Hey! This thing is still a Work in Progress. Files, instructions, and other stuff might change!

MPCNC Tangential Knife

by jamesd256 Nov 3, 2017
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dont know what its good for but nice job! :D

Great design.
How do you home the knife angle?


Home for the knife is pointing towards X positive. You power up the stepper with the knife in this position, and this is 0 degrees.

Thanks for the reply. I have one more question. Have you tried cutting foam board with it? I am wondering if it can be used to cut Flite Test plans on a DTFB?

Yes I have cut foam board very well. However, because of the blade angle, the cut extends further on the top surface. Therefore, if you cut shapes with acute angles, there will be some 'overcut' present.

I'm currently (or was before Christmas!) working on a new cutting head which mounts a 30 degree Olfa craft blade. This will reduce but not eliminate the overcut.

From everything I have read, a needle cutter (see https://www.thingiverse.com/thing:1138627 and its remixes) still offers a great foam cutting option.

MPCNC foam-cutter attachment

I was wondering if X-acto blade would yield a better result for this purpose.
The needle cutter is definitely an interesting option, and generating the gcode for it is easier. The problem with needle cutter is the excessive vibration and noise.

The steeper blade angle reduces overcut, but calls for more rigidity as the shallow angle has more cutting power. Hopefully I'll find time to finish trialling the new head and there can be a big update to the project. Other stuff has changed in what I'm working with now to improve rigidity. Now I'm using a smooth 8mm rod and linear bearings for the A/E axis, and is way less sloppy. There are more details in the MPCNC forum thread.

Nice job on this. I am a little stuck setting this all up in gcodetools. I extracted the file that you supplied and followed the instructions to add it to Inkscape. However when I bring up the tool library and tangent knife none of the options bullet pointed below show up in the green box. Knife lift, 4th axis command etc. Maybe you could point me in the right direction for fixing the problem. I appreciate the help, again good work!

Thanks tjdepuy, I appreciate you joining in with the effort. First of all, I wonder if you ran the create_inx.py (instructions in README.md)?

If you haven't run this, you will find there is no gcodetools-dev sub menu in your extensions list. You may be running the command from the (built in) gcodetools sub menu.

Second, there has been a development in the code. Previously we were outputting angles in radians, but it turns out the firmware is only accepting the first two decimal places for E. This was resulting in inaccuracies in the turn, so after cutting a circle, the knife would be a few degrees behind where it should be.

So the code now optionally outputs degrees, giving enough precision (hundredth of a degree) to calibrate properly. Unfortunately for standard MPCNC Marlin, this now requires a firmware change, because it wouldn't accept E moves over a certain magnitude (long extrusion detection)

I will update the description and code ASAP, and also publish a Github link. The firmware change will be documented.

Very nice. You might consider adding a blade guard on the part of the blade that is not used for cutting as a simple safety measure

Thanks, and thanks for the suggestion. It's a good idea which I will be adopting.