Author Topic: I guess I have an engraver but not a cutter  (Read 2280 times)

crutch

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I guess I have an engraver but not a cutter
« on: May 26, 2017, 05:54:00 PM »
I can cut paper and card stock at about <1 mm

when I try to cut 3mm MDF or hobby plywood I can cut dow to about 1-1.5 mm

I have an A3 2.5W laser.  I have used a wide range of speed setting at 100% power

I have tried cutting many passes but it seems I just scorch the wood on either side of the cut.

what distance from the cutting head should I be using?

I am currently at 20mm- ish.  do these lasers cut better at longer distances or shorter?

or do I just have a lemon for a cutter?

nottingham82

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Re: I guess I have an engraver but not a cutter
« Reply #1 on: May 26, 2017, 06:43:09 PM »
if you can get cut the wood a mm you should be able to keep on going.  That being said, the burned wood in the groove will dissipate the energy and prevent you from cutting further.  You need some high pressure air to remove the ash.  It is called air assist.  There are several different methods on this site for doing that.
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Re: I guess I have an engraver but not a cutter
« Reply #2 on: May 26, 2017, 08:08:47 PM »
Did you focus so that a test line burn is as small as possible?
Your statement: what distance from the cutting head should I be using?
tells me you're not focusing. True or false?

What software are you using to cut?

A 2.5 watt laser is plenty powerful enough to cut completely through 3mm plywood.

Can you line burn at around 30% laser power and then line burn at 100% power and a difference in the two line burns?
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crutch

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Re: I guess I have an engraver but not a cutter
« Reply #3 on: May 29, 2017, 06:40:34 PM »
I use BCL to cut with and T2 for engraving.  are these two compatible?  do I need to update the firmware for each program?

I did not see much if any difference between the 30% line and the 100% line

I ran the grayscale sample in T2 and it produces a solid bar, I see very little shading difference from one end to the other.

have I FUBARed something?

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Re: I guess I have an engraver but not a cutter
« Reply #4 on: May 29, 2017, 07:51:06 PM »
You can use Grbl 1.1e for both programs.

Max laser power must be 255 (not 1000).
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crutch

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Re: I guess I have an engraver but not a cutter
« Reply #5 on: May 29, 2017, 09:24:10 PM »
ok, reset power to 255 and reloaded firmware.

in BCL, I can see a difference between 30% and 100% power.

in T2, I tried engraving the sample grayscale - it was a solid bar across all shades.
I did use the grayscale mode not the b/w or dithered.

I can cut the 3mm ply I have using these settings
100% power
175 speed
7 passes and it will fall out cleanly.  at 6 passes there are hanging 'chads'

I still cant cut through MDF even with 12 passes.

making progress, be patient with me ;)

carolynsdad

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Re: I guess I have an engraver but not a cutter
« Reply #6 on: May 29, 2017, 09:59:35 PM »
Did you focus so that a test line burn is as small as possible?
Your statement: what distance from the cutting head should I be using?
tells me you're not focusing. True or false?

The OP did say he was using 20mm distance [lens mount face to subject I presume]. He could have been at 100mm or anything in between and whilst still able to focus for best spot size will not necessarily be at the optimum distance for the smallest achieveable spot and hence greatest power density. It isn't an easy parameter to measure; I found that trial and error was the only practical method and determined that greatest power density was at 30mm but this will vary from one lens to another as well as the lens mounting. 

I also wonder how close the lens axis is to perpendicular to the face of the laser on these low end items. I suspect this will affect how uniformly the light is gathered hence determining spot shape and power density, to some degree.

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crutch

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Re: I guess I have an engraver but not a cutter
« Reply #7 on: May 29, 2017, 10:24:34 PM »
thats what I was asking about the distance.

if the beam is narrow to begin with, the zone of power at the focus point can be several mms thick from top to bottom.

my setting is about 20mm from lens to target surface.  I will try increasing that to 30mm.

high school physics teaches that the closer the target is to the laser source the higher the power .
you should have more power at 10mm than at 50mm.

I hadnt taken into consideration of how 'fat' the beam is.  thanks

Stonemull

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Re: I guess I have an engraver but not a cutter
« Reply #8 on: May 30, 2017, 04:26:50 AM »
I took this still from my video at http://youtu.be/egc09RVJ2Bk

Light travels in a straight line once it leaves the lens, hopefully all of it passing through a very small dot of space at the focus.

I have looked at the focus point though a usb microscope and tried to use that to focus precisely (I was after a 0.05mm dot if I could get it, photos are somewhere on the forum lol, I think in my pcb etching project thread)  but found that the camera and the eyeball differ on what looks like best focus, best burning power looks like a galaxy shape, it looks like a line with a bright dot in the middle, that dot is the highest power density you are going to get. Getting that galaxy shape though is determined by lens to laser distance, the backfocus distance has an optimum as well. Moving the lens closer to the laser focuses further away.

I found that looking with eyeballs it is hard to see, the dot is so small that it does not reflect much somehow. so either side of that highly focused dot appears brighter.
What I ended up doing is aiming at a piece of paper a few feet away. Experiment with the focus to see what the spot density looked like on that paper and then move a card in between to see what focus distance I now had.

There is math that could I think be done to calculate where the focus point is, not tried it though. the lens exit is a circle of X diameter, the spot on the paper is Y diameter, distance from laser to paper is L.
Then the focus point should be X* L/Y

Unless the beam is not filling the lens aperture .. I think it does though.

If that does work then then you could draw a bunch of circles on piece of paper and write down the calculated focus distance based on the spot size, or just focus till you get the spot suze you need.



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Zax

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Re: I guess I have an engraver but not a cutter
« Reply #9 on: May 30, 2017, 04:34:31 AM »
@crutch do you have a TTL laser (3 wires)?


crutch

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Re: I guess I have an engraver but not a cutter
« Reply #10 on: May 30, 2017, 06:50:33 AM »
yes, the connector has 3 wires to the mana SE board.

I am able to get a fairly good dot for the focus.  the best focus seems to be a line with a dot at the center.

this is all new to me but I am making steps forward, sometimes giant steps, sometimes baby steps

Zax

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Re: I guess I have an engraver but not a cutter
« Reply #11 on: May 30, 2017, 06:59:24 AM »
In that case you should get excellent grey scale results unless you are just burning too much and need to increase the feed rate or step size (resolution value).

pedwards2932

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Re: I guess I have an engraver but not a cutter
« Reply #12 on: May 30, 2017, 07:29:55 PM »
Carolynsdad,  I have my laser mounted in the holes that were cut for the laser and it is much higher than 30mm.  I am getting ready to design a manual z axis adjustment.  Are you saying if I can lower the laser and still get it to focus then I will be able to cut more efficiently or with a finer line?  Sorry to jump in on this thread but I asked this question before.....how do you tell what the optimum distance from the material is?

ggallant571

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Re: I guess I have an engraver but not a cutter
« Reply #13 on: May 30, 2017, 07:48:31 PM »
It might be easier and better for alignment purposes if you raised the bed. I use the extruded diamond steel available at mot hardware stores. Would prefer aluminum but it is not available in my corner of the world.
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Stonemull

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Re: I guess I have an engraver but not a cutter
« Reply #14 on: May 30, 2017, 08:23:20 PM »
In my ramble above I forgot to mention the reason for it ..

Use the larger beam view to determine the best lens position for cutting, then use that as the focal length. I suspect each laser and lens combo is different.
For mine, best cutting is at around 35-40mm.

Further than that away and I lose the 'galaxy' looking spot and I can no longer focus to a tiny tiny dot.

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Hacked Roland DXY-880
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CNC Plasma (work in progress)