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Thread: Hand Scraping - How can it be more accurate than machining?

  1. #1

    Default Hand Scraping - How can it be more accurate than machining?

    I have seen various threads on hand scraping and people wanting to learn hand scraping, but the question I always have is: How can hand scraping ever be more accurate than a machined part or a CNC machined part? When is scraping a better, more accurate technique than using a mill/lathe?

    Thanks.

    - T

  2. #2
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    Scraping if often employed when the resurfacing has to be performed in situ.

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    Quote Originally Posted by MinnesotaHSM
    I have seen various threads on hand scraping and people wanting to learn hand scraping, but the question I always have is: How can hand scraping ever be more accurate than a machined part or a CNC machined part? When is scraping a better, more accurate technique than using a mill/lathe?

    Thanks.

    - T
    Hand scraping is better when the piece of work you have won't fit on the mill/lathe. The bed of your only lathe, for example. Another case is when it is too expensive to machine the part. The bed of your only lathe, to repeat the example.

    Hand scraping 1-2-3 blocks, while an excellent exercise for the student, is probably not going to replace machined and ground 1-2-3 blocks. Certainly there are other examples though where scraping is the better solution for a particular problem.

  4. #4
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    Hand scraping is only as good as the surface you are using as the master.

    In machine grinding the wear on the wheel has to be taken into account as well as the trueness of the slides on the bed which will change with use.
    In hand scrapping the master surface has only minimal wear if any.

    Hand scraped surface plates are made in 3's in which each in turn is used as the master so finally you end up with 3 plates true to each other.

    I assume by now you could get a machine made hardened plate that is better than a scraped one but I expect that you would pay handsomely for it.

    Just my 2p worth

    Peter
    I have tools I don't know how to use!!

  5. #5
    juergenwt Guest

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    Yes we now have grinders that can grind just about any length, but before you can build a grinder or mill or lathe bed you have to have something to work from.A standard has to be set up like a prototype on which all others are based. And like any prototype you have nothing to compare it to.
    Some of the most famous machine tool builders have special clean rooms where they keep the best machines money can buy as a prototype for their product.
    So the "Prototype" is most likely something made by highly skilled craftsmen and built to perfection. Scraping a surface is one of those methods used to achieve the results needed for a prototype.
    As to your question, one answer is: a scraped surface on a machine bed besides being very precise (and costly) has the ability to maintain an oil film much better than a ground finish.

  6. #6
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    Quote Originally Posted by MinnesotaHSM
    I have seen various threads on hand scraping and people wanting to learn hand scraping, but the question I always have is: How can hand scraping ever be more accurate than a machined part or a CNC machined part? When is scraping a better, more accurate technique than using a mill/lathe?

    Thanks.

    - T
    in most cases, there's not much advantage in scraping over grinding - I can grind as well as i can scrape, but both should be more accurate than work in the mill or lathe. The advantage comes in as others have noted, lots of work is too big for surface grinder and if making references, a scraped surface takes the blue better than a ground one.

    End of the day though, probably the greatest appeal for many home shop guys is that very accurate stuff can be done without the need for grinding equipment. I view it as a basic shop skill, sort of like filing; I don't run around all day figuring out what i can file, but as a basic bench skill if you don't have it the work will suffer. There are times when its just right technique to deploy. That, and in small doses, its kind of pleasant zone-out-with-some-nice-music-on work.

    scraping is accurate to the accuracy of your reference, ie surface plate that determines how flat the results are. a good surface place over a couple of feet would be more accurate than the average lathe or mill. Maybe the part you are missing is that the scraper makes a very shallow depth of cut, a tenth or so on light finish passes. Using the blue to identify where to scrape, you gradually bring the surface to the same flatness as the reference. Also, unlike machining, there is no clamping force that distorts the work.

  7. #7
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    Quote Originally Posted by Mcgyver
    End of the day though, probably the greatest appeal for many home shop guys is that very accurate stuff can be done without the need for grinding equipment.
    That's me - no grinder, plenty of scrapers (and time).
    Mark
    What you say & what people hear is not always the same thing.
    www.remark.me.uk

  8. #8

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    Grinding or milling a large/long piece of cast iron is no more accurate than the mounting on the table. Then, the workpiece is removed from the table and moved several times and mounted to a work floor. All sorts of distortions occur during a machine move, especially a long lathe bed.

    Alternately, adjust and level machine in place, shim the mounting as accurately as possible, lock it down and then scrape in situ and WYSIWYG.

    thnx, jack vines

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    Quote Originally Posted by MinnesotaHSM
    I have seen various threads on hand scraping and people wanting to learn hand scraping, but the question I always have is: How can hand scraping ever be more accurate than a machined part or a CNC machined part? When is scraping a better, more accurate technique than using a mill/lathe?

    Thanks.

    - T
    It mostly comes down to time...Grinders these days with CNC able to compensate for inaccuracies in the machine ways are incredibly accurate and very fast compared to scraping with is incredibly slow..

    Machine tools used to be all scraped because labour was cheap, and machines expensive...Now it is the opposite..

  10. #10
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    Mitsubishi Heavy Industries used to have a picture on their machine tools web pages showing one of thier very large horizontal lathes in the finishing stages of production. In a spotlessly clean hall sat this machine with bedways 40 feet long. The only person in sight was a fellow in uniform sat on one of the bedways.

    The caption read something to the effect, "Master craftsman Mr Hiro Mitagami is hand scraping the ways on this machine. It will take as long as he needs to be satisfied the job is done right"
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