Author Topic: Laser Beam Alignment  (Read 2880 times)

ggallant571

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Laser Beam Alignment
« on: March 02, 2017, 07:18:14 PM »
I have been using a not to precise home-brew z-axis mechanism. It is two vertical side plates with groves. A plate with a a nut on one side and the laser heat sink on the other moves up and down in the grove driven by a low power gear head stepper motor. The UP/DOWN accuracy is more than acceptable. However the slop in the slide causes the head to tilt. The error can be lessened by inserting a small spring. The total range is about 30mm but the typical usage has been less than 3mm.

BTW - this problem can exist on systems without the Z-Axis. Most wheel on V-Grove systems have significant wobble on the X laser carriage. Forcing the lower wheels up tight to the aluminum under frame can reduce it but eventually they become sloppy. Also, there is slop in the bolts used to attach the laser to the plate.

The is a another potential cause of mis-alignment, manually adjusting the focus lens. On my machine the focus dot has considerable variance in both X & Y. I do not know if this at an angle or not.

When doing single pass work such as etching it is probably a non-issue. When doing multipass cuts the effect is non-vertical edges and extra wide cut paths. I first observed this problem when cutting 20x20mm squares in 3mm plywood with 6 passes on a factory fresh machine.

The problem that I am trying to solve is implementing a laser alignment mechanism for both the X and the Y axis's. But first I need a method to measure the current angle. So far I have been cutting squares, examining the edges, adjusting, and recutting. Rather tedious but only needs to be performed after observing problems. The thicker the material, the easier it is to measure the alignment.

Currently thinking about mounting a 45 degree mirror on the top end of a rotating axle. To use:
  1. Position the laser at the center of the axle
  2. Emit a low power laser beam
  3. Measure the height of the laser beam at a known distance (~50mm)
  4. Rotate the axle 90 degrees and remeasure

If everything is perfect the heights should be equal. That is a big "if".

Other ideas:
  1. Use a t-square to align the beam with the base. Inject smoke to enhance the beam.
  2. Add leveling adjusters to the bed and using a level on the heat sink.
  3. Get a thick, easy to burn, non toxic, non-distorting foam material.
  4. Make the jig precise enough so that this becomes a non-issue.

Comments and collaborators wanted.
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treinbert

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Re: Laser Beam Alignment
« Reply #1 on: March 02, 2017, 07:43:50 PM »
Hi,

I am thinking of fixing a spring between the top and bottom wheels of the X-carriage to get rid of the spoil. I have mounted a CD/DVD drive mechanisn to my carriage and will be using this as a Z-axis. The maximum run is 37 mm, but I do not think I need that fully.

Without Z-axis I have cut 3 and 5 mm acryl with 99 passes. I also did cut soft plywood of up to 12 mm. I still have to finish the laser with the Z-axis. Hopefully I can publish results next week.
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Re: Laser Beam Alignment
« Reply #2 on: March 02, 2017, 07:58:24 PM »
I used a machinist  square. I started right at the frame and squared that first Then bed needs to be the same distance from the top of the rail all the way around.

I then squared the gantry to the bed the X axis beam needs to be same height on each side, the beam needs to be vertical.

The bed was tested to make sure it was flat. It has a cutting board for a bed and that is bolted to the feet.

Then I squared the Laser module in mount.

To double check this you would cut the material then raise Z and try another cut. See if the track remains true.

The worst issue is the loose focus adjustment causes "wobble". I would wrap the lens module with Teflon tape to tighten it up. Then I would square up the Laser Module!
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Re: Laser Beam Alignment
« Reply #3 on: March 02, 2017, 08:36:42 PM »
My initial thinking on this, if I wanted a way to make sure my laser beam was straight up/down, would be to design a laser diode mounting bracket, with thumb screws and small springs, to adjust the left/right tilt and fore/aft tilt. It would have small bubble levels mounted for each of the two axis to aid in alignment, but I'm not 100% sure how I would mount the bubbles 'square' on the laser diode itself. Need more thinking on this, but I think some kind of design using thumb screws and springs to move the laser diode in two axis.


« Last Edit: March 02, 2017, 08:38:14 PM by Administrator »
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Lob0426

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Re: Laser Beam Alignment
« Reply #4 on: March 03, 2017, 11:37:02 AM »
The important thing is being square to the work. A level will tell you level to gravity. For levels to work you would need levels on the frame and then you would need a couple more on the Laser Module. You would have to set the levels to the beam, that bears some thinking. You can get these small levels for RV's.

Once I squared mine up it seems to be pretty accurate. Between that and the homing switches it is very repeatable. All it should need is the occasional check with the square as long as it does not get banged around. I use a square and dial indicator to "tram" my mill in. It is time consuming but does not need to be done often. Then you check that against your work piece through careful measurement.

