I’ve completed the assembly and testing of a drag knife for cutting thin stock like poster board. All of the parts except the bearings and the mounting shaft were cut from 3/8" plate aluminum with a 30x30" LongMill.
The design is a modified version of the Grunblau drag knife available at http://www.grunblau.com/DragKnife.html. The mods were minor edits to the shape and dimensions of the frame and the way the mounting shaft fits into the router. The bearing pockets and countersinks were all designed and post processed in Fusion 360 and all of the operations on each part, clearing, finishing, and contour, were machined consecutively with the same tool, a Sienci 1-flute flat end mill for aluminum. Machining time was about 58 min for the frame, 25 for each bearing holder, and 10 for the clamp. The holes are clearance and tap drill holes for 1/4"-20 and #6-32 screws that were center marked during machining, and drilled and tapped by hand on a mill. The surface finishes on all parts is much better than they appear in the pics due to reflections. None of the machining marks can be felt with a fingernail.
The finished drag knife fits under the router with no modifications.
I used Fusion 360 for all of the patterns I’ve done so far because there is an add-in available that raises the blade, does a swivel, and lowers the blade at the corners so the blade does not catch in the corners and rip or chew up the material. It reads a Fusion 30 .nc file and writes an edited version that includes the swivels. It works very well once you get the blade height settings right.
In the testing I’ve done so far, dealing with very thin materials is a bit challenging because it is hard to keep flat and can wrinkle ahead of the blade if the cutting speed is too fast. The blade extends about 0.230" inches, so I would think 0.200" thick material is about the limit. I am re-designing the blade holder section of the frame to allow the blade to stick out a little farther for thicker materials, maybe up to 3/8".
I have videos from two angles of cutting the pattern for the dust shoe sealing plate out of 0.013" poster board at 40" min. I can email them to anyone interested.
Have fun. Stay safe.