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View Full Version : Plano convex lens vs meniscus lens



Erik Goetheer
03-03-2017, 3:37 AM
I did a focus ramp test yesterday with the installed 50 mm lens on my RF-9060 and found out that the optimal space between material and nozzle was 10 mm (it's not the focal lenght, but how do we call this space?). That surprised me because the focus tool for this lens is 8 mm, according the written instruction on a sticker on the lens tube. So I checked the lens today (and cleaned it) and I noticed that it is a meniscus lens and that convex side was towards the worktable, so down!! That surprised me too because I thought that it should be up. I checked the internet of course to get some assurance, but I found no clear answer. Some say convex up for a meniscus lens, some say down and some say that it hardly matters.

What is your opinion / expertise in this?

BTW, the 38 mm lens that also came with my machine is a plano convex lens. Don't know why the lens types differ.

Erik Goetheer
03-03-2017, 7:02 AM
Well, I re-installed the lens, this time convex up and I did another focus ramp test and this time the sweet spot appears to be 6.5 mm free space. That's quite a lot less than 10 mm, and still quite much less than the 8 mm focus tool. I checked of course if the lens is indeed the 50mm lens and it is.

I did a test with a small program, a keyhanger with engraving and compared it with the same program that I did yesterday (where the convex was down and focus distance 8 mm). And the new one is better, the engraving is just a bit sharper and the cut sides are smoothier and straight downwards. The difference is not big, but still. I also did the keyhanger with convex up (new situation) and 8 mm distance and this one is the worst. Engraving is the same, but the deeper half of the sides clearly show horizontal lines and the sides are no longer straight downwards. Again, we are talking micromilimeters, but hey, it has to be as good as possible.

So I guess I stick with the lens convex side up and distance between cone and material 6.5 mm.

John Lifer
03-03-2017, 7:56 AM
My 63mm and 40mm both came curve up. Nozzle lengths are different, but both end up at 6mm space. No telling about any other machine! Test as you did and go from there! And yes, everything I've seen has curve up

Kev Williams
03-03-2017, 10:13 AM
Curve should be up but they'll work both ways, and the focal point will be different in each case. I've found with my Triumph that with the 2" lens upside down it will one-pass cut clean thru 1/16" Rowmark with less power. Beyond that I'm not sure what happens because I haven't had the time or the want to experiment :)

As for advertised focus v actual focus, few of my actual's match the advertised. And 2 of the 3 factory focus tools I've used are off quite a bit, with the GCC's tool nearly dead to rights. I just consider a factory number as a roundabout guess.

As for ramp tests, I found when ramp testing a 3" lens in my LS900 a couple of weeks ago that what appeared to be the sweet spot distance on some anodized aluminum was over an 1/8" different from the best focus point I found when test engraving .050 tall characters on white Rowmark..

Erik Goetheer
03-03-2017, 12:41 PM
..... As for ramp tests, I found when ramp testing a 3" lens in my LS900 a couple of weeks ago that what appeared to be the sweet spot distance on some anodized aluminum was over an 1/8" different from the best focus point I found when test engraving .050 tall characters on white Rowmark..

Hm, that's strange. What could cause this?

Kev Williams
03-03-2017, 4:29 PM
Could've been mostly me- I have found in practice that my anodized aluminum likes to be about +.020" farther from focus than does Rowmark or Cermark. Not sure why, but it gets whiter when focused out a bit. I tested the 3" lens with anodized, and I probably misjudged what I thought was the sweet spot. Could be the ramp moved too..

Dave Sheldrake
03-03-2017, 6:48 PM
focal lens on lasers, especially Chinese lens's can be +/- 2.5mm either way of the focal length advertised

Erik Goetheer
03-04-2017, 2:33 AM
And that is why it is always a necessity to do a focus test. :)