Samstag, 16. Februar 2013

Post by Nycon: How to adjust the mirrors for correct beam alignment in a CO2 laser engraver and cutter

Disclaimer: All information supplied in this document has been compiled to the best of my knowledge, nonetheless I cannot guarantee that everything is correct and remains that way. Everybody is responsible for his own actions and suitable safety measures should be complied with when handling class IV laser equipment! 

Usually the laser should be pre-aligned by the manufacturer, however it can happen that the mirrors can get misaligned during transport. Sometimes a small or even larger adjustment has to be made in order to get the best out of the machine.

Caution: Only operate the laser with the lid of the machine closed. Never activate a test pulse with the lid open, especially after having made an adjustment to the mirrors, regardless of how little it has been.  

To obtain consistent engraving and cutting results, the mirrors have to be adjusted in a way that the laser beam always hits the movable laser head in exactly the same spot (see Figure 1). For example, if the mirrors are not correctly adjusted, the entrypoint of the laser into the head drifts with increasing distance to the second mirror. Depending on the magnitude of misalignment a part of the beam may not even hit the opening of the laser head anymore, which in turn means that not all of the laser power actually reaches the material that is to be cut or engraved. In this case the depth of the engraving or cutting decreases with increasing distance to the second mirror.

Figure 1: Schematic of the laser path and the role of the mirrors for correct alignment

It is not absolutely necessary that the beam hits all of the mirrors exactly at their center. It is more important that the actual point of impact is the same for all X and Y positions and that this point is not as close to the edge as to actually “lose” a part of the beam.

Alignment procedure:
Several HowTo’s and manuals advise the usage of masking tape or similar material wich can be directly stuck onto the mirrors. With my bigger G350 laser the manufacturer even included double-sided tape to be used for alignment. This however has the big disadvantage that smoke is produced in the direct vicinity of the mirrors, which can soil and even damage them. In case you have nothing else available but masking tape it is recommended to use several layers of it so that not all layers are burned through and the smoke does not even reach the mirror. Of course this testing is to be done with as little laser power as possible.

The best way to check for the exact impact point of the beam on the mirrors is using thermal paper with very low laser power. Most receipts (e.g. from gas-stations, supermarkets, etc) are printed on thermal paper and can be used for this purpose regardless of how much is printed on them. The advantage: Thermal paper gets a dark discoloration even before it starts to burn!

Stick the thermal paper to the mirror with adhesive tape but take care not to cover the part where you expect the beam to hit with plastic as well, as this will definitely smolder. Adjust the laser power to the absolute minimum and increase it slightly while firing short test bursts. Take care to stop increasing the power once you see that the paper shows discoloration. If the paper is burning, decrease the power again or use shorter bursts, exchange the thermal paper for a new piece. Once the appropriate laser power has been found it is to remain unchanged during the alignment process.

The position of the beam on the first mirror is pretty irrelevant s long as the complete beam is reflected towards the second mirror. It usually is sufficient to test that the beam does not hit too far off-center .If the beam hits too close to the border of the mirror either the mirror mount or the laser tube have to be adjusted slightly. Remove the paper from mirror 1 after this short test.

In the first step of the alignment procedure the aim is to adjust the first mirror in a way that the beam always hits the second mirror in the very same spot (see Figures 2.1 and 2.2). In order to see the point at which the beam hits the mirror, a piece of thermal paper is placed on top of mirror 2.

Figure 2.1: Mirror 2 close to mirror 2Figure 2.2: Mirror 2 further away from mirror 1

The best way to achieve this is to use position the laser head in a way that the distance between the mirrors 1 and 2 is minimized, then firing a short test shot. Then, after increasing the distance between the two mirrors, a second test shot is fired. In a well aligned machine both dots are superimposed on each other. If this is not the case some adjustments need to be made on mirror 1. The adjustment screws have to be turned only very slightly, as depending on which adjustment system is present in your machine, even a whole rotation of the screw can change de angle of the mirror by several degrees. I will explain the procedure briefly using the type of mirror mount present in my machine. The type you might find in yours might differ, but the three-screw design seems to be the most popular one. Some mounts might have screws in different positions, so this guide may not apply exactly to those models. I really urge you to use your brain here and exercise the necessary safety precautions (do not operate the machine with the cover open, wear protective eyewear). I would suggest checking the coarse adjustment using a short distance between the mirrors first and then increasing the distance for a finer adjustment. Please refer to Diagram 3.1 as to how the screws will impact the beam alignment. It usually is necessary to do subsequent tests of short and increased distances with replacing the thermal paper in between, as every adjustment on the mount will affect the positions of the dot at both short and longer distances.

Figures 3.1 & 3.2: Mirror mount and impact of adjustment screws on beam alignment

The mirror mount depicted in Figure 3.1 shows the mirror mount found in my DC-K40, which features three screws for adjustment. Screws A and C make adjustments in one plane only (A horizontal and C vertical) whereas adjustments of screw B will adjust both planes at the same time. The mirror mount in my larger G350 machine were designed differently but had similar placements of the adjustment screws.

When the first mirror is adjusted in a way that the laser beam hits exactly the same spot on mirror 2 regardless of the distance between the mirrors, the same procedure is done to adjust the second mirror in order to get superimposing spots on the in the lens mount assembly. To do this it is sufficient to fix the thermal paper on the hole on the laser head where the laser is supposed to enter. Here again it is tested whether the position is superimposed regardless of distance between mirror 2 and the laser head, best done again with first short and then longer distance. Depending on the type of laser head the third mirror inside can also be adjusted, which is not the case in the small DC-K40 machines. If you happen to have a laser head with adjustable mirror, remove the lens holder and place the thermal paper on the bottom of the laser head and check whether the dot is centered correctly. If not, adjust the mirror in the laser head accordingly.

I have documented the whole procedure in a video, which is in editing right now. Wi will be accessible here soon... [INSERT LINK]


  1. Old post, but was wondering if you ever uploaded that video? Thanks!

  2. What about the position of beam on the second mirror, Should it be on the centre or position doesn't matter as long as the beam strikes same spot in every position of Y axis. I read in some other forums that, beam should strike the 3 rd mirror at exact centre to have a perfect cut other wise edges won't be square.

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