Engraving, Marking and Raster Work

Worked example: the useful resolution

The spot of a 2 in lens is about 94 µm wide (see How Laser Cutting Works and Laser Types). Lines closer than the spot overlap and blend; the finest distinct lines are about one spot apart: 25.4 ÷ 0.094 = 269 lines per inch. Setting 600 dpi on this laser does not give finer detail than about 269 dpi: it only makes the engraving slower and burns the surface deeper from overlap. A larger spot (a longer lens, a defocused beam) gives a smoother fill and a lower useful resolution.

Worked example: a 50 × 50 mm square at 300 mm/s

• At 254 lines per inch (0.1 mm) there are 500 lines; each takes 0.26 s (a 50 mm line plus 5 mm of overscan each side at 300 mm/s, with 5 000 mm/s² acceleration): 2.2 minutes.
• Without overscan the line would be 0.23 s and the total 1.9 minutes, but the ends of each line would be ragged because the head has not reached speed. Overscan is a necessary cost.
• Raise the speed to 600 mm/s and the total falls to 1.8 minutes if the acceleration is high enough; with a modest acceleration of 1 000 mm/s² it is 4.1 minutes (see the effect of acceleration in Power, Speed, Focus and Assist Gas).
• Compare cutting: the outline of the same square (200 mm at 20 mm/s) takes 10 s. A filled area costs far more than a line.
• Reduce the area, not the resolution, where you can: engrave only what shows, and use a pattern of lines where a solid fill is not needed.

Worked example: engraving plywood

• A 30 W beam at 300 mm/s with a 0.1 mm interval delivers E_a = 30 ÷ (300 × 0.1) = 1.00 J/mm². For plywood (E_v ≈ 1.11 J/mm³ and η = 0.5) the upper bound on depth is 0.5 × 1.00 ÷ 1.11 = 0.45 mm per pass; real engravings are shallower (about 0.1 to 0.2 mm) because much of the energy chars the surface rather than removing it.
• For a 1.0 mm deep relief at 0.45 mm per pass you need about 3 passes (more in practice); at double the energy (60 W at 150 mm/s, 4.0 J/mm²) the bound is 1.80 mm per pass.
• Burning and smoke rise with energy. Several faster, lighter passes give a cleaner engraving than one slow heavy one, and refocus between passes (the surface is lower).
• Test on scrap and note the depth for each setting.

Worked example: how many gray levels?

• A halftone cell of 4 × 4 dots can show 17 gray levels (from no dots to all dots). At 254 dpi the cell is 0.4 mm across, so the picture has an effective resolution of 64 cells per inch: gray levels and detail trade against each other.
• A photograph of 3 000 pixels across 100 mm has 762 pixels per inch, 3.0 times more than the 254 dpi engraving: the software must resample it. Resample before dithering, not after, to avoid moiré patterns.
• Prepare the image: increase the contrast, lighten midtones (engraving darkens the picture), sharpen a little, crop, remove the background; test a small area. A dark, low-contrast photograph engraves as a muddy mass.
• Engraved color depends on the material: wood darkens, acrylic frosts white (best seen from the back on a dark backing), leather browns.

Worked example: the scan offset in bidirectional engraving

The beam switches a small time after the controller commands it (the “latency”, about 1 ms in the electronics and the tube). At 300 mm/s the beam is therefore late by 300 × 0.001 = 0.30 mm. On a line going right the marks are shifted to the right; on the next line going left they are shifted to the left: vertical edges appear doubled with a gap of twice that, 0.6 mm. At 600 mm/s the shift doubles to 0.60 mm. Most controllers have a “scan offset” table by speed: engrave a vertical line pattern at several speeds, measure the doubling and enter half of it.

Worked example: a 70 mm tumbler on a roller rotary

• A roller rotary turns the object by friction, so the surface of the object moves exactly as the surface of the rollers does: the software needs the diameter of the roller (here 20 mm), not of the object. A chuck rotary grips the object and turns it directly, so one step moves the surface by π D ÷ steps, which depends on the object diameter.
• For the 70 mm tumbler on a chuck rotary with 3 200 steps per revolution one step is π × 70 ÷ 3 200 = 0.069 mm of surface, and the full wrap is π × 70 = 219.9 mm wide at most. If the software is told the wrong diameter (say 40 mm for a 70 mm object) the picture is stretched by 1.75 times along the surface.
• Cylinders of different diameter need different settings:

• Focus on the top of the cylinder. The surface curves away from the beam at the sides, so a wide design loses focus and power toward the edges: keep it within about 10° each side of the top: the surface falls away from the focus by r(1 − cos θ), which is 0.53 mm at 10° and 4.69 mm at 30° for the 70 mm tumbler, against a depth of focus of only about 1 mm (see How Laser Cutting Works and Laser Types).
• Keep tapered objects level with a support, or the focus changes along the length.

Worked example: marking time and pulse spacing

• At 3 000 mm/s a galvo covers a 110 mm line in 0.04 s; a logo of 60 filled lines 20 mm long takes about 0.4 s plus the mirror jumps: 6.0 to 12.0 s. A CO₂ gantry doing the same fill at 300 mm/s would take ten times longer.
• A fiber marker at 200 kHz and 3 000 mm/s places pulses 15 µm apart. With a spot of about 40 µm (a 160 mm lens, 7 mm beam, M² = 1.3) the overlap is 63 %: a smooth line.
• Color marking on stainless steel: a short pulses and a precise energy grow a thin oxide layer, and its thickness gives interference colors. As a guide, 35 nm looks straw, 55 nm gold, 75 nm purple, 95 nm blue and 120 nm light blue: a laser can write these as colors without removing metal.
• Anneal marking (dark, smooth marks on stainless) and deep engraving (several passes) are other fiber modes; plastics can be marked by foaming or carbonizing.