Question about using granite as reference surface.

[cvriv.charles]

I found a place with scrap counter top granite her in Jersey. I was told $75 for a 2' x 2' UNPOLISHED piece. MY question is for those how have and use them,... does it need to be polished? Or are they generally smooth and flat without polishing. Anyone know? I need this square off my MM pieces.

Thanks.

[liaoh75]

Hi Charles, I did just that when I was welding up my gantry. I clamped both tubes to my marble machined slab to make sure they were on the same plane. My piece was bigger than 2' x 2' but I'm sure it will work just fine for you. In answer to your question, I don't think it needs to be polished, but just machine smoothed with no surface aberrations if you are using it as a reference. Good luck!!

[cvriv.charles]

Well I asked for a quote for a 3' x 3' piece too. i think a 3' x 3' piece would be perfect for all kinds of tasks. 2'x 2' would be good too. but knowing me I'll have something that wont fit on it:)

[KenC]

I bought a piece of 3'x3' polished ceramic tile for the job. I can leterally see my undistorted reflection on the face. Paid a hefty US5 for it...
Wanted to use this as a coffee table top but the welding spatter left some burnt marks on it...

[hennie]

Charles ask your granite supplier for the sink cut out, most of the time it can`t be used and it is ideal for welding your gantry.I have a couple of them for freeeee:)

[liaoh75]

Hennie, I have a large stock of corian counter top stock the same way. I make name plates for people with free stock. The kitchen installers don't know what to do with the stuff.

[cvriv.charles]

how you get them for free? i doubt these guys would give it up for free. I'l see what they say. I bring up the sink cutout in my next reply.

[hennie]

In my trade once the granite is installed we cuck the cutout away.

[cvriv.charles]

That understand but i dont see them just giving it to me. Im pretty sure they have to pay to have all the scrap trashed. so giving it away from free is cheaper. but if anyone thought for a second they could make a buck off it because someone is asking,... i just dont see them giving it away.

[liaoh75]

Charles, just go to your local Kitchen counter installer and start asking from there. They will probably tell you one of two things. One, they have already disposed of what they don't need or already have someone or some company that helps them "recycle" their waste pieces. Two, you can take what whatever they have laying around for free or for a very reasonable price. They really don't have any application for the cut out parts that I'm aware of. The place I get my corian has huge piles of the stuff outside. It's too expensive to make useable again or piece them together and reuse so they just set it outside and once in a while a big truck comes by and picks up all the waste material with a loader and does something with them. Just catch them before that big truck comes by.

I don't know about stone, but at least thats what they do with corian. Good luck! One man's trash is another man's treasure!!

[cvriv.charles]

ok I will do that. thanks guys.

[Johannescnc]

I came across sink cutouts of granite very often in my work in Florida... I had no idea they were so valuable :eek:
I would think they would be flat enough as is to do a small job as you describe...
Have you considered taking a straight edge and feeler gauge to see how flat it is? :rolleyes:

[swatkins]

My granite surface block is flat to .0001" across it's surface.. It's 22 x 16 and 4 inches thick. It was ground to that precision because that was it's intended purpose.. It's 4 inches thick so that it holds its tolerances..

I don't see how a piece of .750 ( or less) counter top, that was made for counter tops, could every come close to the precision of a real surface block.

[liaoh75]

My granite plate is about 2 inches thick and it's about 2' x 3'. It's original purpose was for the side of a building. It's face was machined on a CNC built for stone. Same as surfacing a spoil board. It's tolerances are probably not as good as your "real surface block" but pretty darn close and sufficient to do the job as none of the other components are precision machined. For example, the rails that are hand ground and C-chanels are noot designed to have perfect surfaces. The beauty of Gerald's design is that despite the slight in accuracies, you still end up with a machine that can compete competently with machines far more expensive.

[cvriv.charles]

A slab of granite froma countertop is good enough to be used as a reference surface. Flat enough. Even if it was perfectly flat,... al the instruments used to square off are all inaccurate to a point themselves. I dont know what squares you guys are using but none fo them are actually 90deg. No ruler is perfectly straight either.

[KenC]

I don't really care about flatness measurement.
My old man told me this many dacades ago.
If you can see your reflection, it is polished enough & when you can see undistorted reflection, it is perfectly flat...
Charles, like all thing in life, there are no perfect & absolute in engineering measurement, there is only a range of tolerance., eg, look at tolerance table, there is alway a +/- sign infront of the tolerance.
For me on steel fabrication, 1/8 or 3mm is accurate, for machining part 50 micro (0.05mm) is doable, 10micron is precision.
when you need to measure tolerance within 1/8" a crook ruler will perform enough, but for 50micron, you need a veneer caliper, & at 1~10micron you need micrometer... etc etc etc
for set square, it depends on the application too.
All in all, it is better to have a trust worthy reference surface, & when you know the inaccuracy in your measurering equipment, you will know how to compensate as when it is needed.

[cvriv.charles]

Yes this I know. The advise about the undistorted reflection,... that's good advise. Maybe I should get it polished.