[owhite]

I have a table I'm building that is going to cut sheet metal. It doesnt work yet, but it will use a 100 watt CO2 laser. (Please resist the temptation to post that 100 watters dont cut sheet metal -- they do).

My question is about the platform/support that will hold the sheet metal.

The original top on my CNC table was a 1/2 thick plate of aluminum. I bolted down 24 disc shaped neodymium magnets on this plate with a 4-40 socket head bolts. The idea behind these things is that they'll supply some magnetic force on the sheet metal to hold the sheet in place. The main thing I like about the system is that it is very level. This is important because the "sweet spot" of the beam waist is rather small, and changes in the Z-height of the metal will could result in cases where the beam will not pierce the metal.

The set up as it is now works fine in that they hold the sheet metal quite nicely. The bolt head projects about 1/8th of an inch above the magnet but because its made of steel it transmits enough magnetic force up the post to adhere the sheet metal.

The question I got is if this system will work. One thing I'm wondering is if the beam hitting the aluminum plate will put off undersireable fumes. The other question I have is if there should be more distance between the part and the support underneath. The gap between the sheet metal and the aluminum is around 3/8ths of an inch. I think it'd also be better if I had a grate underneath the part. That would definitely make for better ventilation -- I could put a vacuum system below the part to pull off fumes. Comments are welcome.

This is the table:
http://nilno.com/laser_dir/mag_table_003.jpg

These are examples of the magnets:
http://nilno.com/laser_dir/mag_table_004.jpg

This is a part held on the table.
http://nilno.com/laser_dir/mag_table_005.jpg

owen

[stevelang]

I can't help you on this but would appreciate if you can provide some info on the laser cutting part of your project

[Romos V.]

Owen,

<<(Please resist the temptation to post that 100 watters dont cut sheet metal -- they do). >>
Actually, they don't....

There are 4 kinds of Co2 laser.

1) Continous wave (100w laser of this kind, delivers 100w maximum)
2) Gated pulse (100w laser of this kind, delivers 100w maximum)
3) Superpulse (100w laser of this kind, delivers 100w X 2.5 times)
4) Hyperpulse (100w laser of this kind, delivers 100w X 2.5 times)

G-100 is a Superpulse laser and it has 250w Peak effective power.

Now, about your table... It will be better if gap between the sheet metal and the aluminum is more than 3/8ths of an inch. If this gap is small, the reflected heat will bend the stainless steel sheet.

Now, about your calculations...
(At 1.6mm, if I have a 3x beam expander I get 4.8 mm, which will be 103
micron using a 1.5 inch focal length)
(equation: diameter = .013 * M^2 * (fl/D) where M^2 is equal to 1, and
D is diameter of incoming beam.)

Where did you found 1.6mm?

Here is a table for G-100.

Distance From Laser (mm) vs Beam Diameter (mm)
0 mm distance = 1.9 mm beam diameter
250 mm distance = 2.9 mm beam diameter
500 mm distance = 4.7 mm beam diameter
750 mm distance = 6.7 mm beam diameter
1000 mm distance = 8.7 mm beam diameter
1500 mm distance = 12.9 mm beam diameter
2000 mm distance = 17.2 mm beam diameter

In my case, the focal lens from the laser dinstace is 500 mm.
So, without any beam expander I have...(assume that M^2 = 1.5)

diameter = .013 * 1.5 * (38.1/4.7) = 0.158mm


Romos

[owhite]

Owen,

<<(Please resist the temptation to post that 100 watters dont cut sheet metal -- they do). >>
Actually, they don't....

There are 4 kinds of Co2 laser.

1) Continous wave (100w laser of this kind, delivers 100w maximum)
2) Gated pulse (100w laser of this kind, delivers 100w maximum)
3) Superpulse (100w laser of this kind, delivers 100w X 2.5 times)
4) Hyperpulse (100w laser of this kind, delivers 100w X 2.5 times)

G-100 is a Superpulse laser and it has 250w Peak effective power.

Now, about your table... It will be better if gap between the sheet metal and the aluminum is more than 3/8ths of an inch. If this gap is small, the reflected heat will bend the stainless steel sheet....

Romos,

excellent information, as always. Regarding the reflected heat -- I could put the magnets up higher on 2.5 cm posts, and still use the aluminum plate below.

Do you think that a larger air gap above the plate will be okay or will it still reflect heat and bend the metal?

I remember that you use rows of angle iron that looked like LLLLLL. I can create something like that. I'm trying to decide if using the plate as a support is okay, or if I should make something that will be more complicated.

Owen

[Romos V.]

Owen,

it will still reflect heat but bigger gap will be better. Not only for the heat. When gap is small, small pieces from the melted metal will hit the aluminium plate, and they will go on the back of the metal sheet.

Romos

[owhite]

Romos,

thank you for your comments on my calculations. The distance to my beam expander is 33cm, so using your chart the beam size will be about 3.5mm when it goes into the expander. The beam expander is 3 times the original size so the beam will go to 10.5mm.

Based on the equation:

diameter = .013 * 1.5 * (38.1/10.5) = 0.071mm

This is a great spot size. The problem will be my depth of field. This is based on the formulas shown on this site:

http://www.parallax-tech.com/faq.htm

Depth of field is the distance range that an object can be placed in front of the lens and still get cut. The forumula for depth of field is

DOF = 2.5 x wavelength x ( focal_length / beam_diameter )^2

for our laser it calculates to:

DOF = 0.027 * (focal_length / beam_diameter)^2

I made a chart that gives examples of our two situations.

