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  1. #16
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    Quote Originally Posted by BobL View Post
    There could also be a dip in the middle of the glass plate and I would not detect that.
    I rejigged one of my dial gauges and was able to confirm the ~0.1mm dip in the middle of the plate

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  3. #17
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    Bob, is that toughened glass? It is my understanding that the toughening process introduces stress into the glass. Could this effect the flatness?

    Having said that, the glass I use is toughened, and don’t know that I’d want to try using non toughened glass, as it would probably not last long.

    Lance

  4. #18
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    Quote Originally Posted by LanceC View Post
    Bob, is that toughened glass? It is my understanding that the toughening process introduces stress into the glass. Could this effect the flatness?
    I remember the glass man cutting it on the spot with a score and snap method so it could not have been toughened.
    Attempted cutting of toughened glass this way usually results in pile of glass shards.
    Toughened glass has to be annealed (heated to ~500ºC and allowed to cool slowly) before it can be cut - its is then rehardened by reheating and rapidly cooled (quenched).

  5. #19
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    Actually, $87- is quite cheap and certainly good enough for what you are doing. Hare & Forbes have some bigger surface plates but they're waaay more expensive. https://www.machineryhouse.com.au/Surface_Plates

    I'd get the Carba-tec one if I were you. Glass is very flat but sooo breakable in your workshop. If you use something that's not certified flat, you'll always be wondering if the light you see is because the tool isn't flat or the reference surface isn't flat (or both).

    Quote Originally Posted by yoboseyo View Post
    I just realised that no 2 items I have agree with each other in terms of flatness. I've compared my hand plane, combination square, spirit level and a marble slab. No matter what when I put 2 pieces against each other it lets light through.

    Carbatec has a granite surface plate for $87, but I was wondering whether there are any cheaper options.

    I've heard about float glass, but I'm not sure what float glass is, and it doesn't help that whatever I get that's supposed to be flat I have to trust that it's flat because there's no way I can know.

    BTW, this is for flattening hand tools

    Any tips?

  6. #20
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    Are all stone slabs flat? How are they made flat? That means I can use my kitchen benchtop if I wanted to?

  7. #21
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    Quote Originally Posted by yoboseyo View Post
    Are all stone slabs flat?
    I believe they are pretty flat but have not seen any numbers.

    How are they made flat?
    They're ground flat with a diamond grit grinding machine.

    That means I can use my kitchen benchtop if I wanted to?
    You could use it to check a surface, but if you used it to lap with you will dull and eventually, depending on how much you do, wear away the surface.

    Ideally you should have two flats - one to lap with and one to check on. Eventually when the one you use for lapping gets dips in it you get another.

  8. #22
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    Quote Originally Posted by Dougsshed View Post
    Actually, $87- is quite cheap and certainly good enough for what you are doing.
    Unfortunately 300x300 it's too small for grind a no. 7 plane on ...unless there's a way to do that

  9. #23
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    I've got one of the Carbatec granite surfaces - imo, just get it. You'll never need to worry about 'is my surface flat' again. As long as you don't drop it, it's a lifetime tool. I do abuse mine a bit (metal workers cover your eyes) and use spray adhesive to stick on sandpaper for flattening and shaping really rough irons. Plane soles I do on sandpaper spray fixed to a melamine board that sits on my bench or table saw. It's flat enough for a sole.

  10. #24
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    Surface plates are ground to a very precise level of flatness and they are very thick and dense so that they don't flex or warp. Kitchen bench tops are quite thin (usually only 19mm) and that means that when they are installed in your kitchen, they will flex and distort somewhat because the carcass of the kitchen cupboards will not be perfectly flat. You won't notice this normally but if you put a long straight edge on your bench, you would probably see daylight underneath it. If you don't, maybe its good enough for what you want. However, I assume that you started this thread because you want a surface that is flat to within a very tight tolerance. So, in the end, is $87- really too much to pay for that peace of mind. If it is, use the glass or kitchen bench top and hope for the best.

    Quote Originally Posted by yoboseyo View Post
    Are all stone slabs flat? How are they made flat? That means I can use my kitchen benchtop if I wanted to?

  11. #25
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    In that case, I'd get a piece of MDF as thick as you can and have a piece of glass (also as thick as you can get) cut to match and all long and wide enough for your needs. Glue some raised edging around the MDF to stop the glass sliding around. That's probably as good as you'll get without spending hundreds of dollars. It might have been helpful if you'd said the carbatec granite surface was too small right at the start.
    Quote Originally Posted by yoboseyo View Post
    Unfortunately 300x300 it's too small for grind a no. 7 plane on ...unless there's a way to do that

  12. #26
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    Quote Originally Posted by Dougsshed View Post
    In that case, I'd get a piece of MDF as thick as you can and have a piece of glass (also as thick as you can get) cut to match and all long and wide enough for your needs. Glue some raised edging around the MDF to stop the glass sliding around. That's probably as good as you'll get without spending hundreds of dollars. It might have been helpful if you'd said the carbatec granite surface was too small right at the start.
    I have an aluminium straight edge 72*25*2000. Hollow, of course, and I don't know how thick it is but it's quite light. the only question is whether it's rigid enough. It feels rigid

  13. #27
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    I have a piece of half inch laminated glass out of a shop front that has served me for over thirty years. When dropped it does the concrete floor more damage than the glass.

  14. #28
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    Quote Originally Posted by chambezio View Post
    Molten glass is "floated" on molten tin
    Sorry, my mistake. Same principle, just a different liquid, unless you want to replicate it, of course.

    mick

  15. #29
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    I went around the house just now with my Lapmaster flatness gauge. I took several points on three different granite countertops, a glass table and a mirror, All showed variations in flatness of 0.0002" or more, some were as high as 0.0005". The flattest was one bathroom counter with only a couple of spots that were > 0.0002" out of flat. None were flat enough to produce an optimally flat plane bottom.
    Innovations are those useful things that, by dint of chance, manage to survive the stupidity and destructive tendencies inherent in human nature.

  16. #30
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    Quote Originally Posted by rob streeper View Post
    I went around the house just now with my Lapmaster flatness gauge. I took several points on three different granite countertops, a glass table and a mirror, All showed variations in flatness of 0.0002" or more, some were as high as 0.0005". The flattest was one bathroom counter with only a couple of spots that were > 0.0002" out of flat. None were flat enough to produce an optimally flat plane bottom.
    Even 0.0005" or 0.013 mm sounds good enough for wood working to me, especially if we are talking DIY.
    What level of flatness do you think is required for wood working?

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