Why Does Laser Cutting Leave a Tapered Edge on Thicker Material?

The beam converges to its narrowest point at the focus, then spreads again as it travels down through the material. The top of the cut ends up wider than the bottom. On thin sheet the taper is small enough to ignore. Past a few millimeters it becomes visible, and on a mating hole or edge it can turn into a real fit problem.

Why it gets worse with thickness

A laser beam isn't a cylinder. It's an hourglass shape with a focal waist. The machine sets that waist near the top or middle of the material and cuts through everything above and below at a slightly different beam width. Thin material sits entirely inside the narrow part of the waist, so the cut edge looks straight.

Thick material spans a longer distance along the beam path. More of the cut happens where the beam has already started to diverge again.

Assist gas plays into it too. As the cut gets deeper, the gas has to punch molten and vaporized material out through a narrowing channel. It loses some ability to clear material evenly from top to bottom. That unevenness shows up as taper, and as dross on the bottom edge in the worst cases.

Material and cutting speed shift how much taper you get for a given thickness, but the underlying geometry doesn't go away. It's inherent to the process, not a defect from a bad setup.

What it actually costs you

For most sheet metal parts, a few tenths of a degree of edge taper on a bend or bolt hole doesn't matter. Where it bites is on a hole that has to take a close-fitting pin, or a slot two parts need to nest into. If the top of the hole reads one diameter and the bottom reads another, an inspector measuring at the top gets a different number than a part that slides a pin through the full depth.

It also matters more on stainless and aluminum than on mild steel. Those materials often run at parameters where dross control and taper trade off against each other. Push the cut for a clean top edge and the bottom is more likely to show taper and dross together.

What to do about it

Design around it rather than fighting it. If a hole needs a tight fit through its full depth, don't spec it as laser-cut-to-size at the thick end of what your machine handles well. Flag it for a secondary operation, or open the tolerance enough that taper doesn't matter.

If two laser-cut parts need to nest together, account for the taper direction consistently. Cut both parts from the same side orientation so the tapers match instead of fighting each other.

Where DigiForge's laser cutting fits: our tolerance on the cut is ±0.13 mm, a call on final dimension, not a statement that every wall of every hole is perfectly vertical through the material. If a print calls for a feature that depends on a perfectly square edge through the full thickness, say so on the drawing so it gets routed to the right operation instead of assumed.

Taper on a laser-cut edge in thick material isn't a mistake. It's physics. Design your critical fits so they don't depend on a perfectly vertical cut wall and you'll never notice it.