50MM VS 25MM focus lens

[Jim Reinhard]

Will you get less of a burned edge cutting with a 25mm vs a 50mm lens?

[Chuck Stone]

Not really sure what you're referring to when you say 'burned edge'..
Do you mean that your edges are blaackened and you're wondering if a new
lens would help? (don't think so, but don't know for sure) Or do you
wonder if a shorter focal length lens would remove less material than
the longer lens (yes .. smaller 'kerf' )

I have a problem with burned edges on everything I cut now .. but I need
a new tube. The beam is staying too long in one spot because of the low
output.. but that's the only way I can get through the material.
(1/8" BB .. Power 100%, Speed 0.6%, 30w Mercury! ) S - L - O - W !!!

[Jim Reinhard]

Yes referring to, Will you get less of a burned edge with a shorter focal length lens. I am actually cutting 3m safety walk tape .The white gets a brown edge on it and I am trying to avoid it.

[Chuck Stone]

I see .. then I think that you could get less of a brown edge with a shorter lens because
your beam would need to spend less time in place (since it is more concentrated)
I think most burned edge problems are related to excessive dwell time for that material.
So more power or more concentrated beam (shorter focal length lens) would both allow
your to move the beam away from the burned area quicker, giving you less charring.

But that tape might also be the problem. I'm not sure how those combinations fare under
the heat of the laser. Talc, polyurethane, carbon, polyester .. some will burn, some might
smoke. Are you getting a lot of smoke when cutting it? That might be part of the brown edge,
too. The talc will absorb and retain a lot of heat.. that might also cause problems. If it holds
enough heat, your beam might move a foot away, but that spot you lased might still be
throwing off enough heat to continue scorching the surrounding material. That tape has
a very high mineral content, most of it being talc. Think of the heat retained by a
soapstone oven .. stays hot for hours. Soapstone is also very high in talc.
That's my $.02

[Mike Null]

My inclination is to think that the lens length will not have any effect on the charring. I think, all things being equal, this is more to do with speed, power, frequency and focus.

There are materials that will char/brown in almost all circumstances.

[Kim Vellore]

If you raster the tape at high speed and high power, the color you get would be the best you can do on the edge, if it is brown you will not get better than that on a cut.

Kim

[Chuck Stone]

My inclination is to think that the lens length will not have any effect on the charring. I think, all things being equal, this is more to do with speed, power, frequency and focus.


Agree... but do you think the focal length would affect the speed, since the beam is more concentrated? Or
would the shorter depth of focus slow down the cut? Probaly not an issue on tape, but interesting to know if
there's a tradeoff on other materials. Shorter length, finer beam. Longer length, straighter beam. hmmm...

[Mike Null]

Chuck

I don't disagree with your premise but I think it may have minimal effect in this case. In my 16 years with a laser I have not seriously considered moving from a 2" lens to shorter or longer ones. I know there were times with thicker materials that a longer lens may have helped but I just couldn't justify it. I did try a couple of things with a 1.5" lens and was not satisfied that the result was worth the cost.

I do know others who love their 4" lens though.

[Jim Reinhard]

I ordered a 1" lense and will post back the results when it comes in next week.

[Chuck Stone]

Interesting .. I have been thinking about ditching my 2" in favor of a 1.5 and a 2.5" lens. I'm told the cut is
better with a 2.5" lens without a lot of detail loss. (that's subjective though, I do a lot of small stuff) and
very fine detail with the 1.5" lens.

I can find lenses pretty reasonable online. but not sure about how to accurately mount them.
My lenses appear to be sealed in the metal holder. (glued, perhaps)

[Mike Lysov]

Agree... but do you think the focal length would affect the speed, since the beam is more concentrated? Or
would the shorter depth of focus slow down the cut?

From my experience the longer focal length the slower the speed.

The only way to make charring less is to increase the speed of cutting. Also air assist pressure will help as well as it removes derbies and helps increase speed of cutting.

I use over 30PSI air assist for a 100W laser and over 60PSI for a 280W laser. Before I used to cut MDF with 10-15PSI and speed was slower at least 10% and I had a lot of residue left at the front. Now with 30PSI I have some small residue left sometime only on very small shapes (2-3cm is size) at piercing point. If shapes are bigger than 3cm there is no residue left at all.

[Chuck Stone]

From my experience the longer focal length the slower the speed.
The only way to make charring less is to increase the speed of cutting.

Thanks, Mike. My brain was going that way, but nice to hear from the voice of experience.