Several Common Scanning Methods of Coordinate Measuring Machine

The measurement methods of coordinate measuring machines (CMMs) can usually be divided into contact measurement, non-contact measurement, and contact and non-contact measurement. Among them, the contact measurement method is often used to measure machined products, pressed products, metal films, etc. In order to analyze the processing data of the workpiece or provide the original information of the workpiece for reverse engineering, it is often necessary to use a coordinate measuring machine to scan the data points on the surface of the workpiece to be measured. This article takes the Microxcel Pfx454 coordinate measuring machine of Brown & Sharpe Company in the United States as an example to introduce several common scanning methods and operation steps of the coordinate measuring machine.

The scanning operation of the coordinate measuring machine is to use the PCDMIS program to collect data points in a specific area on the surface of the measured object. The area can be a line, a patch, a cross section of a part, a curve of a part, or a perimeter at a certain distance from the edge. The scanning type is related to the measurement mode, the probe type, and whether there is a CAD file. The "Scan" option on the control screen is determined by the status button (manual/DCC). If the DCC method is used for measurement and there is a CAD file, the available scanning methods include "Open Linear", "Closed Linear", "Patch", "Section" and "Perimeter" scanning; if the DCC method is used for measurement and there is only a wireframe CAD file, the "Open Linear", "Closed Linear" and "Patch" scanning methods can be selected; if the manual measurement mode is used, only the basic "Manual TTP Scan" method can be used; if the manual measurement method is used and a rigid probe is used, the available options are "Fixed Delta", "Variable Delta", "Time Delta" and "Body Axis Scan".

The following is a detailed introduction to the five scanning methods that can be selected after entering the "Utility" menu and selecting the "Scan" option in the DCC state.

1. Open Linear Scan

Open Linear Scan is the most basic scanning method. The probe starts from the starting point and scans along a certain direction and at a predetermined step length until the end point. Open line scanning can be divided into two cases: with and without CAD models.

(1) Without CAD model

If the workpiece to be measured does not have a CAD model, first enter the nominal value of the boundary point (Boundary Points). After opening the "Boundary Point" option in the dialog box, first click "1" to enter the scan starting point data; then double-click "D" to enter the new X, Y, and Z coordinate values ​​of the direction point (the coordinate point indicating the scan direction); finally, double-click "2" to enter the scan end point data.

The second item enters the step length. Enter a new step length value in the "Max Inc" column in the "Direction 1 Tech" column of the "Scan" dialog box.

Finally, check whether the set direction vector is correct. This vector defines the surface normal vector and cross section of the first measurement point after the scan starts, as well as the surface normal vector of the last point before the scan ends. When all data is entered, click "Create".

(2)

If there is a CAD model for the workpiece to be measured, click the corresponding surface of the CAD model with the left mouse button to start scanning. The PC DMIS program will generate a point on the CAD model and mark it with "1" to indicate the starting point of the scan; then click the next point to define the scanning direction; finally click the end point (or boundary point) and mark it with "2". Draw a line between "1" and "2". For each selected point, PC DMIS has entered the corresponding coordinate value and vector in the dialog box. After determining the step size and other options (such as safety plane, single point, etc.), click "Measure" and then click "Create".

2. Closed Linear Scan The closed line

scanning method allows scanning of inner or outer surfaces. It only requires two values: "starting point" and "direction point" (the PC DMIS program also uses the starting point as the end point).

(1) Data input operation

Double-click the boundary point "1" and enter the position in the edit dialog box; double-click the direction point "D" and enter the coordinate value; select the scan type ("Linear" or "Variable"), enter the step size, and define the touch type ("Vector", "Surface" or "Edge"); double-click the "Initial Vector" and enter the vector of the "1" point to check the cross-section vector; after entering other options, click "Create". You

can also use the coordinate measuring machine operation panel to touch the first measuring point on the surface of the workpiece to be measured, and then touch the direction point. The PC DMIS program will automatically put the measured value into the dialog box and automatically calculate the initial vector. After selecting the scan control mode, measuring point type and other options, click "Create".

(2) Closed-line scan with CAD model

If the workpiece to be measured has a CAD model, confirm "Closed-line scan" before measurement; first click the surface starting point to generate the symbol "1" on the CAD model (the surface and boundary points are highlighted when clicking, so that the correct surface can be selected); then click the scan direction point; PC DMIS will give the corresponding coordinates and vectors of the selected position point in the dialog box; after selecting the scan control mode, step size and other options, click "Create".

