Ben's 2.x Laser Build

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18mm lens spacer and adjusting the air nozzle

Postby BenJackson » Tue Jan 03, 2012 6:29 am

My previous adventure with disassembling the lens had me doing lots of test firings with and without the air nozzle. I noticed that the spots fired with the nozzle were ever so slightly elliptical. With very short, low power shots into paper you would see a clean hole with a dark elliptical halo. Today I verified by removing the nozzle that the halo goes away.

To permanently eliminate the halo I did two things:

1. Finally cut a spacer ring (DXF attached) to center the lens in the holder. It's actually a slotted 18-22mm ring cut from 1/16th acrylic. It sort of snaps in past the threads and leaves a perfect 18mm opening for the lens. This is far superior to my previous attempts to center the lens! For one thing I verified that my spot moved slightly when I reassembled.

2. I zip-tied a laser pointer to a dial indicator base and set it up (parallel to the gantry, between the gantry mirror and the carriage) so that the red spot coincided with the test firing of the laser. This allowed me to test-fit the air nozzle several times. With the cheap red laser pointer there was always a "halo" effect around the spot, but I could readily see from that how well centered the spot was. I used 1/4" copper foil tape to shim the air nozzle until the red spot/halo were as centered as I could get. Trying to just aim the nozzle before tightening it didn't work because it would always register itself against one of the mating faces. It actually took quite a few layers of tape (in my case above the screw clockwise from the air input) to get it aligned.

Finally after that I shot a new test spot with the nozzle in place and it's nice and sharp with no halo. I'm curious to see if I pick up some extra cutting power as well.
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Air nozzle verdict...

Postby BenJackson » Tue Jan 03, 2012 8:57 am

...success! I definitely think it was sapping power before. I have been running some full sheets of MDF and after the work above I did some test cuts and was able to run the job 60% faster and with about 25% less power (tried 2.5ms pulses instead of 3ms, which didn't seem to have any effect, and slightly backed down my PPI). Obviously 60% faster is a big win.

I'm pondering how to update Dirk's spreadsheet to take speed into account. His calculations may be true in "lens like" material such as acrylic, but speed is obviously not an independent variable from PPI in wood. As each pulse is "dragged" across the cut it doesn't actually get its full duration to dig into one spot unless the speed is slow enough. I guess you could think of it like running through the rain: The speed can probably be approximated as a slanted beam making the material appear thicker.

Also this MDF is nice and cheap but I haven't managed to get anything but a fairly charred edge out of it. Even weak (non-through) cuts are charring so it's probably just the nature of the glue.
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PPI and the Halo

Postby BenJackson » Wed Jan 04, 2012 6:02 am

Before I fixed my nozzle alignment I tried to prove out Dirk's ideas about low PPI and ultra-low speed to avoid burning the material. This is what happened:
P1000176.JPG


That was a big clue about the nozzle but I ignored it. In hindsight you can also see extra darkening around all of the cuts.

With my nozzle fixed (as described above) I can now do slow (eg F100) cuts without that terrible burning on the surface. What's more, at low enough PPI I can also cut through and merely "toast" the glue rather than burning it. It's good to have that in my arsenal for this MDF, but I had to go quite slow (< F250 at 20 pulse per mm aka ~500 PPI) to avoid blackening the wood.

On the other end of the spectrum I experimented with FSE's "continuous pulsed" mode. Based on the scope trace that Dirk linked a while ago I started with an off period of 250us. That was clearly less cutting power than CW. I kept cutting at F800 which I knew was a through cut with CW and reducing the off period. As I dropped below 100us off (2.5ms on) the cutting power seemed to increase over CW. I ended up at 50us off and I am now cutting through at F1000 and the edges are slightly nicer than the CW cut. I also think the tube itself is glowing brighter.

(note that 50us off is hard to measure without a scope because the EMC2 base period is 37ns -- my true off time is probably either 37ns or 74ns)
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Re: Ben's 2.x Laser Build

Postby educa » Wed Jan 04, 2012 9:38 am

What is F100 ? 100mm/min?

Also could you take a picture of cut quality (cut edge) for the different settings ?

Most cuts on that pic look quite nice
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Re: Ben's 2.x Laser Build

Postby Modelart » Wed Jan 04, 2012 1:32 pm

Hi Ben:
I came across your ppi config for emc2, (I think it's great!). I want to test raster engraving.
Looking in the hal appears that the laser is fired through pin14 (pwm), and pin17 (laser final).
Actually i setup my laser psu like this (custom config - magic z ) :

pwm to PSU pin 2 (ttl high)

PSU pin 2 shorted to pin 4 (wp to ground)

Pot between ground (oin 4) and +5V (pin 6), and whiper to Input(pin 5) to set power to max. 18ma (40w tube, pwm 100% duty)

th | tl | wp | ground | input | +5V
1 | 2 | 3 | 4 | 5 | 6

Surelly is connected the bad way....

