Slot size test

[Robert Silvers]

I have a 2 inch lens. The autofocus was about 0.5mm off. I was able to reduce my spot size a little bit from the auto-focus, and a lot from the included height gauge. Still my spot size is about 0.013" - more than people claim it should be - but I never saw anyone else document their size either.

This is the laser burst - so probably full power. So maybe I would have a smaller spot size with less power.


This is anodized aluminum plate.


http://imageshack.com/scaled/800x600/593/fjmq.jpg​

[Chris DeGerolamo]

Keep in mind that on thicker materials your laser may cut better when focused into the piece, rather than to the surface plane. Good work on your behalf.

[Robert Silvers]

Yes. Now that I found the correct surface focus for engeaving, I am going to cut out a set of gauges to cut 1/2 way into every thickness I plan to cut.

[Dan Hintz]

Still my spot size is about 0.013" - more than people claim it should be

Is your focus lens flat side towards the substrate?

[Rodne Gold]

Your spot size is actually more dependent on your entrance beam diameter than the lens itself , the bigger the beam from the laser to the lens , the smaller the spot size with a given lens. Beam expanders make the laser beam bigger so as to get a small spot
You will do better vector engraving a line to measure your spot size than lasering a spot..the spot size will be the width of the line..

[Gary Hair]

This is the laser burst - so probably full power. So maybe I would have a smaller spot size with less power.

It could be that your spot size is smaller but you are bleaching out the dye around it, not sure if that is possible but it makes sense to me.

Try lasering a line on a piece of material that is angled up from the bed to about 1/2" above the bed. Focus at about the mid point of the material and then run the line. Your focus may be off and this would determine that as well as give you an idea of the spot size.

[Robert Silvers]

Lasering a line at 400mm/sec and 20% power on anodized aluminum made a nice line that was 0.008" wide.

[Robert Silvers]

I made a set of kerf-compensated and verified gauges to manual focus from the honeycomb, which will result in mid way focus in 3, 5, 6, 10, 12, 16, and 24mm materials. The gauges themselves are mid-focused so they are the same size on each side.

[Robert Silvers]

My bed bolts on the four ACME blocks were all unscrewed for some reason. Maybe to prevent binding - though they were too loose. I made the left side tight, and the right side a little loose, to prevent binding (if you make them all super tight, there is some binding - which is to be expected as it would be over-constrained), and used LockTite. I then re-leveled the bed.

[Gary Hair]

Lasering a line at 400mm/sec and 20% power on anodized aluminum made a nice line that was 0.008" wide.

Now angle it so that one end is 1/2" above the table, focus about 1/4" above the table, and run it again.

[Robert Silvers]

Good idea. Then I will know if I am at the best it can be.

[Gary Hair]

Yep, it's really the only way to know for sure where you are in focus.

[Robert Silvers]

I meant for this thread to be called "Spot size test."

I made the line - 430mm long with one side 4mm higher than the other and the autofocus in the middle. Everything in the middle looks about the same. I think I am done with this issue.

Next I will try cutting 12mm acrylic to see if I can.

[Kev Williams]

I'm sure I'm not the only one but how I check my focus is to run a series of straight lines at different distances on some anodized aluminum. The 2 pics are the same, both are darkened, one more than the other, to highlight the white lines. From where my auto-focus started, you can see the +.01 is the brightest line, with +.02 a close second. I figure a bright line is an in-focus line! :)

FWIW, 7% power 30% speed on the 10-1/2 year old Gravograph LS900. My calipers and 10x magnifier ain't the best way to measure, but I keep getting .005 to .006 on the line width, 2" lens...

[Robert Silvers]

I guess if I want a smaller spot size for engraving, then the most practical thing is to have a shorter lens. That gives you more depth of field also. A beam expander will reduce my spot size, but is a probably not an easy add-on, and will reduce depth of field.

[Dave Sheldrake]

That gives you more depth of field also

Other way round

Longer focal length = longer depth of field = less power density
Shorter focal length = shorter depth of field = higher power density

There is also quite a difference between Plano-Convex and Meniscus lens's.

Beam expanders also contribute to aberrations.

cheers

Dave

[Robert Silvers]

Yes, you are right:

http://www.parallax-tech.com/faq.htm

But the spot size is always larger, so it is not a home run.

[Dave Sheldrake]

Shorter focal length = shorter depth of field = higher power density

power density - spot size / input power.

cheers

Dave