3. Patch Scan The patch

scan method allows you to scan an area instead of a scan line. To use this scan method, at least four boundary point information is required, namely the start point, direction point, scan length, and scan width. PC-DMIS can determine the triangular patch based on the boundary points 1, 2, and 3 given by the basic (or default) information, and the scan direction is determined by the coordinate value of D; if the fourth or fifth boundary point is added, the patch can be a square or a pentagon. [page]

When using the patch scan method, select "Closed Line Scan" in the check box to scan a closed element (such as a cylinder, cone, slot, etc.), and then enter the start point, end point, and direction point. The end point position indicates the distance moved up or down when scanning the measured element; the starting point, direction point, and starting vector can be used to define the cutting plane vector (usually this vector is parallel to the measured element). Taking the creation of a quadrilateral patch as an example, several ways to define a patch scan are introduced:

(1) Type in coordinate values

​​Double-click the boundary point "1" and enter the starting point coordinate values ​​X, Y, and Z; double-click the boundary direction point "D" and enter the scanning direction point coordinate values; double-click the boundary point "2" and enter the scanning width for the first direction; double-click the boundary point "3" and enter the scanning width for the second direction; click "3" and then press the "Add" button, and the dialog box will give the fourth boundary point; double-click the boundary point "4" and enter the ending point coordinate value; select the required step size (the step distance between each point) and the maximum step size (the step size between points 1 and 2) for the scan, and then click "Create".

(2) Touch measurement method:

Select the "face scan" mode, and use the coordinate measuring machine operating panel to touch the first point at the desired starting point. The coordinate value of this point will be displayed in the "#1" item of the "Boundary Point" dialog box; then touch the second point, which represents the end point of the first direction of the scan, and its coordinate value will be displayed in the "D" item of the dialog box; then touch the third point, which represents the width of the scan face, and its coordinate value will be displayed in the "#3" item of the dialog box; click "3" and select "Add" to add the fourth point to the list; touching the end point will close the dialog box. Finally, define the data in two directions: scanning line spacing and step length; after selecting the scanning touch measurement type and required options, click "Create".

(3) CAD surface model method

This scanning method is only applicable to workpieces with CAD surface models. First, select the "Patch Scan" method, left-click on the CAD work surface; highlight "1" in the "Boundary Point" dialog box, left-click on the scanning start point on the surface; then highlight "D" and click on the surface to define the direction point; click on the surface to define the scanning width (#2); click on the surface to define the upper scanning width (#3); click "3", select "Add", add additional point "4", highlight "4", click to define the scanning end point, and close the dialog box. After defining the step size in both directions and selecting the required options, click "Create".

4. Section Scan (Section Scan)

Section scanning method is only applicable to workpieces with CAD surface models. It allows scanning of a certain section of the workpiece. The scanning section can be along the X, Y, and Z axis directions, or at a certain angle to the coordinate axis. Multiple section scans can be performed by defining the step size. The boundary points of the section scan can be set in the dialog box. By pressing the "Cut CAD" conversion button, you can find any holes in the CAD surface model, and define its boundary lines in a similar way to open line scanning. The PC DMIS program will automatically avoid the holes in the CAD surface model by the scanning path. The method of cutting CAD according to the user-defined surface is: enter the "Boundary Point" option; enter the "CAD Element Selection" box; select the surface; without clearing the "CAD Element Selection" box, select the "Cut CAD" option. At this time, the PC DMIS program will cut the selected surface to find holes. If there are no defined holes in the CAD surface model, there is no need to select the "Cut CAD" option. At this time, PC DMIS will scan according to the defined start and end boundary points. For complex CAD graphics with multiple surfaces, different surfaces can be cut in groups, and the cutting can be limited to the local CAD surface model.

5. Boundary Scan (Perimeter Scan)

Boundary scanning method is only applicable to workpieces with CAD surface models. This scanning method uses the CAD mathematical model to calculate the scanning path, which is offset from the boundary or outer contour by a certain distance (selected by the user). When creating a boundary scan, first select the "Boundary Scan" option; if it is an inner boundary scan, select "Inner Boundary Scan" in the dialog box; when selecting a work surface, enable the "Select" checkbox, and highlight each surface you select. After selecting all the desired surfaces, exit the checkbox; click on the surface to determine the scan start point; click on the same surface to determine the scan direction point; click on the surface to determine the scan end point. If no end point is given, the start point is the end point; enter the corresponding value (including "value added", "CAD tolerance", etc.) in the "Scan Construction" edit box; select the "Calculate Boundary" option to calculate the scan boundary; after confirming that the deviation value is correct, press the "Generate Measurement Point" button, and the PC DMIS program will automatically calculate the theoretical value of the scan; click "Create".

6. Application Points

(1) The appropriate scanning measurement method should be reasonably selected according to the specific characteristics of the workpiece to be measured and the modeling requirements to achieve the purpose of improving data acquisition accuracy and measurement efficiency.

(2) To facilitate the measurement operation and probe movement, the clamping position of the workpiece to be measured should be reasonably planned; to ensure the modeling accuracy, the probe should be able to complete the scanning measurement of the entire object to be measured at one time when clamping the workpiece.

(3) The selection of scanning measurement points should include the key points of the workpiece contour geometric information, and appropriate measurement points should be added in areas with obvious curvature changes. (end)