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Is countertop granite flat enough for surface plate?

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  • #16
    The few pieces of granite and who-knows-what-else slabs I have check out pretty good on my surface plate. Over about 8 inches wide by about 10 long, I can't detect any rocking at all. If I put one layer of zigzag paper (.001) under any corner, I can feel the rocking that it induces. For me, and for probably 90% of uses, that is more than good enough. Of course, I do have the surface plate to be able to reference to, and I do recommend that a surface plate be purchased anyway instead of relying of some substitute that can't be verified as to flatness and overall accuracy. The plates are not expensive, as others have suggested. Float glass may be decently flat, but it's not something to automatically trust either. Test the pieces you might use before going any further. They might be fine, or you may find some warping. That would not be a surprise to me.

    After I checked my pieces, I was then able to consider using them as sort of 'expendable' flats. If I want to sand something flat, the paper lays on one of these slabs and the workpiece is carefully 'slid around' on the paper. Any non-critical assembly could be done on them as well, where there may be a chance of contaminating the slab. Better these than the surface plate.

    I did check the granite and ceramic tiles at one of our local stores. The granite surface looks rough, though it may be reasonably flat overall. The ceramic tile I considered was way smoother than the granite, but for either I would have to make the purchase, then bring them home for an actual test against my surface plate. I may do that in the new year.

    Some of the inaccuracy figures that have been reported here are way too much- if I had found even .003 of warpage on the slabs I checked, they would be useless, except for my first application which was a base to do soldering on so I wouldn't mark up my desktop.

    Interesting to hear more of experiences on all this-
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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    • #17
      Originally posted by Teenage_Machinist
      If there is a Woodcraft store near you you can do that as well, They sell 9x12 grade B, as some woodworking tools need careful alignment aided by a reference..

      Scraping and toolmaking == NO! NOT PRECISE ENOUGH!
      A Grade B surface plate is flat to within 2 tenths over an 18" surface plate. I think even Forrest, Lane et al would be more than happy with a Grade B plate.

      "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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      • #18
        WHOOOPS! A purchased granite plate is great for scraping and toolmaking, I would not use countertop without measuring with whatever instruments are used. Misled you.

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        • #19
          Originally posted by Teenage_Machinist
          WHOOOPS! A purchased granite plate is great for scraping and toolmaking, I would not use countertop without measuring with whatever instruments are used. Misled you.
          Nice recovery How long before we have to call you Teenage_Machinist_Emeritus?

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          • #20
            I got a piece of granite from a neighbor that was 12 x 12 inches and about 1 1/4" thick
            I set it on my Precision surface plate and graphed the surface contour.
            I first shimmed it so 3 points were flat to my indicator (Plane = 3 points)
            I then used my surface gage and marked the ridges/valleys with a marker pen
            it looked like a typographical map of a mountain range.
            No rhyme or reason, with dimensions =/- .010 from median.

            Perfect for making fudge.
            Rich

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            • #21
              When I had granite counters installed in the kitchen I talked to the vendor about the process. They do all their own finish grinding and sizing. He said they can finish to what ever flatness you need. They make surface plates as well as counter tops. But he said not all granite is appropriate for surface plates and of the type that is only a subset is going to be useable.

              The stuff has a lot of stresses built into it while it's part of a mountain and when it's pulled from there and subjected to changes in orientation, temperature, and humidity (the inside of a mountain can get pretty dry) they can move quite a bit. Some stresses form pressure ridges that continue to relax over time and that may be what you saw in your sample.

              They often have to repair blow-outs with epoxy and stone rabble.

              He said he could make me a 4" grade A plate, uncertified, for a fraction of store-bought.

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              • #22
                I guess it would be naive to think that granite wouldn't have stress in it. So it's millions of years old- what's going to happen when you mine it and cut it up- you might be relieving it of a lot of pressure- ah there's more to this than I care to think about right now. Except for one thing- is there anything industry does to stress relieve it?
                I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                • #23
                  I was doing some searching and found this. Some interesting points here:

                  Sarla Exports, in the span of over one and a half decades has become a one-stop quality source for all major Indian dimensional stones, be it Marbles, Stones or Granites. While the company owns and controls some major deposits and processing facilities, it has strong strategic alliances with few external companies, thus being able to offer the complete range of building dimensional stones from India.

                  At Sarla Exports we always supply merchandise in its original form & never try to push 2nd choice material as the 1st choice. We wish to caution the buyers, of a practice, adopted by many producers of India. Like in any quarry, in India too there is abundant production of material having unpleasing / lighter shade. To push such material as premium choice, an artificial pigment is applied on the Polished Slabs / Tiles that hides all the shortcomings of the natural material. This is however a purely temporary treatment that steals the natural beauty of a natural material.

                  We strictly maintain the shade range of the merchandise as desired by the client. While producing cut to size/ Tiles, careful batching is done even when the material is cut from a same slab/block.

                  We do the repairing / netting to reasonable levels and not to the extent of joining two broken pieces. Moreover, more importantly, we use low viscosity epoxy based resins specially produced in Italy & Germany for stone application. We never use cheaper polyester-based resins, which have a limited life. While sawing, especially Green Marble, we take a lot of care, as it is a very brittle and pasty material.

                  We maintain a very low down feed and regularly monitor the sharpness of diamonds to ensure maximum tolerance of +/-0.5 mm within a same slab.

