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Centre finding

Problems finding centre

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JasonB27/04/2020 07:28:45
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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.

Hopper27/04/2020 07:33:23
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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 Gilligan27/04/2020 09:00:05
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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 brannigan27/04/2020 09:25:05
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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

JasonB27/04/2020 10:03:10
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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.

JasonB27/04/2020 10:07:46
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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.

spotdrill.jpg

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

colin brannigan27/04/2020 10:31:12
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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 Poole27/04/2020 10:52:08
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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

Andrew Johnston27/04/2020 11:22:07
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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:

Finished Differentials

I drilled the circle of holes holding the assembly together by spotting and then drilling. Unfortunately I had the centre back to front. embarrassed 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:

Dividing 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

JasonB27/04/2020 13:02:33
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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?

Andrew Johnston27/04/2020 15:51:34
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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

JasonB27/04/2020 16:09:55
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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 somethingwink 2

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.

Andrew Johnston27/04/2020 17:03:55
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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. teeth 2

Andrew

colin brannigan28/04/2020 10:34:57
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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

Cliff Tandy 201/05/2020 07:31:35
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Hi all, I spent some time last weekend setting the mill up and it made a big difference. All was going well but then my dro display decided to break. Does anyone know can I buy any new display, will it fit the current cables on the scales.

Stewart Wells02/07/2020 20:11:31
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Thanks for sharing this makes for great advice

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