profile calculations

7/29/2019 Profile Calculations

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PC-DMIS Profile Calculation Examples

By Wade Burton

Wilcox Associates, Inc.

The profile dimension has many cases, according to Dimensioning and Tolerancing,

ASME Y14.5M-1994. In the following example from Fig. 6-11 of the standard, profile

dimensions can have a bilateral tolerance, unilateral tolerance (inside), unilateraltolerance (outside), and bilateral tolerance unequal distribution:

A

0.8 A

(a) Bilateral tolerance

A

0.8 A

Unilateral tolerance

(Inside)

A

0.8 A


(Outside)

A

0.8 A


(Inside)

0.2

THIS ON THE DRAWING


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MEANS THIS

(a)

0.8 wide tolerance zone

equally disposed about the

true profile (0.4 each side)

Actual profile

(b)

0.8 wide tolerance zone entirelydisposed on one side of the trueprofile, as indicated

Actual profile

Datum plane A

True profile relativeto datum A

(c)

0.8 wide tolerance zone entirely

disposed on one side of the trueprofile, as indicated

Actual profile

True profile relative to datum A

Datum plane A(d)

True profile relative to datum A

Actual profile

0.8 wide tolerance zone unequally

disposed on one side of the true

profile, as indicated

0.2 0.6


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Bilateral Tolerance

The case of bilateral tolerance can be interpreted two ways This can be the way where you dont care the

position of the tolerance lines, and the curve is allowed to rotate anywhere, but its shape is checked against

an equally-distributed tolerance. In the above example, that means the 0.4 tolerance on each side can be at

any location or at any rotation. In this case, in PC-DMIS, you would create a profile dimension with the

form only option and one plus tolerance value. In versions before V3.6, to allow any rotation, you mustalso create a best fit alignment on the feature before the dimension. In V3.6, this alignment is doneinternally in the dimension to allow this rotation. In this case, the measured value is calculated as the

maximum deviation minus the minimum deviation, and the max and min will be on opposite sides of the

translated/rotated nominal curve. In most cases, the max and min will be the same magnitude.

The other case of bilateral tolerance is where you dont allow the translation/rotation to optimize the curve.

The location of the tolerance bands are being fixed, but they are still an equal distribution. In this case a

profile dimension would be created in PC-DMIS with the form and location option, with two equal

tolerance values. The calculation of this case will be discussed together with the remaining cases.

Other tolerance cases

The remaining cases are actually similar cases: unilateral tolerance (inside), unilateral tolerance (outside),and bilateral tolerance unequal distribution. These cases fix the location of the tolerance bands, and

therefore must be created in PC-DMIS using the form and location option of the profile dimension. The

deviations for these are calculated relative to the true nominal curve, along the directions of the approach

vector for each hit. The max and min values arent necessarily on opposite sides of the curve, and this

certainly affects the calculation of whether or not the dimension is out of tolerance.


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To illustrate, I created a profile dimension inside PC-DMIS with a max of 0.038 and a min of 0.036, with

the deviations completely on one side of the nominal. I used tolerance values of 0.04 and 0.04. When the

deviations are on two sides of the nominal, the total deviation is max min, but when they are on one sideof the nominal, it is the largest magnitude, in this case the max.

Deviation on one side of the nominal

Deviation on both sides of the nominal

Here is where you must remember that PC-DMIS treats form and location profiles different in the sense

that it isnt just checking the measured value against the nominal plus the plus tol and the nominal minusthe minus tol, as it does in the form only case. If the max and min are both positive, it checks that max is

less than plus tol. If they are both negative, it checks that abs(min) is less than abs(minus tol). If the maxis positive and the min is negative, it checks both that max is less than plus tol and abs(min) is less than

abs(minus_tol). If you study the last pictures here you can see that this is how it should be, not just a

blanket 2*max. Thus, in this example, the measured value that is checked is 0.038. Note that this would

have the same result as when the tolerances were 0.04 and 0.00001.

Many people turn off the ability to show the max and min values in their PC-DMIS dimensions via the

FORMAT command. It is important to realize this may cause people to incorrectly interpret their form and

nominal

-0.04 minus tol

0.04 plus tol

max

min

nominal

-0.04 minus tol

0.04 plus tol

max

min

Total

deviation

Total deviation


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location profile dimensions. As shown above, the important checks for the form and location dimensions

involve the max, min, plus tol, and minus tol, not just comparing the measured value. It is possible to have

max and min that violate plus tol and minus tol , yet still give a measured value that is smaller than the plus

tol. This happens when shape of the curve is still relatively close to the nominal curve, but its location is

not close to the nominal curve.

Size ControlPC-DMIS controls the size of a feature with the profile dimension indirectly through its form and location

option. Currently, the form only option does NOT control the size of the feature. However, you can create

a best fit alignment on the feature, which will allow the feature to rotate and translate to find the optimal

position that minimizes the deviations, and then do a profile with a form and location.

Final Note

It is important here to note that ASME Y14.5-1994 does talk about checking for form only or form and

location and to tie that into this discussion. Basically, case (a) of bilateral tolerance with an equally

disposed tolerance zone is checking form only, because it is allowed to shift. All other cases shown here

are actually checking the form and location, because the positions of the plus and minus tolerances,whether it is unilateral or bilateral with unequal distribution, must be fixed to the indicated position.

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