PCS Example #6

Plane - Bisect Points - Circle

In this exercise, we will create a part coordinate system utilizing a plane for the orient and origin, two 2D points from which we will obtain a constructed 2D point through the bisect command then followed by a circle for alignment and origin. The following example assumes the part is placed on your CMM where the cylinder of the GeoWidget is pointing in the -X direction.

figure 1, Datum target layout	figure 2, Complete PCS

Step 1

Request a -Z plane <b>, if auto-direction is not activated, you will be asked for the probing direction which you should select as -Z. You will then be prompted to "Measure -Z Plane", (if auto-direction is active "Measure Auto-Dir Plane"). Capture points A-1, A-2, A-3 and A-4 as shown in figure 1.

Step 2

Select Orient <j>. The orient command will create the primary datum on the axis that is most normal to the machine coordinate system. In your PCS Setup Guide, the Z Axis will be signed off.

Step 3

Select Origin <l>. This will establish the Z origin which is determined by the normal direction of the -Z plane.

Step 4

figure 3, Base plane selection

Select a 2D XY Point <5 + 2>. The selection of a 2D point requires two commands. The first command is activated by pressing the < 5 > key, which will display the 2D and 3D point selection tool, select the PCS base plane you would like to point to be projected into, see figure 3.

After the selection has been completed, Geomet will prompt you to "Measure XY Point". Capture points B-1 and C-1.

Step 5

Select the Bisect command <a>. This command will take the two projected XY points and solve for the bisect point between them. The result will be a 2D XY point.

Step 6

Select Circle <z>, if auto-direction is not activated, you will be prompted for the Circle type: ID,  OD,  IR, or OR, choose ID. Capture the four data points identified as D-ID/4, see figure 1

Step 7

Select Align <k>. This will take the just measured circle center and the previous 2D point created by the bisect routine and perform an alignment. Your ICS will update to include the secondary datum.

Step 8

Select Origin <l>. the Circle, which is 2D, provide two directions when establishing an origin. In our example, the circle can be used for the X and/or the Y direction. The origin command will prompt you whether to use the X and then the Y. For our example you should answer <Yes> to both requests. Upon completion, you will have established PCS 1, see figure 2.

In this exercise, we used a constructed feature, an XY point, as part of the alignment solution. It is important that you choose the correct features to be used for alignment based on the completeness of the ICS. The XY points for B-1 and C-1 were chosen as the ICS lacked an alignment. If two 1D points were chosen, then there would exist an error caused by the part skewed to the MCS base axis. see Measurement Features→1D Points

figure 4, Final Inspection Report

This does not preclude conditions where 1D points could have been used. For example, if we were working in the PCS that was created in exercise 1, then we would have the correct probe compensation vector and we could select -X and +X 1D points for B-1 and C-1 respectively, see figure 5. The resulting bisect would create a 1D point on the centerline of the slot.

However, a 1D point followed by a circle can not be combined to form an alignment. Geomet does not accept this as it causes potential repeatability problems when the slot is not aligned with the current PCS. Therefore you could repeat the bisect of two additional 1D points captured on the width of the part at the other end from the slot points C and D, see figure 5. The resulting 1D point derived from the bisect combined with the previous recalled constructed point could produce the alignment. see figure 5, points C and D, 1D Point bisects

Related Procedures:

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Orient, Align, Origin, Plane, Circle, 2D Point