Laser Maintenance Class/Lesson Plan/2017 Oct

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Overview

  • Laser's default state is "not working"; our job is to give the illusion that it always works.
  • Specific things that go wrong and get fixed: Mirrors out of alignment, mirrors or lens dirty, red laser out of alignment, air hose kinked, smoke extraction not working, table not level, screws loose, limit switches loose.
  • If there are problems, write them down in the logbook. If you do anything to the laser, write it down in the logbook. If anyone says anything laser-related, write it down in the logbook. Check the logbook for recent entries as the first part of any maintenance task.
  • If there is a problem that can't be fixed right away, tape a sign to the laser saying that it's out of service. Using it while it has a problem can damage it further.

Mirror Alignment

  • Used to be the most common thing wrong with the laser; rare now.
  • If cutting is inconsistent across the work area, this is probably why.

Diagnosing

  • How to check: Run "burn-test-paper" at corners, edges, and middle, and check for consistency.
    • Old power supply: 5th line was marked, others cut. New: no 5th line; 4th line is barely cut.
  • If lines are wide and charred, it's not alignment - it's focus. Alignment causes fade-out, with all lines still sharp.
  • Point out locations of mirrors (rear, front, and lens mount).
  • Explain lens mount landmarks: screw is at the top of the slot (mount is all the way down), bottom of mount is almost but not quite resting on the stop (i.e. it's vertical).

Fixing

  • Check alignment before adjusting mirrors. If the laser works, don't fiddle with the optics.
  • Start with the back mirror. Check location of the spot on the front mirror when the rail is at the top of the work area and at the bottom of the work area.
    • Do not go near the dichroic mount. Even looking at this funny is risky, and it's a massive pain to realign.
  • You're looking for consistency, not centering. The dot should be at the same spot on the mirror in both positions.
  • Next, adjust the front mirror, looking for the spot on the mirror in the lens mount. Check the location when the lens mount is all the way to the left and all the way to the right.
    • Put the rail in the middle of the work area's Y range when doing this.
  • Important: there should still be a fair bit of clearance between the spot and the edges of the lens mount mirror. The mirror is inset, and the edges of the hole can block the beam from hitting the lens.
    • If it's impossible to get consistent positioning and adequate clearance, it means the lens mount is in the wrong position (usually too high). Check that.
    • If it's fine and things still can't be lined up, there's a problem in back. Get me and expect lots of swearing.

Practical Exam

  • For each person, misalign both of the mirrors and the optics mount. They fix it and then do an alignment check to test their fix.
  • It doesn't have to be perfect. Let them make two tries, or take two minutes, then move on for the next person. They're trusted to safely practice further on their own.

Cleaning Optics

  • This is now the most common problem, and it's a serious one - it can destroy the lens.
    • Wood is bad for this, leather is horrible for this. Engraving is really horrible for this. If anyone cuts these or does engraving, do a quick lens check afterwards.
    • They're allowed to cut these; it's why the laser's at the shop. We just have to do appropriate maintenance as well.
  • Lens is transparent to IR, but dust isn't. Dust heats up, and lens is sensitive to heat.
  • Mirrors are less sensitive but should still be cleaned if dusty.

Diagnosing

  • If you can see the red dot on a mirror, it's dirty. A clean mirror has a very faint dot.
  • Lens always shows a dot. To check for dust, shine a bright light into the lens mount from above, and look for particles.
  • Also check for scuffing (usually from improper cleaning), for chipping (from over-tightening the mount), and for patches of shininess (indicates that the anti-reflection coating has been rubbed off). If someone let dust get too hot, that results in pitting.
    • None of these can be fixed, but they should be written down. When they get bad enough, it's time for a new lens.
    • The old lens shows all of these; point them out to people. Use good light.

