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Thread: true position

  1. #1

    Default true position

    Hi All,
    Can anyone explain ( or point to an explanation ) of the maths behind a true position calculation, my gd & t knowledge is basic ish, when i put a true position calculation on say, a circle the reported true position error looks like roughly double what I would expect from looking at the component x,y and z errors.
    I,m using cad++ 4.2mr1 btw.

    thanks for any help

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    That is because it is reporting diametical so it is multiplied by two.
    SQRT(xdev^2+ydev^2+zdev^2)


    It is also:

    x(dev)^2+y(dev)^2+z(dev)^2

    or(x(dev)*x(vector))^2+(y(dev)*y(vector))^2+(z(dev)* z(vector)^2)
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    Here is a True Position Calculator
    http://www.engineersedge.com/calcula...sition_pop.htm
    Last edited by Paul Sarrach; 05-16-2008 at 02:50 AM.
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  3. #3
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    In two dimensional take the x squared plus the y squared, fine the square root of the answer then times 2.


    Mathematical Definition of Dimensions
    Version 2.0 Wilcox Associates Inc. 3 of 12
    1.0 True Position
    The true position analysis performs a virtual hard-gauge bestfit of the considered
    feature and datums at material condition. This algorithm works based on allowed
    degrees of freedom (DOF) as determined by the datums that are selected. There are
    six possible degrees of freedom for the bestfit -- translation in x, y, or z, and rotation
    about the x-axis, y-axis, or z-axis. For example, a primary plane datum would be
    used to control rotation about the x-axis and y-axis, and translation in z. A secondary
    cylinder datum would control translation in x and y and a tertiary cylinder datum
    would control rotation about the z-axis. When primary, secondary, and tertiary
    datums are specified with all at RFS, the true position is fully constrained and no
    bestfit is performed. When one or more of the datums is at material condition (MMC
    or LMC), the corresponding degrees of freedom normally constrained by that datum
    can be used by the bestfit algorithm up to the limit defined by the bonus on that
    datum of size.
    The MEAS position of the considered feature that is reported by PC-DMIS in the true
    position report is the bestfit position (not the actual measured position). The TP
    bonus that is reported is the sum of the bonus on the considered feature plus the
    unused bonus after the bestfit on the datum of size that controls location.
    When the considered feature is a cylinder (or other axis feature) or a set of cylinders
    (or axis features), the TP analysis uses the extrapolated MEAS endpoints of the
    cylinder(s) to determine the TP deviation and to "fit" the datums. The extrapolated
    MEAS endpoints are determined by intersecting the MEAS axis with the plane
    formed by the THEO start point and THEO axis vector to get the MEAS start point,
    and the plane formed by the THEO end point and THEO axis vector to get the MEAS
    end point. The user would probably want to select "From WORST end of axis" to
    report the TP deviation. The "Reference Length" is used for cases where a Projection
    Zone is desired.
    When the datum is a feature set, no material condition is allowed on the datum set
    and the centroid of the set is the datum position.
    Starting with V3.7, the bestfit algorithm attempts to zero the deviation on the
    considered feature using as much available datum bonus as necessary.
    Starting with V4.0, true position can be defined using feature control frames similar
    to the callouts on drawings. The user must first define which features are to be used
    as datums by assigning datum letters A, B, C, etc to the datum features. Datums can
    be specified with material condition. The feature control frames also allow the
    definition of composite true position with an upper segment PLTZF pattern locating
    callout and a lower segment FRTZF feature locating callout. The user can specify
    cylindrical or planar tolerance zones. In addition, the simultaneous evaluation
    command can be used to evaluate two or more true position callouts simultaneously
    Mathematical Definition of Dimensions
    (as if they were all one callout) provided that they use the same datums in the same
    order of precedence with the same material conditions.
    In addition, starting with V40, a datum used in a feature control frame can be a set of
    axial features (e.g., cylinders) all of which are nominally parallel to each other. The
    feature control frame user in V40 can also specify a compound datum such as A-B
    where A and B are individual axial datums that are nominally parallel (or coincident).
    Starting with V41, the virtual condition of the datums takes into account any true
    position or orientation tolerance on the datum itself. The bonus on the datum of size
    at material condition is, therefore, the size bonus plus the true position and/or
    orientation tolerance refinement.
    The PC-DMIS true position bestfit of the datum reference frame is in conformance
    with ASME (ANSI) Y14.5.
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    To simplify it, TP is a DIAMETER value, the XYZ deviations make a RADIAL value, 2xR=D

    Quote Originally Posted by AndersI View Post
    I've got one from September 2006 (bug ticket) which has finally been fixed in 2013.

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    Default

    Quote Originally Posted by mrwildebeast View Post
    Hi All,
    Can anyone explain ( or point to an explanation ) of the maths behind a true position calculation, my gd & t knowledge is basic ish, when i put a true position calculation on say, a circle the reported true position error looks like roughly double what I would expect from looking at the component x,y and z errors.
    I,m using cad++ 4.2mr1 btw.

    thanks for any help
    The first 2 guys are correct. GD&T is simple yet complex, it is very important that you learn this to properly make programs, interpret prints and output. Purchase a book and a bottle of spirits and welcome to the nightmare.
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    Be careful of the TP callout it may only call out x or y or z or both or all 3 axis and if MMC is involed grab it. This will help you be inspect. JC

    Jeremy Cavender
    CMM Metrology Engineer
    PC-Dmis 3.2/2010/MR2

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    Another thing to keep in mind, depending on the stuff you check is PERP to CENTER LINE. This will give you a '2-D' TP for a hole in a curved surface. BAsically, what it will do is eliminate the surface deviation gotten from the surface sample hits to simulate what it would 'look' like if you had a fixture and a stab pin.

    Quote Originally Posted by AndersI View Post
    I've got one from September 2006 (bug ticket) which has finally been fixed in 2013.

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    Another monkey wrench to throw in - the "bonus" (or additional tolerance) is supposed to come from the size of the actual mating envelope. that means if your cylinder is off orientation by ANY amount, a gage pin size will not be the true "bonus size", because the cylinder is "acting" different.
    Do your homework on this one...
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    Quote Originally Posted by kbotta View Post
    Another monkey wrench to throw in - the "bonus" (or additional tolerance) is supposed to come from the size of the actual mating envelope. that means if your cylinder is off orientation by ANY amount, a gage pin size will not be the true "bonus size", because the cylinder is "acting" different.
    Do your homework on this one...
    Kevin

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