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CNC plotter with drag knife + CAM software

drzasiek 12570 34

TL;DR

  • A home-built CNC plotter with a drag knife cuts paper decorations for a spouse, replacing a Brother Scanncut that had become loud, slow, and too small.
  • It uses a reinforced OpenBuilds frame, a custom 3D-printed Z axis, a stepper-driven knife, and a push-back spring to set cutting pressure.
  • The machine is built on a 165 x 85 cm frame, and the custom CAM tool CuterCam took about 1.5 months to write.
  • CuterCam loads DXF or SVG files, arranges shapes automatically, and generates g-code in a few clicks for very complicated drawings.
  • The first light-servo Z-axis idea failed, and the frame was considered too flexible for a drag knife, so it was strengthened.
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  • #31 19334617
    yego666
    Level 33  
    Posts: 2175
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    koczis_ws wrote:
    There is no such thing as an arc in digital technology. It is always broken, only it has more or fewer lines depending on the accuracy required. Real bows are analog only.

    If you look at the arc approximation with many "pixels" at the right distance, we get a good approximation of the "analog" arc.
    As for the analog, you are probably forgetting the quantum nature of energy and space.
    From this point of view, there is no "analog", since it is also composed of quanta of space, albeit very tiny.
    The true arc only exists in mathematics that ignores the quantum nature of our world.
    And for painting arcs, the G02 or G03 command is used in the G-Code. In good plotters these commands are always implemented.
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  • #32 19334628
    drzasiek
    CNC specialists  
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    yego666 wrote:

    And for painting arcs, the G02 or G03 command is used in the G-Code. In good plotters these commands are always implemented.

    In the weak too :) Even GRBL supports it. Of course, the grbl itself converts the arcs into sections with the specified accuracy, but the advantage is that the machine does not chop on the curves, making approach and stop to each section, or unfortunately GRBL does not support G64. Throwing a program into the machine that contains only linearly interpolated motions is a feature of a weak CAM program.
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  • #33 19334654
    koczis_ws
    Level 27  
    Posts: 1857
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    Rate: 384
    Okay, let's go that deep into quantum theory.
    As for the G-Code, it was easier for me to process the HPGL file. It was enough to calculate the position of the center of rotation of the knife so that the blade was there and now. The plotter used did not support G-Code. Besides, when I was doing it, and I was doing drawings in AutoCad, the HPGL was practically the only portable vector file. AutoCad did not generate G-Code. However, for other needs (wire EDM machines) I generated G-Code with AutoLispie program.
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  • #34 19334690
    yego666
    Level 33  
    Posts: 2175
    Help: 239
    Rate: 564
    Now it's probably just a tacomat and automatic confessor don't generate G-Code :) .
    What not to touch, you get G-Code on demand. What a time :(
  • #35 19334709
    koczis_ws
    Level 27  
    Posts: 1857
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    Rate: 384
    Now yes, but 20 years ago you had to deal with it yourself and, if necessary, tie your shoes with a shaft :D
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Topic summary

✨ A user discusses the construction of a CNC plotter with a drag knife for cutting paper decorations, aiming to replace an existing Brother Scanncut plotter due to its noise, speed, and size limitations. The new machine features adjustable knife pressure and cutting depth, achieved through a spring mechanism and a motor for lifting. The cutting mat is made of 6mm thick foamed PVC, secured with temporary adhesive. Various responses address technical aspects such as knife control, material handling, and software programming, with mentions of using C++ and Qt for software development. The discussion also touches on alternative methods for holding materials, including electrostatic techniques and the use of temporary adhesives.
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FAQ

TL;DR: Drag-knife plotters cut 170 g/m² cardstock up to A1 size while staying under 60 dB [Elektroda, drzasiek, post #19306777]; “the motor works reliably, quietly” [Elektroda, drzasiek, post #19307243] Purpose-built CAM trims corners, preventing knife breakage. Why it matters: Makers gain quiet, low-cost cutting without lasers.

Quick Facts

• Work area: 165 × 85 cm OpenBuilds frame [Elektroda, drzasiek, post #19306777] • Z-axis: stepper lift, spring-loaded pressure (manual screw adjust) [Elektroda, drzasiek, post #19307243] • Cutting mats: 6 mm foamed PVC with engraved grid [Elektroda, drzasiek, post #19307528] • Control electronics: Arduino + GRBL + CNC Shield + UGS sender [Elektroda, drzasiek, post #19306777] • In-house CAM development time: ≈1.5 months (CuterCam) [Elektroda, drzasiek, post #19306777]

What is a drag-knife CNC plotter and how does it differ from a laser or pen plotter?

