Beam, alignment, ...???

I noticed early on that vector cuts on my Epilog TT had wider kerf on the top side than on the bottom. The cuts were creating a shoulder, like the top was beveled. I always use the manual focus tool, but wondered if maybe I was getting too close so I checked the focal distance using a bit of inclined stock and it seems the tool is pretty much right on, getting the smallest kerf possible. Nobody else seemed to think it was a problem because the shoulder was pretty small, so I left well enough alone at the time.

Recently, I was asked to make small holes (about 0.033 inch, to accommodate a 20 gauge wire) in 3mm BB. The holes are perfect circles on the bottom/back side but on the top side they look like ellipses on something close to a 45 degree angle, and significantly larger due to the shoulder than on the back side. Now people aren't happy with that shoulder, especially since it is asymmetrical. I talked to Epilog about it and was told that's normal (?!). I asked if there was maybe a beam alignment issue that could be adjusted and was told it wasn't the beam but rather a problem with how the x and y motors advance. When pressed, the tech told me to try adjusting the ramp and acceleration compensation settings and tried to explain it as being a problem of the x and y motors not moving in precise synchronization to create a perfect circle. I told him that didn't seem right but I'd experiment more.

I've found that the ramp and acceleration settings had no visible effect on this problem, but the description in the manual doesn't make it sound like they should anyway. It describes these settings as controlling the reversals or turnaround of the head, not axis interaction. Besides, the holes were perfect circles on the back side. As a side test, I tried making squares instead of circles so that only one motor had to move at a time and I got the same oblique distortion... perfect squares on the back side and rhombus shapes on the front, stretched along the same back-left/front-right diagonal.

So, then I tried making a grid of holes, varying speed along one axis and power along the other. When the speed/power settings put low energy on the surface (such as high speed and high power, or low speed and low power), I got a perfect shape (whether using square or circle) lightly etched on the top surface. As the speed/power combination increased energy on the surface, the shape got increasingly distorted on the top surface but was perfect on the bottom. Even at the lowest possible speed, where acceleration seemingly couldn't be an issue, the cut was distorted on the top surface when enough power was applied to burn into the wood. Same thing on plexi. I tried making the holes at all four corners and the middle of the table and got similar top-side distortions everywhere.

The two pictures show the results using 0.03" squares vectored at one speed and 10 different power levels, one picture from the top side and the other showing the back. The "shoulder" and diagonal distortion on the top side are quite clear.

Seems to me like this has absolutely nothing to do with the motors or acceleration; rather it seems to indicate the beam is "hot" in the center but, instead of having a circular profile it is spread out in an elliptical shape on roughly a 45 degree angle, because it marks a perfect figure topside at low speed*power densities, and the bottom is perfect at higher speed*powers but the top becomes distorted as described above.

Any thoughts? Solutions/work arounds? Thanks!

-Glen

Complete n00b but having thought a lot about kerf angles when shopping for a laser we saw these issues in our Epilog demo and although we thought it was alignment, so the wood went away to technical and they still had excessive kerf angles, but this was on larger objects so may not be the same issue.

Just some ideas, don't think for one moment I am trying to tell you how to suck eggs, but am very interested in your thread...

What angle does your air assist point?

If you make a spot with the laser in focus and then make another say a quarter inch too far away do they coincide?

Yeah, it varies with what tech you get as to whether or not they truly understand the problem. Your first pic isn't very clear (dark background?), but at what height are you focusing? On a 3mm board, try focusing at the top, the bottom, the middle, and if your table will allow it, every half-mm in between. See if any of those remove the kerf to a degree you're willing to accept.

You're not going to like me very much, but the tech is right to some degree... it's a power issue, IMO. Lasers are polarized (they cut better in one axis), and special optics are required to get that polarization circular (cuts the same in all directions). When you start getting into small features, this polarization can have a much more evident effect. It's difficult to tell from your pics, but more power along one axis would create an elliptical kerf, and it would be more noticeable on a very small hole.

My suggestion right now would be keep tweaking the speed/power, and don't forget about the ppi.

The number 1 check is to see that the laser beam is actually going through the centreline of the focus lens.
The red-dot will give a fairly good guide, but to test properly you need to put a bit of tape just above the lens (NOT TOUCHING!) and see where the beam burns a hole in it.

If the beam is hitting the lens off-centre, it is fairly easy to adjust the mirror at the end of the beam to correct this.
This test will also show you whether the beam is fairly round-shaped or grossly oval.

Another quick and useful test using the red-dot -
Go into focus mode and focus on the table itself. Mark the place where the red-dot hits on a piece of sticky tape.
Now adjust the table height downwards (as far as it will go).
Did the dot move as the table went down? If so, the movement roughly corresponds to the angle you saw when you cut the wood.

On my machine, the red-dot is not in exact alignment with the laser beam, but close enough that the "table-lowering test" lets me adjust for all but the most critical jobs.

I tried cutting 0.75mm holes in 3mm hardwood last night and to the naked eye they look square enough on both sides. Without a tripod I don't know whether a photo would show it well, but I could see if I can find one or use it on a level surface, and try to get some macros on a white background and see if they are any use?

