I: What is Profile Tolerance?
Profile tolerance is divided into two types: line profile tolerance and surface profile tolerance.
For products, profile tolerance is used to regulate the variation of the actual profile of the measured surface or curve from the designed ideal profile surface or curve. It is also a way to describe the shape and positional accuracy of surfaces or curves.
Profile tolerance, as an important type of shape tolerance for products, actually defines the shape structure and position of products in practical applications. The processing accuracy of free surfaces is measured by its profile tolerance, which is a more difficult geometric element to define. Unlike general regular geometric elements, it cannot be precisely defined with a few parameters, making the inspection of free surface processing accuracy more complex. This is mainly manifested in the inability to directly use the measured surface itself as a measurement reference (as shown in Figure 1), which results in systematic errors in the measurement results due to the non-coincidence of the measurement coordinate system and the design coordinate system.
II: How to Measure Profile Tolerance?
The three most common measurement methods are:
① Profile Template Measurement:
Profile template measurement has significant limitations and can only be used to inspect the contours of products with low requirements. It is not suitable for precision processing contour requirements. Template measurement mainly involves making a profile template according to the drawing contour requirements and comparing it with the actual product. During the inspection process, profile template measurement cannot intuitively display error values; it can only compare the shape of the contour. The common judgment methods are as follows: First, use a small standard feeler gauge or pin gauge to measure the contour gap, and take the value of the largest feeler gauge or pin gauge that can be inserted into the gap as the profile tolerance error. Second, use natural light or soft light for the light transmission method, and choose surfaces with lower surface roughness for measurement to improve accuracy. Judge the gap size by observing the strength of the light passing through, and take the maximum gap as the profile tolerance error.
② Profile Gauge Measurement:
Profile gauges are one of the most commonly used methods for line profile tolerance inspection. They have a wide range of measurement applications, high precision, and can measure various small linear and shape sizes. Based on the up and down movement of the measuring probe, the trajectory of the probe tip is plotted with two methods: arc and straight line. When the probe moves in a straight line or an arc, if the up and down movement of the probe is large, the recorded graph will deviate due to arc deformation. According to the radius of the probe tip, the recorded graph follows the trajectory of the ball center rolling on the measured surface. The equipment compensates for the radius of the probe tip through data processing to obtain accurate measurement values and shape profiles. After completing the measurement operation, the following two methods can be used for shape analysis of the profile: First, manually process the measured data to complete contour measurement, angles, radii, chamfers, R-angles, spacing, etc., and display the values directly. Second, fit the measured data with the design data. When there is a reference for the measured contour shape data, inspect the design value based on the reference. When there is no reference or only the shape is inspected, fit the design data with the measured data.
③Coordinate Measuring Machine (CMM) Measurement:
The CMM profile tolerance measurement method involves measuring a series of points on the workpiece surface and calculating the distances and shape features between these points to obtain the product's profile data. Compared to traditional measurement methods, the CMM profile tolerance measurement method has the advantages of high precision, high efficiency, and non-contact. The CMM profile tolerance measurement method is essentially the same as the profile gauge, which can be divided into manually integrating measurement points and data fitting.
Quality Manager
Yanxiu Ji
Nov 27, 2024