Double Engraving Problem - NOT A MECHANICAL ISSUE

[daveman]

Long story short, I built a custom laser engraver and had double engraving problems. Marco was great, quickly answered my questions trying to resolve to no avail. I simply thought my over ambitious XY Stage was flawed so I built a completely new machine, this time using a PR5030 XY Stage from LightObject. The stage was stellar and everything is moving super smooth. Belts are properly tensioned and it feels real dialed in.

Now the problem, after buying a XY stage, building a whole new machine, I have the EXACT same double engraving issue. The only thing in common between the two machines are the X7S controller, power supply, and laser tube (and I ruled out the power supply and controller). When engraving the first line is always shifted left (the head is moving right) and the next line is shifted right (the head moving left). The faster the speed, the greater the distance between the double image. Either the laser is firing early or the x motor is moving late.

What has been ruled out:
The machine itself, with the change of the xy stage, all new motors and stepper drivers.
Power Supply - swapped with another power supply out of my K40 machine, same issue.
Belt tension - it has been raised and lowered, both with zero change in the problem.
The Controller - Swapped with a old AWC608, sent the same settings to it with the same results

The only things that are consistent:
The laser tube (SPT C40)
LaserCAD (I am running x8.12.21)

Settings:
Both: Falling Edge
X Start Speed: 15
X Max_Acc: 3000
X Max_Speed: 500
Y Start Speed: 15
Y Max_Acc: 800
Y Max_Speed: 400
Stepper Controller Microstepping: 6400 Steps Per Rotation (32 microsteps)

[Tech_Marco]

Please post pictures and video.

Marco

[daveman]

Here are photos and a video.

Included are:
video of engraving operation @ a speed of 300, power 20%

video_cropped.mp4

(18.9 MiB) Downloaded 114 times

photo of the results showing line shift

photo showing scale of issue (almost 4mm, if it is a play/tolerance issue i would have to have 2mm of backlash, don't see that being possible)

photo showing settings in LaserCAD

photo showing current state of machine build

photo showing LaserCAD version

Build Specs:
XY Stage - PR5030
Stepper Drivers - DM420T
Tube - SPT C40
Power Supply: CloudrayLaser MYJG-50 (black case)
Controller: AWC708S (LO X7S), firmware 1.20

[daveman]

Update:

Worked on it a little more this morning with no progress.

What was done this morning:
Verified that set screws were tight on all pinions (the motors that came with the XY stage had press fit pulleys and were missing the set screws, I replaced them)
Retensioned the belt to about 1/4" deflection with the head all the way to one side of the gantry.
Swapped out the laser tube from the K40 to see if the tube was part of the problem, no change.
Verified mirror alignment, everything is dead center at all four corners of the bed, vertical alignment is centered as well checking at a depth of 2" and 6"
Can get a clean single image by using a engrave reverse offset of -3.6mm @ 300 speed in the software (but I shouldn't have to).
Doing the math, having to use a negative 3.6mm offset equates to the laser firing about 6mS early since I have checked every conceivable mechanical cause and have officially swapped out every single component except for the software (although I have tried 3 versions of laserCAD from 6.8 to 8.12 and lightburn)
Dropped stepper drivers to 3200 steps per revolution (16 microsteps) and adjusted uM appropriately.

If I have to operate with a speed defined offset to make it work I will but that to me is masking a problem, I would much rather fix the actual problem. Any advice from Marco or the community would be greatly appreciated. Since I started with this hobby, this forum is always one of the best sources of help around and the crew a LightObject know their stuff.

Thanks,
David

[Tech_Marco]

Hi Dave:

Can you post a video showing the engraving in process? I want to see how the double image happened. It is so strange that happened.

By the way, please change the setting (won't correct your issue but will work better)

Settings:
Stepper Controller Microstepping: 5000~5000
Both: Falling Edge

X Start Speed: 20
X Max_Acc: 15,000
X Max_Speed: 900
Y Start Speed: 15
Y Max_Acc: 3000
Y Max_Speed: 400


User parameters:
Start_Speed:20
Space_speed: 3000
Cut_Jerk: 4 (40,000)
Space_jerk: 5 (50,000
Speed_factor: 1.5
Mini_Acc: 300
Engrave_Acc: 15,000
X/Y Home: 100

Marco

[daveman]

Sorry Marco, I posted it in my second post but it didn't stand out very well. I added it as an attachment this time to see if that works better.

Thank you for the response.

[Tech_Marco]

See my setting added after my post

BTW, the issue on your case seems a mirror or lens issue. Or, the beam was hitting to the laser tip
Clearly the beam is split.

Do burn test after the tube, 1st mirror, 2nd mirror with power set to roughly 10% on the LCD control panel
You need to check each beam spot after each mirror.


