[Clockwork]

I'm getting close to starting in the construction of a machine. The workpiece will sit horizontal on a moving X-Y table under a 50W Chinese CO2 laser tube pointing straight down at the work - no mirrors. Should be able to get the distance between the tube and top of workpiece to as little as 1 inch.

Do I still need a focusing lens at this close distance?

[zax15uk]

Yes, you will still need to focus the beam. Depending on your application I would suggest a minimum 2" FL, and you will probably need air/gas assist to prevent the lens/laser from contaminants.

Does the XY table move up/down for different materials or are you planning to move the whole laser assembly?

Do I still need a focusing lens at this close distance?

[Clockwork]

I thought that was the answer - just looking to have one less assembly to build.

The laser will be mounted on a fixed horizontal gantry and a stepper controlled linear actuator that will provide 7 inches of vertical movement.

I'm building a enclosure for the tube with a couple of small equipment fans blowing in. The enclosure will have a positive air pressure with air blowing out the hole where the laser exits - this should keep smoke away from the tube.

I also have a air assist scheme using a 30psi compressor designed for airbrush applications.

[draekar]

If I understand you correctly, you are planning to mount the laser tube vertical to the table surface? Some of the things you might consider is, are you going to water cool the tube? Most chinese tubes of that size need water cooling. If not the tube wont last long. I can envision the tube mounted verticaly but I have a couple questions. What kind of speed do you expect out of the X-Y table. Laser cutting and/or engraving usualy involves some speed or you run the risk of igniting or melting the material being processed. The point is, if the table is moving at a fast rate and you add the weight of the table to the weight of the material being processed, the enertia from the table changing directions could cause alot of vibration which could cause problems with the tube. Just a thought.
As for a focus lense, yes you will need one to focus the beam to a smaller diameter to concentrate the power. Kinda like moving a magnifying glass up and down in the sunlight to fry ants when we were kids.
Hope this helps

[zax15uk]

It sounds like he is planning to use an X/Y table under the static laser, so inertia or vibration shouldn't be a problem if the laser is mechanically isolated.

If I understand you correctly, you are planning to mount the laser tube vertical to the table surface?

[DaveDoesIT]

Hi Clockwork,

I'd strongly suggest you rethink the mechanics. One of the beauties of the laser is that all that needs to move is the optics and the carriage carrying one axis.

This means that structure can be very light and move very fast. Check out this vid and the laser speed.

[ame="http://www.youtube.com/watch?v=7pQnhB5Jink"]YouTube - Laser Engraved Guitar[/ame]

I have built several lasers from scratch and it makes for much more simple mechanics as you only need to move 2 mirrors and one lens. My current laser uses steppers and is slow by comparison to some modern ones, but even so, it can travel at 16" per second.

The dot size you are burning will be around 0.005" diameter and you will need to overlap 0.0025" to get full coverage. That's 160,000 dots per square inch.

There is a poster on this board who has some vids on a laser he built. Search for mononeuron and I think he is down under in Aussie.

[MonoNeuron]

Hi Clockwork (orange?....couldn't help myself)
I think it is actually easier to build the gantry with a couple of moving mirrors.

If you are going to encase the whole thing and blow some fans inside to use a positive air pressure for the case then you will need a big exit hole for the air to exit otherwise the fans will cavitate and you will have no real air flow to speak of. Try putting a piece of newspaper over a fan with a small hole the size of your thumb in it and see how much air comes out. Not much. But I guess you have already figured that out anyway. If the tube isn't moving then there won't be any real problem with it except getting the water to flow in the right direction. (heat rises etc....)

I tried the small compressor for air brushes and unless you have a hole below the lens that is about 0.010" in size you will get too rapid a drop in pressure and it will be of no benefit. You get pressure but very little volume from them and that is what you also need, not just pressure. Try a high volume electromagnetic aquarium air pump. Much better but you will still only get a few pounds of pressure from it but it will last for years.

If you have too much pressure then you risk actually blowing the material away you are trying to cut and even if you hold it down with several magnets then the forced air pressure will get under the job and make it flap about like a fish out of water.

If you have a moving table then you are restricting yourself to light and small (not tall) objects because you have to move the whole job around with the table as well. If all you want to cut is paper and cardboard then all will be fine but if you want to engrave a 25lb slab of granite then you will have to reduce your speed to overcome the inertia of the whole job plus the table weight and then you will have an all night job on your hands which will get very boring VERY quickly.
All the manufacturers build XY tables and they have the resources to invest in research and development so I think they got it right years ago. I always say "don't reinvent the wheel" but the final choice is up to you so don't forget to take pictures or/and video of the build. They're always interesting for young and old.
Rich.

[MonoNeuron]

Oh, I forgot,
If you don;t use a focusing lens then you greatly reduce your cutting effectiveness. The lens increases the power rating from watts to hundreds or killo-watts at the focal point plus you also have the benefit of a very reduced kerf width of cut instead of the laser output beam size of 4-5mm which isn't good for anything except burning your sweater and charring paper.
The longer the focal length the bigger the spot size will be. A 2 inch focal length lens will give you about 0.006" spot size and a 1.5" focal length will give you about 0.003" size. (If I can remember the sizes correctly but you get the idea).
But the longer focal length will give you a deeper cut size. It's a question of balance and I guess the average size used is about 2" focus length and it's also what I use. I have a 3" one as a spare but I don't think I will need it just yet. A 1.5" one would be more useful for engraving due to the smaller spot size and will give you much better engraving resolution and slightly more power.
Light is inversely proportional to the square of the distance which works backwards for a lens giving you the increased power at the focal point. Not something I am up to speed with but I just remember the formula from school all those years ago. :-)
Rich.
Rich.