changing lens on China laser engraver

[Steve Verran]

Hi
I purchased a 2" focal length lens as replacement for my 11/2" lens that had cracked. I thought I could just replace the lens - reassemble and fire up the laser - this didn't happen - there is absolute minimum laser power flowing the head - the only way to get any laser scan to work is to maximise power - lower speed - this just results in burn. The pulse is operating as it should. Can I do any adjustments to ensure that this longer focal length lens will operate - does the "head" have to be adjusted to account for the longer focal length - prior to this the laser was working fine except for the loss of power due to the cracked lens - I now wish I had kept the old broken lens in - has anyone any suggestions please

[Bill Carruthers]

Hi
I purchased a 2" focal length lens as replacement for my 11/2" lens that had cracked. I thought I could just replace the lens - reassemble and fire up the laser - this didn't happen - there is absolute minimum laser power flowing the head - the only way to get any laser scan to work is to maximise power - lower speed - this just results in burn. The pulse is operating as it should. Can I do any adjustments to ensure that this longer focal length lens will operate - does the "head" have to be adjusted to account for the longer focal length - prior to this the laser was working fine except for the loss of power due to the cracked lens - I now wish I had kept the old broken lens in - has anyone any suggestions please

You will have to adjust your focus distance - -there are numerous posts here telling how to measure and adjust your lens focus distance.

[David Somers]

Steve,

Bill is spot on. You have changed focal length lenses in the same tube. You went from a 1.5" focal length lens, meaning the point of focus for the laser beam is roughly 1.5" away from the lens, to one that has a focal point 2" away from the lens. If you are using the same focus gauge that you used on the 1.5" lens you are now roughly 1/2" out of focus which is huge in the laser world.

Here is a link to a thread on Sawmill that discusses a few approaches. Do a search as Bill suggested and you will find a number of threads discussing this and you can choose the technique that suits you.
http://www.sawmillcreek.org/showthread.php?223720-Focus-Focus-Focus&highlight=determine+focus

Keep in mind that with any of these tests you are NOT determining the focal length of the lens. Your goal is to determine how far the focal point is below the bottom of your lens cone. Because the lens cone is well below where the lens is held in the tube that distance will be considerably shorter than the focal length of the lens. But.....the number you get from any of these methods will give you a measurement you can use to make your new gauge.

Good luck!!!

[Steve Verran]

Thanks for the advice and suggestions - I have refitted the old broken lens and the laser is now working - I did notice that the new lens is "thicker" than the old lens - could this be causing problems with the laser beam - if so how does one find out the correct lens thickness for a particular machine - my machine is China Laser 50watt - please excuse my lack of knowledge

[Bert Kemp]

Steve as mention above the issue is your lens is out of focus. once you find the correct focal distance it will work just find. follow the link Dave posted and read how to find the right focus distance for any lens. Its pretty easy and then your new lens will work just fine:)

[Glen Monaghan]

For starters, I would assume putting a 2" lens into a holder meant for 1.5" lenses would require you to focus approximately 1/2" further away out than you previously did. Try setting the focus as you previously did, then lowering the bed another 1/2 inch (or raise the tube 1/2 inch, or however you adjust the focus on that machine) and see if that is about right.

[Bill George]

And there is a right and wrong way to install the lens.

[Ian Johnson]

Also a longer lens will have a bigger spot, giving less power density. I don't know if there is a big enough change in your lenses to make enough of a difference. I switched my 60w from a 50mm lens to 100mm to see if I could get straighter cuts in 1/4" melamine, and found that it couldn't make it through the bottom layer no matter how carefully I focused it.