaces. And thats how it continued,into full cnc control times with the spotting drills with no flutes and a very stiff flat sided point ,it was just a very high spindle speed and a lighting quick feed that kept things going ,centre drill spossible would not have stood up to the production rate running at 24/7.Since those days I have found that on a good turret mill ordinary twist drills will drill directly in without any form of pilot drilling without wobble , I have some 10mm spotting drills ,though they only get used for boring bar tool bits and deburring holes. I have no problem with centre drills, though even my stock of centre drills is at least 30 years old and made in England perhaps the old ones were ground better than the moder oriental ones. The cause of centre drills wandering and leaving a pip may be the lack of the centre pop before drilling and poor spindle bearings in the drilling machine. I did try some time ago drilling a hole 6mm diameter into flat bright mild steel, In my Fobco the drill tended to wander ,in the Large Elliot Milmor the drill went straight in without any wander,with both spindles in good condition . It just shows how much stiffer and rigid a mill is compared to a drilling machine, I tried this experiment as I had a commercial job where I could save time drilling direct to size without resorting to marking out ,and spot drilling, or making drill jigs.

[Author]

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