Centre Drill Leaves a “Pip” – Sometimes

[Howard LewisParticipant @howardlewis46836]

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

[Mike PooleParticipant @mikepoole82104]

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

[Dr_GMJNParticipant @dr_gmjn]

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.

[VicParticipant @vic]

Posted by Mike Poole on 23/07/2021 17:24:14:

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

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

[larry phelan 1Participant @larryphelan1]

Is there something wrong with a small centre punch ?

[Dr_GMJNParticipant @dr_gmjn]

Posted by larry phelan 1 on 23/07/2021 19:05:00:

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

[duncan webster 1Participant @duncanwebster1]

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.

[bernard towersParticipant @bernardtowers37738]

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.

[Tony Pratt 1Participant @tonypratt1]

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

Tony

[Dr_GMJNParticipant @dr_gmjn]

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

[JasonBModerator @jasonb]

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

[Anonymous]

Posted by Dr_GMJN on 24/07/2021 18:13:43:

……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?

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

Andrew

[old martParticipant @oldmart]

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.

[JasonBModerator @jasonb]

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.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Andrew Johnston on 24/07/2021 18:24:24:

Posted by Dr_GMJN on 24/07/2021 18:13:43:

……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?

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

Andrew

.

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

[Mike PooleParticipant @mikepoole82104]

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

[Dr_GMJNParticipant @dr_gmjn]

I’m only using the centre drill to stop small diameter drills from wandering in contact with the surface of the workpiece. As I mentioned, I use the DRO on the mill, so re-centreing isn’t an issue. Never had it before though, where the centre drill forms a distinct dome within a recess.

[Anonymous]

If small drills are wandering without centre drilling first then either the drill, or technique, are in need of revision.

Andrew

[DiogenesIIParticipant @diogenesii]

In the 'Princess Royal' photo liked to earlier (p2) it looks like quite a lot of the drill is sticking out – if you had the same kind of extension on the centre drill it might do as you describe..

Did the drills that do it come from the same batch / order?

[Dr_GMJNParticipant @dr_gmjn]

Posted by DiogenesII on 25/07/2021 10:37:37:

In the 'Princess Royal' photo liked to earlier (p2) it looks like quite a lot of the drill is sticking out – if you had the same kind of extension on the centre drill it might do as you describe..

Did the drills that do it come from the same batch / order?

The drill was in the Chuck right up to the end of the flutes. I think the centre drills were from Arc, but as I said – never had a problem with those exact drills before.

[Dr_GMJNParticipant @dr_gmjn]

Posted by Andrew Johnston on 25/07/2021 10:22:03:

If small drills are wandering without centre drilling first then either the drill, or technique, are in need of revision.

Andrew

To be honest, there may not even be an issue that requires centering: I did have to use some cheap drills that replaced some broken Dormer ones – possibly I had to centre for those. I’ve then routinely kept on centering for small drills. For the sake of another minutes work it didn’t seem worth risking it. Also, I only started working with castings last year, and I found that small drills could wander on the rough surfaces there, and centering eliminated that.

 

ETA Looking back at my 10V thread, among the first holes I had to drill in castings were those in the sole plate and box bed. There were cast-in dimples, but they weren’t aligned where I needed them. Obviously the pilot drill got displaced by the dimples, so I used a centre drill to obscure the dimples. I just carried on doing this routinely on all other (cast/not cast) parts.

Edited By Dr_GMJN on 25/07/2021 10:58:48

[Tony Pratt 1Participant @tonypratt1]

Posted by Andrew Johnston on 25/07/2021 10:22:03:

If small drills are wandering without centre drilling first then either the drill, or technique, are in need of revision.

Andrew

Pray tell us what is this wonderful technique which guarantees a small drill will not wander on touching the work piece surface.

Thanks,

Tony

[VicParticipant @vic]

Posted by Tony Pratt 1 on 25/07/2021 12:28:18:

Posted by Andrew Johnston on 25/07/2021 10:22:03:

If small drills are wandering without centre drilling first then either the drill, or technique, are in need of revision.

Andrew

Pray tell us what is this wonderful technique which guarantees a small drill will not wander on touching the work piece surface.

Thanks,

Tony

[Anonymous]

Posted by Tony Pratt 1 on 25/07/2021 12:28:18:

Pray tell us what is this wonderful technique……….

Skill.

Using a 4-facet drill helps. Look at the photo I posted earlier in this thread with four holes drilled No.71 (0.71mm) with no previous marking or drilling. The holes mate with 4 pins soldered into a small PCB. The pins are 0.64mm and the PCB plated holes are 0.68mm, so 0.04mm to play with, in theory. The pins on all the boards I tried fitted without a problem. It only works on flat surfaces, machined or extruded. Castings and round surfaces need spot drilling first.

Andrew

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Dr_GMJN on 25/07/2021 09:51:30:

I’m only using the centre drill to stop small diameter drills from wandering in contact with the surface of the workpiece. As I mentioned, I use the DRO on the mill, so re-centreing isn’t an issue. Never had it before though, where the centre drill forms a distinct dome within a recess.

.

Reading Andrew’s recent post, and your mention of castings … perhaps you would have done better to spot a flat surface using an appropriate cutter. There is surely a risk that your centre-drill was deviated by a hard-spot in the casting.

MichaelG.

[Author]

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