How flat are gravestones ???

In a job lot of kit purchased, I've aquired a 2 foot x 3 foot granite surface table in pretty abused condition. I'm tempted to get a local
monumental mason to put it through his gravestone polisher, but how flat are the results? Looking at new granite graves they 'seem' optically flat from the reflected images, but are they? I'm only intending to use it as a marking out table, not the basis for a micron accurate co-ordinate measuring system!
AWEM
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On Nov 4, 10:40 pm, "Andrew Mawson"

Andrew
Can't give you an answer but it's good to know that you've got away from being Bob The Builder for long enough to buy a job lot of stuff that you probably didn't really need. Back to old times eh?
Charles
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wrote:

granite
micron
Andrew
Can't give you an answer but it's good to know that you've got away from being Bob The Builder for long enough to buy a job lot of stuff that you probably didn't really need. Back to old times eh?
Charles
..hehe ... most building work now finished with the exception of the foundry, which will be a 6m x 13m non-inflamable lean too against the barn that is the new workshop. Capital machinery still not back here but all the small stuff is - delay due to being summoned for jury service. Yep - even bought the fork lift truck to shift it all around a bit, but cannot get it delivered until the trial ends.
AWEM
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wrote:

Viewed from the top or underneath ?
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On Tue, 4 Nov 2008 22:40:12 -0000, "Andrew Mawson"

Don't know the answer, but one could always turn up at the local monumental masons with a good straight edge, a matched pair of blocks and a tenths reading dial gauge on a stand. Might be an interesting exercise.
Also easier to do it with a stone laid flat rather than upright in a freezing cold graveyard :-)
Let us know if you get answer.. I've got a 2'x3' Crown surface table that came out of the Reliant works (without help from JS). That looks as if it's had the odd engine block dropped on it. I've not been looking forward to the cost of replacing it.
Mark Rand RTFM
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wrote:

I hope you're not expecting to get it dead flat.
I'll get my coat.
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Perhaps it would remove the need for centre-drills because such stones are normally associated with the dead centre of everywhere?
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On Tue, 4 Nov 2008 22:40:12 -0000, "Andrew Mawson"

Flat enough for most undertakings, no doubt <G>
Regards, Tony
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"Dead Flat"

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Andrew Mawson wrote:

Ignoring the other comments .... This is a 'how long is a piece of string' question. I've been looking after a LARGER polishing machine at my local stonemasons for some time now. 6ft by 12ft bed! The resulting finish can be very good when run slowly, or 'satisfactory' if running a quick polish. A smaller machine there have an articulated arm which is not as ridged as the gantry and there is a couple of thou visible play, but the finished job looks fine. So the answer is - it depends on how the machine has been maintained. If you are only re-polishing then probably worth doing, if you need to take out scratches it may be more of a problem, and not something the mason might want to spend time doing.
--
Lester Caine - G8HFL
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Lester Caine wrote:

Is there some kind of "reference plane", or is the polishing a purely localised action?
BugBear
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On Wed, 05 Nov 2008 13:25:55 +0000, bugbear

I guess for gravestones it must be a much higher plane <G>
Regards, Tony
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bugbear wrote:

The gantry system maintains the height of the polishing head and will take out any surface defects. The articulated arm tends to follow the existing surface, although originally it probably maintained a level action, but it's performance is more a matter of the operators capability nowadays.
--
Lester Caine - G8HFL
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Andrew Mawson wrote:

First the good news - the same models of machines used to grind surface tables are also used to grind monumental granite, granite countertops etc.
Now the bad news - the answer is "It depends" - on the particular machine used, how it is maintained, operated, etc.
I have two 12" x 8" granite slabs from the same mason - one is flat to well within 0.01mm, one is about 1mm high at the edges.
Perhaps the mason would know?
-- Peter Fairbrother
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On Wed, 05 Nov 2008 13:41:23 +0000, Peter Fairbrother
But he'll only tell you if you have the right handshake...
Regards, Tony
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Peter wrote:

Indeed. I think there must be MUCH lesser machines in use, not much better than a manually operated "polishing head"
I have a small piece of polished granite, begged from the local monumental mason.
It's around 8x12x1 (inches) and has a 1/16" convexity on the polished face.
Nice and shiny, not very flat.
BugBear
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<bugbear@trim_papermule.co.u k_trim> writes>Peter wrote:

Just a thought, if you had more than one bit of stone. The 3 item grind. Grind 3 of them mutually, in turn.
Using a suitable paste, grind Stone1 against Stone2. Then Stone1 against Stone3. Then Stone2 against Stone3. Then Stone1 against............ Rinse and repeat until flatness desired is achieved. Optical flatness is possible, just by hand, without expensive, trued up to the n'th degree machinery. It's how astronomical telescope makers do it. The 2 item grind gets you a beautiful CURVED surface.
The only investment is your TIME.
Speaking of time, and reading between the lines, getting a feel for the age of some of this group.... enjoy :)

http://www.youtube.com/watch?v=CRTP0z4jvtw

John
--
JC Morrice
snipped-for-privacy@pentode.demon.co.uk
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JC Morrice wrote:

Yes, that works. I only use that piece of granite as a substrate for sharpening tools, and it's flat enough for that purpose.
I also have a (cheap) chinese 'B' grade 9"x12" granite surface plate that I use "when it matters"
It cost me 14 US dollars, incl shipping :-)
BugBear
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I've never met Andrew, but if he's big enough to grind three marble gravestones together, he must have biceps like two quart cans...
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surface
countertops
particular
grind.
against
repeat until

expensive,
The 2

fee fie foe fum ....... hehe
AWEM
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