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Marc Pod
09-21-2017, 7:21 AM
Hi,

Just changed laser tube and I'm havnig huge problem with alignment of laser beam. It's not forst time changing tube and never had any problems. Here it is
Laser beam on first mirror is in dead center, same on second mirror, dead center on near and far end. But third mirror well it is in dead center going up and down but it is completely off when going from left(near) to right(far). I tried everything and can not figure it out what is wrong. Adjusting mirror 2 just make things worse. Any advice? Thank you.

Marc

Rich Harman
09-21-2017, 7:08 PM
... third mirror well it is in dead center going up and down but it is completely off when going from left(near) to right(far). I tried everything and can not figure it out what is wrong. Adjusting mirror 2 just make things worse. Any advice?

Mirror two needs to be adjusted. If adjusting makes it worse that means you adjusted the wrong way - or the right way too much.

John Noell
09-22-2017, 3:27 AM
I've seen something like this. (I assume you mean it is centered in the y axis [up-down] but not the x axis [left-right].) The issue was that mirror 2 was not exactly centered relative to the movement of mirror 3. Thus, as the distance between the mirrors increased or decreased, the angle slightly changed. From mirror 2 to mirror 3, the beam should be exactly perpendicular to the beam from mirror 1 to mirror 2. (If you want illustration drawings I can do some.) I had to adjust the whole mirror holder, until it was always in alignment with the mirror on the lens tube. (How did it get out of whack? Who knows.)

Rich Harman
09-22-2017, 12:43 PM
... The issue was that mirror 2 was not exactly centered relative to the movement of mirror 3. Thus, as the distance between the mirrors increased or decreased, the angle slightly changed.

The mirror being centered is not important, aside from the importance of the beam not being clipped due to hitting something other than the mirror. The mirror is probably many times larger than the beam, there should be plenty of room for a slight off-center misalignment.



...From mirror 2 to mirror 3, the beam should be exactly perpendicular to the beam from mirror 1 to mirror 2. ...
Yes, the only adjustment for this is by steering the beam using mirror two. Where the beam hits mirror two does not change the angle, only the angle of mirror two relative to the beam will change the angle of the beam.

If, once the beam is aligned so that the spots line up when the X axis is far left and far right, and that spot is not centered on mirror three - then you may need to adjust the location of mirror two. If the spot is low or high you will need to raise or lower where the beam hits mirror one and you may need to raise or lower mirror two, depends upon if the beam still has clearance. If the spot is off left to right then you can either change where the beam hits mirror one or you can move mirror two on the gantry. Sliding mirror two closer or further to mirror three or closer or further to mirror one will have the same effect, depending on your mirror holder you may have only one choice. Either way you would be moving the plane of the mirror either closer to or further from mirror one which will change where the beam hits mirror three.

Kev Williams
09-22-2017, 2:39 PM
The issue as I see it, is the ridiculously small amount of mirror movement(s) needed during adjusting. To move a beam sideways .100" on the next mirror 24" away requires less than 1/4° of mirror angle movement... a .700" diameter mirror pivoting at its middle will move its top or bottom edge only .0015"... this means, as close as Marc was in the original pic, getting any closer to perfect means that only a very tiny bit of movement of mirror #2 needed. There should be no need for changing mirror mounts or anything else. The most important thing needed to finalize the adjustment- is patience! ;)

John Noell
09-22-2017, 5:38 PM
Well, maybe I am missing something. However, if mirror 2 is not positioned pretty darn close to the center of the axis along which mirror 3 and the lens tube move, then as mirror 3 moves closer to mirror 2, the beam no longer hits in the center of the mirror as in the illustration. Or, if you align to hit mirror 2 dead center when mirror 3 is close to mirror 2, it will not hit mirror 3 dead center when it moves further away.

Kev Williams
09-22-2017, 7:16 PM
My 'theory' is based on the OP's statement that this is the second tube change and all was okay after the first change, and that as of now 3 corners are aligned, but having trouble with the 4th corner, which is farthest from the source...
So-
1-2 is aligned full travel top to bottom--
2-3 is aligned full travel top left to top right--
2-3 is misaligned from bottom left to bottom right...

since 2-3 is aligned okay at the top, the basic 90°mirror alignment must be pretty close.
Yet 2-3 moves away to right going toward the bottom right...

logic says if you move mirror-2 horizontally to the left to match the 3-corners burn hole, then the new top right burn should move the same distance.
... does it? Or does it not move at all, or does it move to the right? (no clue, just throwin' that out there! :) )

Seems to me if 3 corners are near perfect but one corner isn't, the problem may be with the corner. I'm kind of wondering if the gantry is a bit misaligned, whereas the X rail isn't exactly 90° to the Y rails. My brain says that if this is the case, 2 and 3 aligning well at the top may be due to the close proximity of 1 and 2, and the farther 1 and 2 move away from each other, the rail's misalignment becomes more pronounced...

