Mirror adjustment

Discussions on optics for laser cutter/engravers

Mirror adjustment

Postby buildsomething » Mon Feb 22, 2010 8:10 pm

I opted to simulate the alignment process by using a small laser pointer mounted where the CO2 lase is to be installed... at least then I would have some knowledge as to the setup procedure and sequencing without hurting myself. After a bit of thinking and some assumptions, I realized that I am assuming the beam will exit the laser dead center of the tube, which may not necessarily be true.

That being said and trying to rationalize my decision, I thought that using the laser pointer was still valid to determine the mirror setup sequence.

I have 4 mirrors in my system, each one that can be adjusted individually.
My co-ordinate system is defined as X being along the longest axis.....in this case 48", while Y axis being along the gantry....15" travel.

The 4 mirrors as I have labelled them are:
1. First to bend the beam 90 degrees after existing the laser
2. Returns the beam back along the X axis
3. Bends the beam 90 degrees along the Y axis gantry.
4. Final mirror before the focusing lens

Because of the type of mirror assemblies that I have, I have the ability to mount a small disk at each entry point with a tiny hole machined in the exact center. I can also see the laser pointer beam as I adjust each mirror. I am using this hole as a target for my laser pointer and try to adjust each mirror to hit the center. These disks will then be used to hold a piece of masking tape over the hole when I power up the laser under very low power. Each mirror ass'y has 3 tiny adjustment screws for mirror alignment.

The sequence that I am using is the adjust mirror # 1 to hit the center of mirror # 2 and them mirror # 2 to adjust # 3 and so forth. The biggest problem that I am seeing right now is that I out about .06" in hitting dead center on mirror # 3 along the full length of the X axis. I must be doing something wrong since I can't get it spot on. The mechanical parts are flat and square......something is definately wrong....or I'm not using the proper setup sequence.

The question to the group is.....how accurate do I have to be in hitting dead center of each mirror in order for the system to work correctly or does it matter? Obviously, dead center is a good starting point, but what about reality and practicality? Is there a proper mirror set up procedure and sequence?

Anyway, I think that this would be a good point of discussion and knowledge exchange on mirror aligment and setup for those who might be considering building a laser cutter or engraver from scratch. Or maybe this is quite easy and I'm just a big dummy and slow learner.

I look forward to healthy discusion and exchange of ideas.

Richard
Attachments
4 Mirrors.jpg
4 mirrors in close proximity
Laser Pointer fixture.jpg
Laser setup.jpg
buildsomething
 
Posts: 69
Joined: Tue Jan 05, 2010 12:54 am
Location: Toronto, Ont

Re: Mirror adjustment

Postby lasersafe1 » Mon Feb 22, 2010 9:04 pm

You don't have to hit anything dead center except for the final lens, but unless the mount is a true gimbal, it will translate the beam as it is adjusted, so it is best to hit the center.

You are right to try it with the laser pointer first to get practice. In the end you will move your tube to the line that your laser is on right now.

Here is a good alignment procedure:

http://www.lowcostlasers.com/index.php?main_page=page&id=5&chapter=20
lasersafe1
 
Posts: 599
Joined: Fri Nov 27, 2009 8:23 pm

Re: Mirror adjustment

Postby buildsomething » Mon Feb 22, 2010 10:07 pm

Well that link to the alignment procedure really does help to explain a few things. I'm still chasing my tail trying to zero it in, but using the small laser pointer will sure help out well at least give me some knowledge without hurting myself.
Thanks again for the info.

Richard
buildsomething
 
Posts: 69
Joined: Tue Jan 05, 2010 12:54 am
Location: Toronto, Ont

Re: Mirror adjustment

Postby lasersafe1 » Tue Feb 23, 2010 12:45 am

I mentioned it somewhere else, but it is worth mentioning again. Save your store receipts.... Not so you can return things, but because they are a good thermal paper that will turn black when heated. If you have a microcontroller or function generator you can program to give a "tickle" pulse, then it won't be enough to burn paper but it will turn the receipt paper black where the spot is hitting. Without the means for a tickle pulse, you actually want a thicker cardboard that will not burn through. It will tick you off the first time you hold paper in front of a mirror and it bursts into flame leaving an ugly char mark on the mirror.
lasersafe1
 
Posts: 599
Joined: Fri Nov 27, 2009 8:23 pm

Still can't align the mirrors!!!