Fixing

  • Side mirrors have caps that unscrew, allowing access to the mirror without disturbing the mount too much.
    • The mount still gets disturbed, so realigning will probably be needed.
  • Mirrors are steel plated with nickel or chromium; they can be cleaned with alcohol or acetone.
    • Use lens paper - otherwise clay particles fog the mirror, or lint is deposited and catches fire.
    • Use acetone, just so that we don't have to deal with two different solvents in the laser area.
  • Lens mount disassembly:
    • Put a piece of lens paper down on the grille.
    • Disconnect the Z datum connector.
    • Unscrew the lens mount part way.
    • Let the lens mount turn so that the lens is on the top, not the bottom. This means a dropped screw won't scratch or chip it.
    • Finish unscrewing the lens mount.
    • To remove the mirror, loosen the mirror clip screw, rotate the mirror clip, and take out the mirror.
    • To remove the lens:
      • Hold the assembly with the lens facing up.
      • Loosen the screws a little bit at a time, going around in sequence, to avoid pressure on the lens.
      • After the screws are out, hold the assembly together by hand, and flip it over so that the lens is facing down.
      • Hold the lens retaining plate, and lift off the rest of the assembly.
      • Carefully pick the lens off the retaining plate, holding it by the edges.
  • To clean a lens or a mirror, soak lens paper in acetone, and wipe the optical element in one smooth motion, starting from the middle. Rotate it, wipe again with a different part of the lens paper. Repeat until either it's clean or you need a new piece of lens paper.
    • Do not wipe multiple times with the same part of the lens paper; that'll smear grease, and scour with dust. Both are bad.
    • Acetone can pull grease with it, and leave it at the edge of the lens when it evaporates. To avoid this, don't soak the entire lens paper; start wiping with the damp part and finish with a dry part to wick away the acetone.
    • The lens will take a lot of careful wiping. Mirrors are usually faster. Don't spend an excessive amount of time on either, though, because there's always at least a little bit of dust present, which will eventually scour and fog the optics.
  • When putting the lens back into the lens assembly:
    • Hold the retaining plate with gasket in one hand. Make sure the gasket is centered; if it overlaps the open area, it'll get burned, which is bad.
    • Carefully place the lens on to the gasket, being careful not to move the gasket while doing so. The lens bulges down towards the bed of the machine.
    • Carefully place the lens assembly over the lens. Hold the two pieces together by hand, and flip it over.
    • Finger-tighten the screws most of the way, then tighten each a half- or quarter-turn at a time, moving around the border in sequence or across diagonals. This avoids putting too much stress on any one part of the edge of the lens.
  • Remember to check that the Z datum connector is reconnected! If it isn't, the optics mount will crash, which throws a lot of the rest of the system out of alignment.

Practical Exam

  • Each person demonstrates disassembly and cleaning of the front mirror, and of the lens mount (mirror and lens). Don't touch the back mirror for the test.

Miscellaneous Mechanical

  • Grab-bag of several topics: air hose, air compressor, extraction system, wiggle check, table check, limit switches, chiller check.

Air Compressor

  • Air system blows air on to the workpiece, sending smoke and soot away from the lens.
    • Without this, the lens gets damaged very quickly.
    • Check airflow by putting hand under the lens assembly; there should be lots of air coming through.
    • Poor airflow usually means the hose is kinked (rubber tube connects to thin copper tube near the lens assembly; rubber tube often kinks).
  • To fix, gently pull the hose connection to the right, towards the end of the copper tube. This gives it enough play to no longer kink.
    • Do not pull it off the tube. This is a pain to fix.
  • Air compressor is noisy, but this can be reduced by having padding or a shim or both under it.
    • If it's really loud, check to see if the shim has fallen out, or if the mountings are wandering. These are problems (the compressor can walk its way off the window ledge if it's free to move).
    • Right now, improvised. Eventually it's worth picking up an isolation mat and mounting/gluing things properly.
    • Past improvised solutions include a teddy bear and a piece of wood. Wadded-up cloth (e.g. rags) is probably the best option if these go missing.
  • If there's poor airflow (or very loud noise) with no obvious cause, check behind the machine to make sure the hose from the compressor is still connected to the laser. Problems are very rare but have happened.

Extraction Fan

  • Extraction check is done by running "burn-test-wood" on a piece of wood. If the smoke moves quickly towards the exhaust, it's working. If it lingers, that's bad (it will damage the lens fairly quickly).
    • Extra check: Put your hand on the housing of the extraction fan to see if it's on. It's very quiet, but you can feel the vibration. If you can't feel vibration, there's a problem.
  • No easy way to fix this if the problem isn't obvious. Sign the laser as out-of-service and contact Marc.

Loose Screws

  • Wiggle check: See if any parts of the translation stage are loose.
    • Easiest to check by trying to wiggle the horizontal beam up/down, left/right, gently.
    • If this is loose, people will be complaining about curves showing discontinuous jumps when the XY stage changes direction. Test pattern is a circle.
  • Fix is to carefully tighten the screws, then do another wiggle check.

Table Level Check

  • The grille sometimes has crud collect under it. This causes focus to vary across its area.
    • This can look like an alignment problem, but isn't. Out-of-focus areas have wider cuts with char around them; in-focus are narrow.
  • The table that the grille sits on can be adjusted, but shouldn't be. I've never seen it go out of alignment in all the years I've been here; it's always been a problem with crud collecting under the grille instead.
  • To fix, carefully remove the grille, remove the support grid under it, sweep out everything with the hand brush, and reinstall.
    • The support grid is missing screws and flops all over the place. Use care and you'll be fine.
    • Do another alignment check after the grille is back in. Z-datum at the middle of the table, and the focus should be good across its entire area.

Limit Switches

  • These tell the laser that its reached the uppermost or rightmost edge of the work area. The laser checks these when it's powered on.
  • If one of these doesn't work, the laser will reach the right edge or upper right corner, then make a buzzing/grinding sound as it tries to continue seeking.
    • Hit the small, square "stop" button on the control panel to stop it.
    • Do not hit the Big Red Button. You don't need to, for this.
  • If the switch is working poorly but not broken, the temporary fix (as documented on the panel) is to use the cursor keys to back it off, click the switch a couple of times with your hand, then hit the "reset" button on the panel to have it try again.
    • If there's a highlight bar on the control panel, it means you're in a menu (and the cursor keys won't respond). Hit "Esc" to get out of the menu.
    • If seeking failed, it'll be in "don't damage itself" mode. Press and hold a cursor key; the first four seconds will be slow, then it speeds up.
  • The limit switches are a standard type, and are attached with screws. There are usually spares in the laser supplies drawer; if not, post to the list and more can be ordered. Make sure to connect the wires when reinstalling one.