A drag-knife plotter moves a small, swivelling blade that trails its pivot. Unlike a laser, it uses no heat, so it stays safe in home studios and generates no fumes. Compared with a pen plotter, the blade must arc through corners and compensate for its offset to avoid tearing paper [Elektroda, drzasiek, post #19310023] Typical 45° blades cut vinyl, paper and thin fabric at speeds similar to hobby lasers—100–500 mm/s [Make:, 2023].

Why was a stepper motor used for the Z-axis instead of a hobby servo or electromagnet?

Tested RC servos proved “too slow and too weak” to lift the spring-loaded head, and internal potentiometers wear out quickly [Elektroda, drzasiek, post #19307243] Electromagnets of similar size overheated during long strokes. The stepper lifts quietly, releases tension during cutting, and needs no feedback sensor.

How is cutting depth and pressure adjusted for different materials?

Depth is set once by screwing the knife farther in or out of its holder; pressure comes from an adjustable compression spring on the head [Elektroda, drzasiek, post #19307243] For new materials you: 1. raise knife flush with mat, 2. advance 0.1–0.3 mm, 3. test-cut a small square, increasing spring preload until clean separation.

What methods hold paper or fabric flat on the bed?

The build uses light tack aerosol adhesive on 6 mm PVC mats; sheets release after cutting and mats clean every few dozen jobs [Elektroda, drzasiek, #19307528; #19309603]. Alternatives include electrostatic beds found in Roland plotters, which “literally sucked the paper to the table top” [Elektroda, CMS, post #19309443] Vacuum tables also work but add noise and cost.

Can soft, stretchy materials such as felt or synthetic wool be cut?

Yes, provided the material is immobilised. Use stronger temporary adhesive or carrier tape and choose a longer, steeper-angle blade (60–70°) to shear fibers. Industrial drag-knife cutters process upholstery and garment fabrics this way [Gerber, 2022]. Cut slower—below 100 mm/s—to stop snagging. Thick felt over 3 mm may need two passes.

What is CuterCam and how does it speed up tool-path creation?

CuterCam is a custom C++/Qt CAM tool that loads DXF/SVG, groups shapes, optimises order and generates G-code in “just a few clicks” [Elektroda, drzasiek, post #19306777] It offers three sequencing modes (inner-first, outer-first, nearest-shape) and matches knife entry angle to the exit of the previous cut, reducing turnaround and breakage.

3-step How-To: Generate ready-to-cut G-code in CuterCam

  1. Import DXF/SVG; auto-detect shapes.
  2. Pick mat size, then drag, copy or rotate parts until the sheet fills.
  3. Select post-processor, choose ‘Internal first’ and click Export—G-code with correct knife offset and arc commands (G02/G03) saves automatically.

Why are arc commands (G02/G03) preferable to many short line segments?

Sending pure linear segments forces firmware to start/stop at each node, causing vibration and longer jobs. GRBL supports G02/G03, then internally subdivides with constant speed, so the machine glides through curves [Elektroda, drzasiek, post #19334628] “Throwing a program that contains only linearly interpolated motions is a feature of a weak CAM program.” [Elektroda, drzasiek, post #19334628]

How much does building a hobby drag-knife plotter cost?

A 165 × 85 cm OpenBuilds frame kit is about US $200, electronics US $40, motors US $60, and misc. plywood/steel sheets US $100. Total ≈US $400–450, excluding spindle tools [OpenBuilds Store, 2024]. Commercial vinyl cutters of similar width start at US $250 but lack large flat beds [SignWarehouse Guide, 2023].

What maintenance is required?

  1. Reapply tack adhesive after 30–50 sheets to keep grip uniform [Elektroda, drzasiek, post #19309603]
  2. Replace blades every 500 m of cut or when corners fray; tips cost US $1–2 each [Roland DG, 2023].
  3. Vacuum PVC dust weekly to prevent roller slip.
  4. Check belt tension monthly.

What edge cases can damage the knife?

Failing to round sharp 90° corners can snap the blade or tear paper because the tip trails the rotation axis [Elektroda, drzasiek, post #19310023] Always let CAM add corner loops or over-cuts. Cutting thick laminate without slowing feed below 50 mm/s also chips edges.

Can an old Roland pen plotter be converted for cutting?

Yes. Design or buy an offset knife holder that fits the pen mount, then use software to add knife offset compensation. One builder exported HPGL from AutoCAD, rewrote vectors for the offset, and re-sent HPGL to the plotter [Elektroda, koczis_ws, post #19334172] Ensure the firmware supports continuous curves or expect stair-stepped arcs.
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