Centerline Guide
thingiverse
Place a #2 graphite pencil directly into the hole, then carefully slide it along the entire edge of the object, creating a precise line down its center.
Download Model from thingiverse
With this file you will be able to print Centerline Guide with your 3D printer. Click on the button and save the file on your computer to work, edit or customize your design. You can also find more 3D designs for printers on Centerline Guide.