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Rotary seams on a fiber laser: causes and fixes

A galvo lens can only mark a small window of a cylinder at a time, so rotary jobs are marked in splits: mark one strip, turn the part, mark the next. The line where two splits meet is a seam. A good seam is invisible; a bad one shows as a gap, an overlap, a sideways jog, or a raised ridge. Each of those has a different cause.

Match the symptom to the cause

What you see at the seams Most likely cause Fix
The pieces on either side don't belong together — the art looks scrambled The part turns the wrong way Toggle Invert direction in Rotary Setup
A small step along the circumference at every seam, all the same way, and a gap or overlap where the wrap closes Part diameter (or steps per revolution / gear ratio) doesn't match the real part Measure the part and correct Part Diameter; check the motor settings
A sideways shift along the part's length that grows from one end The rotary axis isn't parallel to the field Re-square the rotary on the table
The first seam is off but the rest are fine Gear or chuck backlash Turn on Backlash compensation
A raised ridge of material on both sides of each seam, even with perfect alignment Fill line ends stacking at the seam Change the fill angle — see below
Doubled, darker outlines at seams Outlines marked by both splits Set Outlines at seams to Exact seam

Ridges at the seams: line ends, not alignment

A seam is where fill lines end. Line ends are where a galvo mark puts the most heat — the beam slows, the laser switches off, and the melt pool is pushed ahead of it — and at a seam both neighbouring splits end there, so the effect doubles into a visible bead.

  • Change the fill angle. A fill running straight into the seam ends every line at the same place. At 45° the ends are spread along the seam and the bead usually disappears.
  • Check the laser off delay. Too long a delay burns every line end; seams show it first.
  • Measure the diameter with calipers. The part diameter sets how far the part turns between splits, so an error shows at every seam and adds up around the part: the wrap closes short or long by π × the diameter error — 0.1 mm off on the diameter leaves about 0.31 mm at the closing seam. Measure with calipers where the art will be marked, at a few points around the part, and enter what you measure rather than the nominal size. With an accurate diameter and Z height, the splits meet exactly and no overlap is needed.

How FocuZ keeps seams consistent

  • Equal splits. Set the number of splits per revolution and every split is exactly the same width — there's no short leftover strip at the end, and splits that hold no art are skipped.
  • Seams placed to the motor step. Every split is commanded to the nearest motor step, so a seam is never more than one step from ideal. When the split count divides the motor's steps per revolution, every advance is identical and the readout says exact.
  • Backlash-safe approach. With Backlash compensation on, each revolution approaches its first split from the same side as every later advance, so lash never lands in a seam. Turn it on for any rotary that shows lash, however small — it adds only a short move per revolution.
  • Seam-aware splits (optional) move seams into gaps in the artwork where there are any.
  • Outlines at seams — choose Exact seam, a small Stitch past the seam, or share the Overlap.
  • Arc compensation (on by default — keep it on) corrects the curvature across each split, so distances on the part match the design right up to the seam.

Or use the seams on purpose: the 2D Grid action divides every split into a checkerboard of cells, so the seams become lines of the pattern.