Testing a laser with a double image OR fault finding

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baccus61
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Joined: Sun May 23, 2010 6:40 pm
Location: Broken Hill, NSW, Australia
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Testing a laser with a double image OR fault finding

Post by baccus61 »

Hi Guys,
Some people have had problems with a double image being printed when they engrave something and I would just like to show them a little test to see whether it's in the software or hardware. Most times it's the hardware..... :roll: but sometimes it's a little hard to pinpoint where the actual problem is.

Draw a vector square about 20mm in size using "Hairline" width, doesn't really matter how big it is for the test, and un-fill it.

Convert it to a Bitmap

Set your laser to engrave at about 40 dots per inch and then print the square using raster engrave (NOT vector cutting).

If everything is in alignment then all the little dots/lines down each side will be in line.

If anything is out of whack then you will have an alternating staircase effect down both sides of the square and you will have to go hunting for the problem.
Usually it is in the hardware but sometimes it's in software and is easily corrected with the turn time settings on the first tab of the software controller.

This is where the double image comes from. If you are using a high dots per inch then all the lines seem to join together making it look like it is a double image but in reality it is just alternating lines starting and stopping at different places on the return phase of the printing.

Things to check....

Look carefully at all your belts and see if they haven't any broken tension fibers/wires and are stretchy (It's very hard to see so you may have to take them off and check.
Loose pulleys. Easily tightened up.
Collapsed bearings
Belts that don't fit the pulley properly due to wear or damage. I have bought some new ones in the last few months and ended up throwing them in the bin when I got them. They were a very poor fit for an anti-backlash profile. Now I only buy the HTD round toothed belts. A much better fit than the rack shaped ones. 3 or 5 mm are a good size.
Cracked lens. A long shot but it does happen as the ZnSe lenses are very soft and brittle.

That's all I can think of for now.

If your image fades from one side to the other then its usually just mirror alignment. Check and adjust them.
If your start and end points of your job don't meet at the completion of the job then there is probably a loose pulley or belt slop.

If your setting up a new laser table then the center height of your last mirror, above the lens, is the place to measure for height to get all the other mirror heights the same. Then just do your alignment procedure to get the rest fine tuned.

If setting up a red pointer for the first time then get your table fixed in 1 position so you don't bump it and fire a low powered pulse to mark a spot on a piece of wood or card stock about 3 feet/ 1 meter away. Then you can adjust your red pointer to be exactly in the middle of the burnt dot. To fine tune it you will need to do this close to the output coupler and then further away just to make sure it is exactly in the center of the beam all the way along or Co-incident with the laser beam. You may have to drill a small hole in the side of your machine to do this and remove the first mirror closest to the laser tube. OK, drilling holes in your new machine isn't very good but a miss aligned 100 watt laser fired into your face due to it bouncing off the edge of a mirror isn't too good either. Been there done that as I trusted the alignment from the factory. BIG MISTAKE!. I didn't get hurt as the beam was only 30 watts and it was dispersed from bouncing off the edge but it could have been MUCH worse than it was. I felt a nice warm sensation on my cheek before I realized it was the laser. Yes, I did have my safety glasses on!

Normal polycarbonate safety glasses are fine for CO2 wavelength as they absorb 100% of the beam. You don't need to buy $50 laser glasses although they are coated with a special coating, and if you want to feel safer then by all means get them, but they are just not warranted for CO2.

To find your lens focal point make a small wedge of card stock with 1 end folded down about 2-3mm and about 50 mm long so it fits under your lens assembly without hitting it. Set your table/lens height to the middle of the card roughly where it is supposed to be for the lens's focal length then, in Corel Draw, draw a single line about 40mm long and VECTOR cut it at a low power and slow speed, not so slow that it burns through, along the length of the wedge.
Don't use the air assist as it will blow the card away :oops:
It will be easy to see where the line expands and gets smaller and then expands again. Measure the height of the middle of the thinnest bit of the line to a known spot on your lens assembly and mark it or write down the length. It doesn't matter what or where you measure as long as it is at the focal length when you set it up. Dead easy!

I hope this helps some of you get to grips with how to problem solve these things.

Let me know if I need to add any more info on testing as it will help a lot of people and also take a bit of strain off Marco letting him run his day to day business more efficiently. Better for all of us in the long run. :D

Best of luck.

Rich.
P.S. If you like, leave a comment on whether this is clear enough, has enough information, or just let me know if I have done well or not. Any feedback is good. :-)
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