Simplest drag knife?

[garyhlucas]

I wanted a drag knife but they all looked kinda over done. This one has two 1/2” od x 3/16” id ball bearings and a spacer you just hold in a 1/2” collet. The shaft is a shoulder bolt and there is a spring for cutting force. A small piece of aluminum with 2 tapped holes a cutout for the blade.

[Richard P Wilson]

What does it do?

[macona]

You can put it in a cnc mill to cut sheet stock like vinyl. Its how most vinyl sign cutters work. The way the blade sits in the holder causes it to orient the blade into the direction of the cut.

[DR]

Cool, store bought ones are incredibly expensive. How far beyond the center of rotation is the blade tip?

[jcge]

That's very clever Gary - I like the idea of just holding the outer bearing races directly whilst the bolt is free to slide up under pressure

[darryl]

I have a craft type machine that uses a drag knife to cut a heavy paper. You can get some decent looking cutouts to paste into your scrapbook, but I want it to cut my own shapes to the limit of it's capacity. Haven't played with it much, but I did do a bunch of testing with it. The drag knife holder is an interesting mechanism. The next step might be to include some kind of impact hammer that can help facilitate the tip hacking into the material it's trying to cut. Add to that a controlled rotation of the tip to facilitate the tip following the planned cut line. Yikes, this just got more complicated.

I've noticed with my machine that it's not always easy for the tip to rotate as it's being dragged- the result being some tear-outs. A sharp cutter is mandatory. I would find it interesting what is the minimum radius it will work at, and how small an amount of trail is still effective.

[Black Forest]

I made a drag knife also. Mine uses a circular blade. I have two wheels, one on each side of the blade that control the depth of cut. The wheels are 1.5mm smaller in diameter than the blade. I use an air cylinder to supply the down pressure. I regulate the air pressure down very low. It doesn't take a lot of air pressure. I am cutting 1.5mm thick PVC material. The cuts I am making are very simple with no actual corners just a squashed oval sort of in the shape of a big lima bean abot 700mm in the long dimension. Using the wheels means I don't have to have an accurate Z axis.

[garyhlucas]

The point is an 1/8” behind center. I made two, one for me, one for the FIRST robotics team’s router.

[Paul Alciatore]

I am just thinking here, but it seems that it may be better to have the trailing edge of the blade tilted backwards somewhat. Then it would act as a rudder and help in steering the blade. You still want the tip somewhat offset from the center of rotation as the tilt of the cutting edge also brings it closer to that center with thicker mediums being cut.

Of course, I could be all wrong there. But I would try it.

I wonder if that could be used on a 3D printer.

I wanted a drag knife but they all looked kinda over done. This one has two 1/2” od x 3/16” id ball bearings and a spacer you just hold in a 1/2” collet. The shaft is a shoulder bolt and there is a spring for cutting force. A small piece of aluminum with 2 tapped holes a cutout for the blade.