I've had to re-align the gantry's on my old ULS and the LS900 before, a loose screw or a good bump is all it takes--

Rich Harman
09-23-2017, 5:51 AM
Well, maybe I am missing something. However, if mirror 2 is not positioned pretty darn close to the center of the axis along which mirror 3 and the lens tube move, then as mirror 3 moves closer to mirror 2, the beam no longer hits in the center of the mirror as in the illustration...

I suppose that is true but you should never align mirrors in that manner. You always put mirror 3 close to mirror two - fire a mark, then move mirror three away and adjust mirror two so that the marks line up. Making that first mark should be a very good indicator if the mirror's position needs to be changed.

Aligning mirror two isn't about getting the spot centered on mirror three, it is about getting the beam to be exactly perpendicular. In your illustration you have steered the beam to hit the center of mirror three at the far end, instead of hitting the same spot as was made when mirror one was closest to mirror two.

Rich Harman
09-23-2017, 6:05 AM
My 'theory' is based on the OP's statement that this is the second tube change and all was okay after the first change, and that as of now 3 corners are aligned, but having trouble with the 4th corner, which is farthest from the source...
So-
1-2 is aligned full travel top to bottom--
2-3 is aligned full travel top left to top right--
2-3 is misaligned from bottom left to bottom right...

since 2-3 is aligned okay at the top, the basic 90°mirror alignment must be pretty close.
Yet 2-3 moves away to right going toward the bottom right...

I don't think it is aligned full travel top left to top right. However if that was the case then I would suspected a stretched or slipping belt or a loose connection on the torque tube.

The OP says; "But third mirror well it is in dead center going up and down but it is completely off when going from left(near) to right(far)."

To me that means that the mark is centered vertically (going up and down) or it means that the mark is centered on mirror three when it is close to mirror two with the gantry either near or far. Either way it means that mirror two needs to be adjusted.


logic says if you move mirror-2 horizontally to the left to match the 3-corners burn hole, then the new top right burn should move the same distance.
... does it? Or does it not move at all, or does it move to the right? (no clue, just throwin' that out there! :) )

Moving mirror two to the left (or further from mirror one) will move all the burn spots to the right on mirror three, regardless of where it is positioned on the table.



...I'm kind of wondering if the gantry is a bit misaligned, whereas the X rail isn't exactly 90° to the Y rails. My brain says that if this is the case, 2 and 3 aligning well at the top may be due to the close proximity of 1 and 2, and the farther 1 and 2 move away from each other, the rail's misalignment becomes more pronounced...

You can still get a "good" alignment if the gantry is not perpendicular.

Rich Harman
09-23-2017, 6:16 AM
Another thing, it looks like the OP may be using the red dot for aligning. That may be okay, as long as you have verified that the red dot lines up perfectly with where the beam actually makes the marks. I would only use the red dot for getting close, actually fire the laser to get it right on. If the red dot beam is not perfectly collinear with the laser's beam, then it will be a pointless exercise. If the red dot is off a tiny amount it may not become apparent until the beam path is longest.

Marc Pod
09-23-2017, 11:53 AM
Thanx to all. Problem was in 2 loose screws one on mirror 1 and other on laser head. Regarding red dot line. I'm using SPT TR tube and beam is perfectly collinear.

Rich Harman
09-23-2017, 6:53 PM
... Regarding red dot line. I'm using SPT TR tube and beam is perfectly collinear.

Glad you found the problem.

What is a SPT TR tube, and how does it relate to the red dot?

Bill George
09-23-2017, 9:29 PM
Looks like the tube he is using has a built in red dot. Very nice.

John Noell
09-23-2017, 10:22 PM
I suppose that is true but you should never align mirrors in that manner. I fully agree. That was just for illustrating the alignment issue, not how to align. I've seen a situation exactly like that. (How it got that way is not clear.) No way are you ever going to get the alignment right if the mirror is not in the right position. And it can cause the OP's symptoms.

Rich Harman
09-25-2017, 2:04 AM
Looks like the tube he is using has a built in red dot. Very nice.

I looked it up. It looks like a typical red dot beam combiner except it is stuck onto the end of the tube instead of mounted elsewhere. It would need to be carefully adjusted to be sure that it is perfectly collinear, just like the other type. Additionally I think it may be more susceptible to heating than a standard beam combiner, which could lead to problems. I prefer to not have anything impeading the beam path, and certainly not mounted directly to the output window. I don't think the very small convenience is worth the few percent loss of power and potential problems.

368541

Kev Williams
09-25-2017, 3:44 AM
My GCC's red laser is always on (by choice) and always thru the lens- The red laser is mounted just beyond the main tube's snout, and its red beam passes right thru the main beam to a mirror where it's reflected to the main #1 mirror, from there both beams follow the same path to and thru the lens. Whether on or off, I can't discern any power difference. I have a GaAs lens that is completely opaque to my eyes, total black, and a red laser won't pass thru it either, but it's totally transparent to a 10.6 micron C02 beam. I'm thinking the different wavelengths of the 2 lasers make them invisible to each other...