Incomplete Engrave Without Overscan on Monport K40

I am having a strange issue that I can’t work out. I have a new Monport 40-watt co2 and bought the one with the Lightburn board because I love LB. lol

Anyway, when I try to do a fill without over scanning, it powers off the laser early and cuts off the edges. I can fix it with over scanning on at 4mm but is that necessary or did I do something wrong in setup?

The top is with overscan on, and the bottom is without it on. Both are at the same speed and power.

Also, I dont see the minimum power setting except for engraving pictures. So I am confused and looking for some help.


“If it hurts when you do that, don’t do that.”

The overscan region is where the laser head accelerates to the speed you specify for the engraving pattern, so that it can traverse the entire engraving at a uniform speed, then decelerate in the overscan region on the other side.

With no overscan, the head must stop exactly at the border, so it must accelerate and decelerate within the engraving area, causing the problems you see.

Allow a generous overscan region and the controller will use the engraving speed & power you set.

That’s apparently available only for Ruida controllers, not GRBL controllers.

As with most things, not entirely true…

This is my grbl set to grayscale …

@ednisley is correct, generally speaking, most grbl machines can’t handle variable power like this… with vectors or other dithers…


This is solidly accurate.

I’m going to offer that the minimum power setting isn’t available because the cornering algorithm in GRBL adjusts the power up when accelerating and adjusts the power down when decelerating.

So, GRBL does handle minimum power, but it does so internally with its cornering algorithm or by enabling Constant Power.

It’s a little heady… but here’s the calculus for the cornering algorithm.

The TL;DR version is the rate of photons you throw at the work while moving along either axis is the Speed and % Power you command. When you change direction you make a hypotenuse or a radius and the laser speeds up and needs to throw more light. when stopping and reversing it throws less light to make up for the lower speed. This is why we use overscan for image engraving.

It’s exactly the same as painting a car - you go past the edge of where you’re spraying before you release the trigger so you don’t get too much paint on the end of the panel.

Which the Ruida does, too, between the limits set by the Min / Max powers while above the enigmatic Start speed and Jumpoff speed. It seems as though it’s applying linear interpolation at the intersections.

@jkwilborn showed plots of how the Ruida applies all those values, but the GRBL circular cornering algorithm seems to come closer to a good solution without requiring quite so many arbitrary parameters.

1 Like

Guess that’s my problem with the painted car, didn’t adjust for overscan :rofl:

Thanks for the clarification… and the cornering link…


1 Like

Thanks, everyone. I have a much better understanding of what is happening now. I appreciate all the information, and if I ever need to paint a car, I will be able to do that now as well. lol

This topic was automatically closed 30 days after the last reply. New replies are no longer allowed.