Laser alignment

[Chris Clayton]

Hey peoples. I hate to ask about laser alignment because I know there is a lot of info out there, but I have a quick question.

I'm trying to help a guy fix up his big Chinese machine. I've made sure the machine is level and attempted to align the mirrors. If the Y-axis is straight (center of 2nd mirror at closest to tube and furthest point), and the 3rd mirror on gantry is aligned at furthest points from tube (closest to operator on left and right side), then shouldn't the two corners closest to the tube also be perfectly centered?

I can get the x-axis straight when closest to me, but as the gantry moves closer to the tube it starts going way off and hitting higher. The previous owner had washers under the rail and I'm wondering if I should start messing with the actual machine or if it sounds like adjusting mirrors could still fix problem.

I'm just thinking of the x-axis is perfect on one end of the machine, it should be the same on the other?

Sorry if this is a stupid question and thanks for any input. I'll try to add photos later if that helps.

[Chris Clayton]

I'm so lost. Can a co2 beam shoot out at different directions by any chance? I don't know if the mirror adjustment screws are moving or what, but I had the x-axis centered, moved gantry towards tube and the beam hit higher, then moved gantry back closest to me and the beam now hits lower than center. I'm assuming this might be why the previous owner had some kind of white silicone on the back of the screws...

[Kev Williams]

I haven't done this too many times but what works for me is this procedure---

1- make sure laser is hitting mirror one in the center.

2- move the gantry rail all the way down towards you, and the laser head as far from mirror 2 as possible.

3-check burn for center on mirror two. Move mirror one until beam is centered.

3- now move the gantry all the up, and check mirror 2 again for center. Goal is to have the beam hits the mirror as close to the same spot on mirror 2 with gantry full up as full down. If it's not, tweak mirror one until the beams match. Absolute dead on not necessary, but if you can't get it very close, the tube will need to be adjusted. (hope not )

4- Once mirror two is set, repeat for mirror 3 over the lens, adjust mirror 2 until beam hits mirror 3 in the same place from full left as full right.

5- once that's done, set up mirror 3 to hit dead center on the lens.

There should be 2 sets of mirror screws on each mirror, one set of 3 spring loaded for adjusting, which should stay put, and a set of 3 thumbscrews with a jam nut or knurled ring. You don't TIGHTEN the thumbscrews, as they also act as adjusters. When the mirror is where you want it, if any thumbscrew is loose, just turn it in only until it makes contact with the mirror frame, then tighten all the jam nuts.

For what it's worth, on my Triumph some of my thumbscrews are loose, and pretty much none of my jam nuts are tight (I just checked! ) and I've only had to make one minor adjustment to mirror 2 in going on 4 years, and that was right after my broken stepper wire caused some rather violent vibration in the drive train...

[Rich Harman]

It sounds like either something is loose and shifting, or the two Y axis rails are not lying on the same plane. I had the problem with the rails not being planar. I had to shim up the left rail nearest mirror one in order to get it close. It is still not absolutely perfect. If I put the third mirror at the bottom right, fire a mark, middle right then back right - the middle mark is slightly higher than the other two. That indicates that one of the rails has a slight bow to it.

Rigging up a red dot laser to the output of the main tube so that it passes through all the mirrors was quite enlightening. You could see how the dot wandered around the target as the laser head was moved to various positions around the table.

Something else I did when I first got my machine, I made a small triangle bracket to stiffen up the sheet metal base that the second mirror assembly is mounted to. Light finger pressure on the second mirror would cause it to deflect - didn't think that was a good thing. That was six (?) years ago and it seems to have worked out well. Someday I will take this thing completely apart and rebuild it, without the imperfections.

[Rich Harman]

...Absolute dead on not necessary, but if you can't get it very close, the tube will need to be adjusted. (hope not )

If the beam is hitting mirror one in the center there will be no reason to adjust the tube. The only exception is if mirror one needs to be raised or lowered, then you would have to adjust the tube so that it once again hits mirror one in the center. In other words, if you are having trouble getting the two spots to line up on mirror two - it isn't the tube, it's mirror one.

What I do;

1. Put two pieces of tape over mirror two on the side facing the incoming beam, move the gantry to be as close to mirror one as possible and fire a mark.
2. Move the gantry as far away as possible from mirror one (pull it towards you) and using a fine point sharpie, draw cross hairs centered on the mark.
3. Fire another mark, just enough to be barely visible. If it is off then make a small adjustment and fire again. You can make several adjustments without having to re-fire the initial mark. You can even put another piece of tape on and transfer the cross hairs to the new piece and continue fine tuning. It is much more important that the two spots line up than for the beam to be in the center of the mirror. What we are doing here is steering the beam so that it is exactly parallel to the Y axis rails. If the beam hits the mirror slightly off center you could even consider that an advantage - you'll be able to rotate the mirror in it's holder to expose new parts to the beam, saving having to buy a new one.
4. When satisfied that you cannot improve the alignment, put new tape on and start the process over. If the two marks are in perfect alignment then you are done with adjusting mirror one. If not then go to step 1. Generally only one round is required. If I have moved the tube or replaced mirror one, then it may take two or three rounds. Each round only takes a few minutes.

