K40 retrofit lens

[brian257]

I just bought the DSP retrofit kit for a K40 laser that comes with the new air assist head. The head does not come with a lens. Can someone tell me the part number for the lens I should use. Thanks.

[Tech_Marco]

You may use 18mm ZnSe lens for that laser head. Even 20mm can fit into it
Then, you decide what focus point you like. There is 38.5mm, 50.8mm, 63.5mm, 68.5mm....and so on
For K40, most folks will use 50.8mm. It is OK for both cutting (4mm or less) object or pretty good quality on engraving.


Marco

[brian257]

Thanks. Would the GaAs lens be better than the ZnSe?

Can you give me a brief explanationon the advantages and disadvantages of the longer or shorter focal lengths.

[Tech_Marco]

Personally I haven't ran a test to compare the difference. Some said that performance from a GaAs lens would be better.

Shorter focus lens do better on engraving because the focus pointer is sharper. But it is not very good for doing cutting because it bevel affect more on cutting edge. Longer focus lens do better on cutting as the bevel angle is less but it won't be good for engraving as the beam is a little "fuzzy". Plus, it take more power to support a longer beam.

So, if you need to do two things: cutting and engraving, you better to have two lens ready.


Marco

[brian257]

OK. Thanks.

[Vegaman_Dan]

I'm in the final stages of a DSP conversion using the kit from Marco. While there are a few hangups that I'll address once I get the initial calibration done, I do wonder about the lenses.

The kit comes with a new air assist holder that you move the lens from the original K40 mount to the new one. No problem. But I see the poor quality of the focusing lens and can foresee wanting to get a replacement. The topic thread here suggests you can use an 18-20mm lens, but I'm not so sure. When I measure the inside diameter of the original and new lens holder, it only measures 12.5 mm, or 1/2". Even the smallest lens offered is 14 mm.


Can anyone point me to the correct lens to choose for a K40 setup?

[Tech_Marco]

That's something I hate the most. They (manufacturers) are keep changing things in intention of trying to cut cost. Our laser head will fit 18~20mm lens. For 18mm lens it may get lose a little but it can be solved by using your machine to cut a ring using pipe wood. Or, if you have some O-ring, you can use it

In your case, you may want to post a picture of your lens holder and the lens. Put a ruler/calibrator next to it so that I can see what you have. The smaller lens I carry is 14mm


Marco

[Vegaman_Dan]

A comparison of the current generic K40 lens assembly to that of Light Object's offering:



The lens included is only 12.5 mm. I disassembled the Light Objects lens holder fully and found how it can mount a lens size from 12 mm to 20 mm. I now know what I can order safely. For those interested, I've made up this graphic as well to help address any confusion.

[brian257]

I would recommend the 38.5mm focal length for engraving if you do any small text or fine detail. I have been getting much better results than with the 50.8mm. I have been doing a lot of .020" thick lexan engraving material and I use this lens for engraving and cutting.