Granite slab - how thick?

[Allan Speers]

I have a hankerin' to get a hunk of granite, to use both as a blade- sharpening base and also as the top of an assembly table. It's probably overkill, but you know....

For woodworking, even for sharpening blades, I doubt I need some A-grade machinists' reference plate, plus, I already have a small piece of marble for flattening most blades, this top would primarily just be for assembly. - but of course there's no point to this at all if I don't get something "reasonably" flat.

But does anyone know how thick a given size of granite must be, to give long-lasting, dependable flatness? Let's say I'm looking for a 4' X 2' top. (the maximum I'd ever consider.) One sees a lot of countertop cutoffs for sale, but they are typical 1/2 - 1" thick. A reference plate even HALF that size would be 4" thick or more. - but I can't deal with that kind of weight or cost.


So.... what should I be looking for?

or is it just a dumb idea all around?

[Bill Adamsen]

Most granite slabs I see are 1-1/4" to 1-1/2" ... that's what I use for sharpening with wet or dry sandpaper. I keep thinking of getting a large slab as you've described above. Instead mine are quite small (perhaps 4" by 12") so I can fit them in a a tray of water. That's the minimum length I find useful ... any shorter and there's not enough room to use the rolling sharpening guide and the 9" Wide paper (cut to strips).

[Stan Calow]

Allan, I got a broken piece of granite kitchen counter top at my local Habitat for Humanity Restore. They literally have tons of remnant pieces, and you could find one as big as you want. They gave mine to me since it was too small (18"x24") for any real use, but cheap enough anyway. If its stiff enough for a countertop, I'd guess its stiff enough for most purposes. Granite would likely crack before sagging.

[Allan Speers]

..... mine are quite small (perhaps 4" by 12") so I can fit them in a a tray of water. That's the minimum length I find useful ... any shorter and there's not enough room to use the rolling sharpening guide and the 9" Wide paper (cut to strips).

Yep, I find the same. My marble slab is only about 3" X 10", and it's a bit frustrating with large planer blades, and useless for flattening the sole of a nice hand jointer.

I know I could build a torsion box assembly table, but why not use granite, if possible? The only downside is it would be hard to sink a T-track into granite, though I suppose even that is possible.

[John TenEyck]

My kitchen and bathroom counters are 3 cm granite, roughly 1-1/4". They are flat. Not machinist reference table flat, but two pieces will suck together like plate glass. For an assembly table however I don't think I'd want granite. Yeah, it's flat and glue won't stick, but it will chip if you drop metal on it, which would happen the first time I used it, and you can't screw or nail into it. I often do that with my assembly bench in order to hold something. It's an old maple lab bench that I flattened and then skinned with a piece of 1/8" Masonite. When the Masonite gets all ratty, I just pull it off and brad nail on a new piece. It has stayed flat for 25 years.

John

[Mike Schuch]

I have often dreamed of picking up an optical bench as an assembly table. They are stainless steel, very flat AND have .25" threaded holes every 1" or 2". I have seen nice surplus ones go for as low as $100.

[Chris Padilla]

We shopped quite a bit for granite countertops for our bathroom recently and they were all mostly 2 cm (~3/4") thick with a few at 3 cm (~ 1 3/16"). Granite weighs around 17 lbs for a 1 foot by 1 foot by 3 cm (~1 3/16") piece so you're looking at ~140 lbs for a top of 8 ft^2. Maybe you can find a remnant that large but moving that sucker around wouldn't be fun so hopefully it stays in one spot. You could still build a torsion box but top it with the slab. Sounds like fun not to mention a dead flat surface to build upon and reference to but I think you'll find it frustrating not being able to clamp anything to it except near the edges.

Why not extend the craziness and have a bunch of holes like an MFT cut into it and then you'll have something really nice to use! :)

[Jim Dwight]

Granite tiles are readily available. They are flat and rigid. 12x12xmaybe 3/8.

[Andrew Hughes]

I bought a granite surface place for 185.00 bucks about five years ago.Its 4inch thick x 2ft x 4ft weighs about 350 lbs.Its really handy,In my area the local CL have lots to choose from.Its kinda sad I know someone made a living use them for important machinery work and here I am scraping glue and dried finish off mine.Here a pic of mine. Aj

[Allan Speers]

Nice, Andrew !


I still like the idea. I'm not worried about damaging tools, wince it would ONLY be for assembly, but I'm feeling more & more that the lack of clamping options would stink, so combining several ideas, above:

Suppose you got, say, THREE smaller pieces, and fled them to the top of a torsion box, but with two t-tracks glued in-between? Add some extruded channel around the perimeter, and you'd be in pretty good shape, clamping-wise.

You could do a 2' X 3' table with reasonably- weighted slabs, each individual slab would still be dead-flat for sharpening, and you'd surely be able to get the entire table as flat as required for any cabinet work.

Does anyone see any potential negatives to this idea?

And again, what thickness do you think is needed? Is 1/2" too thin to make the whole project make sense, or would that still work? (With the torsion box underneath)

[Rich Riddle]

I like 3 cm thick. You can usually get remnants fairly inexpensive.

[Jebediah Eckert]

Is regular countertop granite flat enough for an assembly table? I don't have any in my house to check with a straight edge.

[Allan Speers]

Is regular countertop granite flat enough for an assembly table? I don't have any in my house to check with a straight edge.


That's a good question. I would think so.

[Ellen Benkin]

I think Woodcraft sells a slab to be used for sharpening.

[Bill White]

My surface plate is a granite sink cut out. 3 cm. Price? Free! Flat? Yep.
Bill

[Tom M King]

If you live in Maryland or close, Community Forklift has a lot to choose from, including whole kitchen counters with sinks even, for not too much. I bought a 9' piece with undermount stainless sink for 175. Builders carry stuff there that they take out of houses so they can get a charitable deduction rather than have to pay to dispose of something. They also have hundreds of all sorts of windows, and other such useful building materials.

[Scott Hearn]

Hit up the granite counter top places in your area and ask them about a sink cutout. They may just give you one. You can cut it with a circular saw and a cheap diamond tile type blade if you want to trim it.

[Joe Jensen]

I can't imagine that regular granite would not be flat enough. I suspect regular granite is more flat than nearly any bench anyone has. It would certainly be flatter than what one can do with winding sticks.

I don't see that crazy flat is necessary for assembly. My garage floor is not flat enough for assembly but every bench I've made (without excessive focus on flatness) is fine for assembly.

[mreza Salav]

don't expect that a piece of granite cut for cabinet work is very flat. If you take a straight edge to most of them they are not.
I once bought a piece of 30"x24" 1.25" thick one to use as a flat surface and it was out of flat by about 1/32-1/16"