I was trying to find the exact center of the root rib on an Aquila XL so I could replace the tiny 1/4" rods with a decent 1/2" carbon joiner. Trying to get the measurement just right with just a rule, wasn't that easy. I had a Robart center finder that has really helped me find centerlines for hinges and such. The only problem is that it is too small for a lot of my needs. Soooo... I made a quick and simple center marking gauge large enough to fit the thicker ribs on many of my planes.
(2) 1/4 inch by 3/8 inch by 3 inch block (you can vary the sizes however you want)
(1) 5/8 inch by 3 inch block (the longer this block the thicker the thing you want to find the center of can be)
(2) Machine screws ( I used 8-32 stainless because that's what I have handy)
(2) "O" rings (optional)
(1) Scribe insert (optional)
1: Cut out the blanks and mark for the holes.
2: Drill the appropriate end holes in the three blocks. I counter sunk for the screw heads, but it is not necessary.
3: Assemble the unit (I chose to use knurled nuts and "O" rings to make it work like I wanted).
4: This is a critical step to get the exact center. Take and push the blocks together and scribe a line on the main block. Flip the outer bocks the other direction and scribe again. This gives you an "X" right in the center of the block.
5: Drill the center hole. I chose to use a center scribe insert, since I had several in my tool chest, but you can drill a small hole for a pin or a larger one for a pencil.
Knurled Cap Nuts and "O" rings
Scribe Centerline right and left
Using Pen or Pencil
Shown on Gemini root.