20mm lens, 150W Power, Which Focal Length?

[TurkishDelight]

Hi, I spoke to my "Foreign Sales Manager" in China and asked which lens and mirrors my laser cutting machine uses. I was thinking to buy a better lens and mirrors to improve the cutting performance. So he said to me that it's a 20mm diametered Zinc lens with 100mm focal length. I was confused, since the "Focal Stick" they gave us with the machine to adjust the height of the laser head was 52mm(or 50.8mm) long. I'm using a 150W Reci Laser tube, so is this lens suitable for cutting acrylic with a 150W laser? Do I have to adjust the laser heads (lens) height to 100mm? I'm really looking hard for the answer, since there is a 3 day long Labor-Day Holiday in China, and I'm using this machine for over 2 years now. If I couldn't use my machine at full performance for 2 years beacause of the wrong "Focal Stick" I'll be mad as hell.

[gfacer]

The machines are usually shipped with a 50mm lens, sometimes a 40mm, from what I have read (those numbers are approximate, I think it's all really in inches converted).

So, while it would make sense for you to have a 100mm lens with your 150w to allow more straight cutting in thicker materials, probably was not what it came with.

Take a look at the diagram attached to see what is best for you with what you cut. I'm sure there is a trade off between best for some things and the best all around.....I'll find that out myself once my machine arrives.

Also, I gather there are techniques to test the focus other ways (ramp test I saw one poster refer it it as), so you can look up those and try to check to see what you have.

I don't remember where this picture attached came from, looked for a link first but could find it again.

Good luck!

[RossMosh]

Buy 2" through 4" and see what you like. You're talking about an additional $200-300 if you buy extra laser heads to keep the lenses in. Grand scheme of things, it's not that expensive. If you went and did something similar through a major US brand, you'd be looking at $150 for the lens and another $50-100 for the holder. Figure in the shipping and all the other nonsense you'd be looking at easily $350-400 for one option.

[Melody-gweike]

Hi, I spoke to my "Foreign Sales Manager" in China and asked which lens and mirrors my laser cutting machine uses. I was thinking to buy a better lens and mirrors to improve the cutting performance. So he said to me that it's a 20mm diametered Zinc lens with 100mm focal length. I was confused, since the "Focal Stick" they gave us with the machine to adjust the height of the laser head was 52mm(or 50.8mm) long. I'm using a 150W Reci Laser tube, so is this lens suitable for cutting acrylic with a 150W laser? Do I have to adjust the laser heads (lens) height to 100mm? I'm really looking hard for the answer, since there is a 3 day long Labor-Day Holiday in China, and I'm using this machine for over 2 years now. If I couldn't use my machine at full performance for 2 years beacause of the wrong "Focal Stick" I'll be mad as hell.

Hi, TurkishDelight,

I agree with you. I also think if your major work is only for thick cutting, your laser head height also needs to adjusted longer to match the focal length.

Melody
li@wklaser.com

[RossMosh]

I read the post wrong.

Put down a piece of material (wood, acrylic, lacquer coated steel, whatever).

Set the material 4" from the laser head. Engrave a 12" line and some text. Mark the material noting it was at 4"

Bring the material .25" closer (making the distance from laser head to material 3.75"). Engrave a 12" line and some text. Mark the material noting it was at 3.75".

Do this until the line gets very fine and then starts getting wider. When the line is at it's finest, that is very close to the focal point.

To fine tune the focal point, simply redo the process but instead of adjusting every .25", do it every .05". It will take a bit of time but you'll get your actual focal distance quickly enough. Once you're done, take the final measurement and cut a focal stick.

Another method is to find a piece of material cut at a long steep angle. Run a line down that material and you'll see where ever the spot is smallest, that's the focal point.

[TurkishDelight]

Seems like I figured it out! The reason I was thinking that the focal stick was at fault, is that the focal stick is 52mm long, but the lens is more upwards than the focal sticks end. So the focal length is actually 100mm and the diameter of the lens is 20mm. My Chinese "Foreign Sales Manager" said that the material is Zinc and one side of the lens is convex, while the other side is concave. I assembled the lens so that the convex side faces down and the concave side faces upward. Seems right.

The bigger problem I'm facing now is that the laser tube might not be aligned with the gantry. As seen in this topic I might have to disassemble the whole tube.

[gfacer]

I think your lens is upside down the way you describe.

The convex side should be up.

[TurkishDelight]

Now look at this picture. The lens on the right bottom is supposed to be my laser lens. So the laser beam should come from left to the right, which means the concave side is upwards and the convex side is downwards. Am I right?

[Exsecratio]

Your lens is the top left one and should be fitted convex upwards toward the incoming beam.

OPLaser support forum • View topic - Final Lens Orientation (The Definitive Answer) for an explanation.

best wishes

Dave

[TurkishDelight]

Your lens is the top left one and should be fitted convex upwards toward the incoming beam.

OPLaser support forum • View topic - Final Lens Orientation (The Definitive Answer) for an explanation.

best wishes

Dave

I was looking for that article, but couldn't find it again after reading it once. Thanks!