Postby buildsomething » Sat Mar 06, 2010 12:47 pm

Well I am totally frustrated at this point in time trying to line up the mirrors for the beam to be dead center.
I have NOT been able to line up the X axis as of yet and still about .06" off center over the full length of travel. I haven't even attempted to line up the Y axis yet.

I have tried aligning the beam so that it hits dead center when the carriage is at its extreme movement and then driving it home to see where the beam enters and its off by .06". If I try to align the beam dead center when the carriage is home and the drive it out to the extreme, the beam is out....way out. The only thing I can think of is that there might be a mechanical / dimensional problem.

I must be too close to the problem because I just can't see the solution as of yet. I have even started to remeasure everything to make sure that I machined all the pieces correctly. One problem that I did discover was that the center line between the 2nd and 3rd mirror were slightly off in the x and z axis and that was corrected....but still no joy.

As you can all see....I'm just venting right now...time to take a break and watch the snow melt...almost as good as watching paint dry. Maybe its time to clean up the shop, sweep the floor and put the tools away and hopefully I will be able to see a solution. Cracking open a cold one will also help.... :D

Richard
buildsomething
 
Posts: 69
Joined: Tue Jan 05, 2010 12:54 am
Location: Toronto, Ont

Re: Mirror adjustment

Postby lasersafe1 » Sat Mar 06, 2010 1:33 pm

If you've never aligned before, it can be a challenge and a frustration. Let me make a suggestion: Concentrate on one plane at a time. Lets say that you know that your final mirror that points it down through the lens is 5.00" above the table plane. Next ignore all other mirrors and make sure your laser tube output is also 5.00" above this plane. Since you are lucky enough to have a pair of mirrors before heading off to the sweeping axis', then you only need to make sure that the final stationary mirror is at the right launch height. The key thing to remember is that the beam that travels toward your sweeping Y and sweeping X mirrors must LAUNCH at EXACTLY 5.00". If it is "working its way to the correct height along the plane", then it will not work. I usually tape a piece of thermal paper to a ruler at the necessary height and move it around the table to ensure the beam is on the plane. You must work your way from the laser toward the target one step at a time.

Once you have the single plane clear in your head, you then need to think about the other directional plane in the same way. Look at the distance from your Y rail to the centerpoint of the Y mirror. The beam that travels toward your sweeping Y mirror must LAUNCH from the final stationary mirror at the same distance away from your Y rail, so you may need to physically move the base of that last stationary mirror inward or outward so the launch point is correct. When your beam finally hits the sweeping Y mirror at centerpoint along the whole travel of Y, you need to make sure that the centerpoint launch toward you sweeping X mirror is at the same physical distance away from the X rail.

Its gonna work!
lasersafe1
 
Posts: 599
Joined: Fri Nov 27, 2009 8:23 pm

Re: Mirror adjustment

Postby bdring » Mon Mar 08, 2010 2:55 pm

I have done it from scratch about 3 times now, due to changes and re-assembly. It have it down to about an hour now. The first few attempts were trial, error and tweak. Now, I spend a lot more time adjusting things things before I ever turn on the laser. You really need to make everything is square and on the same planes. If you don't, you spend a bunch of time getting it to aim to a certain position, then find it does not work in a new postion. As soon as I see that, I stop aiming and go back to squaring up.

If I ever build a bigger laser I am thinking about putting the laser tube on the back of the gantry. This would really simplify the alignment. It would have the same number of mirrors, but way fewer alignment issues. The thought of doubling or tripling my beam travel is scary.
Bart
"If you didn't build it, you will never own it."
bdring
Site Admin
 
Posts: 2966
Joined: Sun Nov 22, 2009 7:33 pm
Location: Chicago, IL, USA

Re: Mirror adjustment

Postby buildsomething » Mon Mar 08, 2010 4:01 pm

I'm still working out the little bugs...ie alignment issues.

