true position

[mrwildebeast]

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

[Paul Sarrach]

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)
__________________



Here is a True Position Calculator
http://www.engineersedge.com/calcula...sition_pop.htm

[pniven]

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.

[Matthew D. Hoedeman]

To simplify it, TP is a DIAMETER value, the XYZ deviations make a RADIAL value, 2xR=D

[in search of zero]

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.

[JCLASTOUTLAW]

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

[Matthew D. Hoedeman]

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.

[kbotta]

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

[Paul Sarrach]

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

Kevin, this Noobies are very skitish, a little at a time or they will run away