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Jim Priest
05-02-2014, 1:18 PM
Can someone briefly explain the use of the various focal length lenses (1.5 inch, 2.0 inch, and 4 inch) used on CO2 lasers.

Thanks

Dave Sheldrake
05-02-2014, 1:59 PM
Longer focal length = bigger spot size = lower power density = straighter cut on thick materials and visa versa for shorter focal lengths.

1.5 inch = engraving or cutting paper
2 inch = general cutting and less fine detail engraving
4 inch = long depth of field for cutting material that is above 8mm thick or so.

cheers

Dave

David Somers
05-02-2014, 2:00 PM
Morning Jim,

First....just a fast explanation of the focal length in case you aren't clear on that. As your laser "beam" comes through the lens all the light converges on one point at a fixed distance from the lens. That distance is the focal length....the distance from the lens to the point where all the light is focused as well as it is going to. Visually the light beam would look like an X. The light converging on the point of focus at the narrowest part of the X, and then diverging again at the bottom of the X. There is a very limited distance above and below that point of focus where the beam has enough energy concentrated to do any meaningful work. That distance is called the Depth of Field.

Think of when you played with a magnifying glass out in the sun burning leaves as a kid and you will have experienced all of these terms and effects. The lens of a laser system is pretty much a magnifying glass. With a magnifying glass you move the glass up and down to get the point of focus on the object you are trying to toast. With a laser you essentially do the same thing, except you raise and lower your object in relationship to the lens to bring it to the point of best focus.

So a 1.5 inch lens has a distance of 1.5 inches from the lens to the point of best focus. And a 2 inch lens has a distance of 2 inches, etc.

Sorry if I am being too basic, but just wanted to be sure you knew this part of it.

Now....with the longest lens, the 4 inch, you will find that center of the X is longer. You could draw an X on paper that is 1.5 inches from top to center and then draw another X with the same top width but 4 inches from top to center and see the difference. A 4 inch lens will give you a greater depth of field than a 1.5 inch lens because of this.

A 4 inch lens also puts the point of best focus further from the lens than a 1.5 inch lens. So if you were trying to cut or burn something that was in a fairly deep object a 4 inch lens might allow you to do it, where a 1.5 inch lens might not. The lens on a 1.5 inch lens simply has to be too close to the surface to work inside a deeper object.

4 inch lenses tend to get used for cutting more than anything. And 1.5 tends to be used more for fine engraving, and a 2 inch lens is a nice compromise. Which is why they tend to provide that to you on most laser systems.

Most folks on the forum have questioned the need for a 4 inch lens for most uses. If you do a search you can find lots of threads discussing this. It really depends what you want the system to do. It seems that for most people a good 2 inch lens is just dandy and very versatile.

Does that help?

And an oops to Dave Sheldrake. Looks like I posted this just as you did yours. Only I babbled a whole lot more! <grin>

Dave

Bill Stearns
05-04-2014, 12:29 PM
Hi All -
This thread brings up 'question I've had for 'long time. I have always used the "auto focus" procedure, and I mean for eight years. My Epilog came with some sort of metal v-shaped part - used for manual focusing, think I remember. But, have never used it. Have I been missing doing something important that would make some sort of difference? (does the day ever come that you know all there is to know 'bout this business?)

Bill

Michael Hunter
05-04-2014, 2:23 PM
Bill :

It depends on how you work.

If you have a 1.5mm Rowmark day and then a 6mm MDF day, etc., then using the focus tool once for the first job each day saves some time.

If every job is on a different thickness material, then autofocus comes into its own.

Bill Stearns
05-04-2014, 5:16 PM
Michael - thanks.
Guess many times we just stick with what we know 'n understand, uh? So, guess I'll stay with auto-focus. Was just wondering what manual focus - that V-shaped thing - was all 'bout.

Bill

Jerome Stanek
05-04-2014, 5:18 PM
I like how my auto focus is set up. You place the piece on your table and hit auto focus and the table moves up till the material trips a micro switch and then backs off after you set the tube height then every time you change material all you have to do is is place the material on the table and hit auto focus.

Glen Monaghan
05-04-2014, 6:06 PM
I stopped using autofocus early on, and eventually removed the autofocus plunger. More often than not, I don't have anything located at the Epilog's autofocus location so I had the occasional issue with activating autofocus without first temporarily positioning the substrate or a substitute under the plunger... At least when I manually focus, nothing crashes or gets wedged... And it gives me the opportunity to tweak higher or lower for certain substrates and jobs (such as when vector engraving and I want a slightly out of focus beam to soften the line).

I took the plunger off about the 3rd time it hit a raised portion of a 3D object that I was engraving, knocking the object askew and ruining the job.

Dave Sheldrake
05-05-2014, 5:07 PM
I stopped using autofocus early on, and eventually removed the autofocus plunger. More often than not, I don't have anything located at the Epilog's autofocus location so I had the occasional issue with activating autofocus without first temporarily positioning the substrate or a substitute under the plunger... At least when I manually focus, nothing crashes or gets wedged... And it gives me the opportunity to tweak higher or lower for certain substrates and jobs (such as when vector engraving and I want a slightly out of focus beam to soften the line).

I took the plunger off about the 3rd time it hit a raised portion of a 3D object that I was engraving, knocking the object askew and ruining the job.

I'm with Glen, autofocus gives you an average for the lens, each lens will have variations in it's focal length but the system will be set to the stated focal length hence not that accurate. I've seen plenty of them fail over the years jamming machines and damaging rails etc to the point I won't have them on any of my machines.

cheers

Dave

Michael Hunter
05-05-2014, 5:45 PM
... autofocus gives you an average for the lens, each lens will have variations in it's focal length but the system will be set to the stated focal length hence not that accurate.

Epilog allows adjustment of the autofocus, so it can be set properly for the actual lens fitted to the machine.

I don't use it often, but when I do need it, autofocus seems just as good as using the gauge - I haven't needed to adjust it at all.

Jerome Stanek
05-05-2014, 6:04 PM
I use it but also have gauges that I cut for different materials