How do you laser mark round, curved, and stepped parts?

A flat nameplate is the easy case. The moment a part is round, stepped, or oddly shaped, the real questions are whether the mark fits inside the lens's field and whether the surface stays in focus while the beam moves. Laser marking sorts into a handful of part-shape categories, and each one has a known way to handle it: flat parts that fit the field, flat parts too big for the field, round parts marked partway around without rotating, round parts rotated to mark all the way around, stepped parts at different heights, and irregular shapes that need true 3D marking. Once you know which category your part falls into, you know most of what the job will take.

What Part Shapes Can a Fiber Laser Handle?

Every part-shape category comes back to those two facts. The lens defines a flat plane where the beam stays in focus, and that plane has a fixed size. As you choose a lens with a larger field, the lens sits farther from the part and the focused spot gets larger, which lowers power density at the surface. So the strategy for any awkward part is to keep the surface you are marking inside the lens's field and inside its focus, and to move the part or the beam when you cannot.

The rest of this comes down to which of those categories you are in and what has to move.

What Is the Marking Field, and Why Does It Limit Your Part?

An f-theta lens is a flat field lens, which means it focuses light across a plane rather than at a single point, and that plane sits perpendicular to the lens. Anything you can draw as a two-dimensional vector file can be marked inside it, as long as the material matches the laser wavelength. The practical range runs from small fields around 4 to 5 inches square up to about 12 inches square.

Bigger is not automatically better. A larger field means a larger focused spot and less power density, so a small-field lens is the better choice when you need deep engraving or fine detail. One distinction trips people up: spot size and kerf width are not the same thing. Spot size is a calculated number, the beam diameter at the focal point. Kerf width is the real width of the line the laser cuts, and it changes with the material because some materials burn wider than others.

If the design is larger than the field, you tile it, which brings us to round parts and big parts.

How Far Around a Round Part Can You Mark Without Rotating It?

As the beam moves down the side of a cylinder, it also moves away from the lens's focal point. The focused spot grows, power density drops, and before long there is not enough intensity to give the mark a uniform look. That distance is the depth of focus, and how far you get depends on the material as much as the optics. Some materials are very sensitive to focus and limit the wrap to a small angle. Others are forgiving, and the limiting factor becomes distortion instead.

Geometric distortion is the second problem. A circle marked on the barrel of a pen comes out as an oval, because the surface is no longer flat under the lens. This is correctable in the marking software with a projection-correction feature that compensates for the curve, so the circle reads as a circle and the edges stay inside focus. With the right correction and a forgiving material, marking around 160 degrees of a part without rotating it is possible, though that is near the practical edge.

How Do You Mark All the Way Around a Cylinder?

Steered-beam markers draw lines like a plotter, not pixel rows like a dot matrix printer, and that shapes how rotary marking works. Some systems wrap a bitmap around a cylinder by printing one row of pixels, indexing the part by one pixel height, then printing the next row. It works, but it is slow and limited to bitmaps. A steered-beam system does better by breaking the vector image into tiles and rotating the part to bring each tile under the field, the same idea as tiling a long image on a flat part.

The hard requirement is precision. The marking software has to control the rotary indexer, and the indexer has to land each tile within a laser line width of the previous one. Miss by more than that and the seams between tiles become visible. At Jimani we run a Sherline rotary indexer for this work, and for high-speed rotary applications we use a DCA high-speed rotary with Prolase 10 DCA software. Rotary and linear tiling can also be combined to wrap a long image around a circumference and down the length of the part.

How Do You Mark Parts With Surfaces at Different Heights?

Stepped parts are the common version of this. When each mark sits at its own height and the differences are larger than the depth of focus, you reposition the Z axis between marks so the surface being worked is at the focal point. Fiber lasers are small and light enough that motorizing the platform the laser sits on is not complicated. The Z drive has to be perpendicular to the marking plane and repeatable to a few thousandths of an inch, and the software steps it to each height as the job runs. The Jimani Hybrid desktop and enclosed workstations give 19 inches of Z travel for exactly this kind of range, though we do not offer this option at this time.

That approach works when each height has its own discrete mark, and nothing moves mid-mark. It is the wrong tool for marking one large image across a continuously irregular surface. That job belongs to true 3D marking, which changes the focal point on the fly using a third focus axis, either by moving a focusing lens or by feeding a relay lens, with the Z information carried inside the marking file itself.

Round, stepped, or just plain awkward parts are routine in our job shop. If you have one that has stumped another vendor, send it to us and we will tell you how we would hold it, focus it, and mark it.