Beam alignment oddity

[Martin Berriman]

While cutting some 9mm ply today, I noticed that my cuts are are not vertical when cutting in the Y axis, if you imagine looking down from above, the beam is pointing slightly towards the left; they are pretty much perfectly vertical when cutting in the X axis though.

I immediately suspected mirror alignment and I checked in the 4 corner positions to see where the beam was hitting the final mirror. I got a consistent result as shown below.

on to final mirror.jpg

It shows that the beam is hitting the final mirror a bit high - which in theory should mean that the beam is travelling down the lense tube slightly on the left hand side.

I took off the lense tube and tested the position - see the target marker in the image below - and it seems to be on the right hand side, not the left.

after final mirror.jpg

I also test fired, without the lense, on to the piece of masking tape stuck to a piece of cork with the table high and with it low. The mark is in the same position on the tape (see the bottom right dark dot on the tape) at both high and low points which would seem to indicate that the beam is travelling vertically down the tube.

I then refitted the lense tube and without moving anything did another pulse. This resulted in the lighter dot more to the left on the tape. This seems to indicate that the lense is bending the beam towards the left and this would appear to agree with the 9mm ply cut I observed. I think this also makes sense because the beam is off-centre towards the right.

My confusion therefore is why the beam is off-centre in the lense tube in the opposite direction to what I expected.

Unless I am missing something, I need to lower the head assembly so that the beam hits the final mirror higher up in order to bring it nearer the centre of the lense tube. Does that make sense?

[Pete Cyr]

Are you sure it is not reflecting off the inside of the tube?

[Martin Berriman]

Are you sure it is not reflecting off the inside of the tube?

Not thought of that but I'd be surprised, apart from the slant, it's cutting well

[Philipp Pap]

Maybe have a quick look at the images next to "A bit of theory" here http://makestuffnow.com/set-laser-focus . If you are lucky you can set the 3rd mirror by turning all screws in or out, but yes, this is the hardest alignment but worth it.
In my case the laser has to hit the 3rd mirror in the middle front to back but 3 mm above the middle up/down to make good, vertical cuts.

[Martin Berriman]

Maybe have a quick look at the images next to "A bit of theory" here http://makestuffnow.com/set-laser-focus . If you are lucky you can set the 3rd mirror by turning all screws in or out, but yes, this is the hardest alignment but worth it.
In my case the laser has to hit the 3rd mirror in the middle front to back but 3 mm above the middle up/down to make good, vertical cuts.

Thanks - yes, that images correspond to what I was trying to explain. I spent quite a bit of time fiddling with the mirrors yesterday and made a lot of progress but I was still having trouble.

One of the first things I did was tweak mirror 2 very slightly - the beam still hits above centre of the mirror hole (which is not a problem) but I moved it a miniscule amount to be more central vertically.

I then went back to mirror 3. I found that my bed moves slight side to side as it raises and lowers therefore it was not possible for me to raise and lower the bed height to check for a vertical beam since the target did not stay directly in the same spot - I therefore made a tower type jig where I could insets targets at different heights to see which way the beam was slanting - see photo here.

tower.jpg

This was really useful and I could easily see the slant from the top target to the bottom target. I found that I could adjust mirror 3 to be able to hit the same spot on the top and bottom targets and be pretty well centred however when I refitted the lense tube, I found that the slant returned!

This was very confusing and seemed to indicate that the lense was the problem. I tried tweaking the mirror 3 with the lense in place and found that I could get it more vertical but that it then started hitting the side of the nozzle - i.e. I had the bean vertical but off-centre.

This again led me to suspect the lense so I decided to remove the lense from the tube. To my surprise, I found that the lense was not a good fit in the tube and could slop around, side to side by around 1mm! I think therefore that the lense was simply off-centre in the lense tube and that was causing my problem all along. It was however getting late so I left it and will return the the problem later today now I have a clearer idea of the real culprit. I need to find a good war of centring the lense in the lense tube...

[Doug Fisher]

Nice job with the tower solution! Please keep us informed about how your mission progresses.

[Martin Berriman]

Ok, finally I am happy. The lense seems to be a 2" meniscus lense and the outer diameter is 18mm. The lense tube has a diameter of 18.8mm therefore the lense can slop about by 0.8mm ish. I therefore found a piece of soft wire around 0.4mm diameter and formed it in to a circle to hug the outside circumferance of the lense. I dropped that in to the lense tube and then dropped the lense in - it sits nicely inside the ring of wire which takes up the space and ensures the lense is central.

Once the lense securing nut was screwed in, I rechecked mirror alignment and found I had to adjust mirror 3 again to get a vertical beam and then adjust the Y position of the head on the linear rail to get the beam in to the centre of the lense tube. Once that was done, I refitted the lense and did some final checks which looked near perfect, both central and vertical.

I've just cut some test squares out of 9mm birch faced ply and the edges in both X and Y directions are pretty much vertical - there is a very small taper due to beam divergence but it is the same in both X and Y so I am happy that it's cutting vertically now.