Laser cutting at an angle?

[Brett Winston]

Hello,
Hoping someone can help with this problem. I’m cutting 1/4" cherry and getting through the board with no problem using many different speed and power setting combinations. The problem is that the resultant profile of the cut piece is not straight (viewed from it’s side). It does not seem to be a focal length problem (beam widening) because in my testing I’ve changed focus from one extreme to the other – laser nozzle sitting right on top of the board, to focused on the top plane of the board, to 2mm out of focus above the board. In every instance (over 50 test cuts), the profile is exactly the same (see attached image). It just looks like the beam is shooting at an angle. A pulse test with tape over the nozzle shows the beam is perfectly (seat of eyeballs) centered and a pulse through the board cuts a nice tiny round hole right through. Metrics follow:

Machine: Chinese G460 60w, Ruida Controller
Speed: Tried combinations from 10mm/s to 18mm/s
Power: 12mA to 18mA

Profile of cut piece:
346646

Thoughts?
Thanks!
-Brett

[Jerome Stanek]

I am running into the same problem and when I pulled the lens holder I saw the mirror was set to far back so in order to get the beam in the center it would be reaching the lens at an angle. I am in the process of moving the mirror so it fires straight down.

[Bert Kemp]

try taking the cone off and see if it still does it, check and make sure your table is level at all 4 corners. R U sure the laser head is not at an angle? and make sure you have the proper focus. for the lens your using

[Kev Williams]

This is actually fairly typical of cutting, I believe due to beam refraction or whatever it's called.

The beam hitting the cone center only means it's in the right place at the cone. Me, I don't use cones because I've found that no matter how centered the beam is, part of the beam path hits the cone. I've proved this to myself at least with wood cutting tests with no difference other than cone off or cone on. The beam at the cone tip is still fairly wide because it's still 1/2" or so from actually being focused. If the cone stops part of the beam, then it seems logical that it may steer the beam off on an angle.

And even if no part of the beam is hitting the cone, it could still be hitting the lens and the work on a slight angle but just happens to be centered as it leaves the cone. Once the beam hits the work on a slight angle, refraction may cause the beam to angle farther. Adjusting mirrors can help, but that could get tedious.

Lots of conjecture on my part I'll admit :) --However, my LS900 doesn't cut straight down, never has. I don't do much cutting so it doesn't matter to me. My Triumph cuts wood pretty straight, but not thick plastics.

Take the cone off and see if that helps even a little bit...

[Brett Winston]

I just took off the cone. No joy, same cut profile. Looking at the lens though now for the first time. It appears to have been installed "hastily", go figure. Looking for some bent needle nose to act as a spanner wrench and will investigate further. Lens also looks to have a spot or two on it. Time for a cleaning at the least.

[Brett Winston]

Lens was installed with convex side down - towards table. I thought it was supposed to be convex side up towards laser and flat side down towards table (?).

[Brett Winston]

Convex side down seems to have given slightly better results but I'm going to run convex side up for a while to do more testing. Table is dead flat. Not sure how to re-angle the beam other then to shim it where it mounts to the bottom of mirror 3 but I don't have anything that thin to use.

I think Kev's idea of mirrors being off a bit is probably a good next step to investigate. Thanks guys. Will keep you posted.

[Gary Hair]

If you cut a square from a piece of acrylic does it have the angle you show on two sides or all 4? If it's on all 4 sides then it's just the shape of the beam, however, if it's on 2 sides then it means your beam isn't hitting the center of the lens. You should also check to see if it does the same thing in all four extreme corners of your bed. If not, then you have alignment issues as well as missing the center of the lens.
This isn't something you have to live with, it can be corrected with proper alignment.

[Bert Kemp]

yes convex up



Lens was installed with convex side down - towards table. I thought it was supposed to be convex side up towards laser and flat side down towards table (?).
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[Brett Winston]

If you cut a square from a piece of acrylic does it have the angle you show on two sides or all 4? If it's on all 4 sides then it's just the shape of the beam, however, if it's on 2 sides then it means your beam isn't hitting the center of the lens. You should also check to see if it does the same thing in all four extreme corners of your bed. If not, then you have alignment issues as well as missing the center of the lens.
This isn't something you have to live with, it can be corrected with proper alignment.

