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Basic dimension

Shane-o
BASIC DIMENSIONS USE G D &T TOLERANCING, THE QUESTION IS I HAVE BASIC DIMENSIONS COMING FROM X & Y AXIS. AM I SUPPOSED TO "CUT" THE TOLERANCE IN HALF WHEN DIMENSIONING THE BASIC DIMENSION? example X AND Y ARE BOTH [2.077] THE TRUE POSITION CALLOUT IS ALLOWING UP TO .015 SHOULD I USE .0075 FOR X AND .0075 FOR MY Y TOLERANCE?
  • +1,000,000

    There is a correct mathematical formula for calculating true position RFS. Simple and quick. Square deviation of X and Y, add them together, take the square root of that number and multiply that by 2. That would be the formula you give your operator.

    mmmmmmmmmmmmmm.... lasagna
  • You obviously have no idea about how basic dimensions work (or even that there is a button marked 'caps lock' on your keyboard...), so I'll give you the advice that I nearly always give those who either can't or don't understand the mathematical theory behind it:

    Take your "true position" value, and divide by 1/3 for the safe amount of tolerance in either direction. This will in effect be over-tolerancing the position, but will keep even the least experienced operator safe from creating scrap.

    This is far better, and correct.


    There is a correct mathematical formula for calculating true position RFS. Simple and quick. Square deviation of X and Y, add them together, take the square root of that number and multiply that by 2. That would be the formula you give your operator.

    NUGUY,

    Thank you for supplying the correct information to someone that is learning.
    Much better than teaching them an incorrect method that they'll start using and be embarassed when someone calls them out.

    All of us were new at one time or another. Someone taught us.
    It's our obligation to teach others.
  • +1,000,000

    There is a correct mathematical formula for calculating true position RFS. Simple and quick. Square deviation of X and Y, add them together, take the square root of that number and multiply that by 2. That would be the formula you give your operator.

    Or, as I tell the imagineers when they get uppity:

    Position is calculated as double the root of the sum of the squares of the axial deviations. Slight smile That usually shuts them up for a few minutes while they try to sort it out in their heads.

    You can also draw a circle bisected vertically and horizontally with a line drawn from the intersection of the bisectors to a point on the circle which is the point created at the intersection of a line drawn perpendicular to one of the bisectors, and show them that the triangle created can be solved by Pythagoras' theorem, and then point out that the hypotenuse is the radius, which is half the diameter, which is denoted by the number in the FCF (if preceded by the diameter symbol).
  • Basic dimensions by nature are a theoretically exact value; however, the feature(s) of a part they define as ideal or exact do need to have tolerances to permit acceptable levels of imperfection during manufacturing.

    When a feature is defined with basic dimensions, the tolerance for that feature must be expressed through a geometric tolerance. Most often, the geometric tolerance is indicated directly to the feature or feature of size on the face of the drawing; however, some companies include a general geometrical tolerance (such as a position tolerance or profile of a surface tolerance) in the drawing’s general notes. This can be an effective tool when the note is carefully written.

    Drawings based on ISO standards frequently use a class of general geometrical tolerances standardized in ISO 2768-2 :1989. The three classes are identified through the use of the upper case letters H, K, or L after the ISO 2768 indication on the drawing. You would need a copy of the standard to interpret the amount of tolerance available.

    If you are absolutely certain that a geometrical tolerance has not been applied to the basic dimension (check all drawing views), then the drawing must be considered as incomplete. Without a tolerance, selecting a manufacturing process for the considered feature is impossible to do with any amount of certainty, and there is no acceptance criteria to compare measurements to during the inspection process.


  • There is a correct mathematical formula for calculating true position RFS. Simple and quick. Square deviation of X and Y, add them together, take the square root of that number and multiply that by 2. That would be the formula you give your operator.

    I assume that could work in an educational setting. Around here, though, most machinists can't even spell "machinist" correctly, and you expect me to be able to teach them Pythagorean theorem? Even if I had better than a 10% chance of getting any one of these pikers who pass for operators on the floor to even use the equation properly, I've found that even a lot of educated people forget to multiply by 2 when doing a position calculation. No, I'm sorry, but I've TRIED doing that. I even had a supervisor translate my instructions into Spanish, Vietnamese, and Ukrainian, and STILL nobody did it correctly.

    No, I'm sorry, but I'm just going to keep telling people to go 1/3 of the total in any direction. It's safer that way.
  • I assume that could work in an educational setting. Around here, though, most machinists can't even spell "machinist" correctly, and you expect me to be able to teach them Pythagorean theorem? Even if I had better than a 10% chance of getting any one of these pikers who pass for operators on the floor to even use the equation properly, I've found that even a lot of educated people forget to multiply by 2 when doing a position calculation. No, I'm sorry, but I've TRIED doing that. I even had a supervisor translate my instructions into Spanish, Vietnamese, and Ukrainian, and STILL nobody did it correctly.

    Being able to spell has nothing to do with being able to square, add and multiply few numbers, especially if the people do not speak English as their first language.
  • Being able to spell has nothing to do with being able to square, add and multiply few numbers, especially if the people do not speak English as their first language.

    You mean they operate a manual lathe and mills or are they just button pushers.
  • I assume that could work in an educational setting. Around here, though, most machinists can't even spell "machinist" correctly, and you expect me to be able to teach them Pythagorean theorem? Even if I had better than a 10% chance of getting any one of these pikers who pass for operators on the floor to even use the equation properly, I've found that even a lot of educated people forget to multiply by 2 when doing a position calculation. No, I'm sorry, but I've TRIED doing that. I even had a supervisor translate my instructions into Spanish, Vietnamese, and Ukrainian, and STILL nobody did it correctly.

    No, I'm sorry, but I'm just going to keep telling people to go 1/3 of the total in any direction. It's safer that way.

    This is why the correct formula is written on the dry erase board in the inspection lab. Along with a number of other formulas people need at our place.
    Some will learn. Many will if treated with dignity.
  • "You mean they operate a manual lathe and mills or are they just button pushers. "

    About 1/3 - 2/3 split.

    "Some will learn. Many will if treated with dignity."

    And the corollary to that: In the mean time, expect the scrap rate to remain elevated......
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