Line Profile Tolerance in GD&T How CMMs Measure and Evaluate Complex Curves

Line Profile Tolerance in GD&T: How CMMs Measure and Evaluate Complex Curves

Line profile tolerance in GD&T explains how complex non-circular curves are controlled, measured, and evaluated using CMMs, trigger probes, scanning probes, and PowerDMIS software.

Geometric Tolerance Type

1. Overview of line contours

Line profile is the condition in which an arbitrarily shaped curve maintains an ideal shape on a given plane of the part.

Line profile tolerance refers to the amount of change allowed by the actual contour line of a non-circular curve to the ideal contour line, which is used to control the shape error of the cross-sectional contour of the plane curve on the part.

Non-circular curves of common parts in production include cam contour lines, turbine blade cross-sectional contour lines, non-circular curve templates or die-by-die working surfaces, etc. The shape of non-circular curves is relatively complex, and the theoretical correct size is usually used to determine its ideal shape.

According to the functional needs of parts, there are two types of requirements for line contours.

1.1. Line contour without benchmark requirements

It means that the measured contour line is a single element, that is, only the shape tolerance requirements are given to itself, and there is no reference requirement. Its tolerance zone is the area where the diameter is equal to the tolerance value t, and the center of the circle is limited by the two envelopes of a series of circles on the theoretically correct geometry

1.2. Line profile relative to the reference system

It means that the measured contour is an associated feature, that is, its shape has position requirements relative to the datum. This ideal contour is the ideal position relative to the datum. The tolerance zone is also a region limited by the two envelopes enveloping a series of circles with a diameter of tolerance value t, but the centers of the circles are located on lines with a theoretically correct geometry relative to the datum of the ideal location. This tolerance requirement is a directional or positional tolerance.

1.3. Line contour characteristics:

For Each Line Feature

For each line feature;

Tolerance Zone

the two-dimensional area where the tolerance zone is uniformly distributed along the theoretically correct contour of the measured line elements;

Theoretical Contours

Theoretically correct contours can be defined by mathematical models or two-dimensional views;

Different Cross-Sections

It can be used for parts with different cross-sections, such as wings, etc.;

Fixed Sections

It can also be used for parts with fixed sections that do not require control of the entire contoured surface (such as rotary parts).

2. Line contour evaluation method

Line contour evaluation: Curve contour detection by scanning method can obtain the test results accurately, quickly and intuitively.

When the contour has a reference, in the calculation and evaluation process, the position of each point on the line or surface is required to be within the tolerance requirement range.

When CMM is measured, there is only one way to evaluate and calculate its contour, that is, after comparing the point coordinate value with the theoretical contour coordinate value, take twice the absolute value of the largest difference as the contour measurement result of the line.

Generally, when measuring the curve profile with a trigger probe, the evaluation line profile of the feature group is constructed after measuring N points, and the deviation between each point and the theoretical value is calculated.

The compensation of the two-dimensional curve is very simple, just compensate the radius along the vector direction of the point.

The calculation formula of line contour is 2 times the maximum distance from the point on the extracted line profile to the fitted line profile according to the ISO standard (GB/T national standard).

The core formula for line profile calculation:

According to ISO standard (GBT 1958-2017</b2 0 >Product Geometry Technical Specification (GPS) Geometric Tolerance Detection and Verification), the calculation formula of the line contour error value is:

Measured value = maximum deviation absolute value×2 (MEAS = MAX{|max|). ，|min|} ×2)

Where the maximum deviation refers to the maximum distance from the point on the extraction line contour to the theoretical (fitting) line contour.

3. Line contour evaluation standards

3.1. ISO 1101 2017 and ASME Y14.5 2018 reports:

(1) In the evaluation results of ISO 1101 and Y14.5 2018, there is no negative tolerance in the tolerance band;

(2) The ISO evaluation results use the maximum deviation value 2 times the output report, the ASME evaluation results also use the maximum deviation value2x output result. This result meets the requirements of the specification definition and the evaluation result is correct.

3.2. PowerDMIS line profile evaluation

PowerDMIS software is ISO 1101 2017 compliant and: ASME Y14.5 2018Standard shape and position tolerance evaluation.

PowerDMIS GD&T evaluation, users only need to fill in the blanks according to the drawings, and there is no need to manually disassemble the meaning of the shape and position tolerance annotation.

(1) Contour without datum

Only the shape of the curve is constrained, not the position of the curve.

The point on the measured contour must be in the envelope area with a diameter of 0.1, and the center of the circle must be on the theoretical contour line, and the actual curve as a whole can be translated and rotated in a two-dimensional plane. (No benchmark: shape error).

A. Trigger probe CP100T

When the trigger probe measures the curve contour, the feature group evaluates the contour after measuring N points, and calculates the deviation of each point and the theoretical value.

A number of measurement points are selected on the features of the measured contour, and a multi-point feature group is constructed to evaluate the line contour.

There is no benchmark contour evaluation, and the deviation of each point from the theoretical contour is calculated by fitting 20 points as a whole, and the vector maximum and minimum deviation value is used to obtain the contour value.

B. Scanning probe CP500S

For scanning probes, the curve can be scanned directly with the scan function for evaluating the profile.

Without reference line contour evaluation, the measured curve is best fitted with the theoretical contour, and the line contour is twice the absolute value of the maximum deviation.

(2) Contour with datum

At the same time, the shape and position of the curve are constrained. When measuring, it is necessary to ensure that the absolute coordinate value of each point in the datum is within the tolerance value of the contour.

Example 1:

The point on the measured profile must be in the envelope area with a diameter of 0.1, and the center of the circle must be on the theoretical contour line, and the actual contour as a whole is based on the actual measurement datumABCFixed. (with datum: position tolerance).

There is a reference profile evaluation, the specific position is determined by the datum, and then the deviation between the point and the theoretical contour is calculated, and the deviation value of each point is obtained to determine the contour.

Example 2:

The point on the measured contour must be in the envelope area with a diameter of 0.1, and the center of the circle must be on the theoretical contour line, and the actual curve as a whole is based on the actual measurement datumABThe fixed part can be translated up and down as a whole for the actual contour. (There are benchmarks: shape and position).

Example 3:

The point on the measured contour must be in the envelope area with a diameter of 0.1, and the center of the circle must be on the theoretical contour line, and the actual curve as a whole is based on the actual measurement datumAFor the fixed part, the actual contour can be panned up and down, left and right. (There are benchmarks: shape and position).

3.3. Composite line profile (American standard ASME Y14.5)

The composite line profile tolerance is to represent different profile requirements for the same measured element with two frames, and the contour of the upper frame is the requirement of the measured profile relative to the reference system,The lower box is the further requirements of the measured profile for a certain baseline, both of which must be met at the same time.

The contour requirements of the upper and lower frames are qualified, then the product meets the drawing marking.

Upper grid: Compare benchmarks A, B and C.

Bottom grid: The lower grid 0.02 tolerance band is in vertical A(direction constraint) and direction B (parallel), it can be up and down in a tolerance band of 0.1, Pan left and right.

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Article referred from:

PowerDMIS in ISO 1101:2017 Line Profile Tolerance Verification

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