Constructing Janus, by Dirk

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Suggestions for a new lens

Postby dirktheeng » Wed Sep 07, 2011 1:57 am

All,

Since I scratched my lens, I have to look for a new one. I can go back and get the same lens, but I was thinking that it would be good to get a longer focal length lens. A longer focal length has a few advantages:

1) it is more forgiving with focus
2) It is possible to generate a thiner kerf
3) It keeps edges more square
4) it keeps the work farther from the lens and reduces changces of geting smoke on the lens

Does anybody have a longer focus lens? What do you think if you do have one? I'm having trouble finding a lens that is coated to reduce reflection losses at 1045nm like the ones we have now.

Any thoughts or recommendations?
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Re: Constructing Janus, by Dirk

Postby bdring » Wed Sep 07, 2011 2:07 am

Technically you should never touch ZnSe with your bare hands. It is allegedly poisonous.
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Re: Constructing Janus, by Dirk

Postby dirktheeng » Wed Sep 07, 2011 2:25 am

bdring wrote:Technically you should never touch ZnSe with your bare hands. It is allegedly poisonous.


MSDS says toxic if swollowed or inhaled... skin absorption is not a problem... just wash your hands and you'll be fine. Also, the toxisity levels require a lot of it. Main concern is swollowing it as it reacts with strong acids to form H2Se, which is toxic. Don't put it in your mouth or lick it. Main concerns would be powder forms too not solid pieces. Both Zn and Se are nutrients you need in small quantities. This is a safe chemical for the most part.

Just for a comparison, water is listed as a skin and eye iratant on MSDS's. They tend to be very conservative.
Last edited by dirktheeng on Wed Sep 07, 2011 2:30 am, edited 1 time in total.
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Re: Constructing Janus, by Dirk

Postby bdring » Wed Sep 07, 2011 2:27 am

FYI: Longer focal length equals larger spot size.
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Re: Constructing Janus, by Dirk

Postby dirktheeng » Wed Sep 07, 2011 2:56 am

bdring wrote:FYI: Longer focal length equals larger spot size.


Do you mean at the same distance from the lens or at the equivalent optimal focus?
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Re: Constructing Janus, by Dirk

Postby dirktheeng » Wed Sep 07, 2011 3:01 am

nevermind... I had the ratio mixed up in my mind...

do=2fn/D
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Window Vent

Postby dirktheeng » Sat Sep 10, 2011 8:22 pm

All,

I designed the removable window vent. I also received my fan. I'll probably use my laser without the fan to cut the plywood. There isn't much to cut and I'll put a fan in the window to help exhaust in the meantime.

Here's the design:

LaserWindowVentAssmbly.jpg
Window vent


I have expandable sides on each side so I can keep the fan centered above the outlet. there is a ridge in the window that I can sneak a 1/2" wood frame into. I'll double up 1/4 to make it fit. I have some adhesive backed foam door sealer that i can put around the edge to seal it. Then I just put the window down on top to hold it.

Here's a pic of the fan I got:

DSCN4173.JPG
Exhaust Fan


Here's the fan in front of the window that it will be exhausting through:

DSCN4174.JPG
Fan in front of window


Now i'm going to run to lowes to before the football traffic gets out.
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Got the table installed

Postby dirktheeng » Sun Sep 11, 2011 1:44 am

In preparation for making the window venttalation system, I figured that I should get the table installed. It worked out well. I got an expanded aluminum flat plate from mcmaster and the diamond pattern matched up very well with the table frame.

Here's a picture:

DSCN4179.JPG
Table top
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Cut Quality problems

Postby dirktheeng » Mon Sep 12, 2011 2:12 pm

All,

I set up the laser to cut some of the plywood again yesterday, and I am getting dissapointing results! I am getting a lot of charing and I'm having trouble cutting all the way through (it just barely makes it through in a few spots with the 1/4" stuff). I have a feeling that it is due to the scratched lens. I haven't gotten a replacement yet.

I also think that the plywood from lowes is a problem. It has a core in that looks like balsa or fiber board. After doing some reading online, I think that this may be a big part of the problem. Plywood, apparently, is not a very easy thing to get right and the wood (especially the glue) have a huge role in how well it cuts. My understanding is that it should cut without producing enough charring that it leaves black marks on your hands when you pick it up. It may look black, but it doesn't leave your hands black when you touch it. It is also apparently very hard to produce edges that look just browned and not black, especially with stuff thicker than 1/8".

The charing with the stuff from lowes was bad enought that it noticably seems to change the dimensions of the part, especially at corners.

I also tried acrylic. It seems to cut much better than plywood. However, the edge isn't as nice as I had hoped for. It is a bit wavey. It's almost like the laser power isn't consistant as it cuts and the kerf width is changing or maybe there is a vibration in the optical system somehow (I tried turning off the air assist with no change in results). I guess I thought it would look like glass after being cut. Also, the power/speed of the cut seems to play a big roll in how square the edge is.

Also, I need help finding the right focal depth.

Can people give me some thoughts and/or advice?
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Re: Cut Quality problems

Postby twehr » Mon Sep 12, 2011 2:42 pm

dirktheeng wrote:All,

I set up the laser to cut some of the plywood again yesterday, and I am getting dissapointing results! I am getting a lot of charing and I'm having trouble cutting all the way through (it just barely makes it through in a few spots with the 1/4" stuff). I have a feeling that it is due to the scratched lens. I haven't gotten a replacement yet.

I also think that the plywood from lowes is a problem. It has a core in that looks like balsa or fiber board. After doing some reading online, I think that this may be a big part of the problem. Plywood, apparently, is not a very easy thing to get right and the wood (especially the glue) have a huge role in how well it cuts. My understanding is that it should cut without producing enough charring that it leaves black marks on your hands when you pick it up. It may look black, but it doesn't leave your hands black when you touch it. It is also apparently very hard to produce edges that look just browned and not black, especially with stuff thicker than 1/8".

The charing with the stuff from lowes was bad enought that it noticably seems to change the dimensions of the part, especially at corners.

I also tried acrylic. It seems to cut much better than plywood. However, the edge isn't as nice as I had hoped for. It is a bit wavey. It's almost like the laser power isn't consistant as it cuts and the kerf width is changing or maybe there is a vibration in the optical system somehow (I tried turning off the air assist with no change in results). I guess I thought it would look like glass after being cut. Also, the power/speed of the cut seems to play a big roll in how square the edge is.

Also, I need help finding the right focal depth.

Can people give me some thoughts and/or advice?


Plywood cutting success is pretty much dependent upon the glue used to hold the plies together. In general, construction grade plywood has a dark glue that is nearly impossible to cut with the power levels we have available to us. Craft plywood, on the other hand, is made of Baltic Birch with a white or clear colored glue that cuts like butter. Yes, you are going to get black/dark brown edges, but it cuts nicely. If you are using 40 watt, you still probably will not cut 1/4" in a single pass, but 1/8" will work at about 3-5 mm/sec. I run 5 for raw wood and 3 for wood that has been stained or has a hand-rubbed poly finish.

As for the focal depth - if you are referring to getting the right distance for your material from the lens, do a search of the forum for "ramp test". I think you will find help there. Ramp tests are the best way to get it right.
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