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Author Topic: Laser Focal lens selection method  (Read 2045 times)

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Laser Focal lens selection method
« on: July 05, 2012, 01:17:14 AM »
When my laser focal assembly fell on the floor and the lens inside broke in small pieces, i was really wondering if it was possible to find a replacement lens. When i got onto Ebay, i  was astound that this little piece was readily available.
My problem is now that there are different lenses with focal sizes and colours.
How do i know what the focal length is. The lens that broke was dark grey in colour. Is there a reason for the lens colours ? I have seen orange one`s and my one that broke is grey in colour. Does the colour mean anything ? What focal size do i go for? I have seen 40mm, 50mm and so on ..i`m confused. How do i make this selection? ???

The lens size in diameter is 21mm, so i have that one covered, but what about the rest. The method for getting the focal distance correct when the laser was still working was to put a small piece of perspex 2mm thick under the cutting nozzle. The laser has a big tube (1,8 meters long) that has a spec of  100Watts.

I was thinking of measuring the distance between the last mirror that shoots the beam down up to the focal mirror position, and take this as the Focal distance measurement..

Any info will help.


Offline Tweakie.CNC

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Re: Laser Focal lens selection method
« Reply #1 on: July 05, 2012, 06:58:53 AM »
Hi Andre,

The lens focal length is measured from the centre of the lens to the work.

Line up you machine with your Perspex gauge then measure from the work surface to the position that the old lens was fitted and then, if possible, obtain a new lens with a similar focal length to the distance that you just measured.

There are two common types of lens material used with CO2 lasers. The yellow colour lenses (seen on ebay) are made from Zinc Selenide (ZnSe) and are the cheapest – the grey colour lenses are made from Galium Arsenide (GaAs), are usually quite expensive but are considered to be more hard wearing.

I have lenses of both types but the ZnSe is the one I use all the time.

Remember to fit your new lens with the convex side towards the laser tube and the flat side towards the work.

Re: Laser Focal lens selection method
« Reply #2 on: July 05, 2012, 07:10:11 AM »

Yes, the actual one that came out is GaAs lens. On Ebay they go for about $60.
So what you saying is that once i have measured the distance from the lens down to the workpiece surface , that is more or less the distance. So it will either be 38.1 , 50 , or 60mm , as this is what i have seen on Ebay.

I will do some measurements tonight. I have not even thought about that the lens need the the round side up and the flat side down, but now that you mention it, i have seen it like that when i took the lens holder out for cleaning.

Thanx for the info Tweakie
much appreciated