One thing that I am still wondering about the tolerances required to line up the mirrors. Obviously dead center with no movement of the beam over the full travel of the gantry would be perfect. That being said, because the beam is about 6-8mm wide and marking a thermally sensitive material to adjust the beam would have some tolerance issues...I would assume. How does one measure diameter of a burn mark...or am I make this more complicated than I need too?

I haven't turned on the power to the laser yet, but still learning alignment procedures using a small laser pointer as an indicator. Over a 48" travel I am out in the Z axis...(vertical) about .050" which I feel is still a huge error.
If my object that I will be cutting is very small...ie 3" or so, the any error in the final product would be small or insignificant. However because I will not be marking but cutting over a 48" x 15" area, the cutting beam postion must be quite accurate in order for the final parts to fit over the full cutting area....well thats my thinking anyway. Some one said, well just try it and see what happens....but with safety being a paramount issue and that this is first time that I am doing this, I feel that I would rather learn on a laser point rather than playing around with a live beam.

I'm probably making this more complicated than it should be.

Still frustrated.

Richard
buildsomething
 
Posts: 69
Joined: Tue Jan 05, 2010 12:54 am
Location: Toronto, Ont

Re: Mirror adjustment

Postby bdring » Mon Mar 08, 2010 4:38 pm

I have a roll of that thermal paper, but I prefer to use a piece of wood. I usually use a piece of 1/16" thick plywood. The paper gives me a bit of a fuzzy edge, where the wood gives me a crisper circle. I can also free stand the wood up on its edge. I slide the wood over a bit for each position and visually compare the position of the circle.

Once the beam is focused, you can measure the results much more accurately on a scrape work piece. If I draw a 12" long line and it measures +/- 0.03" I call it done.

You might want to move forward with "close enough" at this point in your build. Often, I spend too much time in an early stage only to find another issue down the road that needs addressing first (the "polishing a turd" syndrome).
Bart
"If you didn't build it, you will never own it."
bdring
Site Admin
 
Posts: 2966
Joined: Sun Nov 22, 2009 7:33 pm
Location: Chicago, IL, USA

Re: Mirror adjustment

Postby buildsomething » Tue Mar 09, 2010 9:33 pm

Well I must say that .030" is way too much out of spec for my requirements. :(

Most of my cutting will be done in area bounded by 3"-4" x 36"-48" and the relationship between the end points must be precise. If I was to only cookie cut areas of 3" x 3" for example, then I would be out of tolerance by about .007 to .008" which is acceptable...barely. However if I was to cut a piece that was 48" in length then I would be out by at least .120" which is way too much.

Secondly, as I'm adjusting the mirrors I am noticing that the focal point of the beam is also moving around...as to be expected. In some cases it does not even make it out of the air assist chamber of which the opening is .125" in diam which is like a barn door in relative terms. :D Time to review my measurements and see if I machined anything out of spec.

Mechanically the machine is sound and square...but the alignment of the mirrors is becoming a bit of a concern for this application. During an engraving operation, one isn't as concerned with the final product mating up with another piece. While in cutting, I must be able to cut pieces out precisly so that individual pieces can mate up with other ones without any significant gaps. There are obviously differences between engraving and cutting and yet some similarities as well. I just have to sort out the two in my mind and determine what is acceptable and what is not.

I guess coming from a manufacturing career in high speed automation and robotics, tolerance buildup is paramount in my thought process.

I think I will set the project aside for a couple of days, regroup my thoughts and see where I might have gone wrong or misguided in my design. Only time will tell :D

Time to crack open a cold one :lol:

Richard
buildsomething
 
Posts: 69
Joined: Tue Jan 05, 2010 12:54 am
Location: Toronto, Ont

Next

Return to Optics and Lasers

Who is online

Users browsing this forum: No registered users and 6 guests