Can you tell me how PP pins 14 and 17 are connected to PSU, for your setup?
Thank for your time
Regards
Rick
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Re: Ben's 2.x Laser Build

Postby sliptonic » Wed Jan 04, 2012 4:29 pm

I noticed an undesired side-effect of "magic Z" control. My job runs fine unless I interrupt it and restart it. Then I get a bogus cut on my material. I found that my HeeksCNC post processor is doing a G0 move to the starting location before moving to the safe height. That should certainly be fixed in HeeksCNC and I'm working it. But it raises the question why one would ever want the laser on during a G0 move (or off during a G1).

I'm not sure how much time it takes to move down from 0 to .01mm at very fast plunge rate, but it must take some time and during that time the beam is on.

Instead of using 'magic Z', would it make sense to tie the laser on/off directly to G1/G0 respectively? That would have the side benefit of letting me return the Z axis to table height control.
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Re: Ben's 2.x Laser Build

Postby bill.french » Wed Jan 04, 2012 4:38 pm

sliptonic wrote:Then I get a bogus cut on my material

Ben suggested i add a M65 P0 to the start of my gcode.
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Re: Ben's 2.x Laser Build

Postby BenJackson » Wed Jan 04, 2012 5:12 pm

bill.french wrote:
sliptonic wrote:Then I get a bogus cut on my material

Ben suggested i add a M65 P0 to the start of my gcode.

The equivalent for his magic Z issue would be to put "G0 Z0" somewhere before any "M3" in the code. The M3 master means it won't unexpectedly fire the laser at the start of the job, so you have a window for your prologue to set up. If Heeks is moving in X/Y before setting Z clearance or if it's starting the spindle (with M3) before moving to the Z clearance height that's definitely a bug. You can imagine the kinds of problems it would cause with a router or mill as well.

By the way, you can simply jog Z up from Axis before you start your job. If you used my included axisrc (which puts the table U on pg up/dn) then Z is moved to brackets [ ]
Last edited by BenJackson on Wed Jan 04, 2012 5:21 pm, edited 1 time in total.
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Re: Ben's 2.x Laser Build

Postby BenJackson » Wed Jan 04, 2012 5:19 pm

Modelart wrote:Can you tell me how PP pins 14 and 17 are connected to PSU, for your setup?


Pin 14 ("PWM") goes to "input" in your diagram (labelled "IN" on my laser PSU). On Bart's board there's a jumper to select 14 vs 15.

Pin 17 ("laser-final") goes to TH I believe. That also happens on Bart's interface board so I'd have to double check, but I think TL is only the front panel test-fire button.

You can use the pot to set power and ignore pin 14 if you want. Bill French is doing that in his configuration. With PPI 100% power will be your usual choice (just reduced PPI) but you will still want to set it lower for things like paper.

For raster engraving you will need to turn the pot down because your laser probably can't move X fast enough to engrave at 100% power. For rastering you'll pick the highest X speed your laser is capable of and then set the depth of engraving with power.
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Re: Ben's 2.x Laser Build

Postby sliptonic » Wed Jan 04, 2012 7:52 pm

BenJackson wrote:...The M3 master means it won't unexpectedly fire the laser at the start of the job, so you have a window for your prologue to set up. If Heeks is moving in X/Y before setting Z clearance or if it's starting the spindle (with M3) before moving to the Z clearance height that's definitely a bug. You can imagine the kinds of problems it would cause with a router or mill as well.

By the way, you can simply jog Z up from Axis before you start your job. If you used my included axisrc (which puts the table U on pg up/dn) then Z is moved to brackets [ ]


I can think of a couple ways to both work around and actually fix the bug in the Heeks post processor. That needs to be done regardless. But I'm still wondering whether using G1/G0 to control laser on/off wouldn't be safer, more intuitive, and more broadly compatible with different g-code generators than using the Z position.

My bed is completely manual right now, but I can imagine adding a stepper to allow for automatic touch-off focusing and maybe stepping down to change the focus on multiple passes but HeeksCNC doesn't do anything beyond 3 axis control.

If there's a reason to do a G1 without the laser being on (and I can't think of one), it can be done by preceding it with a M5 to turn off the spindle.
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