I think that getting mirror one perfect (meaning that I can't see a difference is spot position) is very important. If it isn't, whatever inaccuracy it has will be amplified as the beam travels further. If mirror one isn't "perfect" then when you adjust mirror two - that adjustment will only be correct for whatever position the gantry was in when you aligned it. Also, the larger table you have, the more critical alignment becomes.

Adjusting mirror two is the same process. The difference is that it is fairly important to hit the third mirror in the center. If the beam hits mirror three off-center then you would need to adjust mirror three to steer the beam to the center of the lens. Which means that the beam would be entering the lens at an angle, which means the beam will be coming out of the lens at an angle. The amount of this angle will depend upon the distance between mirror three and the lens, shorter distance means greater angle.

To correct for being off centered (left to right) on mirror three you must move mirror two - meaning you must loosen the mirror mount and slide the mirror holder. If the mark is high or low then you must adjust the tube to hit mirror one higher or lower. It may require raising mirror one and two also. This is assuming your third mirror cannot be moved higher or lower, if it can then you would move it instead.

You shouldn't have to resort to moving the mirrors, they should have been set up correctly at the factory, you should only need to align them. However, on both machines I have imported it was necessary to move the mirrors. The setup was so poor it was laughable. On the second machine the beam was literally clipping the hole where it passed through the case after mirror one.

[Chris Clayton]

Thank you guys for your input. I'll be back in a few hours to write a proper response.

[Rich Harman]

...I had the x-axis centered, moved gantry towards tube and the beam hit higher, then moved gantry back closest to me and the beam now hits lower than center.

That behavior is perfectly consistent with mirror one not being aligned properly. As the beam leaves mirror one it is on a downward slope. The longer the beam path becomes, the lower the spot goes.

[Chris Clayton]

Thanks for the assistance Kev!

[Chris Clayton]

I definitely think the y-axis rails are not on the same plane. Thanks for your thoughts. I was thinking it might be easier to make a new machine as well haha.

[Chris Clayton]

Really appreciate the time you (and everyone) spent replying to my S.O.S. I had a feeling yesterday that once the beam was aligned on mirror 3 (on left and right sides), I would need to adjust mirror 2 mounts to make mirror 3 centered (because it was aligned on left and right side of machine but not centered). When I first went to help this guy with his machine, the beam was hitting the metal circle thing as well.

[Chris Clayton]

What confuses me is that mirror 2 was/is aligned and hits the same spot closest to mirror 1 and furthest from it. If the beam is slightly off and mirror 1 is adjusted to be straight with the y-axis, that's all that matters correct?

[Chris Clayton]

ok so I'm going to see if I can add some images. I have the tube leveled and hitting mirror 1 pretty close to center, not perfect but I'm more concerned with the beam hitting it straight on (so I can see what's happening with the other mirrors). On mirror 2, the beam hits the exact same spot when the gantry rail thingy-ma-bob is closest to the tube and when it's furthest away. So I'm thinking that means the beam is going straight down the y-axis? Even though it's not as perfectly centered as I'd like (in a perfect world). Then I get to the 3rd mirror. When gantry is closest to me (furthest from machine) on the left side and right side, I can get it centered. I'm assuming I'll need to play with the 2nd mirrors mounts and tube height to get it perfect, but I'm not aiming for that yet.

So we have y-axis completely straight and x-axis completely straight when gantry is closest to me. As I move the gantry away from me on the right side (back of tube side), it starts acting really wonky. In the middle, beam hits mirror 3 towards lower left. Keep moving it back about 3/4 of table and beam hits higher. Move it to back right corner and it hits even higher to the left. I'm thinking the y-axis rails are off, but I'm not 100% sure. I'm not a mathematician but if everything (rails etc) was level and on the same plane with each other, I would think the back right corner would be the exact same as the front right corner, correct? The worst part is that when I move the gantry to the back right corner and then move it back to the front right corner, the position changes on the 3rd mirror. Before moving it, the 3rd mirror (x-axis) is perfectly lined up on the front left and front right corners. Then I move it to the back right corner and it comes back hitting way lower. I really hate this thing right now haha

Thanks again guys!Mirror1.jpgMirror2_straight.jpgMirror3_straight.jpgMirror3_wayOFF.jpg

[Bill George]

Big Chinese machine, hmm do you suppose the rails and such are parallel with one another? If you can't keep mirrors aligned properly or to begin with that is what I would check first. Once your assured the machine is level, square and straight then the mirrors should not be that hard to do.

[Chris Clayton]

Oh and don't get me wrong, I understand the beam going into the 3rd mirror needs to be dead center. For now I just need to figure out what's throwing it off so much when y-axis is aligned (even if off center of mirror slightly) and when x-axis is aligned towards front of table. I'll do some more tinkering this week and please let me know if anyone can make sense of the photo showing the beam hitting 3rd mirror in the center when gantry is towards front of machine, lower when in the middle of table, then jumps higher at back right corner.

*shakes fist at China*

[Bill George]

The machine needs to be square and rails, gantry parallel before your going to solve the mirror issue.