[Ciencias]

Dear all,

Trying to cut 6mm plywood i notice the following symptoms:

1) I need to use high power (150W - 180W laser tube at 80%) and low speed (15%) to be able to cut it.
2) The edges of the piece cut are divergent. With this i mean that if i turn the piece arround, the back side looks like scaled up.

I understand by my research that when the laser does not hit the focus lens in the middle the sides of the cut will not be square, but in that case they are all sided to the same direction, and this is not the case.

I have got a meniscus focus lens with f = 2.5' and a laser bean diameter of aprox D = 9.5 mm at the focal lens.

My guess is that with that f and D, the Depth of Focus is too narrow (acording to my research it would be arround DOF = 1.8 mm, and that would be the maximun depth I could get a square cut on any material.

To solve my problem i found two possible solutions, and here is where any advise would be very much appreciated:

1 - Change the focus lens for another one of a larger f value. According to my own calculations, that would be f = 7.5' to get with my current D to a DOF of arround 10 mm; that is my depth target with a square cut.

2 - Buy a beam expander, in my case to reduce D, so as to get a larger DOF. I estimated if I could reduce D to half of it's current value, with a 3.5' focus lens I would reach arround 9 mm of DOF.

Option number 1 seems to be impossible for me, as my head can not go as up as needed to make focus, so I have to make the desition of buying the beam expander or not.

Ok, finally here are my questions:

a) Does the above reasoning (besides the numbers in calculations) sounds logical to you?

b) Does anybody have any experiense with beam expanders?

c) If that is what I need, any idea of where to buy them?

Thank you for your time,
cheers!

Luis

[pippin88]

Can we have a picture?

If the angle on the sides is /---\ (like a pyramid with the top cut off) this may mean problem with focus / depth of field.
If the angle on the sides is \--\ the laser beam is not hitting directly perpendicular to the lens (the mirror that sends the laser down through the lens is not aligned properly)

[Ciencias]

Attachment 195302

Can we have a picture?

If the angle on the sides is /---\ (like a pyramid with the top cut off) this may mean problem with focus / depth of field.
If the angle on the sides is \--\ the laser beam is not hitting directly perpendicular to the lens (the mirror that sends the laser down through the lens is not aligned properly)

Yes, in my case it seems to me I am on the pyramid scenario. (/---\ )

Here are the pictures:

[CUTCNC]

Hi Luis, I think the low power laser will always have that problem, high power laser more than 1000w will better, but it is rather expensive, maybe you show consider the milling cut method. Please know more about it in below link: Steel rule die board router machine
Attachment 196546

[cncadmin]

Use a 4" FL lens on thick material, works good for me.

[Ciencias]

Use a 4" FL lens on thick material, works good for me.

Thank you Paul, I am on the process of doing that.

Regards,
Luis

[Exsecratio]

2 - Buy a beam expander, in my case to reduce D, so as to get a larger DOF. I estimated if I could reduce D to half of it's current value, with a 3.5' focus lens I would reach arround 9 mm of DOF.

Beam expanders make the final spot smaller not bigger, they will reduce the DOF by making D input bigger. If you try to fit them the opposite way round (assuming you have ARC both sides) you stand a good chance of blowing up mirrors due to the big increase in PD.
Beam profilers may help as will reflective phase retarding mirrors but as Paul said, longer focal length lens are easier and less expensive

cheers

Dave

[Ciencias]

Beam expanders make the final spot smaller not bigger, they will reduce the DOF by making D input bigger. If you try to fit them the opposite way round (assuming you have ARC both sides) you stand a good chance of blowing up mirrors due to the big increase in PD.
Beam profilers may help as will reflective phase retarding mirrors but as Paul said, longer focal length lens are easier and less expensive

cheers

Dave

Thank you Dave. What you explain sounds very logical to me. Maybe i have over reacted with my research and ideas ;-) . I have already calmed down and ordered longer focal length lenses.

cheers!
Luis

[Pplug]

It looks like you have some nasty burning on the sides. If you have ppi, I suggest you use it. I can cut through 1/2" poplar or oak without severely burning the sides with an 80w reci tube. They get brown but not turned to charcoal.