Rotary engraving stretched or squashed: finding the right setting

Guide · Published 1 October 2026 · Worked examples are calculated with the laser rotary tumbler calculator

If a rotary engraving comes out longer or shorter around the cup than you drew it, one rotary setting is wrong, and one ratio fixes it: engrave a line of known length, measure it, and scale the setting by intended ÷ measured (or the inverse, for a diameter). If only part of the design is distorted, or the bottom is narrower than the top, the cause is the taper instead. This guide covers both.

Why it happens

Your laser software turns a distance in the file into a rotation of the cup. To do that it needs to know how far the surface moves for each turn of the rotary motor, which it works out from one setting: mm per rotation (or steps per mm) for the axis, plus the object diameter on a chuck rotary or the roller diameter on a roller rotary. If that setting is off by 7%, every distance around the cup comes out 7% long or short. Distances along the cup (across the rotary's axis) are not affected, which is why a circle turns into an oval.

The test

1. Wrap a strip of masking tape round the cup where the design will go. On a tapered cup, put it at the height whose diameter you typed in.
2. Engrave a horizontal line (or a square) of a round length, say 50 mm, at low power so it marks the tape.
3. Peel the tape and measure the line flat with a steel rule, or measure it on the cup with a flexible tape.
4. Work out the corrected setting below, change it, and repeat until the line is within about 0.5 mm.

The laser rotary tumbler calculator makes a ready-made masking-tape test file (SVG or DXF) for your cup: a square and a circle pre-warped for their place on the cup, and a strip one full turn long with 10 mm ticks whose ends should just meet.

The correction

The first kind of setting says how far the axis moves per command: if the line came out too long, the axis moved too far, so make it smaller. The second kind tells the software how big the cup or roller is: a line too long means the software thought the cup was smaller than it is (so it turned it further), so make the diameter bigger.

Worked example: a 50 mm line that came out 53.5 mm

The line is 53.5 ÷ 50 = 1.070 times too long (7.0% stretched). Depending on which setting you correct:

Change one of these, the one your setup actually uses, not several. On a roller rotary in LightBurn the Object Diameter does not change the output; the roller diameter and mm per rotation do. A squashed result (line too short) works the same way, with the ratio below 1.

Stretched at the top, squashed at the bottom? That is the taper

On a tapered tumbler the rotary turns the whole cup by the same angle for every row of the design, but the cup is narrower further down, so the same angle covers less surface. A design that is right at the diameter you typed comes out narrower below it. Calibration cannot fix this, because it is not one wrong number but a different scale on every row.

For a 20 oz cup (87.6 mm at the rim, 71.4 mm at the base) with a 2.25 in logo starting 0.5 in below the rim and the chuck set to the rim diameter, the logo's centre row comes out 4.7% narrow and its bottom row 8.0% narrow. The fix is to warp the design so each row is stretched by the right amount (×1.015 at the top to ×1.087 at the bottom here), either with your software's taper correction or with a pre-warped file.

• If a line at the typed diameter is the right length but lower rows are short: taper. Warp the design.
• If a line at the typed diameter is already the wrong length: calibration. Fix the setting first, then deal with the taper.

Other things that look like a wrong setting

• The cup slips on the rollers: the error changes from run to run. Add weight, clean the rollers, or level the cup.
• The wrong axis is set as the rotary, or the rotary is not enabled for the job: the design comes out tiny or huge, not a few per cent off.
• A roller rotary with the cup riding on its narrow end: the cup turns by the diameter that touches the driven wheel, so measure the test line there.
• Both the software's taper correction and a pre-warped file: the stretch is applied twice. Use one or the other.

The maths here is checked digitally, not on a machine. Always run the tape test before engraving a real cup. For printed sublimation wraps rather than engraving, see tapered tumbler wrap size.

Keep a bench reference

Print the rotary troubleshooting decision tree for symptom-by-symptom checks and a one-page PDF.