There has been some interest in discussing settings for the Harry Potter signs and the Rowmark 2-toned acrylic. Since the designs should stay in the FLD section and, settings have to go in BTM, this is the related thread.
For the Harry Potter signs on the two-toned Rowmark acrylic, I engraved at 1000 speed, 20 power and 270 LPI with one pass.
I experimented with scoring around the engraves to smooth the edges. That changed the appearance of the text in a way that I didn’t like. So, the only place that survived was the inside of the border on the Diagon Alley sign. Scoring was one pass at 500 speed and 20 power.
Cutting was at 150 speed and Full power (on a Basic machine – Pro machines might be a little faster or lower power).
As I mentioned elsewhere, there is some “orange peel” texture to the engraved areas but, if anything, I feel like it added to the feel of the signs.
I’m wondering, what is the logic of unfocused engraving? I’m wondering so that I can come up with a constant formula for various thicknesses of acrylic. Thanks.
Unfocused engraves smooth out the engraves and reduces the visible engrave lines. The formula is not really needed. The Glowforge focus range is 1/2" if you defocus by 1/4" in either direction, it will work. Specifically, by focusing 1/4" above the surface or 1/4" below the material surface the beam will be wide and unfocused when it hits the material.
Un/de-focusing makes the laser beam physically wider.
Edit: I should have continued “at the point the beam hits the material.” For the more detailed, more precise answer, see the post from @jamesdhatch right after this one.
You could easily enough but might not find it varies enough to matter. Speed and LPI will also affect the effect of the beam being out of focus.
The beam is an hourglass shape with the pinch point being “in focus” that’s typically wanted to be on the surface of the material. So if you’ve got 1/4" material the lens slides up in the head so that the pinch point is 1/4" above the bed (or below the head because it’s got 1/2" of room between the head & bed.
If you “defocus” you’re telling the GF that the material surface is different than it is. That will place the pinch point of the hourglass above or below the surface and the surface will be in the broader portion of the beam.
If you have your 1/4" material and tell the GF that it’s 1/2" then the pinch point (or focal point) will be 1/4" above the material and the surface will be in the lower cone shaped portion of the beam. Likewise if you tell it your thickness is .01" then your focal point will be virtually on the bed and the surface will be in the upper cone of the beam.
The beam size never changes, where your material is within the hourglass just puts it in a different part of the beam. The pinch point of the hourglass is where the maximum power is in the smallest space.
The further your focus is above/below the material’s surface, the wider the beam and thus the wider the burn. Usually I only find it useful at the extremes as I either want a fast thick line or the default. The slower you go the thicker the line and if you’re doing engraves, the higher the LPI the more lines are squeezed into a particular space and defocusing can mimic the apparent width or in the case of acrylic smooth out the ridges at lower LPIs.