Before it I double checked the alignment, including vertical alignment by raising and lowering the table so that the burnt spots and red dot position were aligned at all vertical positions. If it makes any difference, the air assist is inside the cone.

I have done a lot of micro machining with Solid State UV lasers and that sure looks like an oval spot. As mentioned above check your raw beam with tape and a single pulse if that looks perfectly round next check beam centering on the focusing lens.

ernie

FWIW, I'm including a new picture of the top/front side of the squares with back lighting. Since it seemed like there was around 45 degrees of angle to the kerf, I ran a "cross hair" at 45 degrees as shown in the 2nd photo and, sure enough, the top left/bottom right diagonal is narrow with virtually no shoulder and the bottom left/top right diagonal exhibits the shoulder effect. Seems pretty clear that the beam hitting the surface isn't circular but rather is spread along the top left/bottom right diagonal.

I tried using the red dot to determine if the beam is vertical by turning it on and watching its location on the table as I raised and lowered it, but the dot is so bright it was hard to tell. Still, it did appear to drift some with table height.
Instead, I burned parts of a "cross hair" onto a piece of bb at minimum focus distance (table as high as possible), manual gauge's focus distance, and with the table about 3/4 of the way down. The 3rd photo shows this, with the central area being minimum distance, the middle region being "best" focus, and the fuzzy outer area being table 3/4 way down. I'd say the beam is moving up and right as the table gets farther away.

Looking at the lens holder, it doesn't appear possible to get a piece of tape directly over the entire lens due to the shape of the holder so I'm not sure how to further check whether the beam is hitting the center of the lens. Seems pretty clear that the beam on the surface of the substrate isn't circular, but not sure how to tell if it's a problem of lens/mirror alignment or inherently non-circular beam.

Just found the pdf manual and there's nothing in it about adjusting the lens or aligning the beam for a Legend TT...

-Glen


-Glen

you have a problem in your optics someplace...Mirror..lens..alignment..How old is your tube? Could be time for a replacement.. But this is not normal for a TT.. I've been running mine for 8 years, and have never had anything look like that, except when the tube was on it's last legs.

That first pic makes it look like you have a secondary (and wide) beam, not necessarily an elliptical path. Metal cartridges rarely lose alignment of their internal optics, but I suppose it's possible, and that would cause an elliptical beam. But I think it's something farther up the chain... a loose mirror/lens, misalignment, etc.

Take a test shot at each mirror and make sure the beam is round there, for starters, and let us know...

Have you checked the output lens of the source itself , this can get dirty or dusty .. do not clean it like a lens , you need a photographic puffer type brush thing or clean dry gentle air.

For the archives, I realigned the laser and saw a very slight improvement but still had basically the same problem. I also noticed during the final stages of alignment that there was a spurious "extra" red dot about a centimeter over from the "main" red dot, so I called Epilog and wound up talk to a different tech, who told me I needed to pull out the tube, take out the periscope, and remove the mirrors for inspection and possible replacement. I got the tube out but couldn't get the periscope loose so I called again and got a third tech, this time Ian. When I told him what I was trying to do and why, he pulled up the pictures I'd sent and, after a while, told me to forget it all, put everything back, and pull the final mirror/focus lens off to get a better look at them. The mirror was slightly streaked, almost like there was a faint fog on it, and there were 5 or 6 tiny spots, perhaps .001", scattered over the surface but nothing apparent to me at the center. Based on my description and the age of those optics, Ian suggested replacing them would be a very good idea. He also said the combiner optic for the red dot laser was the reason for that extra red spot, it was a common enough thing but usually not noticed, and to not worry about it.

So, today I replaced the final two mirrors and lens, ran through the alignment yet again (much easier this time!), and lo... problem completely gone. Beam appears essentially round, .03" holes are round, and initial impression is that it's cutting a bit better. Having the old lens easily accessible, I examined it more closely, and in good light saw that the coating was damaged right at the center. Presumably, that was the source of the beam problem. Third tech was a charm, so to speak.

-Glen

Glen

Glad you have solved your problem and thanks for the detail. I must admit I have never seen results like you posted so I was interested in the solution though I suspected it was the optics.

What is interesting as well is the center photo where the disc appears to have cut as it should have.

Mike,

I looked back at the photo of the disc and was a bit puzzled myself that it looked like it was cut without the shouldering. Turned out the disc itself was sitting at the base of my monitor and I took a good look at its edges. Looking at it edge-on and rotating to view the circumference in profile, I can clearly see the shouldering on the "LB" and "FR" sides, and straight cuts on the "LF" and "BR" sides. I suppose the closeness of the lens to the disc, along with the macro zoom I used, hid the edge effect.

I'm guessing I damaged the coating when cleaning the lens sometime. Now that I know how to remove the lens holder, and know that it is keyed to allow being taken off and replaced without messing up the alignment, I plan to remove it for cleaning from now on. Removal makes it immensely easier to see what needs cleaning and how you are doing! Case in point: after alignment, I took off the lens holder simply to verify that removing and replacing didn't mess up alignment and, in the process, noticed that the final mirror had a bit of fog across the top half. Must have been some smoke residue from the alignment targets. Anyway, no way to see it without removal, and easily cleaned out of the machine.

-Glen