24" plate master scraping question
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  1. #1
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    Default 24" plate master scraping question

    I posted this over in the metrology forum, but it was suggested that I might get a better answer here to the scraping part of the question.

    Some years ago I bought a surface grinder. With it came a 24" cast iron surface plate that some muppet had 'reconditioned' lightly with a flap disk!
    It was free, I had a trailer, and I needed a welding plate that was 'flat' for some level of flat, so I took it.

    I also have a 30x48 Cast iron table, which looks unmarked - original planer marks are on it, only minor scratches. I have no idea how flat it is, it also is use a lot as a bench (covered with 3/4" sheet of MDF + anti corrosion paper to protect it).

    I have no intention of scraping the 30x48, although I would like to understand how flat it is at some point. It seems useful to have a large flat thing for as yet unknown reasons (it was £50 and is a useful 'bench')

    So I am going to buy a 450x450 (18") plate, probably granite and set it on its own stand. Better suited to the general size of things, and not a bench.

    However the 24x24 seems like an ideal flat thing to start on, except I have no 24x24 or larger known master.

    Can I scrape a 24x24 flat using an 18x18 as the master, but moving it around? something like do 18x18, then move and cover the last 6" or similar.

    Comments - even 'no you cannot' type welcomed.

    Dave

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    Dave,

    Yes you can.

    I remember an article about an old guy (a pro) re-scraping a large jig borer
    with just a 24" prism master. Richard King talked about that article in one
    of his posts.

    Paul

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    Can it be done? Yes.
    Could I do it? Very doubtful.
    Could you do it? Equally doubtful.

    I did once scrape a long part with a shorter reference. It worked out great until I bought a longer reference then I found it was dished. It's like asking a first-year apprentice bricklayer to build a vaulted arch.

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    I've never done a large cast surface plate but would like to get one eventually and I'll need to be able to check it. My understanding is that you can use a smaller calibrated surface to print and check the whole plate in sections, but then you need to determine that it isn't twisted or bowed overall, and for that you can use a long straight edge, preferably at least as long as the plate, and use it to check the plate length ways, diagonally, and short ways. All of the individual prints have to be considered together and you need to understand how each print hinges and see if one side is printing heavier than the other. It's the same methodology you would use when using a smaller standard to print a bigger machine way.

    When they calibrate big granite plates, they do it the same way. Use one method to determine overall linear flatness, then use another method to determine small area "local" flatness.

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    It's not the ideal way to start scraping, but it is do-able.

    Overlaps put a premium on being able to "read" the prints, and on checking "hinging" a lot. I think it is still easy to get in trouble.

    Your case is almost ideal, since you have a 2/3 vs 1/3 split on the overlap. Even so, I'd bet you get fooled on the overlap prints, and have to end up re-doing the scraping to correct a dip around the 18" mark.

    Given the little I have done that way, I think I would suggest "roughing" all of the plate first, then going back to refine it. Getting 18 x 18 good, and trying to make the rest match I think will be misleading and get you in trouble. At least it has gotten ME in trouble.......

    Interested to hear how you do.

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    I'm not sure why you don't just get a 24" x 24" granite and be done with it. It's just a little bigger and would save headaches working on the cast iron plate (although a bigger reference would be better still).

    Heck, get fancy with your stand so you can somehow have a counterbalance pivot between the two so you can alternate which is in use at any time, but maintain the same footprint.

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    I seem to be in the camp of buy as big a an A grade granite plate as one can and use it to recondition smaller cast iron plates as they become available then use the cast iron for day to day work when it’s small enough. Seemed logical at the time. And I find iron plates easy (not quick) to rescrape.

    Not sure if I’d want to rescrape a 30” x 48” iron plate even if I had access to a larger master plate. Lifting gack can be rigged up easily enough, and the scraping would just take time, but I’d imagine hinging something that heavy would be difficult. Maybe I’m wrong?

    L7

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  10. #8
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    24” granites are close to twice the price of 18” ones. Given most of the work I do is watch sized 18” seems big enough. I did consider a 12”, but I think that might end up being just a bit cramped.
    I’m only considering scraping the 24” cast iron plate because I have it already, and it was free.
    It’s been kicking around in the workshop for 10 odd years...

    Dave

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    Dave,
    How flat/good are you looking to get the cast plate. If you just want a reasonably flat surface to work from the answer is yes you can scrape it with a smaller master. But you will be doing a lot of back tracking (correcting errors you made) and it will be time consuming. In the end if you are careful, have some skills and patience you could have a reasonably flat surface. I say go for it. You will gain a new skill and appreciation for the art of machine tool scraping. good luck

    Ron


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