best place on laser bed to do engraving?

Discussions on optics for laser cutter/engravers

best place on laser bed to do engraving?

Postby educa » Thu Dec 06, 2012 1:23 pm

Whilke reading about laser post size and depth of field etc... I was thinking about the following.

Am I right if I state that the best engraving (actually the smallest dots) will be possible at the place where my laser beam i the widest possible before hitting the lens? And or course using a small focal length lens instead of a large focal distance.

So on my quite large bed (1200x600) I would get the best engraving results by laying my material in FRONT RIGHT on my table (the place where the laser beam is longest before hitting the lens and therefore also widest beam diameter?)


Kind regards,

Bart
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Re: best place on laser bed to do engraving?

Postby orcinus » Thu Dec 06, 2012 2:06 pm

Both factors (beam diameter and lens focal length) influence both the depth of field and the spot size (circle of confusion would be the equivalent in photography parlance, i believe).

I've got no direct experience with laser cutting, but i take that to mean that it's always going to be a balance between the two you need to settle on. Decreasing the focal length decreases the spot size, but reduces the DoF as well. Increasing the beam diameter will decrease the spot size, but make the DoF shallower as well.

Edit: http://www.parallax-tech.com/faq.htm#depth - (note that one equation has a linear dependence on f/D while in the other it's quadratic)
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Re: best place on laser bed to do engraving?

Postby educa » Thu Dec 06, 2012 3:23 pm

If you take the calculator at http://www.buildlog.net/cnc_laser/laser_calcs.htm then spot size DECREASES when you have larger beam diameter.

And I do understand that DOF decreases too, but thats why I asked this question specific for engraving (no real dof needed here)
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Re: best place on laser bed to do engraving?

Postby orcinus » Fri Dec 07, 2012 1:40 am

educa wrote: spot size DECREASES when you have larger beam diameter.


That's precisely what i wrote :)

Increasing the beam diameter will decrease the spot size


If you don't care for DoF, then yes, increase beam diameter, reduce the focal length.
But not too much or i'd imagine you'll have a harder time refocusing when changing to a thinner/thicker stock.
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Re: best place on laser bed to do engraving?

Postby BenJackson » Sun Dec 09, 2012 1:22 am

I just did a test where I modified the raster sweep to also sweep the table slowly so I could see the the effect of focus placement. I'm trying to find the best settings for LaserMAX, which is an acrylic product with a very thin foil color layer on top. I wanted to see how focus depth would change the quality of the edges and of the engraved area. I think you'd need to do a similar test on each material you wanted to experiment with. If you can't ramp the bed, just place one edge of the workpiece on a shim so the piece is slanted.

educa wrote:So on my quite large bed (1200x600) I would get the best engraving results by laying my material in FRONT RIGHT on my table (the place where the laser beam is longest before hitting the lens and therefore also widest beam diameter?)

I think your assumption about beam diameter is wrong: My understanding is that the rear mirror of the CO2 lasers we're using is very slightly concave, such that the beam converges over a distance of about 15m. That information is probably on some other thread right here in this forum.
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Re: best place on laser bed to do engraving?

Postby educa » Sun Dec 09, 2012 1:54 pm

@Ben , I got these information directly from RECI and they do state very clearly that the exit beam diameter is aproximately 6.9mm with a 2.3mRad divergence, so if you take into acount that my laser beam can travel upto 2.4 meters before reaching the lens at farthest point, then that diameter is really not a joke.

I also understand that a beam expander could make my beam better over the overall distance, but by expanding the beam my cut depth of field will allready be worse at the upper left corner, because it will be consistent over the total tabloe, but be worse then I had before on the upper left.

Its all a tradeoff and until now I must say that in the upper half of my bed (which is still 600x300mm) I get very nice cut quality. I guess by playing with power and ppi settings I will manage to get better results in the other areas too. That should work. I'm just looking for a reason why it doesn't work so nicely.

I currently have another problem with resolution too, but that is stepper-related and I'll post that in another post.
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Re: best place on laser bed to do engraving?

Postby Guldberg » Sun Dec 09, 2012 8:10 pm

The lose in DOF can be compensated with, by using a longer focus lens. I just got a 2x beam expander from lightobject along with a 100mm focus lens. I do however think I can do with a 75mm lens. Will post some results when I have had time to do some testing.
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Re: best place on laser bed to do engraving?

Postby educa » Sun Dec 09, 2012 8:28 pm

Is it this one?

http://www.lightobject.com/2X-Beam-Expa ... -P767.aspx


How do you mount that one? I know it comes between the laser aperture and first mirror, but with what kind of holder do you mount is ?

I allready have a 100mm lens + head on spare here so that will probably be a nice solution to get nice results over the whole table. Just combine 100mm lens with 2x beam combiner.


So... more info would be more then welcome.

Kind regards,

Bart
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Re: best place on laser bed to do engraving?

Postby macona » Mon Dec 10, 2012 5:07 am

Beam expanders typically are screwed into a kinematic mount with X/Y capability. You need to be able to adjust for the center of the lens as well as make sure it runs straight though. This is the one I have for my laser cutter but didnt used (Lenses were toast, too much power loss)

Image
IMG_2426 by macona, on Flickr

I have a new beam expander I need to try now. It actually bolts down.
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