Chiller

  • The chiller must always be on, and should always read a relatively cool temperature (usually 23 degrees C).
  • If the chiller is off, or is reading warmer than 30 degrees, turn everything off, sign the laser as out of service, and post to the list. If the laser tube gets too hot, it'll destroy itself, and it's quite expensive to replace.

Practical Exam

  • Without the students watching, the air hose gets kinked, some of the XY stage screws are loosened, a wire from one of the limit switches is disconnected, and a foreign object is placed under one corner of the grille. Each student has to identify and fix these problems, with different variations per student.
  • Students can watch each other; they just can't know where the problem spots are when it's their turn (so that they have to diagnose it).

In Back

  • Two systems of note: the dichroic (which combines the red laser beam and the IR beam), and the laser tube.

Dichroic

  • The dichroic mount holds two parts: A coin-shaped optical element (the dichroic itself) that is transparent to IR but reflects (some) visible light, and a red diode laser that shines on to the dichroic, reflecting in the direction of the machine's IR beam.
  • The red laser has to be realigned whenever this is adjusted, and realigning the red laser is an incredible pain in the tail. Avoid adjusting the dichroic mount if possible. Avoid going anywhere near it if at all possible.
  • The dichroic itself should be lined up with the IR beam. It can be positioned by loosening the nut holding it in place, moving it, and tightening the nut again.
    • The dichroic mount is held on by friction and wishful thinking. Jostling the machine can throw it out of alignment, resulting in 30-60 minutes of swearing while I or another maintainer tries to realign it. Avoid jostling the machine.
  • The IR laser's position and pointing direction can both be changed. Both will need to be changed to line it up properly. The laser has a soft plastic shell, and there are five (formerly six) screws that hold the shell and the laser in place. Adjusting these screws changes where the laser starts from and where the laser points.
    • Because the shell deforms, and not all screws are in contact with the shell at any given time, there is no obvious relationship between how the screws deform and where the laser starts and is pointing. If you have to adjust this, good luck.
    • Moving the dichroic mount side to side (slightly), up and down (slightly), and bending it forwards and back (gently) will also adjust position, in more predictable ways. You'll still need to mess with the screws, though.
  • To test red laser alignment, place a card in the path of the beam, and push the "laser" button on the control panel. Observe the position of the hole or burn mark, and the position of the red dot. If the red dot goes through the hole, the beams are aligned at that location.
    • Use a piece of tape to hold the paper in place, so that it doesn't move during the test or while you're checking the spot position. Opening and closing the lid will cause enough air motion to move the paper.
    • I normally move the lens assembly to the lower right of the work area, then check the red laser at the front mirror (at the lower left) and at the optics mount (at the lower right).
    • If the red laser is misaligned, you'll need to check at least two points along the beam path to figure out whether position, direction, or both need to be changed.

Laser Tube

  • This is a gas discharge tube. A high-voltage discharge makes the gas glow, mirrors bounce the light back and forth so that lasing can happen, and a water jacket keeps everything cool.
    • This is a sealed system. We can't repair or clean it. We can't even clean the output mirror, so cover that opening if doing anything with the cover open that might make dust.
  • Make sure the power is off before messing with the tube. The high voltage supply has exactly the right traits to kill you dead: enough current to stop the heart, not enough to restart it, and enough voltage to deliver that current.
  • The tube is on adjustable support posts. Do not adjust these.
    • They can only be coarsely positioned, and it's easy to point the tube in a direction that makes it impossible to align the laser properly.
    • Moving the tube will also require realigning the red laser, which is an incredible pain in the tail.
    • Misaligning the tube enough that the laser no longer passes through the dichroic will give us a CO2 beam that's bouncing off of all of the other metal surfaces back there. This is very dangerous and very bad.
  • The tube will eventually burn out. The main symptom is that it loses power. Alignment will be perfect and it still won't cut.
    • This has only happened once, over many years. Usually the problem is something else (like the power supply, an interlock switch acting up, or some similar problem).

Practical Exam

  • Have each student do a card test to measure the alignment of the red laser beam with respect to the IR beam.
  • Do not misalign the red laser or have them fiddle with it. The alignment setup is atrociously bad, and it'll take 30-60 minutes to get it back in alignment, mostly by sheer fluke rather than skilled manipulation.
    • If we ever replace the red laser and dichroic mounts, then alignment can be added to the practicum.

Conclusion

  • Class members are now authorized to diagnose and fix problems with the laser on their own initiative.
  • Everyone gets a fob or card to show that they've passed the course and are authorized to do this.