Drilling holes in slate and granite

What kind of drill bit do I need for a) slate and b) granite? (I have a small drill press)
How do I proceed - one hole, or several - drilling out gradually to increase the diameter of the hole? I need to drill out a hole of about 8mm.
Any difficulties I should forsee, like slate splitting?
Do I need water cooling? Depth of slate would be either 10mm or 20mm. Is this fairly fast or very slow and laborious? How is granite compared to slate?
Thanks - never drilled into these materials before. Andy
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Eusebius wrote:

Slate is easy and very forgiving - just use a masonry bit. You won't need cooling. A pilot hole will make it easier to get the hole exactly where you want it, but is unnecessary if you can clamp the workpiece. It should only take you a couple of minutes.
Granite is much, much, much harder. I'd use a masonry SDS (assuming the same depth and hole size), starting with a 4mm and working up to an 8mm. But you need to support the granite very well to avoid breaking it.
--
Grunff

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Grunff wrote:

I wouldn't.
an 8mm HSS would be more suitable or even an old wood bit.
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But you need to support the granite very well to avoid breaking it. >>
In terms of splitting, is granite better or worse than slate? I'm considering using one or other for a face plate for a diy hi-fi amplifier. If there's a risk of splitting, I presume I need to use an aluminium plate behind it for structural reasons, and glue the slate to the alu (???what glue). Prefer to avoid it - what do you suggest?
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On 6 Sep 2006 05:13:02 -0700, "Eusebius"

Ebonite's rather nice for front panels.
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Frank Erskine
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Frank Erskine wrote:

surely thats been unobtainium for a long time. Does anyone still make it? Also I'd think slate or granite much nicer. Marble can be pretty too, though its weak stuff.
NT
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On 6 Sep 2006 15:29:45 -0700, snipped-for-privacy@care2.com wrote:

If you do a search on Google you'll find a number of suppliers, both home and abroad.
I have a quantity of it in stock.

They aren't really a natural choice for panels though...
I mean, slate's for roofing, and granite for tombstones.
--
Frank Erskine

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On 7 Sep,

We had switchboard panels made of slate at work (many years ago). They looked fine too.
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B Thumbs
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Commonwealth war graves in France have slate headstones from the Wynncilate quarry. Saw pallet loads of them last time I was there 'cos a bunch of French oiks had smashed up a load of Tommies' resting places.
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On Thu, 07 Sep 2006 00:06:14 +0100, Frank Erskine wrote:

Slate comes in many grades, roofing slate is very soft and splits cleaning and easyly (thats how they make 'em). Some slates are very dense, hard and don't split. We have some 3cm think lumps of Cumbrian Green Slate for the hearth, tough as old boots.

And building and worktops, though I can't see why people want granite worktops unless you are a pastry chef.
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Cheers snipped-for-privacy@howhill.com
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Dave Liquorice wrote:

Anything that's survived going through a volcano has a better chance of surviving my wifes treatment of work surfaces.
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good thing you write under a nom-de-plume :-)
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Dave Liquorice wrote:

I have never understood the need for granite or marble pastry surfaces in a kitchen. After all, they will be at ambient room temperature either way.
Now if you were to put a marble slab into the fridge before use, I could understand that for rolling out pastry.
Dave
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Which is a lot warmer than my hands. So as I warm the pastry by working with it the slab cools it down - and better than a plastic worktop as it has a higher heat capacity.
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Eusebius wrote:

They are totally different and break in different ways. Slate delaminates, splitting along the layers. Granite just breaks.

Presumably your 8mm holes are for mounting pots, switches, indicators etc? Which means you'll need a few of them? I would use slate, without a doubt. It'll be much less work. No need for a backing plate.
--
Grunff

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Eusebius wrote:

Granite is dead tough..it make break out a little at the back tho. a
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Grunff wrote:

I found a drill that is a diamond tipped core drill for drilling ceramic tiles and stones..I may be using it tomorrow in anger..but diamond is about all that will touch granite.
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Eusebius wrote:

They're chalk and cheese.
Slate will make holes if you stare at it hard. Your problem is to avoid it splitting. Granite is really hard work and you need just the right tools. Slate is also highly variable - Lakeland slate is almost like granite, South Wales slates is flakey junk and a nightmare, North Wales slate is somewhere in between -- prone to splitting, but reasonably hard and stable.
For slate I'd suggest non-hammer drilling with an abrasive drill bit for large holes, or a simple glass drill carbide leaf bit for small holes. Any drill should cope, even hand drills, but watch out for wobble causing splitting. Slate should be supported underneath with soft scrap wood or heavy corrugated cardboard over wood if uneven
For a small number of 8mm holes in softer slate, I'd use a sacrifical twist drill and expect heavy wear on it. One of the brad-point wood drill bits with twist flutes but outside spurs as well, ideally TiN coated, will drill slate well and gives a good edge. Use your drill press for rigidity (those brad points are hard to start by hand, until the points sink). You can drill it with normal metalworking twist drills too, but expect heavy wear.
If you use a masonry drill and non-hammer action, then sharpen it first. Most masonry drills are sharpened with impact chisel edges, not cutting edges. For slate it will matter.
Granite can be drilled with an SDS masonry drill and hammer action, more slowly with a non-SDS hammer drill, or non-hammer with a diamond core drill for large holes. So you probably can't use your bench drill for any of these. Diamond drilling of granite may need coolants, extra slow speed, or high torque (with a safety clutch). Ask the drill bit maker.
You can also chain drill granite, then break out between holes with a chisel. This is for 1 1/2" and upwards though, not small holes.
For 8mm holes in granite, then I'd use a standard masonry bit and SDS. There'll be chipping around the edge though, maybe 15mm affected radius. If you _must_ have a smooth hole, go with wet diamond. Try to drill from both sides, as you'll see even more chipping on the exit hole.
Wet coolants need to be generous, or else they just make a sticky paste that grabs onto the drill. I probably wouldn't bother, except for diamond in granite, production in slate, or if your diamond drill maker recommended it.
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Slate's dead easy. Use plenty of water or you'll end up with dry clayey mud gluing the whole thing up. No hammer, unless you want to end up with it all in bits.
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Eusebius wrote:

Try your local plumber's or tiling or kitchen/bathroom retailer/merchant. I had to drill through porcelain and granite tiles on my last job and the 'spade' shaped bits were useless. I ended buying a diamond crusted bit which worked beautifully... no stressing or straining... no burning or 'glowing' of the bit or material.
have a look here... http://www.toolshopdirect.co.uk/shopscr3098.html
HTH, deano.
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