Squaring things takes careful thought. I had to learn how to do it correctly from a machinist. You start at the base and work up!
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ggallant571

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Re: Laser Beam Alignment
« Reply #5 on: March 03, 2017, 01:25:27 PM »
There are two components to this problem, measurement and adjustment.

I agree with thump screws and springs for adjustment. Probably want lock nuts.

I moved my laser to the unheated garage yesterday to cut some 1.6mm PVC plate. Came out quite poorly. Temperature dropped to 16F this morning so the laser is getting a rest. Working and a design with linear bearings on 8mm rod. Hope to eliminate some of the problems associated with the wheels. Want to have the measurement system in place for this build. Keep thinking!!!!!
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ThothLoki

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Re: Laser Beam Alignment
« Reply #6 on: March 03, 2017, 01:29:24 PM »
Thank George. Where you from? I woke up to 12F this morning. Should see 60F tomorrow.
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ggallant571

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Re: Laser Beam Alignment
« Reply #7 on: March 03, 2017, 02:29:23 PM »
Central Maine USA. It has warmed up to 27F and the wind has stopped. Almost swimming weather.
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ThothLoki

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Re: Laser Beam Alignment
« Reply #8 on: March 03, 2017, 04:19:00 PM »
Ok. Minnesota here. We had a high of 32 today.
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mrehmus

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Re: Laser Beam Alignment
« Reply #9 on: March 29, 2017, 10:21:06 PM »
If these laser diodes are built like CCD arrays were way back, the diodes are not square with the enclosure in the first place. So what one needs to do is 'merely' turn on the beam and find the beam centerline at several vertical intervals from the table to the lens. It might be possible to start out with a piece of paper on the table and then raise it exactly vertically and see if the burn circle moves sideways. Another trick might be to see if paper held at a slight angle receives a larger or smaller burn spot as the papers are introduced around the beam at differing angles.
What we need is a Collimator to insure the diodes are emitting a vertical beam. (https://en.wikipedia.org/wiki/Collimator)
We were making the first digital cameras and had to bond the CCD array in a carrier where the receptors became aligned with the carrier reference surface.
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ROSS

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Re: Laser Beam Alignment
« Reply #10 on: March 30, 2017, 12:19:50 AM »
Just a thought..Those old oak Victorian (and some modern)   type cameras  that use either glass (!) plate or sheet film had a rack and pinion arrangement that allowed the lens to move up and down RIGIDLY,  The sideways movement was just the lens board in a grove with sides and a screw stop.....
« Last Edit: March 30, 2017, 08:10:02 AM by Administrator »
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ggallant571

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Re: Laser Beam Alignment
« Reply #11 on: March 30, 2017, 05:03:27 PM »
I we had sensitive X &  Y adjustment screw it might be possible to burn a line test line (say 50mm) long and continuously raise the beam by a fixed amount (5 mm). Measure the variation, adjust the alignment, and repeat till the results are satisfactory. Then preform the test on the other axis. If it dialed in good and lasted a while it might be usable.

That is something I can experiment with (no adjustments) at the cost of a few pieces of cheap cardstock.
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Re: Laser Beam Alignment
« Reply #12 on: March 30, 2017, 05:40:24 PM »
You're the man George  :)
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pedwards2932

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Re: Laser Beam Alignment
« Reply #13 on: March 31, 2017, 03:44:12 AM »
Just thinking out loud here.  I wonder if you made an alignment tool that would be say a 1 inch thick piece of acrylic with a very small hole drilled in it vertically.  Then you put the alignment tool on the bed and aligned the laser with the hole in the top of the acrylic then if it was in perfect alignment with the hole it would burn a dot a the base of the alignment tool?  If it was out of alignment then it would hit the side of the hole instead of going directly thru.  I am not sure how well the laser shoots thru acrylic but if it would go thru the acrylic without the hole then you could put a cross hair on either side of the acrylic then align the laser to the top cross hair and see if it burns a dot exactly in the center of the lower cross hair?
The reason I thought of this is I used to do boat shaft alignment using lasers and we had a clear acrylic cylinder that was about 5" long and you would put it in the strut.  It had cross hairs on either end and you would focus the laser on the first cross hair and then see if it was centered on the other side.
I found some 1" diameter clear acrylic rod that should work.....something about 4 or 5" in length.  Need to figure out how to do an exact cross hair.
« Last Edit: March 31, 2017, 06:48:36 AM by pedwards2932 »

ggallant571

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Re: Laser Beam Alignment
« Reply #14 on: March 31, 2017, 11:25:53 AM »
It is great to hear ideas. I am going to a friends shop today and he has precision tools extensive experience with optics. Will forward your idea and see if we could make a prototype. Even it it doesn't cut, a non vertical laser beam may be visible in the plastic.
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