What general tolerances do you scrape to.
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  1. #1
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    Default What general tolerances do you scrape to.

    I have been scraping the compound, and carriage of a Rockwell 14 and have so far achieved alignment and flat to about .0003 at the worst and in some cases a little better. Is that common practice. I realize it depends on the size of work and type of machine. What should I be aiming for?

    Edit: I meant to write "achieved alignment and flat to about .0003" However first typed .003" A much different number
    Last edited by lowCountryCamo; 07-01-2014 at 08:05 PM.

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    I am not sure the size of a Rockwell 14. Take some close up pictures and how bad was it when you started? Take some pic's of your scraper and straight-edges, so much easier to answer when I have some visuals and dimensions. Rich

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    Quote Originally Posted by lowCountryCamo View Post
    I have been scraping the compound, and carriage of a Rockwell 14 and have so far achieved alignment and flat to about .003 at the worst and in some cases a little better. Is that common practice. I realize it depends on the size of work and type of machine. What should I be aiming for?
    You're scraping for alignment, therefore you would scrape to the point that the machine is within the tolerance you require. Sorry that may not be the "instant gratification" answer you're after, but I think it's important to always keep in mind what you're trying to achieve rather than the process used to get there.

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    I will take some pics. The lathe is 14 swing 40 centers about 2000 lbs. The ways were concave by .002 in the center and slides convex like a rocking chair also about .002. I am not doing the bed as the ways there are hardened and don't show much wear. Also my longest SE is 18". I am using shop made carbide scraper and made a CI lapping plate on a motor with green diamond compound. I have a Collins micro flat 18 by 6 by 3 granite square, a Chinese B 12 by 18 plate, and .0001 and .0005 indicator. My SE is a shop made from a rib in a piano sound board. After milling it I stress relieved best I could in a fire to a red heat and then covered by coals, ashes and fire brick allowed to cool for 24 hours. The reason I believe I am flat to .0003 is because that is the tolerance of my plate and I verified that best I could with a tenths DT. This is my first big machine scraping project with some experience scraping smaller tooling. I have been reading closely here and Machine Tool Reconditioning, and I am learning much. Thanks for the help guys.

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    Is it one of these?
    Page Title

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    Quote Originally Posted by Pete F View Post
    Is it one of these?
    Page Title
    Yes sir. Just like that.

    I just edited my original post. I meant to write " achieved alignment and flat to about .0003" but first typed ".003"

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    These are the tolerances and process you will need to go through for your lathe, scroll down to the section on lathes as it covers all machine tools. However the first part of the book is worth reading before any material is removed. Careful reading at that! I'd also suggest "Machine Tool Reconditioning" by Connelly.

    http://www.totallyscrewedmachineshop...hlesinger).pdf

    I don't know if that's quite the answer you were hoping for, but in my honest opinion I do feel it's the appropriate answer. Again, you're scraping for alignment, not to make the part look pretty, so the above is what you're aiming for. Your original post was too vague in relation to the end goal to say whether you're getting close to the end result.

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    I would guess when I am scraping, I would look to make it wear in before it wears out, meaning you deliberately scrape something not flat.. Take a saddle for example, it will wear at the ends first before the middle.... So scrape the middle a tad hollow... Rich often mentions for example grinders where the table overhangs the ways, he scraped the bottom middle hollow, so the table slightly rises at the extremes of travel to counter the fact it droops due to gravity affecting the unsupported end of the table...

    More to it then just how flat can I get it...

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    Right on RC!! As a rebuilder you need play detective as plot out the wear and figure out why it wore that way. Look at the original surfaces and measure from them to bring the machine back to the original spec's. The manufacture 99% of the time did it right and you can follow his work to speed up your rebuild. i use to tell my men that I would want them to read the manual and get a "game plan" before loosening a bolt. Then when dismantling it clean the parts and look for bad ones and write up a parts list even when their hands were dirty. i would make copies of the parts manual so he could mark the copy and not worry about getting it greasy. I can recall when a guy would pull apart the machine and toss the parts in a box an 6 months later discovering a part that he forgot was worn out and we had to wait a while to make or order one. Now it is so easy to take digital pictures when dismantling it too. Or take pics, clean as you take apart, use plastic zip lock bags to put the parts in and mark the bag with where the parts came from to make assembly fly bye.

    I will read now and let RC and the other Craftsmen teach now. I will add something if I see something that needs to be tweeked. Rich

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    I was thinking of something that I use for a general rule of thumb:
    Conventional machines: That would include your lathe, other lathes, regular mills, standard surface grinders, standard cylindrical grinders etc.:
    .0002" per 12" with 12 to 20 PPI (Points per Inch) with 50% contact.

    Precision Machines: That includes all CNC Machines like VMC, HMC anything with a ball screw in general and Jib Bores,jig grinders, Studer and Drake type Cylindrical grinders, surface plates, Straight-edges, etc. .00005 per 12" 30 to 40 PPI with 50% contact.

    The spec's are for scraping and alignment. I was going to include squareness too, but we leave some area's out of square on purpose to compensate for tool pressure, sag, wear as RC mentions. This technique to concave the shorter bottom side can be tested using a precision level setting in the table top and as you push the table out you watch the bubble to check for sag or if the table bends. I use this mostly on Excello boring machines or Heald ID Grinders that have thin and long tables the bend like a noodle.

    Also remember to leave the center 40% of one side, it depends and not always the short or moving slide to compensate for wear because the dirt works it way in under the wipers.

    You have to investigate and think about which side. Like the bottom of a short table of a long bed grinder would be relieved. Or bottom of a short bed grinder with a long table moving over it. One needs to check to be sure if the machine has long travel that the moving part does not fall into the relief. This amount varies depending on the machine. But for the most part scrape the high spots in the middle 2 to 5 scrapes or .0004" to .001" low. Some machine builders relieve with a end mill and it might be low by .030" I usually don't do that because dirt and coolant can work its way in. If you ever need help figuring this out please write me as sometimes I miss questions asked on here. Rich

    Edited: The info on relief of 40%. and concave scraping.
    Last edited by Richard King; 07-05-2014 at 07:08 AM.

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  14. #11
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    Quote Originally Posted by lowCountryCamo View Post
    Edit: I meant to write "achieved alignment and flat to about .0003" However first typed .003" A much different number
    I'm glad you made the correction. Twice while replying I typed in a reply that questioned the original number, but erased is as I thought surely I must have misunderstood, but was the reason for my comment "Your original post was too vague in relation to the end goal to say whether you're getting close to the end result."

    I thought that was a little more diplomatic than my original thoughts, which were along the lines of "0.003! Jee-zus, what are you using for a scraper man, an axe?!!!"

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