It's definitely more pronounced on two sides. The other two sides appear to be splayed outwards, opposite each other at the bottom of the piece looking down, which I would think would be indicative of getting outside of the focal range. However, that test piece was pretty thin. I don't have any thick acrylic at the moment. I'm leaning towards beam alignment at mirror 3. Will test shortly.

[Joseph Shawa]

Is lens seat flat? If the lens was an angle I think the beam would leave at an angle. How far does the spot move when you are close vs far from your target?

on my machine I found that the beam was getting cut off going into the head before mirror 3. I machine it out on the top part of the hole and raised the beam so it would fire straight down the tube BEFORE I even put the lens in.

[Joseph Shawa]

how do you keep focus lens clean?

[Jerome Stanek]

I now have it where it almost hits dead center at the end of my tube without the lens in I tried a quick cut and it seems now that it is cutting pretty straight. Before I had to shim my tube to cut straight down.

[Kev Williams]

The lens will work convex up or down- My Triumph lens came convex down, ran it for several days before figuring that out!

Funny thing, upside down it would cut 1/16 Rowmark with less power. But the focus range was very small, and the actual optimal focus distance was like more than 1/8" closer upside down...

Upon further experimenting, I found the lens performed MUCH better 'overall' right-side up.

With the beam leaving the convex side, the convex itself may be playing a role in the angled cut. SO, put the lens in right, and run a ramp test to find optimal focus, because it WILL be different!

As for the beam hitting and/or leaving the center of the lens, I've always been perplexed about this: My LS900's red dot hits all the mirrors in a totally different place than the laser does. On the last mirror I got the laser almost dead-centered, but the red dot hits the mirror just above center, but off towards 3 o'clock about .040". Looking down from the front of the machine, the beam hits the lens off center, low and left slightly (about .040" both ways), the red dot hits the lens up right and left, close to an 1/8" inch...

Yet, across the whole table, the light and beam focus on the work within .003"-.005" of each other... All I got for an answer to that is, these lenses much be fairly forgiving... ;)

[Rich Harman]

It is very easy to test if the beam is at an angle. Fire the laser to make a dot on some thermal paper (like what receipts are typically made of) that is taped down to the table, then without moving the laser head lower the table as far as you can and fire the laser again. If the dot is centered within the darkened ring made by the second firing then the beam is perpendicular to the table, if not then it is angled.

[Brett Winston]

It is very easy to test if the beam is at an angle. Fire the laser to make a dot on some thermal paper (like what receipts are typically made of) that is taped down to the table, then without moving the laser head lower the table as far as you can and fire the laser again. If the dot is centered within the darkened ring made by the second firing then the beam is perpendicular to the table, if not then it is angled.

I used this method (with plain copier paper - all I had) and was able to see the problem. Thank you! I leveled the tube and realigned the mirrors and am getting a straighter cut. Not perfect but better. One issue that is consistent is the beam location when hitting mirror 3. When the head is at the furthest x and y location I have to adjust mirror 2 down to meet the mark made at x=0. It appears this is working at mid-table but is off a bit when returned to x=0. Not sure how to compensate for this.

[Richard DiMaggio]

I have had this problem before too. I built my own system and found that when the beam is not hitting the focus lens in the middle, it would cause it to deflect at an angle. If your X/Y relationship is not 90 degrees, the beam will move across the focus lens as the distances between the mirrors changes too. 2 cents.

[Joseph Shawa]

In my case X/Y was right on in all locations right up to the head where it would enter perfectly horizontally and hit mirror 3.
If the beam lever was at the center of mirror 3 it would not be centered in the tube and lens so would get cut off at the cone.
If the beam was raised so that it would hit the center of mirror 3 then it would get cut off by the head BEFORE it even hit mirror 3. Just bad design of the head or maybe made for a smaller beam. My machine came with a 60 W tube and I replaced it with 150 W.

By machining the top of the entry hole into the head the beam could hit high on mirror 3 and be reflected nearer to the center of tube. Cuts great now. I might have kept the 60 W because I think most of my power issues were due to power loss at the cone tip and I didn't know that I should hit mirror 3 high. Live and learn.