Chinese Laser Focusing

[Brian Leavitt]

I lost the focusing tool for my Chinese Laser. I know you all use the "angled piece of Anodized aluminum" method of finding the focal point, but if I know I have a 2" focal length lens can I not simply measure 2" from the bottom of the lens to determine what size to make the new focusing tool?

[Gary Hair]

The short answer is no.

The longer answer is still no but for two reasons: first - because the lenses aren't ground exactly the same every time and the focus will be slightly different with every lens, second - because the focus point on the lens may not be on the bottom of the lens. You must do a ramp test to determine the focal point for your particular lens. Also - the shorter the lens the more critical the focus. ie. a 1.5" lens has a very short focal range compared to a 4" lens and if you are off 1/32" on a 1.5" lens it may mean the difference between sharp enrgaving and fuzzy, where it won't have much impact on the 4".

[Dave Sheldrake]

The focal point is from the upper side of the lens to the surface of the material not from the underside. As Gary said, they can also vary by +/- 1mm (more than the depth of field of a 50.8 / 2 inch lens)

[Ian Johnson]

You can do the ramp, or you can also get a thick scrap of wood and run the laser across the edge at a slow speed. That will often create a burn in the side of the wood in the shape of the beam so you can see where the focus point is.

[Kev Williams]

My "focus tools" are just pieces of scrap 1/4" thick Cintra or plex.

first do the ramp test Gary mentions. When you find the best focus spot, position the lens head directly over it. Now take some thin cardboard, and cut a piece about 1/4" wide and 2-1/2" long (or 1" if using a cone). Start snipping it shorter until it just fits between the piece you engraved and the lens HOUSING or CONE. Now cut yourself several pieces of scrap plastic the same length and mark them as your 'focus tools'.

I have 3 or 4 at all times because I loose them regularly. :)

[David Somers]

Kev,

Just checking. Is Cintra a mispelling of Sintra? If yes, isn't that a PVC material? When you cut it to size as a focus gauge you mean cut it mechanically and not with the laser due to its being PVC? I ask not for your sake....I know you know better, but for the benefit of others who may not be familiar with it?

[Bert Kemp]

Kev I super glue a small rare earth magnet to my focus tools and just stick em on the laser case works awesome
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My "focus tools" are just pieces of scrap 1/4" thick Cintra or plex.

first do the ramp test Gary mentions. When you find the best focus spot, position the lens head directly over it. Now take some thin cardboard, and cut a piece about 1/4" wide and 2-1/2" long (or 1" if using a cone). Start snipping it shorter until it just fits between the piece you engraved and the lens HOUSING or CONE. Now cut yourself several pieces of scrap plastic the same length and mark them as your 'focus tools'.

I have 3 or 4 at all times because I loose them regularly. :)

[Kev Williams]

Sintra then if I spelled it wrong. Can't remember the last time I actually saw the word...

And I saw it with a safety saw. The only reason I use "it" is because I've had tons of scraps of the stuff here since 1991 after doing some sign work for BTS.

I forget that most people here aren't 'tool' engravers, so like Dave says, DON'T laser cut the stuff!

In addition to my 'stub' focus tools, I have a few longer pieces that are sized to fit between the work and the gantry. I use those for focusing to tiny things in clamps (like the ends of 1/4" rod stock) so I don't have to get the lens in just the right place, all I have to do is slew the Y to get the whole gantry anywhere over the part. Easy peasy...

[David Somers]

That is a cool idea Kev! Good thinking!!!

[Brian Leavitt]

Thanks, guys - even though you killed my dreams. :D

I did the ramp test last night and made a new focusing tool. Afterwards, I had to open up the back of the laser so I removed the junk that was sitting on top of it and... found the original focus tool... :rolleyes:

Where I determined the best focus spot to be was actually .062" longer than the original tool, which I found odd.

[Kev Williams]

not odd at all--

My original LS900 tool is a 3/4" diameter piece of delrin. I had to stick a piece of 1/32" plastic to it... :)

[David Somers]

Brian,

Like Kev I am not surprised. Those lenses are not high quality optics. They are likely to vary enough from production run to run to cause that kind of variance. And every lense tube probably has a variance in the position of the lens in relation to the bottom of the cone as well. And I sincerely doubt that the laser company is performing a ramp test on each machine they make and creating a custom focus gauge to match it. So I would always be tempted to do a ramp test anytime you work with a new lens. Now that you have done it you see it doesnt take much time, and there is a clear benefit in now having an accurate gauge compared to what was provided to you.

[Bill Carruthers]

Brian,

Like Kev I am not surprised. Those lenses are not high quality optics. They are likely to vary enough from production run to run to cause that kind of variance. And every lense tube probably has a variance in the position of the lens in relation to the bottom of the cone as well. And I sincerely doubt that the laser company is performing a ramp test on each machine they make and creating a custom focus gauge to match it. So I would always be tempted to do a ramp test anytime you work with a new lens. Now that you have done it you see it doesnt take much time, and there is a clear benefit in now having an accurate gauge compared to what was provided to you.

Dave - just thought I might share what I find useful in a focus tool - Having two machines and regularly changing lenses in both of them, and constantly losing the focus tools, I decided to make my own and came up with the type of thing pictured - you can cut them out of any scrap (one is from an old credit card) and I knock up about 3 or 4 at a time which might last me for a year or 2. By making them up to 20 mm I can focus both +/- 50mm and 38mm lenses in each machine and I can measure pretty accurately how much I might take something out of focus (when necessary). I purposely made the shoter area in 1mm steps and the higer area in 2mm steps because I found that the longer lenses greater depth of field made the 2mm "close enough" for most of what I do. If I have to recreate something in between - I make a note that such and such a job works best at a "tight" fit 7mm or a "loose" 6mm. Just an FYI. Cheers, Bill


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[Brian Leavitt]

Yeah I know the lens and mirrors are junk. The mirrors are of the silver variety. I intend to replace them sooner rather than later. I have 20mm mirrors and an 18mm lens, and it looks like Lightobject has both of these. I'm also looking to replace the laser tube with an actual 60 watt, higher quality tube. I was trying to find the EFR 1200CL from a US distributor, but I have been unable to locate one. The only ones I can find in the US are SP tubes and I don't know anything about them.