Marco

[daveman]

Thanks Marco I really appreciate it. I have done that but will go over it with a fine toothed comb again.

On my inspection yesterday, mirrors are secure and I aligned each in order starting with the tube mirror, x axis, then the head mirror. All are centered at all locations throughout the bed. The laser head mirror was centered as well with a dot burned at 2" and the bed dropped to 6" a larger out of focus spot made and they remained centered and inline. I used a small machinist square to ensure that the beam was exiting the head in the center. I initially suspected a reflection too, so went to great extremes to rule that out. As shown in the video, I did the test without the air assist cone on to minimize the opportunity to reflect.

If the video is slowed down, it is clear the left image is burned when the head is moving right and the right image is burned when the head is moving left. They are burned separately, alternating the bi-directional scan lines. If I do unidirectional and not bidirectional I get a single image, but it is shifted to the left. I will add some photos in a bit that demonstrate some of this.

Again, thank you for the prompt reply. I will get back to you after some further testing.

[Tech_Marco]

If all things didn't work, then it could be a bad controller card. Please return it for diagnosis

Marco

[daveman]

Thank you sir, before I take your valuable time, I will put the card in my K40 machine to see if it does it there. My K40 has been rock solid for years and if the issue moves with the card, then I will send it in. If the problem does not move with the card I will continue to diagnose my machine. Thanks for being so responsive! The main reason I have bought 2 controllers from you all. Great customer service!

[daveman]

Marco, thank you for your responsiveness. I feel like I have wasted some of your time by thinking about a digital problem in a very analog way. I have been somewhat fixated on it being a timing issue, but I seem to have forgot everything about how CNC works after years of being in the hobby. The laser doesn't fire at a specific time, it fires at a specific place based on cartesian coordinates. Since I have ruled out mirror alignment (I don't think I have EVER been able to get such perfect alignment on any of my machines as easy as this one thanks to the excellent build quality of the PR530, great job on those by the way) and ruled out backlash (again thanks to the PR530 build).

That leaves it being 1 of two problems, either the head is not where the controller thinks it is when the laser is fired or the controller is telling the laser to fire in the wrong spot. Since I swapped with a old AWC608 and had the exact same results, it has to be the head is not where it should be when the laser fires leading me to look at the stepper drivers. While the drivers were totally different in the two machines experiencing these issues, they were both the DM series drivers (DM556Ds for the larger machine and DM320Ts for the smaller one, both probably knock offs from Amazon). These appear to be only slightly better than the low budget TB6600 drivers. I think I am going to jump in and invest in some higher quality drivers, even if they aren't the issue it is still likely a good investment from a reliability standpoint.

Thanks again and I will update this thread once I give that a try.

[daveman]

After beating my head on the desk repeatedly on this issue across two separate builds, it turned out to in fact be the stepper drivers. I was using DM320T drivers and decided to break out some of my cheapo TB6600 drivers and the problem went away and the alignment is dead perfect with no double engraving. The motors are super whiny and noisy because TB6600s are terrible for that, but they are at least working correctly and I know the design and build works as it should. Now I can just order some good drivers and I should be all set. Again, thank you Marco for being so attentive to the issue I am ordering the recommended drivers through LightObject soon. They are DM series as well, but look to be much higher quality that the knock offs I have now.

[Tech_Marco]

Excellent!
I was pulling my hairs and couldn't figure out what could be the issue. Now you bear me and found the "bad guy" congratulations

Marco

[daveman]

Question for the forum in general. After finding the source of my problem I have now been able to try multiple drivers and have found that ANY updated digital stepper drivers that I have cause the double engraving issues. I am just guessing but all of these newer digital drivers have features like autoconfiguration, soft start, and acceleration and deceleration. I am guessing these features are ramping up the stepper meaning the head is lagging behind where the controller thinks it is. It gets to the right spot eventually but a fraction of a second later than the controller thinks it does. Going to put the oscilloscope on it this weekend to see what the waveforms look like but thought I would pose it to the forum.

Drivers Tested:
DM320T - Double Engrave
DM542S - Double Engrave
DM556D - Double Engrave
TB6600 - Works Perfect (no advanced features)
M415B - Works Perfect even under driving the motors amp wise (no advanced features)
Really Old TB6560 - Works Perfect (no advanced features)

Am I missing something, I really don't want to run on TB6600s but they seem to be the only ones that work so far and I am hesitant to buy any more. Can someone share their driver success and failure stories?

I appreciate it!
David

[Tech_Marco]

Those drivers you mentioned were very good one , except the 320. You may change the "falling edge" to " rising edge" and give it a try.

Marco