Centre finding

[not done it yetParticipant @notdoneityet]

Ron,

Although considerably stiffer than centre drills, there is also the angle of cut.

A centre drill will cut to the centre of a rotating bar in the lathe – slightly off centre, even – the hole will just be larger than the drill. As long as the cone for the centre, used afterwards, is central, it matters not if the initial pilot hole is oversize. Actually, with a revolving centre Being used mostly (these days), there is less reason for the pilot – it is useful for lubrication with dead centres and provides a clearance for the point of the revolving centre ( easy enough to do without that point?). Well, that is how I see it.

I only use a dead centre if the revolving centre gets in the way – and it is then usually the half-centre which gets pressed into service. Carbide centres are also available and would not wear like a softer unlubricared centre.

Edited By not done it yet on 26/04/2020 15:28:08

[Former MemberParticipant @formermember32069]

[This posting has been removed]

[Mike PooleParticipant @mikepoole82104]

I think the whole point of centre drilling is to clear the chisel point of the drill and give a true start to the cutting edges of the drill, surely starting with a greater angle than the task drill will put the chisel point back in play even if it is now at the bottom of a depression. If you need to guarantee a hole position then boring will be required and time to get the toolmakers buttons out, I suppose a decent DRO and accurate edge finder will get pretty close though.

Mike

[Clive FosterParticipant @clivefoster55965]

Ron

The number of drilled holes started with perfectly adequate accuracy using centre drills over the years is way beyond convenient or even inconvenient counting.

Accurate hole starting needs a short, stiff drill with minimal chisel point width, or 4 facet shape, to minimise wander. How its done is immaterial.

Centre drills do a perfectly adequate job but are inherently more fragile than a spotting drill due to the small diameter tip.

Spotting drills came into general use primarily as a result of the CNC revolution where something without the smaller tip was more durable and able to be run both harder and longer. CNC consumption quantities made them relatively affordable and easy to buy. Hence if a more durable alternative to the common centre drill can be got for similar money it makes sense to buy the stronger device. Pre CNC a spotting drill was more a toolmakers tool for use in high accuracy work jig boring et al. The ordinary manual machine shop would have centre drills by the handful but spotting drills were rarer. In retrospect I'm a little surprised that shops with tool grinding facilities didn't routinely convert broken centre drills to spotting drills. I guess the cost of labour meant it wasn't worth it although a jig to make it trivially fast would seem simple.

Claims of more accurate starting and hole positioning are irrelevant unless the drill is accurately sharpened with equal length and equally (very) sharp cutting edges. Unless you have new or lightly used good brand drills or a proper commercial standard sharpening system drill point errors will outweigh any difference between centre drill and spotting drill.

A drill doctor or any of the inexpensive sharpening systems will not produce a sufficiently accurate point to see the difference. I wasted several years before getting the drill sharpener for my Clarkson T&C grinder going as the combination of Picador and PlasPlugs devices seemed to give perfectly adequate results. The professional standard Clarkson is so much better its almost untrue. A time served toolmaker pal did some by hand which looked good and were sharp but seriously substandard on the accuracy side. I doubt if any home shop hand sharpening guy would do as well as he did.

If my lifetime supply of centre drills runs thin then maybe spotting drills will be my new normal.

Clive

Edited By Clive Foster on 26/04/2020 15:42:03

[HopperParticipant @hopper]

Posted by JasonB on 26/04/2020 13:09:40:

Hopper do most people go right in to the 60deg angle on ctr drills when just using to locate the position of a hole?

Yes.

[Paul LousickParticipant @paullousick59116]

No, I only make a small dimple. Similar to that produced by a centre punch.

Can we have a servey of what people use ?

Paul.

[Paul KempParticipant @paulkemp46892]

Dinosaur I am afraid, centre drills, spot drills were not around when I was in school and I don't have any. I also only use the small tip to give a location to start a drill unless I have a good reason to go deeper.

Paul.

[Ron LadenParticipant @ronladen17547]

I have always drilled deeper than the pilot when using a centre drill to start a drilled hole, but always keeping below the diameter of the finished hole size otherwise you end up with a small csk.

i.e. For a 6mm hole I would use a BS3 and drill deep enough to produce 4.5 – 5. 0 mm (ish) entry. My thinking (probably wrong) is that gives the drill a better start than the 2.5mm pilot hole, just the way I have used centre drills for drilled holes.

Edited By Ron Laden on 27/04/2020 05:36:33

[Thor 🇳🇴Participant @thor]

Posted by Paul Lousick on 27/04/2020 00:00:21:

No, I only make a small dimple. Similar to that produced by a centre punch.

Can we have a servey of what people use ?

Paul.

I have only used the small tip of the centre drill to give a location to start the drill. A while ago I found a supplier that had spotting drills at a reasonable price and bought one, it works well and is what I use most often.

