To Create a Midline Line
By using the Midline option, you can create a Constructed Line midway between two previously created Constructed Lines. The reference lines can be at any angle to each other.
1. Click > 
Line/Axis. The CMM Construct Step dialog box opens.
Line/Axis. The CMM Construct Step dialog box opens.2. Select the parent reference Csys from the Parent Csys drop-down list.
3. Select Midline from the Type drop-down list.
4. Select 2 measure or construct steps of the following types:
◦ Line
◦ Cylinder (axis)
◦ Cone (cone axis)
5. Click OK.