Dragknife thickness and jittering

I have a handful of questions for anyone with drag knife experience:

I recently designed a drag knife holder based off of the work of others (see picture). I made it out of 3D printed housing material with bolts as the shafts (one bolt is sleeved in plastic to make it fit the skate bearings)

I tested the drag knife on 1/8" foamcore cutting a ~3 inch circle with a very small stepdown, but with poor results.

My questions:

  1. How much does the distance between the center of rotation and the vertical face of the knife affect performance? I designed it to have 1mm, but am wondering if more distance would help as it tends to follow in a jerky fashion rather than smoothly, and occasionally drifts sideways.

  2. Can drag knives cut thick materials well? I’ve seen professional setups work on foam core, and I’ve seen amateur versions work on paper and vinyl. I had hoped to do foam core with some thoughts toward building up a model of a church the local congregation is raising money to build. If drag knives aren’t the way to go, I might need to look into needle cutting.

  3. I haven’t done any backlash tuning on the new SO3 XL. Should I be doing that with a permanent marker square before doing any of this, or is backlash not likely to be an issue at paper-cutting scale, and more a problem for getting seamless transitions in hard materials?

Thanks for any pointers!

1 Like

The drag knife should cut foam core well, and of course with good depth control you’ll be able to do very nice rabbeted joints.

The big thing is toolpaths which allow one to cut corners — the Donek folks have a spreadsheet which is supposed to help with that and dxf2gcode is supposed to support drag knives as an output option.

I would recommend calibrating for belt stretch — it’s important if you’re going to cut things in varying orientations and have them fit (see: Design into 3D: Boxes: Magazine storage for an example).

I’ll go through the belt calibration. Thanks.

I’ve read through about the corner tool paths, and plan to use the Fusion add-on at https://apps.autodesk.com/FUSION/en/Detail/Index?id=3603732473616813300&os=Win64&appLang=en for the more complicated profiles, but until I sort out cutting a circle of large radius (the easiest thing besides a straight line) I’m not going more complicated.

Post a photo of a cut and a file?

I picked up a drag knife a while back (and did a design even further back) and could test the cutting.

I printed another drag knife, increasing the blade offset from 1mm to 3mm and tried again with a greater depth of cut. It slices the foamcore smoothly with a 0.1" depth of cut or on a full 0.2" depth of cut, but on both the second pass of 0.1" or on the full 0.2" pass I still get some side-ways dragging of the knife blade. Is this an artifact of my 3D printed drag knife (too much flex) or something else? Has anyone else used a plastic drag knife setup successfully?


I did a simpler design which included a metal bar for holding and to make it more rigid: