Which one cuts faster, short lens or long lens.

[Bruno Piancatelli]

Hello. I have this doubt.
as the tittle says I want to know, given the same power setting, If I was to cut 3 mm MDF, which lens would allow me to cut faster.
I´m us¡ng a 100mm lens right now, that came with the machine.
From what I´ve read the shorter lens concentrate more power on a smaller surface, which should mean faster penetration of the beam
But I was told by my supplier that the wider cut that the long lens produces gives a faster cut, because of the air flowing better through the cut makes for a better cleaning of the cut. or something like that. But I have my doubts..
What are your experiences on this?
Thanks!

[Rodne Gold]

Short answer , shorter (50mm) lens if you don't have oodles of power to spare..

[matthew knott]

Short answer , shorter (50mm) lens if you don't have oodles of power to spare..

I would go with shorter lens, but there are so many variables that the only way to be sure is physically try it.

[Chuck Stone]

As I understand it, the shorter lens is a more concentrated beam, smaller spot size.
That concentration gives you more power density at the spot, cutting faster.
However, it also has less depth of field, so that increase in power density is only
better at cutting in a shorter depth.

Picturing the beam as ah hourglass, the center is where you focus for the best
power, as that's where the density is highest. Above and below that spot there
is still SOME power and reasonable focus, but as you get away from the center,
the power density decreases.

On a longer lens, that hourglass is taller. Bigger beam size, less power concentration,
but more depth of field. On a shorter focal length lens, the hourglass is shorter. More
power in the center than a longer lens, but less depth where it cuts as efficiently.

So there must be a 'sweet spot' where it will cut faster. But without specifications
on the laser, the lenses and the materials, (and more understanding than I have)
I wouldn't know where that sweet spot is. Maybe it will be faster on 1/4" material,
or maybe only faster on 1/16". I just don't know.

[Bruno Piancatelli]

My thoughts were not so wrong then.
I have a 60watt chinesse. I do some cutting on acrylic and MDF usually not thicker than 3mm (1/8)
there is also the possibility for a 63mm lens that could be somewhere in between.
Right now im cutting both acrylic and MDF at about 12mm to 14mm´per second´s respectively, using no more that 65% tube power.
I´m gonna buy a new lens anyway but just haven´t decice if 50 or 63mm´s

[Rodne Gold]

Go for a 38.5 or 40mm lens , even better than the 50mm for 3mm acrylic and mdf. Both for engraving and cutting.

[Bruno Piancatelli]

OK Rodne.
Thanks a lot!

[Dave Sheldrake]

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.

PD (Power Density) goes, area of spot / input power

"Useful" depth of field has been arbitrarily found to be 1.4x beam waist diameter in the +/- Z axis so basically 2.8x spot size.

Although an exact science there are a lot of variables that affect the above from quality of lens, abberation, heat distortion and so on.

On 3mm MDF / acrylic / ply a lens of 38mm to 45mm will give best all round results, moving up to the 50.8 and 63.5 if you need more clearance for tipping parts (as the clearance gets bigger remember your air assist will be less effective unless you extend the nozzle or move the air blast point.

cheers

Dave