Centre Drill Leaves a “Pip” - Sometimes

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

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

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

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.

Drills ground with four facets hardly need centring.

But you are unlikely to be able to grind four facets, to drill accurately, freehand. Even two requires practice and skill, particularly on smaller sizes.

A sharp centre drill should have a chisel point rather like a four facet drill.

Since a centre drill is not required to produce a hole of exact size, a small difference in lip length should not matter too much. The important thing is that it is sharp and cuts!

Maybe, in addition to ensuring that the centre drill is sharp, use a smaller centre drill, so that the tapping drill locates on the cone rather than the bottom of the blind hole.

Howard

Spotting drills were never mentioned in the old books I used to read on engineering although they were not that old when I read them. Centre drills were the standard method of starting a drill as taught to me in metalwork at school then tech college and the factory training school. As NC and later CNC were quite new when I started work, that is it was barely 20 years old I suspect a lot of tooling has been created to meet the needs of automated machining and so was not on the curriculum when many of us were taught workshop methods. We should not be shy of embracing new things but if your current technique works there is no need to buy new tooling for the sake of it. Being aware of new options can as Andrew mentions save time and money in commercial work although pottering about in our own workshops for fun, time often doesn’t matter too much.

Mike

Appreciate all the info guys.

So even pretty much the simplest operation imaginable, using a drill that’s called a centre drill can’t be relied upon to drill a centre hole…Cant wait to try between centres boring for the first time this weekend ha ha.

Such a wide range of fun ways to fail in this hobby!

Then again, it’s a learning process, and when things finally go right, it’s great.

Cheers.

I was never told about spotting drills either. I started using them about twelve years ago when I was disappointed with the results from using a centre drill. I’ve tried to tell others but some just come out with the old “I’ve always done it this way” remark about centre drills so I let them get on with it.

Number 73

Is there something wrong with a small centre punch ?

I was drilling holes to secure some feet to a pair of cylinders:

**LINK**

For anything like that, I use an edge finder in conjunction with the DROs on the mill. I can get everything central, and re-centre for subsequent drilling or tapping very quickly and easily. It may or may not be the most appropriate method to use, but it’s more than accurate for my needs, so I just use the DROs/co-ordinates whenever possible and be done with it. With my current level of skill it’s one less source of position errors as far as I’m concerned

So why are spotting drills so expensive, best I can find for one off 3mm is ~£5 whereas I can buy a set of 5 different centre drills for that price.

I’ve taken to using 3 mm carbide drill shanks with 3 facets ground at 45deg leaving a sharp point at the end, it gives you a lovely small divet for the drill bit. Cancer machines use this method for small holes and you should see the price of the bit or not.

For small drills you can just use a sharpened pointer to make a starting indentation in the work piece.

Tony

As per Emgee’s comment, I watched a thing on YouTube that said the spotting drill should have a 130 degree tip, when used with a 118 degree hole drill. The reason being the tip of the hole drill will hit the apex of the spotted hole first, and not get deflected by the edge. Seems legit? I can find some 90 degree sporting drills for a decent price, but not 120.

Edited By Dr_GMJN on 24/07/2021 18:14:49

I've always used 90deg and not had any problems plus they will also chamfer and CSK.

Have you seen centre punches ground to 120deg? always seem to be ground to 90deg.

Does pay to hunt out ones with a good point, for example the one on the left will leave a flatter ended cone which the end of a very small drill may skid about on, the one on the right which has better web thinning will be better if drilling small holes.

Edited By JasonB on 24/07/2021 18:23:49

It's the subject of some controversy, even on professional machining forums. Like Jason I use 90° without any apparent problem.

Andrew

Most of my spotting drills have a 90 degree point, but I have got one with a 130 degree tip. It stays in its tube, as I found its drawback, you don't get a good view of exactly where the end is. I also have some centre drills with the 90 degree flank rather than the standard 60 degree. Not much good for use with a centre, but good for when a drill is to be used.

I tend to set my holes out using the DRO so being able to see the end of the spot drill or anything else for that matter is not really an issue.

.

I recall reading a warning [which seems logical] that carbide drills are at risk of chipping if they are started in a ‘spot’ that has a smaller angle than the drill.

MichaelG.

.

Edit: __ found it :

https://www.model-engineer.co.uk/forums/postings.asp?th=163412&p=3

https://www.model-engineer.co.uk/forums/postings.asp?th=157991&p=4

https://www.destinytool.com/spot-drills.html

Edited By Michael Gilligan on 25/07/2021 08:13:44

If drilling by eye then marking a box round the hole can help with getting the start positioned accurately. A dot punch sharpened to 60° helps to get the initial dot accurately positioned and will feel the scribed lines so the intersection can be found by feel, once the dot is positioned to your satisfaction then it can be enlarged if required. The job can be positioned for drilling with a sharp point in a centre finder or a sticky pin. A CNC machine is not hampered by visibility issues and using a DRO makes life easy once a datum is established and a spotting drill will save punching and with a clear head marking out can be skipped.

Mike