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View Full Version : Double Laser Dots! Please help!



Mike Audleman
08-10-2014, 2:57 PM
Oh boy. I may have pooched the screw.

Yesterday, I had a sheet of ply in the bed weighted down (it was slightly warped). I had hit the "Frame" button and the gantry was showing the cut, it was nearly the whole sheet. Unfortunately I had one of the weights too close, and it got a hold of it while framing. It shoved it against one of the z-axis bed screw rods. It knocked the gantry off square, the laser focus tube off kilter and the machine groaned madly and skipped teeth on the right timing belt before I could hit the panic kill button.

So, I spent 4+ hours loosening screws getting the timing belts loose to re-square the gantry. I can only eyeball the tube for vertical as there are not two surfaces that I can put a triangle on to adjust squareness. I held a drafting triangle up to it and got it as close as I know how. Hundreds of 3" squares of paper later, I got all the parallax out and they were square again instead of parallelograms. At least as close as I can measure by eye and drafting triangle. I cut squares from the 4 corners. They all looked and measured perfectly identical. So I went to bed.

This morning, thinking I got it repaired last night. I slapped a sheet in and ran a project only to find out, not so much. Its square and everything but the cut lines have a shadow low and to the left. And even at 100% power and 10mm/s speed it took 4 passes to cut through 3mm plywood and the cuts looked horrible...but only in the lower right corner of the work area. Upper right, straight through at 15mm/s and 75% power, like it used to. The scan etch also looked ok but it was in the upper middle (250mm/s 40% power).

Upon further investigation, it seems I have a beam refraction somewhere, I know not where. The second dot, just from outward guessing, is about 25% of the main beam's power.

I did pulse tests (no gantry movement) at 20%, 50%, 75%, and 100% powers. I briefly hit the pulse button while the gantry was stationary. I moved the workpiece between power changes so the gantry remained in the identical same position between power tests. I then ran it to the lower right and repeated the test with another scrap block.
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Full Size (http://www.wolfiesden.com/android/2014-08-10_12-59-11_188.jpg)
The left block was done in the upper left corner of the bed, the right one in the lower right (extremes).
Notice the double dot? It seems more pronounced in the lower right block but its present everywhere. This didn't show up when cutting paper squares last night as the power was very low and the speed was fast the second dot didn't have a chance to scorch the paper.

You can see the shadow effect of the cut on the top of the right block. The masking tape is burned away just below the horizontal cut across the top. Also on the right edge of the left block, same story.

I have checked the mirror alignment (tape over holes and pulsing at extremes) at both the mirror at the gantry and also on the down mirror at the head. The dots on the alignment tape line up darn well and are mostly in the middle of the optic hole. Of course the beam is not in focus at those points so its hard to tell if there is a double dot or not. But it sure looks the same. That points to the focus tube. I think.


Can anyone help me get this corrected? I just don't know where to look or what to look for.

Chuck Stone
08-10-2014, 4:42 PM
Don't know what you have for equipment, but I'd be looking for a hairline crack or chip
in either the lens or one of the mirrors. You have both a mirror and a lens in the
gantry? I would check those first, might want to use a flashlight to see any stray
beam. (might not be visible otherwise)

Mike Audleman
08-10-2014, 6:45 PM
Don't know what you have for equipment, but I'd be looking for a hairline crack or chip
in either the lens or one of the mirrors. You have both a mirror and a lens in the
gantry? I would check those first, might want to use a flashlight to see any stray
beam. (might not be visible otherwise)

Hey! I like that flashlight idea! Can't see the infrared of the laser but I can see a flashlight! Gonna try that later. Thanks for the idea.

And yes, I opened the tube and inspected the focus lens. Dirty but not cracked or anything. Cleaned with microfiber lens cloth and DSLR sensor cleaning solution. Spiffy clean now. While there I did look through the mirror (with tube out). It looked clean and clear, no cracks there either.

I suspect its the tube not being exactly perpendicular to the worktable. I have no idea what to do to align it. Thoughts?

Gary Hair
08-10-2014, 11:17 PM
If you don't have a cracked lens or mirror then your beam is probably hitting something and bouncing off. My laser has anodized aluminum tubes and it's pretty easy to mis-align the beam and hit the tube, it looks fine except for the split beam.

Chuck Stone
08-11-2014, 12:34 AM
I suspect its the tube not being exactly perpendicular to the worktable. I have no idea what to do to align it. Thoughts?

Without knowing what equipment you have, it's hard to even say where to look

Mike Audleman
08-11-2014, 3:01 AM
I got it figured out with the assistance of a PM from Bill Carruthers (http://www.sawmillcreek.org/member.php?78865-Bill-Carruthers). It was the tube not being perfectly perpendicular with the bed. Apparently I didn't get it perfectly aligned by eyeball. It appears he has a similar issue a while back and this was the cause so I figured I needed to get it closer than eyeball.

It was working sufficiently enough to cut a jig. I cut a piece of luan (1/4") with forks that would go against the tube between the knurled knob that holds it to the head, bridge the red dot pointer bracket and another finger that sits between the dot bracket and the air assist/lens ferrule. The back side (parallel with the tips of the forks but offset to clear the knurled rings) of the jig gave me something to put a triangle against. And sure enough it wasn't square on either the x or the y axis! Loosening the brackets and applying gentile force, I got it square on both planes. Took a couple times to get it right. Torquing the screws down moved it slightly so I had to over compensate so when I torqued them down it brought it back square again.

After getting it as close as I could this way, the shadow dot was almost gone but not quite. Short pulses yielded no shadow but a long pulse as might happen on a slow cut (5mm/s @ 100%) showed it was there..barely. Enough to scorch masking tape but not burn it. I finished with adjusting a couple screws on the head mirror. Didnt' take even 1/16th turn on either screw and now the dot is a nice little symmetrical pin prick. Its nice and round and small, even holding pulse down for a couple seconds with no gantry movement (obviously making a bigger hole due to continued burning).

So, there was the answer. Even though it was close to square...close doesn't count in lasers, only hand grenades :)

Thank you Bill for the tip that lead to the solution! Probably would have been guessing at solutions for quite a while without the info in the PM.