Thor

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Paul Lousick on 27/04/2020 00:00:21:

No, I only make a small dimple. Similar to that produced by a centre punch.

Can we have a servey of what people use ?

Paul.

.

For me … It depends upon the drill:

Conventional twist drills [*] have a ‘Chisel Edge’ **LINK**

https://www.researchgate.net/figure/Twist-drill-geometry-16_fig1_320926557

The diameter of a pilot hole, or the size & shape of a dimple, needs to clear that.

… otherwise, that edge will ‘scrub’ instead of being guided.

Beyond that … any diameter smaller than the drill will suffice.

MichaelG.

.

[*] other styles are available

[Ron LadenParticipant @ronladen17547]

Something else I have learnt I didn't realise there are various angles available with spot drills. Mine are 90 degree and the supplier didn't list anything else, with Barrie mentioning 142 degree I have just looked at a couple of sites and see there are 90 degree 120, 147 etc.

[JasonBModerator @jasonb]

When I used to use ctr drills I only used the very end to make a dimple for location

Only times I use them now is if I want to use ctr support or am cutting a 60deg CSK for a steam pipe nipple or similar.

My thoughts on the angles of spotting drills is that in a commercial or high accuracy application where the part may have to mate with something many miles away then the flatter angled ones are the best option and the business can afford to buy several sizes in each angle such as they work Barrie and Andrew do..

For the Hobby user who going buy the replies each time spotting drills come up bulks at the cost of even a single one then they are better served with the 90deg. I will adequately locate the end of their drill be it a name brand or cheap as chips import, it will serve to remove the burr from around the edge of a hole in one op rather than having to deburr afterwards, it will act as a countersink so saving having to CSK the hole after drilling and also handy to use for small CSKs where there may not be room to get a larger one in or it's simply not pointed enough for small holes. They can also be used at a push to put an chamfer along an edge and for light engraving duties.

[HopperParticipant @hopper]

147 degree is for drilling sheet metal so you are not poking the nlnose through the other side before the main cutting edges have started cutting fully. Less grabby that way.

[Michael GilliganParticipant @michaelgilligan61133]

Some interesting notes and warnings about [carbide] ‘Spot Drills’ here:
**LINK**

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

MichaelG.

.

Edit: ___ and [somewhat contradictory] here:

http://www.harveytool.com/secure/Content/Documents/Chart_SpottingTheRightAngle.pdf

.

Edit: ___ I note with interest that neither of these sources mentions the chisel edge of a conventional drill … So perhaps further investigation of manual vs CNC drill geometry is required.

Edited By Michael Gilligan on 27/04/2020 09:13:33

Edited By Michael Gilligan on 27/04/2020 09:28:26

[colin branniganParticipant @colinbrannigan54160]

Okay, so it seems after using centre drills for fifty four years as a toolmaker I have now read there is a better way of starting a hole, every day is a school day, so can someone tell me what is the preferred size of angle (90, 120 etc) spotting drill to buy for general hobby work and where to buy them from.

Regarding centre finding, on the my mill my fixed vice jaw is my Y zero point and a stop plate bolted to right side of vice is my X zero point, (checked regularly). Trial hole is drilled and checked usually within plus or minus .1mm, good enough for me.

Kind regards from sunny Gower

Colin

Edited By colin brannigan on 27/04/2020 09:39:43

[JasonBModerator @jasonb]

Posted by Hopper on 27/04/2020 07:33:23:

147 degree is for drilling sheet metal so you are not poking the nlnose through the other side before the main cutting edges have started cutting fully. Less grabby that way.

Probably was back in the day but now used for high performance and difficult material drills which often have a point that is flatter than the common 118Deg, expect that is why Barrie uses them as he works with some fancy metals and composites.

[JasonBModerator @jasonb]

Colin see my post earlier this morning for my thoughts an angle for all round hobby use. Don't know what size work you tend to do but 1/8" and 3/16" (3mm and 5mm) cover my work. As for sources MSC Direct, APT, Cutwel, Drill Services and good old e-bay to mane but a few. I think Tracy may also do a small range and JBcutting tools have also had them on their stand but not on website.

Also worth pointing out that not all spot drills are created equal, in the forground is my usual KEO brand and the one behind I think was Interstate, the KEO has a lot more web thinning so the chisel is smaller and makes for easier spotting.

Edited By JasonB on 27/04/2020 10:16:00

[colin branniganParticipant @colinbrannigan54160]

Thanks Jason for the info. I shall buy 3 6 and 8mm from Tracy always find them great to deal with.

