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View Full Version : Focus is different between two machines with the same lens...??



David Rust
12-23-2014, 9:22 PM
Hi All,

I have two Epilog laser machines, a Helix and a Mini. Both are practically the same (except power)... run from the same driver, both have the same firmware loads, etc...
I set up a piece of anodized aluminum on a ramp (3" long piece of aluminum raised on one end to ~.75") to verify my focus tool. I set up a 2.5" hairline to vector engrave.
The line engraved nice in the middle area and faded in/out on the ends (70S/5P/1000F). However I did not see a distinct "thinness" or "very clean" area around the center, it was pretty uniform.
The distance between the "faded areas" was ~2"... So I split the difference and set the focus tool at that height.
I expected the two machines to have similar focus heights (using a 2" lens on both)... but they were different by 0.200"... Not what I expected!

Anybody have a similar experience? Should I have seen a distinct "thinness" at the center? Is there typically a focus variation of this much between two of the same type machines?

Thanks,
Dave

Dave Sheldrake
12-23-2014, 10:46 PM
Both are practically the same (except power)

Incident beam diameter is different due to different powers, ergo final spot size will be different even if the focal lengths are the same. Lens focal lengths are a bit of a miss mash sometimes, it's very rare to find a 2 inch lens that actually focusses at 2 inches, they can be as much as +/- 1mm up or down depending on the manufacturing tolerances among other things.

cheers

Dave

Richard Rumancik
12-24-2014, 12:56 PM
Dave Rust: you are probly not getting a really distinct indication as the shape of the beam as it passes through the lens is not like an "elliptic cone" i.e. pointed. The shape is more of a hyperboloid where it gradually "necks". So there is a gradual change rather than an obvious best point. But I would not have expected a .20" difference between the two machines with the same lens installed. These two machines use the same reference point so I would have expected better agreement. I think maybe it is just a problem of getting a good "read" on the best part of the line.

Dave Sheldrake: I agree that true focal lengths of lenses are not identical to the actual, they are sold as a "nominal" size. Each lens has to be calibrated. But in this case this should not a contribution to the discrepancy between machines as he is using the exact same lens in both machines. So if it is a 1.95 or 2.0 or 2.05 or 2.20 lens it should behave the same in both machines i.e. the optimum point should match. And shouldn't the optimum point also be identical even if one machine has a smaller spot size?

Dave Rust: Different people use different methods to find the optimum. Personally I like Kim Vellore's suggestion way back, where he proposed vector marking parallel lines aligned with the y-axis, on an inclined piece of aluminum. If you do this you could probably see visually which line is the thinnest rather than trying to find the thinnest point on a horizontal line. Plot the lines as close to each other as you need for best resolution.

Kim's sketch is in this thread:

http://www.sawmillcreek.org/showthread.php?134863-Epilog-Manual-Focus-tool

Dave Sheldrake
12-24-2014, 1:55 PM
Dave Sheldrake: I agree that true focal lengths of lenses are not identical to the actual, they are sold as a "nominal" size. Each lens has to be calibrated. But in this case this should not a contribution to the discrepancy between machines as he is using the exact same lens in both machines. So if it is a 1.95 or 2.0 or 2.05 or 2.20 lens it should behave the same in both machines i.e. the optimum point should match. And shouldn't the optimum point also be identical even if one machine has a smaller spot size?

The focal depth isn't actually a spot, it's a point +/- where the spot is 1.4x the smallest diameter of the beam. If the lens is being swapped between machines then the focal point should be very much the same however I got the impression he is using 2 x 2 inch lens's and is wondering why their focal point is at different Z heights.
Depth of field is a product of the focal length of the lens as well as the incident beam diameter, if the lens's vary by 0.1 focal lengths and the incident beams vary by 2mm on the diameter the focal *point* will differ by 0.25 easily. It also assumes both lasers are operating at M^2 = 01 when there is little chance that is the actual case.


I expected the two machines to have similar focus heights (using a 2" lens on both

A 1.9 inch focal length and a 2.2 inch focal length will give the 0.3 inch difference without any other contributing factors.

If it's the same physical lens in both machines ie: it's being swapped between machines then a 0.25 difference is a LOT but not hugely unusual given that even matched pairs of lab lasers differ at the most basist of levels.

cheers

Dave

David Rust
12-24-2014, 3:45 PM
Dave, Rich,

Great conversation... yes, the machines have their own lenses that I am comparing, same specifications...
Off to read the thread that Rich linked to...

Thanks!

Dave Sheldrake
12-24-2014, 3:59 PM
yes, the machines have their own lenses that I am comparing, same specifications...

There is your answer David :) Focal lengths are *nominal* and as yet I have never seen two that focus in the exact same Z position

cheers

Dave

Richard Rumancik
12-27-2014, 10:50 AM
. . . yes, the machines have their own lenses that I am comparing, same specifications...

Dave S: Ok, since we are taking about two different lenses in two different machines, your answer was correct. Somehow I mistakenly interpreted that David was moving the lens from machine to machine.