Two identical end pieces provide a center point and a drawing point, which are connected with a PLA arm or a 1/2" square wooden arm.

You can place marks on the work at the desired spacing using the CM scale on each arm.  

Each end piece can be used to hold the center point or be used to house the cutter or pencil.  The cutter can be either an Exacto Craft Knife or an Exacto replacement knife without the handle.

Two 6 mm bolts secure the knives, pencil and center pin on each end.  The bolts can be purchased or printed.

Two center pin designs are provided.  One has a flat that lays on the print bed and the other is round and prints vertically.  The flat design provides a larger surface interface with the bolt.

Because the holes and threads in the end pieces are perpendicular to each other, supports are appropriate for printing.  I used tree supports, which removed easily.

I had to debur the arms.