                  At Sarla Exports we have carefully controlled this very common and major problem. We never let the sawing result in slabs with a curve/bow, which makes the fabrication/ installation very difficult.We always maintain flatness of slabs to the permissible tolerance limit.

                  We polish the Marble Slabs & Tiles on automatic multiple head line polishing machines from Breton spa & Terzago spa of Italy. We never supply merchandise polished on "locally made table polishing machines".

                  We achieve the gloss between 75°-90° and guarantee an excellent flatness, proper pre bed preparation and suitable life of the gloss at international Quality Standards.

                  We maintain perfect squareness in cut to size materials, thanks to the automatic bridge cutting machine from Breton SPA, Italy. We never use locally made bridge cutting machines or conventional hand driven edge cutting machines as it is far than difficult to maintain size and squareness accuracy on such machines.
                  http://www.worldstonex.com/en/SF/Sarla/AboutUs.asp

                  Heh. So, this Indian stone products supplier thinks their Indian produced machine tools are of inferior quality. What a surprise.
                  Free software for calculating bolt circles and similar: Click Here

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                  • #24
                    Had to go back to page one to see what the original question was, gee Gregl, if you want to sell your home machined parts to NASA you're going to have to spend lots of dollars/pounds to get the accuracy required (and don't forget to cost in recession tax). Thought I saw the key word FREE in the original question? T'aint free cos you already paid for it in the kitchen mod, but if you put it in the dumpster and someone else "Discovers" it, it will be the "Find" of the centuary.

                    In the mother country we used to recycle marble washstand tops and lumps of 1/2" thick plate glass and be exstatic. Oh how times have changed?

                    Regards Ian.
                    You might not like what I say,but that doesn't mean I'm wrong.

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                    • #25
                      I think the bigger question is will Enco's $25.00 12X18 grade B Surface plates with free shipping work well as counter tops?

                      I actually considered using them as paver blocks out to my shop.
                      - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
                      Thank you to our families of soldiers, many of whom have given so much more then the rest of us for the Freedom we enjoy.

                      It is true, there is nothing free about freedom, don't be so quick to give it away.

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                      • #26
                        Stoned

                        [QUOTE=Your Old Dog]I think the bigger question is will Enco's $25.00 12X18 grade B Surface plates with free shipping work well as counter tops?

                        I actually considered using them as paver blocks out to my shop.[/QUOTE]

                        So THAT'S wot John Stevenson used for that berluddie big paving job he had with those pallet-loads of pavers.

                        I've never seen John mention that he had or used a surface plate. Perhaps he has been saving them up.

                        I guess after it was finished both he and the paving were more than just slightly "stoned".

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                        • #27
                          An i'll bet the air was slightly "Blued"
                          You might not like what I say,but that doesn't mean I'm wrong.

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                          • #28
                            I actually considered using them as paver blocks out to my shop.
                            Sure would make a nice hearth.
                            Free software for calculating bolt circles and similar: Click Here

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                            • #29
                              Originally posted by darryl
                              I guess it would be naive to think that granite wouldn't have stress in it. So it's millions of years old- what's going to happen when you mine it and cut it up- you might be relieving it of a lot of pressure- ah there's more to this than I care to think about right now. Except for one thing- is there anything industry does to stress relieve it?
                              Dunno, but my wife just showed me a blowout on our kitchen counter. A dime size shallow divot showed up under an area we keep our cook books in. No abuse possible there, it just let go.

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                              • #30
                                Some comments on granite

                                Granite is basically a cast silicon dioxide and aluminum oxide with some other minerals thrown in. Grain size, shape and color is driven by the composition and cooling rates. As a result, granite has all the issues normal for a casting. Granite is solidified liquid magma and the grain size and residual stresses are going to depend on the thickness and cooling rate of the original slab when it solidified. I've seen some writeups on granite surface plates that talk about using specially selected granite from specific locations. I have to assume that a particular quarry is tapping into a specific granite "casting" that just happened to come out better than the rest. Arkansas sharpening stones are similar in that the type and quality was determined by the quarry they came from in Arkansas.

                                I've made some tools for scraping and measurement using countertop material. A lot of what you can use depends on what you need from it.

                                I made two 12" straightedges from emerald pearl granite and the results were OK. I was told that emerald pearl comes from a specific location in Norway and is selected for large grains (up to an inch across) angle of reflection and color. I polished it mostly with harbor freight diamond plated hones and used yellow oil based artist color thinned with WD40 as the transfer spotting compound. I also made a 6" x 6" flat from paradiso granite sample, which is fine grained and pinkish red. The top surface was initially flat within about .002 and finished at about .0001 flatness.

                                Cutting was done using a 4-1/2" wet diamond saw blade (also Harbor Freight) on an angle grinder with a trickle of water put in at the entry to the cut. Messy, but effectice. A tile cutting saw would probably work better. I also use a 7" saw blade under flood coolant on a tool and cutter grinder.

                                You would be really surprised at how quickly the diamond plated hones in water will flatten granite. It's basically the same process as scraping, but you are snading rather than scraping. With the plastic back removed from the hone, the metal mesh domes slightly where you press aginst it, which works well for removing a local spot of material. With stiff backing, the mesh is stiff enough to avoid rounding over edges.

                                After taking the scraping class from Forrest last June, This has launched me on a path to see how accurately I can scrape and then measure to verify the accuracy. So far that's been limited to the accuracy of my best dial indicator and the measuring tools i've made to use the indicator.

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