Tools - The Essentials, Straight Edge Rulers

Here's what most of us start with, the cheap wooden ruler. That thin strip of metal set into the edge won't last as a metal straight edge. After you've cut out a few fins, the thin edge will slip out of the slot. Once it's gone your knife will cut into the wood ruler that remains.

Here's a metal ruler that's good but not ideal. The 1/32" markings are a little hard to read.
This metal edge would work to cut straight lines and fins using a hobby knife.

Notice the rounded off left end. There is a border margin before the measurement markings start.

Get a 18" long metal straightedge with a squared end.
You can double check the spacing of a centering ring. Rotate the ruler around tube at the base of the centering ring before the glue dries. Here the ring is set 3/8" from the end of the engine tube.

The cork back helps the ruler grip. If you were using Rapidiograph pens it raises the edge so the black ink won't run underneath.

TIP: If you use a straightedge to cut out parachutes, flip over the ruler with the cork side up. A raised ruler edge can allow the plastic to flex and you could end up with wrinkled cuts.

TIP: Here's a great ruler for marking, manufactured by Incra.
Those small holes are made for a metal scribe but a .5mm mechanical pencil will fit.
To learn more: CLICK HERE