4 inch vs. 2 inch lens

[Brian Robison]

Hi Gang, can some one tell me the down side of using a 4 inch lens?

[matthew knott]

Hi Gang, can some one tell me the down side of using a 4 inch lens?

In a 'nut shell' a bigger spot size , a 4 inch lens wont focus down to a smaller spot as a 2 inch, this means less power density and engraving detail not being as fine, on the plus side you get a greater depth of field meaning the focus is not as critical.
4 inch can be useful for certain things like cutting thicker material or engraving something without a uniform height but its a compromise.

[Scott Shepherd]

4 inch can be useful for certain things like cutting thicker material

Really? I've found the 4" horrible for cutting. Especially acrylic. The spot size is so large and the air is so far away when using a through the nose cone air system that it either catches everything on fire or blisters the backside of acrylic horribly, which, if the paper is on, will also start a fire really easily. Can you give some examples of what you would cut with it that you think it does a better job on? I'm only asking for reference. Maybe I just haven't tried the right material yet, but so far, my experiences with cutting using a 4" lens haven't ended well, not a single time.

[Brian Robison]

maybe there is another solution to my issue. On the Rabbit laser, the nose cone is too close to
the material I'm engraving and seems very sensitive about going out of focus.

[Kevin L. Waldron]

Brian,

4" is all we use on our large machine. Love it....... spot size in acrylic is really not critical for basic template work......... possibly photo engraving...... even then I'd probably use a 3"....... seldom if ever do we use a 2" or the HPDFO lens.......don't like them that close to the acrlyic.

Scott,

We have a Universal laser and we cut acrylic almost exclusively.... something in your setting are not right......... ( usually set density at 4 try 400 dpi for marking... speed at 70 and power is 28% and.... 800 dpi for cutting 35 speed and 70% power this will cut 3/16" with out problems we have two sets of tubes and those settings are for the 120 watt set... two 60watts we also have two 75 watt tubes.... and power or speed is reduced/increased accordingly ) We do use a cone and you have to make sure it's sized for the lens..... air pressure around 45psi and sometimes in rear cases we run nitrogen gas if we don't want a flame.

Blessings,

Kevin

[Scott Shepherd]

Scott,

We have a Universal laser and we cut acrylic almost exclusively.... something in your setting are not right......... ( usually set density at 4 try 400 dpi for marking... speed at 70 and power is 28% and.... 800 dpi for cutting 35 speed and 70% power this will cut 3/16" with out problems we have two sets of tubes and those settings are for the 120 watt set... two 60watts we also have two 75 watt tubes.... and power or speed is reduced/increased accordingly ) We do use a cone and you have to make sure it's sized for the lens..... air pressure around 45psi and sometimes in rear cases we run nitrogen gas if we don't want a flame.

Blessings,

Kevin

We don't have it for the Universal, but for the Trotec, it puts the head 4" or so from the work and the nose cone for that is short and fat, so any air you have doesn't even come close to reaching the cut. I would imagine with air assist on it, that's closer to the work, it might work, but to my knowledge, they don't offer any long nose cones. I'd much rather vector cut with my 2". I don't know of any advantage to vector cutting through material with a 4" lens, other than thick material where it gives you a wider sweet spot on the cut. But for 3/16", I can't see any reason to use a 4" over a 2". What benefit would there be for thin materials?

[Brian Robison]

I think the lens I have is 55 mm, I see there is a 75 mm, this may be a good compromise for me.
I'm also using the little air pump, maybe I need to use the compressor when I'm cutting? No way
I'm getting 45psi out of the little pump.
Oh, and howdy neighbor!

[Dave Sheldrake]

A 4" lens will have a straighter focal point for a bigger distance but as Matt said, a lower power density.

It's a constant battle on HX type machines getting clearance Vs air assist efficiency (not that it is any fault of the machine just one of those annoying problems)

There is a modification (that costs under $1 and is reversible) that works on Rabbit type machines, if you drop me an email I'll send the pictures over :)

cheers

Dave

[Kevin L. Waldron]

The advantage of the 4" lens for us is that the material is further away from the head, belts, bearings, wheels etc. Don't know about the rest of you but there is a certain amount of blow back from the flame and the plastic as it metals/cuts. When we were using the 2 inch lens our traveling exhaust is almost on top of the material and several times we have actually caused a fire to start because of the additional vacuum/exhaust air velocity over the area. Longer lens everything is further away and the bellows effect is not as great. Other reason is the nature of the large Universal it has a tendency to pull dust into the head even though it has positive air pressure, when it's down close to the material it like a vacuum on the head pulling the dust in. We have 5 different focal length lens for the Universal but for overall performance/ ease of use we prefer the longer focal length of the 4" and our cones are not stock cones. They make and sell 256 different color crayons for a reason......
Blessings,

Kevin

[Scott Shepherd]

The advantage of the 4" lens for us is that the material is further away from the head, belts, bearings, wheels etc. Don't know about the rest of you but there is a certain amount of blow back from the flame and the plastic as it metals/cuts. When we were using the 2 inch lens our traveling exhaust is almost on top of the material and several times we have actually caused a fire to start because of the additional vacuum/exhaust air velocity over the area. Longer lens everything is further away and the bellows effect is not as great. Other reason is the nature of the large Universal it has a tendency to pull dust into the head even though it has positive air pressure, when it's down close to the material it like a vacuum on the head pulling the dust in. We have 5 different focal length lens for the Universal but for overall performance/ ease of use we prefer the longer focal length of the 4" and our cones are not stock cones. They make and sell 256 different color crayons for a reason......
Blessings,

Kevin

Thanks for the explanation Kevin, looks like it's not cut and dry, but rather machine specific. Our Universal lens vector cuts a lot of acrylic and I check the lens about once a week and it's almost always still clean (the 2.0 lens). So apparently there's more to it than just the lens size, but also has to do with exhaust flow, airflow in the cabinet, etc.

I have seen some machines that do exactly what you are describing, they billow up a lot of smoke and it comes high in the cabinet, which is bad, but we haven't had that issue on our machines, so I haven't seen the need to switch to the 4" to get rid of that issue because we don't have that issue.

Interesting though, thanks for explaining it. I learn something new every day :)