Rate of metal removal/hr by scraping
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  1. #1
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    Default Rate of metal removal/hr by scraping

    I don't have the computer skills to pull the text out directly.

    But if you go to this Okuma site, there are some Q & A's about their practice. Good on them for promoting an essential part of their operation, and one that many "lay people" dismiss or are unaware of in today's manufacturing world.

    Hand Scraping

    If you scroll down to the bottom, one of the Q's is:

    What surface area can a trained craftsmen scrape per hour?

    They respond that on a 1.5" x 18" gib, "up to 540 sq inches/hr" IOW 22.5 iterations, or 1 iteration every 160 seconds

    smt

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    in my companies old Standards book there is a mention of surfaces to be hand scrape must be machined good enough that .0004" "onion skin paper" put between 2 pieces must be tight held.
    .
    so if 2 pieces are accurate to one another already to say 0.0004" per foot i would think the hand scraping would go a lot faster than if the surfaces were 2 or 10 times farther apart in contact areas. i am just saying how good the parts are you start with probably influences how long it will take.

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    You are saying how long a "certain part" might take from start to finish; which is a function of "effort" (DOC) and number of iterations.

    Okuma seems to be saying regardless the number of iterations, you should be able to do "up to" 540 square inches/hour.
    Actually, IME, the closer to finish, perhaps the slower the iteration due to more spots to hit, and harder to see and more care in marking. But the effort (other than eye strain ) is less.

    smt

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    Default

    Quote Originally Posted by stephen thomas View Post
    I don't have the computer skills to pull the text out directly.

    But if you go to this Okuma site, there are some Q & A's about their practice. Good on them for promoting an essential part of their operation, and one that many "lay people" dismiss or are unaware of in today's manufacturing world.

    Hand Scraping

    If you scroll down to the bottom, one of the Q's is:

    What surface area can a trained craftsmen scrape per hour?

    They respond that on a 1.5" x 18" gib, "up to 540 sq inches/hr" IOW 22.5 iterations, or 1 iteration every 160 seconds

    smt
    I copied and pasted the text here:
    "There are a lot of factors that go into calculating how much surface area a trained craftsman scrapes in one hour. It all depends on the size of the part, of course. Let’s take for an example a gib for a cross-slide that measures 1.5” x 18”. In one hour, he should have taken up to 540 square inches off that gib, given that all other factors are correct. Scraping is just like any other job, if all factors are correct, it will take less time to scrape, but if there is one factor that is not correct then it could take up to several hours to scrape that same gib."
    That seems mighty fast. On the other hand, the first few rough passes might go pretty fast. Not answered is how many PPI they are shooting for on that part. That could make a big difference, too. I suppose, on a gib they might not be trying for super fine contact. Still seems fast whatever degree of "perfection" they are going for. Thanks for pointing out the link.
    Denis

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    If you are pushing hard on cast iron figure .0007" per pass with a biax and sharp carbide blade.

    I am not positive on this but it has been a best guess.

    ??

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    Default tolerance

    Quote Originally Posted by DMF_TomB View Post
    in my companies old Standards book there is a mention of surfaces to be hand scrape must be machined good enough that .0004" "onion skin paper" put between 2 pieces must be tight held.
    .
    so if 2 pieces are accurate to one another already to say 0.0004" per foot i would think the hand scraping would go a lot faster than if the surfaces were 2 or 10 times farther apart in contact areas. i am just saying how good the parts are you start with probably influences how long it will take.
    .
    my point is some surfaces milled or ground are already to tolerance and any scraping is more for oil retention or decorative and this might take less time.
    .
    we always try for 0.01mm per meter or 0.0004" per 40" on a CNC milling machine and modern finishing carbide inserts can leave a smoother finish than Blanchard grinding. supporting parts on 3 points at the Airy points. using Tram blocks machined to .0002" prior to mounting part. using a torque wrench. checking bottom of part on tram blocks to confirm it is sitting on clean tram blocks and not on dirt. using a 0.0001" indicator to check parts after for movement or deflection when machined. These are all done by a lot of places now a days to save time hand scraping and or eliminate the need for it. I often find a part off 0.0005" per 40 " and take another 0.001" off so it is flat parallel to within 0.0002" per 40" This at 10" per minute feed might take 4 minutes while still on the CNC machine. I assume hand scraping would take much longer than that.
    ........ When the Quality lab says a part we did is out of tolerance (say 4 foot long) we often setup again and remachine the surfaces they say are in error. if it takes me 2 hours total i am assuming the is faster and cheaper than hand scraping the same part to a less than 0.0004" per 40" tolerance


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