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Thread: What is close tolerance work?

  1. #1
    Join Date
    Jul 2001
    Posts
    533

    Post What is close tolerance work?

    Here is a question that is of no value other than to show the differences in the way we think about machining.

    What do you consider close tolerance work.

    I was talking to a man at the Mazak facility about machining and some of the things he and I had done and seen. I told him about a part we had been making in my shop that had a bore tolerance of +.0004 -.0000. I was quite sure he would think this tolerance was damn tight I sure did. He worked in some type of R&D shop for Ford in Detroit and told how they worked with tolerances measured in microns and I am sure he new that is real damn tight.

    Anyway that is where the question was spawned?

  2. #2
    Join Date
    Apr 2001
    Location
    Maine
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    6,700

    Post

    The man I consider my mentor, Dan Fay, used to say that people in home shops frequently didn't give themselves credit for the tolerances they achieved in fitting parts. They might not be fitted to a *specific* dimension, but the one-off fit of, say, a piston to a bore in a home shop can be within a couple ten-thousandths. As long as you control both parts and you don't have to achieve "interchangable parts" tolerance, which implies specific dimensions, it's amazing what you can achieve in your basement.

    As far as making something to a specific dimension goes, I think your +0.0004, -0.0000 is about as close as I'd want to worry about on a regular basis. Measured accuracy to under half a thou starts to get problematic.

    Another story: years ago I took a night class at the local voc-ed school. The instructor was a machinist at MIT's Draper Labs. He regularly worked to a tenth or less, and said when an occasional job came along with a tolerance of a thousanth, it felt like he almost could do it with a hammer and cold chisel. It all depends on what you get used to....
    ----------
    Try to make a living, not a killing. -- Utah Phillips
    Don't believe everything you know. -- Bumper sticker
    Everybody is ignorant, only on different subjects. -- Will Rogers
    There are lots of people who mistake their imagination for their memory. - Josh Billings
    Law of Logical Argument - Anything is possible if you don't know what you are talking about.

  3. #3
    Join Date
    Oct 2001
    Posts
    126

    Cool

    My favorite quote about work tolerance:

    A house framer works to the closest 1/8".
    A cabinet maker works to the closest 1/32".
    A boat builder works to the closest boat.

    Nuttin' to do with nuttin', but I love the opportunity to say it...

  4. #4
    Join Date
    Jun 2001
    Posts
    484

    Post

    Apples and oranges. You have to know what you are comparing.

    Your .0004 inches is 10 microns, see you are working in microns also and didn't know it.

    A micron is .001 of a Millimeter I believe.

    A young fellow I know whom worked in a bearing company used to spout micron tollerances to me, I'm sorry my brain functions to the English system of measurement, and I would have to translate inch measurements to him.

    .0004 tolerance is close for common work, try to get much finer and you will have to start using air gaging and temperature control-compernsation, even at .0004 you have to watch the temperatures.

    Work with a fellow whom worked for a hydralic pump manufacturer, he says, most tolerance I ever had was 2 tenths, but he was running a hone and a lap, Sheffield air gaging, optical flats, surface profilometers and a inspection department at his disposal. This same fellow couldn't use a set of telescope gages and mics to a thous.

    .0004 is close for ordinary methods, think of it this way, plus or minus .0002, dern, closer than you thought, why thats only 5 microns.

    I just love these metric people.

  5. #5

    Post

    .....and I worked with the Brit engineer that called everthing metric "mills".....
    (for "millimeters", I guess....)

    I think the point behind Gizmos quote is that it is all relative to the type of work you do. The shop I work in has been building inspection gages, but environment-wise, we're probably running the ragged edge in being qualified to inspect our work. For a gage lab it would be a different story.

    Anybody can hit close numbers....there's a trick (technique) to everything. It's simply a matter of figuring it out (or being taught by one who knows).




    [This message has been edited by BrianH (edited 10-19-2001).]
    b

  6. #6

    Cool

    FYI Guys,

    Metric system "rules" in science at least. Makes unit conversion so simple your dog could do it.

    Hardinge HLV lathes have a spindle runout of .oooo25" or .ooo635mm or .635 microns or 6,350 Angstroms (about the wavelength of HeNe Laser light).

    Starrett/Webber Chrome Carbide Master (grade 0.5) blocks have a tolerance of +-.000001" (1 millionth) or .0254 Microns or 254 Angstroms.

    It is good to know craftsmanship it not dead yet...

    Dave

    Also 1" = 1000 mil


    [This message has been edited by Thrud (edited 10-19-2001).]

  7. #7

    Post

    Hi,Me again.
    I agree that what you need is what to work toward. More time is lost due to over specified work than can be measured. With computer drawings going to CAM no tolerances are specified for a prototype shop. We have to make drawings for the manual machines. The draftsman isn't a machinist and every dimension is in "tenths" (four decimal places) on the drawing! We in the shop have no access to the overall drawing and do our best but, if we knew where the part was going we could save hours by dropping back to +or- a few in most places and putting the accuracy where it is required.
    Lautard tells the story in a "Bedside Reader" of a fellow who got a drawing of what looked to be an indexing plate of brass. He did it right proud!! Then found it was to be a cover for a floor drain..Need I say more!
    toff
    To know by reading is different than knowing by doing. OR:
    What you have going into a situation is knowlege..What you have coming out of that situation (providing you survive!) is wisdom.

  8. #8
    Join Date
    May 2001
    Posts
    462

    Cool

    To answer the original question, I try to work to +/- 0.0000. Since this is unacheivable, +/- .001 or .002 usually works just fine for most parts. For precision work, I try to get as close as possible, but +/- .0005 is acceptable error and is probably the best I can do. Was talking to a tool and die maker once, told him I can get by being off a thou or two and he said that that much slop in his work would be junk. So I guess it's all relative.

  9. #9
    Ron LaDow Guest

    Post

    I think Toff's got a point.
    Given time, most any machinist can hit a couple of tenths. But does this widget need that kind of fit?
    I've wasted time on fitments that really should have been looser, but that mandrel sitting in the main saddles right over there better be within a couple of tenths.

  10. #10
    Join Date
    Oct 2001
    Posts
    154

    Smile

    I think most toolmakers talk a better game than they play. At least in my experience. Yep, I used to do that stuff too. Metal stamping die work mostly. Work quality varied enormously depending on who did it. Rarely was anything held to .0001. Plus or minus a half was the basic tolerance I guess.

    The factory shut down so I had to take a job as a machinist. The closest work I do is with mating parts like a bearing that presses on a shaft. Sometimes they'll tell you: "Just make it a peck fit." I don't think they realize how close that is!

    Steve

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