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Hey guys, I need your help =) I bought a 2 foot square marble tile for a sanding board and thought that was it...
...Yet checking with a 12 inch rule and feeler gauges, it is flat to within .04mm (.0015”) at any portion. And when I checked it with a 24 inch rule, the marble is flat .10mm the whole length left to right, and is concave by .3mm (.012”) the whole length up and down. I imagine it’s like the side section of a cylinder with a big big radius.

Should I dish it, or is it still flat enough for sanding side rims and fretboards? Thanks in advance

All my sanding boards are 20 or 25mm MDF...a bit easier to lug around than a hunk of marble.

kiwigeo38966.2270601852

   The decision is yours. I like granite plates as they are flat to .0005 or less. The .0015 is ok I guess. That can be tightened with clamping. I would say to do a test piece and see what the glue line looks like .
    I think that the accuracy may be within acceptable limits for wood
yours
john

thats fine for rims or fretboards - 1.5 thou over 12 inches is nothing, its wood remember, any little bit of rock as you sand will be worse than that ... and like I tell students, the glue has to go somewhere.

_________________
Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

The tolerance on polished cabinet top grade granite and vs. precision ground and polished granite is huge. Mostly in the allowable front to back tilt and allowable out of flat. For most sanding we do a cabinet grade or floor grade tile will work but if you need true precision like a necks fb mounting surface. I would not trust one. I posted a set of specs on floor tile and cabinet grad slabs back about a year ago. I got the specs from a mill that makes head stones, counter tops, floor tile and machinist precision granite bases.

I agree with John. For general wood sanding chores, I suspect it will do fine. My sanding board is 3/4" ply and not as flat as your marble tile length-wise, but pretty flat width-wise, but that doesn't stop me from getting good joints.

I scavanged a scrap Corian sink cutout a few years ago. It's about 34"x22". I tested it for flatness with a 24" rule and a 0.0015" feeler gauge and was not able to fit the feeler gauge anywhere between the ruler and the surface. I wouldn't use it for a machinist surface plate, but it's plenty good enough for using as a flat surface when I need one for wood glue-ups, or for checking for flatness.

Best,

Michael

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Live to Play, Play to Live

I'll also point out that float glass is generally "pretty flat", or at least that's
my understanding. Flat enough that it's popular for scary sharp setups.
Might be easier to find than a precision-ground granite plate. Just a
thought in case your marble turns out to be inadequate for the tasks you
throw at it.

. (first OLF account, and it doesn't have my real name which turns out to be a forum no-no . i'll be grateful to the mods if they could delete this account. thanks Harry Faner aka harryf which i'll be using hereon.)   altec38967.001099537

[QUOTE=ecklesweb] I'll also point out that float glass is generally "pretty flat", or at least that's
my understanding. Flat enough that it's popular for scary sharp setups.
Might be easier to find than a precision-ground granite plate. Just a
thought in case your marble turns out to be inadequate for the tasks you
throw at it.[/QUOTE]

Thanks. My very next thought actually, but this time i'll carry a rule and check before buying . (or is this unncessary for float glass? my conjecture is that, due to the manufacturing process, it's impossible to buy float glass other than flat-enough... but i don't want to find myself wrong again hehe.)

a 3/8 thick should do it.

Just a thoguht, but how flat is the rule you are using .. most (none ??) are NOT precision ground, and even decent straight edges (got the new LV catalog last night) are only .001 over 2 feet, aluminum ones are .003

_________________
Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

excellent point Tony. The typical 36" s.s. rule is not very flat. that is to say the edge is not that straight in regards to the plane of the edge. This is because most are sheared or plasma cut. They are fine for measuring with in 1/64" and some even 1/128" over a 24" length but the longer the rule the more they deviate from a constant plane. Sterrit's are fairly good about being straight. but for a plane that is true flat with in .005" or better over a 24" span you need a precision ground straight edge to be able to confirm a flat surface with a straight edge. If the slab you are checking is out of flat in near the same proportions as the rule you are checking with you would show to be flat when in fact you may be far out of flat.

This is far to picky for what we do. But none the less always keep in mind that tolerances stack up. If you have 4 components that are built with in tolerance of + or - .0625", and all to the high side of the tolerance. Then the total out of desired dimension of the combined components is 1/4" over size. The same principle applies when measuring to confirm flat. If the scale is out then your retrieved data is out.MichaelP38967.4027662037

I used to design gaging for checking machined parts. The rule of thumb that we used was: The tolerances of the gage your using should be 10% of the tolerance of the part your checking. That is, if your checking a plate or whatever to be withing .005", your straightedge should be straight within .0005". Might be a little over-kill but you shouldn't run into trouble that way.

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Horton, MI

I recently bought one of those grizzly grainite slabs in 24 x 36 and I use it all the time. I love it.

IIRC it was pretty cheap too.

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Brock Poling
Columbus, Ohio
http://www.polingguitars.com

[QUOTE=Brock Poling]
I recently bought one of those grizzly grainite slabs in 24 x 36 and I use it all the time. I love it.

IIRC it was pretty cheap too.

[/QUOTE]
Granite plates are the best way to go, and, thanks to the industrious Chinese, they are affordable to woodworkers.

The only downside...uh, Brock, how much does that 24" X 36" rock weigh?

400+ lbs. I had it on a fridge dolly, and I thought... "Oh, I can get this down my basement stairs by myself"... bad idea. It was like I was water skiing. I think I had the muscle for it, but not the mass I guess. Oops.

I got 2 neighbors to help me set it in the stand... now they run and hide when they see the big truck pull up to the house.

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Brock Poling
Columbus, Ohio
http://www.polingguitars.com