A friend built me this drag knife:
There is no separate bearning, it sits just in the collet of my router. In the video everything works as it should.
Today I wanted to cut a circle into cardboard (same type as in the video). It started to cut and the from one second to the other the knife turned 90° degree against the direction of travel and the tip of the knife broke. I had once had the same behaviour while cutting wood. There I tought it might be a problem with the wood grain or something, but with cardboard...?
Any idea why this happens?
Cheers Christoph
CNC drag knife
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twizzstyle
- Posts: 2207
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- Location: Kenmore, Wa USA
Too much of the blade ahead of the center of rotation is contacting the material. The thicker the material, the more likely this is too happen.
I made my drag knife by grinding down a carbide drill blank by hand. I made the "blade" pretty far back from the center of rotation, so I've never had problems with the blade not staying true to the direction of the cut. But it does mean I can't do very sharp corners, because there is such a large offset. For ski bases (the only thing I use it for), its fine.
If you can slide the blade slightly down, parallel to the edge of the blade, such that more of the blade is behind the center of rotation, that should fix your problem. But you only need a tiny amount.
I made my drag knife by grinding down a carbide drill blank by hand. I made the "blade" pretty far back from the center of rotation, so I've never had problems with the blade not staying true to the direction of the cut. But it does mean I can't do very sharp corners, because there is such a large offset. For ski bases (the only thing I use it for), its fine.
If you can slide the blade slightly down, parallel to the edge of the blade, such that more of the blade is behind the center of rotation, that should fix your problem. But you only need a tiny amount.