I’m looking to replace my 18mm 1.5" lens and mirrors but not sure what to go for, mostly use to etch glass and engrave/cut 4mm acrylic, would like to cut 4mm ply but not had much success with the setup I have, any thoughts would be gratefully recieved.
If your mirrors and your lens are intact and clean, then there is no need to change them for the tasks you are referring to.
When I cut 4mm plywood or 6mm acrylic without problems with my K40, you should be able to cut much better / faster with your 80 Watt machine.
Have you done a ramp test and found your correct focus point?
Hi Bernd
Yes, I set the laser up through all 3 mirrors and did a ramp test, about 2mm out from the acrylic tool sent with the machine, I removed the 1.5" lens and noticed some clouding on it, I know it is a relatively cheap Chinese machine and probably sub standard mirrors and lens, I though I would look to replace these with something of a better quality and possibly 2" focal lens.
Yes, it is understandable to swap with something better. But I just mean that there is something that does not fit with either your machine or your settings. If the mirrors and your lens are not very dirty or cracked, then the original lens should have a focal point of approx. 0.1 to 0.15 mm, maybe max. 0.2mm. it is fine enough for engraving fine details and cutting 4mm plywood at 1000 mm / min and something equivalent to approx. 75% of your max. power. My dataset for this material is 600 mm / min with 16 mA (40Watt tube!).
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… forgot to write that the mentioned laser focus point applies to a 1.5 “lens, a 2” lens gives a longer focus area but also a larger focus point. It is better for cutting but not better for engraving.
I can achieve a fine line when engraving just not a very good cut in 4mm plywood, I have tried several different settings in both speed and mm/sec, I will try 16mm/sec and 75% power and see what happens.
Make sure you do not exceed your max mA!
I’ll be carefull ;0)
I have tried lenses from 1.5" to 7" focal length. Cheap lenses from ebay, clouldray lenses and us made meniscus. I have even tweaked some with compound lenses.
Quite simply you have to change lenses to match what you are currently doing. If you are removing massive amount of material with little detail (ie large sign letters) you will want a .012 to .015 spot so you can move on. If you a ablating a .002 mask to reveal a .005 color layer you want a short focal length lens of good quallty. If you are cutting .5" thick material for packaging, load up a long focal and jack the power.
If you need fine detail, its going to be a meniscus/compound lense.
All lenses I have used seemed to be fine quality wise and the focal length is always Really close. There will always be a difference in the cutting/engraving quality when you raise/lower the focal distance, but with a few tests and experience you figure out how much and which direction to cheat it for better results. I would recommend testing up and down from your calibrated focal distance 1mm increments and cutting and engraving some test pcs over a 2-3mm range. You will quickly see if the results change.
Mirrors… Man them gold over silicon are sexy. And they pass some more power…Buuut…forget polshing them back up. that being said I’m running gold in one machine and moly in other machine and its like 1% difference… If you mirrors are damaged or bent or have scatter replace them with what you can afford. if you have the moly you can polish and retry.
I settled on 3 inch focal length meniscus. gives me about a .007 dot. cuts up to .5" foam .25" acrylic. and will engrave letters .03" tall. Plus I have about a .375 range of height I can engrave over without adjusting Z/U. And I have good clearance.under head to avoid fixtures and have travel to drop down up to .75" to engrave inside an object. This is red 60watt omtechs that are capped at 22ma…50w.
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Thanks Maccass< I need to read through your reply carefully as there is a lot of information that you have given freely that I need to understand, thank you for taking the time to reply, 
I agree with Maccass- you might need more than one lens so you can fit the lens to the job. I have bought the lens set from American Photonics that has an integrated cone so that the focus distance is the diameter of a AA battery for all of them. They use glued in lenses, but remove very easily for cleaning. If you get the whole kit, it may be more focal lengths than you need. Here is a link to the products. Realize that there are other tube diameters to fit the OMTech. I have 16mm tubes and 4 of these lenses. "Benny Kit" Retrofit your Boss(R) laser head to use our Alignment Tool – American Photonics
Dear friends, first of all Clive needs to find out why the original mirrors / lenses do not work properly and then he can “upgrade” with new mirrors and lenses.
Interesting info above. I also have a 60w OMT laser - blue. However, I was told via forum that it is really a 50w. My issue/question. I am told by OMT that these machines come with 2.5" lenses and can cut 1/4"/6mm. With that lens (aligned & lens & mirrors clean) I can cut 3mm baltic birch (plywood) 10mm/sec 25%/7ma- 25% - acrylic 10mm/sec 12ma- 45%.
