Distance between air assist and target

Anything to do with air assist, vacuum tables, fume extraction and related...

Distance between air assist and target

Postby buildsomething » Wed Jan 20, 2010 3:44 pm

I am just in the process of designing my focusing lens assembly and I was looking for some guidance as to how far I should keep the end of the focusing assembly/air assist away from the part that I will be cutting? My focusing assembly/air assiste will obviously hold the lens and will have an air going coaxially around the beam and out toward the target. Should be bottom of my assemlby be say .100" away from target or .25" or further? How much clearance should I have between the beam width and diam of the exit hole of the assembly?

What are problems associated with it being too far away or just too close? Is a large air assist hole better than a smaller one? I've never done this so any quidance would be helpfull.

I plan to have the ability to move the focusing assembly up and down manually. The focal length of my lens is 2.5".

Richard
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Re: Distance between air assist and target

Postby bdring » Wed Jan 20, 2010 4:54 pm

I have no idea what is best, but here are my design thoughts when I made mine.

I had the same questions. I wondered about the internal design as well. I wanted the air to exit like a jet of air and not diverge much until it bounced of the work piece. Ideally you want the length of the final tube leading to the outside to stay a consistant diameter for about 4-6 times the diameter. In other words if the diameter is 1/8", the length of the final tube should be at least 1/2". I worried about mirror alignment sensitiviy too. You don't want the beam to contact the sides of the tube. The beam is converging so it could be a little fat at the top.

My nozzel can adjust up and down a little, but I like to keep it at the upper end so there is about 1/2" between the tip and the work piece. I think mine is really only good for keeping the lens free of smoke and keeping the beam clear of smoke to the work piece. I don't think it does anything inside the cut. The cut is so darn thin, you are lucky to see light through it, so I doubt any air gets there.

I made my fitting on the rear of the enclosure a quick change. I thought I might want to have the option of a special gas to keep the flames down when cutting thin stuff. I have not tried it yet with anything but air.

Attached is the drawing of my nozzle. I actually made the inside a little different with an extra step down and a slightly longer final tube.

nozzle.JPG
nozzle.JPG (15.78 KiB) Viewed 15410 times
Attachments
nozzle.pdf
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Re: Distance between air assist and target

Postby buildsomething » Wed Jan 27, 2010 10:15 pm

Well here is my focusing assembly that I just finished making. The lens that I am using is smaller than most but seems to work ok on my test bench. The lens is held in place on a machined surface on one side and an "O" ring on the other. There seems to enough pressure via the "O" ring to hold the lens in place. I've seen this technique used in other applications and I hope it works out well here...only time will tell I suppose. The lens and "O" ring are not shown in these pics.

I have about 5/8" of travel that I can adjust. The air assist is going through the side and will be plumbed up using standard silicon fuel line tubing using in model aircraft fuel lines. Internal diam of the air assist exit is .125" and the beam is focused at .25" below that. A fiend of mine who cuts commercially has a gap of about .25" between the end of the nozzle and the material being cut.

The overal length of the nozzle is 2.625" long with a diam of .9375"

Replacement of the lens is quite easy and if I wanted to use a 1" lens, then it just a matter of machining up a new nozzle. I just had to use the lenses that I had.

The ass'y is a bit of boiler plate design, but it won't break.... :lol:

The project is getting closer to getting finished.

Richard
Attachments
Focus Ass'y.jpg
Focus ass'y and air assist
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Focus Ass'y 1.jpg
Top View
Focus Ass'y 1.jpg (46.81 KiB) Viewed 15360 times
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Re: Distance between air assist and target

Postby bdring » Thu Jan 28, 2010 2:35 pm

I like it. it looks beefy.

Is the oring held in by friction alone? If so, you should test it with your assist air running full power. The pressure might try to move the lens.

One thing I don't like right now with my air assist is that it comes from a diaphram pump that has no reservoir this causes pulsing in the line. Even my bigger, air tool, compressor has some pulsing. I have to make sure my lens is tight or it would probably loosen.

:!: Safety note: Depending who you ask, the lens material is calssified from toxic to highly toxic. Avoid touching it with your bare hands and definately wash them immediately if you do. There are several MSDS sheets on the Internet regarding this. Some are quite scary.
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Re: Distance between air assist and target

Postby buildsomething » Thu Jan 28, 2010 3:28 pm

The "O" ring is held in place by friction alone. That being said, depending upon how much pressure will be built up and if there is lift off, then I would machine a slight recess inside the tube to "snap" the "O" ring in place. I haven't measured the back pressure going through the nozzle, but I can do that a bit later. My little pump can deliver up to 30+ psi when it fully restricted, but personally don't think that the back pressure will be of any significance....but we shall see.

At present, I don't have a resevoir inline, but I do have one that I found in my junk box that I can use if need be. What would be the final effect on the product of a pulsing air assist? I can see if the final ass'y is flimsy, then a pulsating blast of air going through a restriction "could" cause some vibration....but in my case.......nothing moves... :lol: Boiler plate design. :ugeek:

It does take a lot longer to design and build a robust system, but what the heck....its only a hobby... :D

If I was to do this at a commerical level and put on my engineering hat, I don't think that I would design the system like the far east boys do....cheap and easy, but rather a bit more robust and solid. Its all a matter of cost and what one can afford. Since we are in the throw away and discard society, cheap and easy is the way to go...........not for me!!! I like to build it once and rarely touch it again...but thats just me.....I guess the 30 years spent at IBM is starting to
show :lol: Better resale value as well if you build it solid. :D . Well thats my story and I'm sticking to it :lol:

Back to the basement for me.

Richard
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Re: Distance between air assist and target

Postby lasersafe1 » Sun Feb 14, 2010 5:15 pm

Well after a couple days of playing with the machine I'm seeing a problem with the air assist. I have a design that is very similar to the "nozzle.jpg" posted by Bdring. I have about 1/8" clearance from the bottom of my nozzle to the focal point. This makes it easy to eyeball the appropriate standoff distance. The problem occurs when engraving or cutting wood with a high sap content. The sap spits back up toward the nozzle. The air flow keeps it from going all the way up to hit the lens, but it does stick to the lower part of the nozzle and solidifies. Eventually the sap builds on itself to make it large enough to actually touch the wood where it will either rub or break off.

I am going to make a couple of changes. First I will shorten the nozzle by another 1/8". Second I will make the end of the nozzle into a point shape so the sap will not see a flat bottom. I guess there may be some advanced polymer that this could be machined from that is non-stick?? Nylon, Frelon, Delrin ??

When I rebuild the ULS, I may not use any air assist at all. I'm not sure yet. One thing I will do is modify it to accept a window beneath the lens. I just bought a lot of 10 windows for $15 each from ebay. A window will completely protect the lens and is much cheaper to replace. The guy I bought from just told me that he has many more if anyone is interested. He also sells lenses and other CO2 stuff. http://shop.ebay.com/guyovad/m.html
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Re: Distance between air assist and target

Postby bdring » Sun Feb 14, 2010 6:12 pm

My distance from tip to work is about 0.45". My tip is darkened and sticky, but no real buildup yet.
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