Lens focal length pro's and con's?

Laser Cutter Help
#1

Hi, wondering what more constraints there might be on selection of focal length of lens for a cutter?

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With a longer focal length, on the upside you get less divergence of the beam, so presumably the kerf stays thinner - so hopefully neater back side and cut faces closer to parallel with the top face. The tradeoff seems like it would be that a longer focal length means you can’t really do air assist, and any vibration or slop of the carriage with the tertiary mirror and lens in it is going to be magnified a lot - plus you need a higher gantry.

I guess also any imprecision in the lens would also be magnified more by the longer distance, so the size of the spot at the focal plane will be proportionally larger - but are there some other effects that I’m missing?

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#2

well, having had a go with the longer lens, it’s definitely a bit more ‘dotty’ when doing test runs scribing paper, but that smooths out on anything else, and does seem to work pretty well;

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#3

Even without a close-up air assist nozzle, it works pretty well on 20mm PE foam;

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#4

At some point I’ll put the 2" lens back in and have another go at this cardboard test just to compare - 4" lens, front and back of single walled cardboard;

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#5

An interesting set of experiments and observations. Thank you for sharing them.