FAQs: Laser Marking Round Parts

Laser marking round parts presents even experienced laser system operators with some challenges. Ideally, when marking a round part, the entire mark will be made without moving the part. However, sometimes this isn’t possible. Successful marking, regardless of movement of the piece, is best accomplished when the laser systems operator has experience and a deep understanding of the system they are working with and the material they will be marking.

With that in mind, let’s take a shot at answering some of our most frequent round part marking questions.

How Far Around the Circumference of a Part Can You Mark?

The beam quality of your laser and the characterics of your optical system are going to be the biggest determiners of how far around a part you can mark without moving it. As the laser beam "wraps" around the diameter of the part, the size of the focused spot changes and that causes a change in the power density of the laser on the part. A significant change in power density will cause the appearance of the mark to be different or non existent on portions of the part.

While not always the case, a good rule of thumb is that you can mark about 60° around the circumference of a part with a 1” diameter without rotating the part. This rule of thumb, however, supposes that the wavelength of the laser and the material being marked are a good match. If they aren’t, the circumferential marking distance will be reduced.

How Large of a Mark Can You Make Without Rotating the Part?

This is probably the most commonly asked question about marking round parts. The answer, unfortunately, isn’t as straight forward or simple as many would like. There are three factors that affect how far one can mark around the diameter of the part without rotating that part. Those are:  The material composition of the part, distortion in the mark as the laser beam moves out of the focus plane of the focusing lens and the depth of focus of the laser and beam delivery system.

Depth-of-focus is the major limiting factor in the size of the mark. This is because, as the laser beam passes through the optimum focal point, the focused spot size grows. As the focused spot size increases, the laser power density applied to the part decreases and this causes a non uniform appearance in the mark. Because power density decreases as the square of the increase in laser spot size, it doesn't take much of a change from the optimal focal point of the lens to cause a large change in power density applied to the part.

How far you can make a uniform mark is greatly influenced by the material you are marking. How sensitive a material is to the change in the focal point, and the change in laser power density as a result of moving away from that optimal focal point, limits the size of the mark you can make. For materials that are particularly forgiving of the change in laser power density, the limiting factor of size then becomes distortion.

Geometric distortion occurs whenever you make a mark on a surface that is not a flat plane. A circular mark on a flat surface, for instance, would result in an oval mark on a round surface. This type of distortion should be accounted for and corrected by the laser marking software you are using.

You can see an example of geometric distortion in the left-hand image above. As the marked image wraps around the radius, it becomes distorted. As the marking continues further around the part, the laser power density is insufficient to mark.

The right-hand photo shows the mark once the geometric distortion has been compensated for.

Even if the material and depth of focus would allow a proper looking mark over the entire 180 ° of the part visible to the lens, marking more than 160 ° is just not practical without rotating the part.

Can You Mark Greater than 160 °?

Yes, although to mark farther you will be forced to rotate the part.

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