Different Focal Length Purposes?

[Peter Meacham]

Can someone explain the purpose of using different focal length lens'?

What is the need to purchase different length lens - clearance over objects being lasered, finer detail, ...?

[Rodne Gold]

There is a lot of info and many discussions on this very subject in the forums , search focal length and related terms like lens , focal point , depth of focus 2" 1,5" etc.
You will get a great understanding of what its all about reading thru the threads.

[John Noell]

The short answer is that sometimes you may need more depth of field (e.g., for curved objects or bowl-shaped pieces) or sometimes you might want a smaller more intense dot-size for the beam (i.e., more detail).

[Bruce Dorworth]

So, I am using a 2" focal length lens now, and when I cut .5" acrylic I notice the edge tapers somewhat. Would a different focal length help correct this, and if so which direction would you go, 1.5" or a 2.5"?

Bruce

[john banks]

Longer focal length will cause less taper if the taper is due to depth of field. It won't fix alignment issues but these can sometimes look like they are due to short depth of field. There is also an effect on thicker acrylic where if you can see it as it is being cut with safety glasses then the bottom edge cuts later than the top edge as it looks like it is melting, so cut direction and speed can influence taper too.

[Rodne Gold]

1/2" + thick acrylic is a bear to cut IF you want perfect edges , no matter what the laser mnfgrs show , you will struggle to get it right without a LOT more power. We max out at 8 or 10mm with our RECI 80w tube... 1/2" works , but its not saleable.
Using a longer lens might not work anyway , as acrylic is a waveguide and doesnt really "deflect" the beam , it keeps it straighter as it goes thru than lets say , wood would.
the other issue you face is that a longer lens has a larger spot size so 1, your cuts will be thicker and 2, you might actually have too little power density to achieve a good vaporisation and 3, the probability of a much bigger heat affected zone is greatly enhanced.

[Dan Simone]

When you say it is not saleable what does that mean for how the cut looks? Is saleable only an edge that looks fire polished? I've been trying to figure out what is acceptable because I can't seam to find a setting on my machine that doesn't cause what appears to be pulse marks. Of course these marks can only be seen if you hold the acrylic up to the light just right and the edge still feels smooth.


1/2" + thick acrylic is a bear to cut IF you want perfect edges , no matter what the laser mnfgrs show , you will struggle to get it right without a LOT more power. We max out at 8 or 10mm with our RECI 80w tube... 1/2" works , but its not saleable.
Using a longer lens might not work anyway , as acrylic is a waveguide and doesnt really "deflect" the beam , it keeps it straighter as it goes thru than lets say , wood would.
the other issue you face is that a longer lens has a larger spot size so 1, your cuts will be thicker and 2, you might actually have too little power density to achieve a good vaporisation and 3, the probability of a much bigger heat affected zone is greatly enhanced.

[Bruce Dorworth]

Thanks Rodney, I knew you would come through with an answer>

Bruce

[Rodne Gold]

Dan , depends what you using stuff for , for an award or some sort of recognition item , it's really got to be perfect , for industrial or other uses , maybe you get away with it. Flame polishing will help a huge amount to get rid of the striations etc.

[john banks]

Dan, the striations seem to reduce by reducing power and speed but this may not be economical or work well for thicker stuff. We tried dot mode and that helped, but by the mechanism that it just reduced power and speed.