Laser Printer vs. Laser Cutter: Alignment & Scale

The setup for cutting the Tektronix Circuit Computer desks looks like this:

Tek CC - Bottom Deck cutting setup
Tek CC – Bottom Deck cutting setup

Four neodymium bar magnets hold the corners flat against the honeycomb and the neo disk magnet pins the center down, thus ensuring the red alignment laser meets the cutting beam at its focal point on the surface.

The triangular shapes mark the OD of the perimeter (177.8 mm) plus twice the cut margin on each side (2×2 mm), with the tick mark in the upper right ensuring I slap every deck down in the proper orientation. Aligning the two right marks to the edge of the honeycomb frame (with a straightedge for some offset) aims the deck’s 0° index along the cutter’s X axis.

The cut pattern origin is, naturally enough, the center point of the deck, so aligning the red dot to the center cross should put the OD cut at the place all around the perimeter. For confirmation, I fire the laser (“A single ping, Comrade.”) and verify the hole is in the middle of the cross.

Before cutting the deck, the laser also marks the corner shapes, so this may come as some surprise:

Tek CC Middle Deck Corner Targets
Tek CC Middle Deck Corner Targets

The laser printer (a venerable HP LaserJet 1200) produced the dark triangles and the laser cutter (a new OMTech 60 W) burned the light brown marks. The picture is a composite of the four corners, with the blank center removed to concentrate on what’s important.

The scrawls give the edge-to-edge distances in both inches (because that was the scale at hand) and converted to millimeters (because that’s how it’s laid out), with the L suffix for the laser marks.

What’s of interest is that you can’t overlay the two sets of marks by a combination of scaling and rotation with the centers (not shown) of the two patterns pinned together.

The laser measurements differ from the ideal 181.8 mm by 0.1 mm vertically and 0.4 mm horizontally. This may require dinking with the scale factors in the firmware, which I recall having weird values.

The LaserJet is definitely not a precise instrument, off by 0.4 mm vertically and a millimeter horizontally, with considerable variation. I think this comes down to unrealistic expectations for toner stuck to a flexible sheet wrapped around rollers and heated enough to melt dust into the fibers.

More study is indicated …

7 thoughts on “Laser Printer vs. Laser Cutter: Alignment & Scale

  1. Paper shrinks quite a lot when you laser print it. Not too surprising, really: it gets hot enough to fuse toner, so it also drives out moisture in the paper. Also, it’s much harder nowadays to have your printer driver print to the size you specify.

    I used to manage a printer that we’d carefully calibrated for printing book proofs. Managing paper humidity was the hard part. The only advice I have is, don’t be responsible for managing a calibrated proof printer if you can possibly avoid it.

    1. Yep, the thermal/humidity cycle through the fuser really does a number on the “accuracy” of the finished room-temp output, even if the internals are putting the photons in the right place. And then it continues to change as the paper ages and equilibrates to whatever the conditions in the Basement Lab are…

      1. Aaaaand it goes through a hot laminator before the cutting starts. It’s a wonder the marks stay on the same side of the paper …

    2. Back when I had a hulking Lexmark duplexing dual-tray laser printer, I kept track of paper grain orientation and stuff like that. Nowadays, I feed cheap paper into the LaserJet and It Just Works.

      Ah! This surely justifies two product lines: CNC-plotted for “high accuracy” and laser-printed for “economy”.

      TIL: marketing!

  2. I’ve found that laser printers are never dimensionally accurate. I think they use movable mirrors? My inkjet, on the other hand, is spot on, which you would expect since it must lay down a microscopic mist of ink in precise patterns.

    1. There’s at least a spinning mirror in there, in addition to the rollers / fuser / all that jazz. AFAICT, it works fine for all practical purposes, which certainly doesn’t include precise matching with another bit of machinery with its own collection of quirks.

      It’s actually Close Enough™ that it took me this long to notice it’s off by enough to be mmmmm noticeable. [grin]

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