Beam Path
Last Updated: 24th May 2009
The image above shows the ideal path of the laser beam and the thing to note is that until the beam hits the last mirror, the beam is moving entirely in the horizontal plane. Alas this is not how it is on my machine.
On my machine the beam travels downwards between the 1st and 2nd mirrors and the upwards between the 2nd and 3rd i.e. the 2nd mirror needs to be higher but there is no means of adjusting this. It's as high as it will go using the installed mechanisms. Consequently the beam hits the last mirror in different positions depending on where the head is located. This results in the beam hitting the lens, and thus the work piece below the head, in different positions depending on the position of the head i.e. it causes inaccurate cutting. Over the distance of a few centimetres this is hardly noticeable but over the full extent of the cutting bed it results in an inaccuracy of a millimetre or two.
I mentioned on my page about set up that I had removed some padding from under the laser tube in order to lower the point at which the beam hit the first mirror and having measured, as well as I can using the simple tools at my disposal, the diameter of the laser tube and the arcs on the metal blocks on which it sits, it is my belief that designer intended for the tube to be installed without padding, and that the padding is an afterthought (which raises the height of the tube). Certainly it seems to be the case that the supports for the 1st and 2nd mirrors are at the highest possible position to which they can be adjusted.
If this is the case then re-machining the tube supports to allow for padding should help to improve the beam path. However as mentioned on my page about the cutting bed, I am also considering the idea that raising the beam path would increase the working area that I have available for cutting sheet materials. This would necessitate altering the mountings for the mirrors however I don't see any particular problem in doing this.
The first mirror is mounted on a platform whose height relative to the chassis is determined by three bolts. I've had a nut off one and they are M6 types. The bolts appear to be 30mm long (I haven't had one out to make an exact measurement), but replacing them with longer ones will allow this platform, and thus the mirror, to be raised up.
The second mirror is mounted on a bracket attached to the end of the arm along which the head moves. Ever wonder what that little round cover plate on the left hand side of the machine is for? Well so did I until I came to thinking about how I was going to access the screws holding that bracket in position. 
Upon inspection I see that the bracket is positioned as high as it will go. However with a little cutting, it should be possible to modify the existing bracket to get something like another 5mm of height. 5mm would probably be enough to rectify the alignment problem on my machine however I'll need a bit more if I am to extend the usable area of the bed. My current thinking is to make a rectangular plate which will attach to the end of the arm. The bracket will then attach to the plate such that we gain extra height.
The third mirror is on the head assembly and is attached to a plate, fixed to three pillars by three screws. These are 8mm long M3 machined screws and replacing them with longer screws, and spacers under the platform, will allow the height of the whole head assembly to be raised up.
Watch this space. ;-)