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View Full Version : How much more focus tolerance would a 5" lens have over a 3.2" fiber?



Seann Fordham
09-20-2016, 9:38 AM
We are considering getting a 5" lens in the near future and was wondering, since fiber is less forgiving then co2 when it comes to focus, how much more depth of focus would you gain on stainless steel for example?
With the flexx lens at the moment I can go out of focus by roughly 2mm until it starts annealing.

We do lots of personal items and such, most are reasonably flat like zippo lighters, etc. But then there are those curved items that can be a bit more awkward to engrave.

I've searched the forums and the only posts I've found are about galvo's, which doesn't really help a lot since galvo's have a completely different delivery method for the laser beam. There are also those few occasional jobs where the extra clearance would come in handy as well.

Rodne Gold
09-20-2016, 12:21 PM
I cant answer your query specifically as i have a galvo fiber, but along with the longer lens , you will lose energy density and then 20w might become marginal for some applications

Dave Sheldrake
09-20-2016, 12:53 PM
D = Diameter of input laser beam from tube.
F = Focal length of the lens we are using.
W = Wavelength of the laser tubes beam.
DOF = Useful depth of field for a given optical set up.

The equation goes as follows:

DOF = 2.5 x W x (F/D) squared.

Seann Fordham
09-21-2016, 5:29 AM
I understand that it will lose some power because of a larger dot size, but I'm not really sure how much. Is it enough to have to reduce the speed of the machine by 30% to get the same result as the smaller focus lens?

I wasn't after an exact answer, just what other people have experienced with different fiber lenses, etc.

Dave Sheldrake
09-21-2016, 6:21 AM
reducing speed with increase dwell time but not power. Power density is a product of power/area

A 1mm spot is 0.79 SqMM, a 1.5mm spot is 1.77 SqMM, same power spread over 2.24x the area so in effect 2.24x less effective radiated power.