The first row is what happens with your laser. You have a focal length of 38 mm, beam diameter of 4.7, you get 158 micron spot size, and 1.8 mm depth of field. My current situation I have a 10.5 beam diamter, a 71 micron spot, and a small small depth of field of less than a milimeter.

I tried to other variations. One would be if I had no beam expansion. My beam diameter would be around 3.5 (close to yours) and a slightly larger spot size. This might work, but it also may not be enough power density to cut metal.

The last row of the chart is where I use a focal length of 4 inches with the beam expander. If I do that, I get a spot size of 190 micron, and 2.5mm depth of field.

I know that since I may be cutting other materials I will want to have more than one focusing lens anyway, so maybe its time to look into purchasing them.

owen

[owhite]

Owen,

it will still reflect heat but bigger gap will be better. Not only for the heat. When gap is small, small pieces from the melted metal will hit the aluminium plate, and they will go on the back of the metal sheet.

Romos

Okay based on your comment I think I'll raise the height of the magnets for now and run a fan to blow air away from the cutting. I want to get something set up just so I can do some testing before making the most perfect and complete solution. One of the things that's frustrating is that I will be able to make a honeycomb support -- once I can cut sheet metal. So I want to get it working so it can start bulding parts to complete the project.

owen

[sendkeys]

hiya owhite

i recognize your servo's
If you dont mind me asking some questions
How do they run?, power?, heat? what voltage did you go with?
What controller did you use? , working good??

Looks like you're running the sevos 1.1 ratio. how is that working out?

THanks

[owhite]

hiya owhite

i recognize your servo's
If you dont mind me asking some questions
How do they run?, power?, heat? what voltage did you go with?
What controller did you use? , working good??

Looks like you're running the sevos 1.1 ratio. how is that working out?

THanks

Here are some specs on the motors:
http://nilno.com/laser_dir/2003_12_0...28957759165454

here are some calculations for its power supply:
http://nilno.com/laser_dir/2003_12_0...48459727349689

This is a thread on cnczone that was about changes I made to the toroid:
http://www.cnczone.com/forums/showthread.php?t=2924

The motor drivers are gecko G320s, the motor controller board is called a Universal Stepper Controller, found at:

http://pico-systems.com/univstep.html

I like the UCS, the geckos, and the motors just fine. I'm told by Mariss, grandfather of gecko motor drivers, that they are oversized for my application. The problem with oversized motors if somehow the encoder were to fail, and the motor just ran at full speed, you damage everything. Now since nothing ever breaks in CNC work, the encoders couldnt possibly fail on me >>insert big eye-roll here<<

owen

[sendkeys]

Thanks for the links and info

[owhite]

So I made a new rack for the top of the CNC table. Its made of 3/4 inch aluminum bar stock. Its held together with threaded rod. Aluminum tubes were cut 1 3/4 inch for spacers.

There are neodymium ring magnets attached with small bolts in on each bar. The neodymium rings came from ebay. If you search on neodymium you'll get a million hits. Nice stuff, but you cant machine it, so I purchased the ring variety for easy bolting.

owen

[Romos V.]

Owen,

I made some testing for you, and I took some pictures.

Case 1 (I don't have beam expander)

lens distance from laser: 500mm
focal length: 1.5"
Material: Stainless steel 2mm

Samples are 2 and 3cm height.

There is no buildup of material on the back of the cut

[Romos V.]

Case 2 (I don't have beam expander)

lens distance from laser: 500mm
focal length: 2.5"
Material: Stainless steel 1mm, and acrylic 10mm

Samples are 2 and 3cm height.

There is some buildup of material on the back of the SS.


Romos

[owhite]

really nice information. thanks. the quality of the edge of the ss metal is amazing. I bought some extra optics so I can try focal lengths of 1.5 inch and 3.75 inch. right now I am installing the gas lines. I have a question -- do you have a filter on the oxygen line before it goes into the cutting head?

thanks again,
owen

[ViperTX]

Haven't thought about lasers in a long time....How much to get started with the 100W and what is the cost of the replacement lenses....how are these protected from splatter? Thanks, Paul

[cncadmin]

You may need compressed air to push the melting metal through like a plasma cutter.

[owhite]

You may need compressed air to push the melting metal through like a plasma cutter.

I have both oxygen and nitrogen on hand and I tried 'em both.

o.

[Romos V.]

Owen,

2mm distance between metal sheet and nozzle is ok.
Try with 3 bar oxygen pressure.
Adjust the distance between nozzle and focusing spot (not nozzle and metal) at 3mm. (look the attachment)

In the picture you can see 3 cases.

Case 1...the focusing spot is under the metal. (wrong case)
Case 2...the focusing spot is above the metal. (wrong case)
Case 3...focusing spot is in the metal. (correct)

[owhite]

Romos,

I think I have it at the right distance. In the picture I have attached, it shows a test wher I used thermal sensitive paper at different lengths away from the nozzle. The numbers that I wrote are based on an adjusting system on the side of the cutting head and are in 1/100ths of an inch. (They dont just really reflect the actual distance from the lens to the part, just how much the head is moved from a relative position of zero).

What the paper shows is very similar to your picture where first I started a little bit too far away from the paper, and moved in closer, and then around 625 it was too close. It looks like from this experiment that the best distance was around 650-700, which is what I used.

Still no cut, which is frustrating, but I will also try changing the beam width going into the focusing lens, and a different lens with a focal length of 3.75 inch. I forget right now what the beam size is when it goes into the lens but I think its larger than yours, which might be part of the problem.

owen

[Romos V.]

Owen,

Are you sure that your beam is centered?
If not, a part of the beam will reflect in the nozzle creating a secondary beam.
You can see it if you make short shootings on a clear acrylic.

It will look like the pattern in the picture.