Hello. Im new to the site. I have a Nova 35 with a 100 watt laser tube. I’m having trouble getting the 4” lens to work. I can’t get a clean cut and the laser will not cut though material. I installed the larger air cone, increased the air flow, tried different focal lengths for 7mm to 13 mm. I have no issues with the 2” lens and the HR lens. Has anyone else had success with this lens cutting thicker material ? Thanks Richard
Check the orientation of the lens and be sure the outward curved side (convex) is facing up.
1 Like
tried different focal lengths for 7mm to 13 mm.
What do you mean? a 4" lens needs ~100mm from lens to surface.
What thickness of material are you trying to cut?
The beam cone needs to fit through the nozzle. If you are too close or to far away from the nozzle opening, you could be hitting the inside of it.
Are you sure you’re focused? Are you saying here that you’ve adjust the distance between nozzle and work over a range of 7 - 13 mm? That’s almost certainly way too close.
For a 4" lens, the focused distance will be with the lens about 4" above the work. That’s 101.6mm.
If the lens is 1" up from the bottom of the nozzle, then the nozzle will want to be about 3" above the work.
This also is a very good point.
My 101 lens is set 93.5mm from the bottom of the nozzle up in the tube to lens bottom and my space between nozzle and work piece is 7.5 mm I cut 3/4 acrylic cleanly
my space between nozzle and work piece is 7.5 mm I cut 3/4 acrylic cleanly
My laser head has the lens at the bottom, not the top, of the tube. A 63.5mm lens means about 2" between nozzle and surface. Not all heads are the same. 
I realize that but if hes lens tube does not have provisions for a 101mm lens he will need to buy the proper tube and nozzle I get mine from cloudray
Could you advise what type of tube you run in the head? As @Ram48 Mentioned there are some heads that have a lower and upper place to install the focal lenses, If you have a 101.6mm Lense in the lower spot its not going to work right with the cone on probably. Do a ramp test with all setups, I just installed the same FL lenses and have had impressive results for my application.
To help clarify, some lens holders are designed to mount lenses in either an upper portion (for longer Focal Lengths) or a lower portion (for shorter focal lengths), so that the convergence of the point occurs where intended and that the width of the beam (in the case of longer focal lengths) clears the air assist cone.
opening up / unscrewing the lower portion of the lens holder reveals:
If I place a 63mm lens in this lower section:
viewing into the upper portion of the lens holder you find:
If I place a 101.6mm lens in this upper section:
5 Likes
Great visual of what we are trying to explain 
Hi everyone. Thanks for the great response. To help clarify I had an Epilog Zing 24 - 40 watt for 10 years. I needed more power and a larger cutting area. I couldn’t afford to purchase a larger machine from Epilog so I recently purchased a Thunder Laser Nova 35 - 100 watt. I’ve been happy with the machine so far. The machine came with a 2” lens. I purchased a larger 4” lens to cut thicker materials like pallet wood. I also purchased a high definition lens. The only issue I’m having is with the 4” lens. The focal lens is in the bottom of the head assembly. I couldn’t get the beam to focus properly. I performed a ramped burn test On a piece of plywood to find the optimal focal point which seemed to be around 13 mm (nose to work piece). The factory setting is 9 -11 mm. The original nose cone had a very small opening so I used a larger one. I also hooked up the airline to my 20 gallon compressor. All this has helped, but laser beam is still cutting at least twice as wide as the cut on the 2” lens. It seems to me that I should have a better focused laser been. I now have dialed it in to cut a 5/8” piece of purpleheart. I’ll look at everyone’s suggestions and continue to work on it. Thank you for all of the suggestions. Richard
OK, so how far is it from the lens to the work piece? 4" ?
If it’s not somewhere in the vicinity of 4" then it’s not focused.
1 Like
Hi my understanding on this is that a long focus lens keeps the focus longer allowing for straighter edges but does not focus down to as small a minimum spot. This optic is appropriate for cutting thick foam but not thick wood. A 4” lens has a large spot size, when focused, which prevents it from being ideal for cutting wood and acrylic. However It’s ideal for engraving from a distance for irregular objects like the bowl below or for cutting thick, non dense foam like the gun case stuff.
I’m pretty sure I’m right when I say that it will not cut pallet wood.
Cheers
Sasquatch
You cannot have a 4” lens in the bottom holder unless you enlarge the air assist hole thus messing up your air assist.
1 Like
That’s not how it works in reality. I understand that’s what is always referenced in write ups about lens focal lengths, but I am telling you from experience its bogus. The point and detail I find to be just as good with my 101mm lenses as my 63mm lenses. I stopped swapping my pre-loaded lens holders out because it was wasting my time.
Oh, and I just use 101mm now…
1 Like
Yeah, this is certainly going to be a problem. Having a 4" lens that close to the nozzle means the nozzle needs a huge exit hole and also that the nozzle is probably 3" from the work piece.
What you want for cutting thick wood is a fairly small orifice located very close to the work so that the air velocity is very high to effectively clear smoke and debris from that deep kerf.
Not gonna get that with a nozzle that’s a furlong away from the work and has a sewer pipe sized opening…
1 Like
)I’m 100% certain what I saw with my own two eyes that the spot size is larger on a 4” lens vs a 1.5” lens.
Here is a calculator from some pencil 
necks that agree with me as well and they use that “new math”
Cheers
Sasquatch
I see the math states a 200 micron difference in spot size. I guess since I am not doing white tile Jarvis double Dutch jump roping art of elephant ivory contraband electric boogaloo, I cannot discern the 7 thousandths of an inch difference in the type of projects I am used to. 
But I still think that “new math” is BS. 200 Microns is 3 times the width of a human hair, and I am sure that is not the difference.
That’s it, I am putting that fancy fangled eye loop to the test!