# CO2 laser that cuts sheet metal

## Step 4: Laser Optics

Sheet metal cutting requires a power density of 106 watts/in2 (source: Mike Klos @ laser mechanisms)

Converting to millimeters, that's 1550 watts/mm2. (using equation: 1in2 = 645mm2)

A 100 watt laser can achieve a power density of 1550 watts/mm2 in a spot size that is 0.6452mm2

A spot size that is 0.6452mm2 has a diameter of of .28mm or 280 micron (using area = pi * (d/2)2)

280 micron! If I can deliver 100 watts to a spot of 280 micron, I should be able to cut metal. That's too easy.

Why? Well, how big a diameter can I expect with my optics? The information on my beam diameter varies. I have read it goes anywhere from 1.6 to 2.3 mm.

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 * M2 * (fl/D) where M2 is equal to 1, and D is diameter of incoming beam. See this site.

If I substitute in an M2 of 1.5, I still get a diameter of 150 micron. So according to calculations I should be able to deliver a power density needed is 106 watts per square inch.

Note: I'd like hear from anyone who could verify that 106 watts per inch is the power density I need.

Note: the reason I bought the microscope was to be able to measure in micron -- hopefully I can use it to check my beam diameter

Romos gave me some excellent feedback on my post about beam sizes. He points out that the expected beam size can be taken from this table for the G100:

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 his case, the focal lens from the laser dinstace is 500 mm. So, without any beam expander I have...(assume that M2 = 1.5)

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

The distance to my beam expander is 33cm, so using that 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 the G100 laser it calculates to:

DOF = 0.027 * (focal_length / beam_diameter)2

The optics for the laser
The beam delivery system is composed of a bend mirror, a processing head, a cut quality enhancer and circular polarizer, and a beam expander. In order to attach the cut quality enhancer to the G-100 two adaptors were machined out of aluminum (1, 2). The cut quality enhancer improves the shape of the G-100 beam, and the circular polarizer prevents the beam from reflecting back into the laser head. Both parts came from Laser Mechanisms. The cutting head was manufactured by Haas LTI.

The principal of the cutting head is that the beam enters the top of the head and is directed to a focusing lens that is found in the center of the cutting head cavity. A focused beam exits through the bottom of the cutting head nozzle. Gas, such as oxygen, is fed into the side of the chamber below the focusing lens. This gas exits the nozzle along with the beam and the laser beam/oxygen combination serves to vaporize the steel for cutting.

The optics for the laser
The beam delivery system is composed of a bend mirror, a processing head, a cut quality enhancer and circular polarizer, and a beam expander. In order to attach the cut quality enhancer to the G-100 two adaptors were machined out of aluminum. The cut quality enhancer improves the shape of the G-100 beam, and the circular polarizer prevents the beam from reflecting back into the laser head. Both parts came from Laser Mechanisms. The cutting head was manufactured by Haas LTI.

The principal of the cutting head is that the beam enters the top of the head and is directed to a focusing lens that is found in the center of the cutting head cavity. A focused beam exits through the bottom of the cutting head nozzle. Gas, such as oxygen, is fed into the side of the chamber below the focusing lens. This gas exits the nozzle along with the beam and the laser beam/oxygen combination serves to vaporize the steel for cutting.

Alignment
Originally I thought this was going to be voodoo engineering because you cant see the beam of the laser. It turns out that its not that hard. First set up a system to mark circles or edges of your beam path with cross hairs in the center of scotch tape.

The place your targets on the beam path. If the item that gets the tape can be threaded into place it makes it easy to mount the target.

Using this system, I started with a target on the cut quality enhancer, and then moved on to the elbow that points the beam towards the floor. The elbow has allen head screws that allow you to microadjust the mirrors in the beam. This took a little while to figure out the impact of changing these screws and where the beam lands, so for a while I would take to shots on one piece of paper, and view the where the beam moved after making a change. After I got the hang of this, I went back to the targetting system to adjust the beam as best I could to be on center.

The cutting head has a nozzle on it with a port that is roughly half a millimeter in diameter. If the beam is not exactly on center, it gets reflected off the side when it comes out of the nozzle and forms a characteristic pattern that looks like this.

Another alignment method I used to cure this problem was to remove the nozzle, and shine a short pulse on thermally sensitive paper. The issue is to carefully adjust the beam so that it produces the same spot shape with and without the nozzle to ensure it is going directly through the port of the cutting head. A friend also recommended that acrylic works as an alternative to the thermal paper.

Here is a nice picture of laser beam height adjustment. The issue is that the beam forms a waist and the most power of the laser occurs at the minimum waist diameter. The sweet spot of the beam waist can be placed in path of the beam by adjusting the height of the cutting nozzle.

To find the best height for minimum beam diameter, I used the thermally sensitive paper and looked at the beam diameter as a function of height. The markings on the card are based on 100ths of an inch are relative; they do not reflect the actual distance of the focusing lens to the paper. What you can see from this experiment is that the beam size gets smaller down to a distance of 650ths of an inch and then starts to increase in size.

I would not claim that is a good method to determine the beam diameter. I dont know if there is a way to determine what the beam size is, however, it was still interesting to look at the spot under the 100x microscope.
This is a picture of my smallest possible spot on the thermal paper. The microscope was focussed on the stainless steel below the paper. You can see burnt edges around the hole. The burns are not a result of reflection as shown here, at least they dont occur like this repeatedly. It seems more like the results of a heat flare coming off of the beam.

This is the underside of 1/8th inch thick plywood which was cut at 10% power levels on the laser. The kerf width is roughly 200 micron as well.

Remove these ads by Signing Up
dirceu2 years ago
the secret pro metal cutting laser 150W co2?
are the mirrors, lens, polarizer cut quality enhancer, or the collimator, or the laser tube?