Stock air supply (not much air). I have a horrible time cutting 6mm ply or 6mm acrylic. The acrylic requires several passes and sloppy results. The ply I at times can get a single pass cut but is is very charred and heavy smoke debris. Masking helps but still the heavy edge soot on both the 3mm and very heavy on the 6mm.
I decided to buy a 5" Ten-High18mm FL. 101.6 (5") lens. I installed it the the same tube that was supplied with the laser. Realizing there would be no air at cutting surface I proceeded with my focus test. 4" stilt to bed level over 16" resulting in apx 14deg angle. Proceeded to make several cuts on 3mm ply.
The above result showed the focal point apx mid way through the ‘cuts’. However, even using 6 & 8 mm/sec 90% 24ma (max output). On this machine 90 to 98% pwr all result in 24ma. Seldom ever go over 70% 21ma.
I never did cut through the ply. Some of the cuts were The bottom cut was 4 passes at above max pwr 6mm/sec. It still wouldn’t full cut that line. I can see some light through part of the cut.
With a 5" lens: I was expecting/hoping to clean cut 1/4" ply & acrylic; narrow kerf; a lot less edge soot.
Very disappointing so for.
Pssst… 101.6mm = 4"
ooooops. 4" That’s what happens when you get my age.
I went with a 4 inch fl lens II-VI in the original 12mm diameter. I rearranged the optics to go thru the lens first then the mirror (#3) and down to the work. I have the base K40 which probably started as a 30 or 35 watt. I measured a true 28 watts prior to the lens. Moly mirrors, have been running this way for 2 years or so. As others have said, the down side is a (slightly) bigger spot size, the up side is a nice depth of field cutting range. I cn engrave a fairly curved surface with little loss of sharpness, the limitation is the distortion of the image.
Thanks everyone for your help, very much appriciated, I have ordered a 1.5" meniscus lens to try, hopefully it will be the slightly cloudy lens I have in it at the moment which is causing the issues.
OK. I am going to beat a dead horse. Air, Air, Air. Air management is the key to beautiful and successful work.
If I am engraving information on stuff I run little to no air. And I really mean that. I just run enough air to keep smoke out of the lense cone. If you an feel the air coming out of the cone with your fingers it is enough to degrade the quality of your engraving.
Cutting…it takes a good bit of air and it has to be in the right place. When successfully cutting you will be amazed at how much air it takes. Use tissue paper, chalk, paper or whatever you have to to make sure you have a good solid stream of air on the cut
Also when cutting you have to adjust your focal point to be halfway into the material. If you are cutting .25 inch material. focus on table and then drop table .125" or 3.1mm this puts your focus .125 out at the top and bottom and will be the best you can do. iy you focus on the top, at the bottom of the cut you will be getting too far out of focus range.
OK. I am going to beat a dead horse
I understand, and so far have been using the provided air pump housed inside the laser, which just pushes a lower amount of air. If the case is smoke filled then that smoke is passed through the air tube past the lens. I need to get that issue taken care of.
During testing this new 4" (not 5") lens, and the lens being placed in the 2.5" lens tube there is 0 air at the material level. However, I did expect to cut and realized the result would be sooty etc.
I decided to do some more testing. Using a somewhat steep slope over a long distance I would get the focal point on the material. I was surprised to see no cut through running 95% power on 3mm baltic birch. The max depth of cut was 1.32 mm less than 1/2 of the ply. As best I could measure the kerf was 0.69 mm wide.
I though maybe I had problems with the output of the tube. I measured that. At pre-3rd lens I was measuring 52w, 24amp at 98%. Decided to test more… 75% (think safe zone) 21 amp, 48w, 50% 15amp 40w, 25% 7amp 26w, 15% 5amp 14w and 10% 4amp 4w. Not knowing much about all this I am pretty confident the power measured is getting down to the lens.
Unless air is that critical I am wondering if the problem is the lens itself.
Yes, the problem is the lens itself.
The problem is that although a 4" lens has a greater depth of focus, which is desirable for cutting thick material, the spot size is also substantially larger than say a 2" lens. And larger spot size = lower power density. The spot ain’t as hot.
So long lenses become a tradeoff between depth of focus and power density and I think for cutting a 4" lens on a 80w machine is kind of past the point of “maximum goodness” in that tradeoff.
I also have a 80W OMT and I also have disappointing results with a 4" lens. I get better results cutting with a 3" than with a 4".
And I’m a believer in lots of air and often run 30-40psi air with a nozzle that puts a good high velocity stream close in to the crack. A 4" for cutting still sucks with all that air.
For cutting I get better results with a 2-1/2 or 3" lens every time.
Love a 4" lens for engraving bricks…