Thanks again Colin

[Mike PooleParticipant @mikepoole82104]

I think we are in danger of generalising a technique for drilling when in reality the way you mark out, the size and type of drill and the machine you are going to use will affect what you do. If you mark out conventionally and can centre punch accurately then small drills will start happily in the punch mark, larger drills will benefit from starting in a centre drilled hole as the chisel point grows larger it will not locate in a punch mark, for even larger drills it will be best to drill a pilot hole to relieve the effort required to keep the drill cutting. To check your hole is in the right place it was common practice when marking out to box the hole so you can see the position is correct after you have drilled the centre mark away. Four facet drills have the useful feature that they tend to self centre and not do the chisel point dance. Using a mill with a DRO can minimise marking out but until you measure the hole position you will get no feedback on where the hole is. Drilling in the lathe will always want the centre to be drilled as even the tiniest pip will hinder a clean start to the hole. As long as the hole finishes up the right size and in the right place then how you do it is up to you, it is useful to be aware of the options available. A drilled hole can vary from a hole that only needs to be big enough to get a bolt through and it’s position doesn’t matter within a few millimetres to dowel holes that must match others and cannot be drilled together. Drilling can be just a waste removal task to be followed by boring or reaming.
Mike

Edited By Mike Poole on 27/04/2020 10:55:05

[Anonymous]

Posted by Paul Lousick on 27/04/2020 00:00:21:

Can we have a servey of what people use ?

I rarely mark out and centre pop. In general only for sheet metalwork where I'll be using a hand drill. For probably 80% of my drilling on flat surfaces I don't use anything, just drill direct with a 4-facet drill. For holes up to about 8mm I use one drill, no point in faffing about going up in small steps.

I have 2, 3, 6 and 10mm 90° carbide spot drills. I use them in three circumstances. One, for non-flat, mostly round, surfaces. Two, for rough (cast) surfaces or three, where I want more accurate location of the holes.

I don't know what accuracy I achieve by spot drilling first, but anecdotally probably a thou or two. When I made this differential consisting of two ring gears and a centre casting:

I drilled the circle of holes holding the assembly together by spotting and then drilling. Unfortunately I had the centre back to front. I turned it over and all the holes still lined up perfectly. I drill 6mm for a 6mm bolt, so I estimate my location accuracy must have been a thou or two.

The last time I used a centre drill before a twist drill was on this special dividing head plate:

And that was primarily because the existing countersunk screws had 60° included heads.

Ideally I'd do some experiments, but even in lockdown, with no work available, I seem to be busier than ever and the round tuit list is expanding exponentially.

Andrew

Edited By Andrew Johnston on 27/04/2020 11:28:31

[JasonBModerator @jasonb]

Posted by Andrew Johnston on 27/04/2020 11:22:07:

I have 2, 3, 6 and 10mm 90° carbide spot drills. I use them in three circumstances. One, for non-flat, mostly round, surfaces. Two, for rough (cast) surfaces or three, where I want more accurate location of the holes.

&

But surely if the tip of the drill needs to seat in the spotted location the angle of the spotting drill needs to be larger than the following drill. Which is the opposite of saying it doesn't matter, and also contrary to JasonB.

So Andrew are you also saying that a 90Deg spotting drill will do as that seems to be what you use or should the spotting angle be greater than the drill angle?

[Anonymous]

Posted by JasonB on 27/04/2020 13:02:33:

So Andrew are you also saying that a 90Deg spotting drill will do as that seems to be what you use or should the spotting angle be greater than the drill angle?

I don't know what the answer should be, or even if there is a 'correct' answer. Superficially one would say that an angle greater than the following drill is correct so that the tip starts first. But my spotting drills have a chisel point so the centre isn't going to be clean nor will it constrain the following drill. For a 4-facet drill it might even be worse than starting on a flat surface.

Using a 90° spotting drill seems to work; but that's anecdotal rather than proved by measurement.

Andrew

[JasonBModerator @jasonb]

Posted by Andrew Johnston on 27/04/2020 15:51:34:e following drill. For a 4-facet drill it might even be worse than starting on a flat surface.

Using a 90° spotting drill seems to work; but that's anecdotal rather than proved by measurement.

Andrew

Careful, we seem to agree on something

Took a look at a 5mm carbide spotting drill I bought for the CNC and could not see any chisel on the end of that, or if there was it was far less than those pics I posted earlier, will post a photo in a while.

[Anonymous]

Posted by JasonB on 27/04/2020 16:09:55:

Careful, we seem to agree on something

Bother, I can assure you it's purely coincidental.

Andrew

[colin branniganParticipant @colinbrannigan54160]

Taking Jason's advice (thank you) I placed an order with Tracy Tools for some spotting drills yesterday order placed at 10 45am and they duly arrived today at 10 15am, that is wonderful service, even more wonderful considering the conditions the country is in at the moment, and the postage was only £3.

Kind regards Colin

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