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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.

Height adjustment.
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.

<p><strong><em>Congratulations</em>, Sir!</strong> Your perseverance is well rewarded.</p><p>Living next to the world's workshop (<strong>China</strong>), in <strong>VietNam</strong>, I opted for the cheaper solution of simply buying our laser cutters BUT you article triggered some D-I-Y thoughts I am going to explore.</p><p>I cannot emphasise safety too much. It is essential to <strong>wear goggles</strong> any time the laser is on as even reflected laser light can gave a deleterious effect on eyes and body! My employees being somewhat lackadaisical weren't using these goggles (<strong><em>only </em>USD$10</strong>) so I fitted very small temperature sensors that are connected to the laser controls to ensure goggles are worn before the power can be applied.</p><p>Your <strong>D-I-Y</strong> surgical endeavours can be prevented by adding two safety buttons, either side of the work space, with protective rings, to prevent cheating the switches, so at least you know where the fingers are!</p><p>We also use CCTV and an closed off area for our workshop, it's amazing the stupidity of some people around dangerous machinery.</p><p>And <strong>OXYGEN</strong>, around oil, can be very dangerous!</p>
<p>I love you! </p>
<p>luv u too</p>
<p>I already have a CNC setup for cutting wood. I would like to add a laser head. Am I right in understanding that the laser you're using in this instructable is $27k? Awesome writeup btw.</p>
I believe the link to &quot;this is a comprehensive diagram&quot; is broken.
where is the laser tube ?
ok, here comes a stupid question., -Why can't you just laser cut by hand-?
The machine would be much more accurate. Besides, how would you wield a 250 Watt Laser with 2500 Watt water coolant?
what if you had it mounted like this but with no motors and used like a bandsaw <br>
I agree, Thanks.
Congratulations, it's a great work and perseverance sample. I would like a CNC of those at home.
This is very cool. The kerf on my machine is 1/8&quot; running a CNC plasma torch. What is the kerf (width of cut) with your laser here?
the secret pro metal cutting laser 150W co2? <br> are the mirrors, lens, polarizer cut quality enhancer, or the collimator, or the laser tube?
Just a little bigger then we will need it in the military. <br />Psssssh Ahhhh wheres my wing!!!
Link near top - &quot;comprehensive diagram&quot; - is 404 Not Found... :-(<br><br>Great project - been thinking about building such a thing (after finding out that TechShop here in the Bay Area doesn't have a metal-capable laser cutter... :-(<br><br>(Hint, hint, Jim, Ridge, et al... &gt;;-)
Lovely project - but expensive.<br>And it doesn't have to be.<br>Less than $350 can get you 3 axes *with* controlling electronics and motors.<br>(see the reprap project for a great example, although with a moving head. a moving table is just as easy and maybe even cheaper though.)<br>You don't even need 3, but if you use 3 you could save on laser power and thus on laser cost a lot by using a reciprocating laser setup which adjusts focus during the cut.<br>A quick look on ebay gives me $1375 (that's with worldwide shipping) for a laser with power supply included.<br>Your transformer setup requires a large, bulky, line frequency transformer which gives you horrible regulation and high cost. A computer power supply or some other form of SMPS will give you much better regulation at a lower cost. The $350 figure already includes the power supply with the electronics though.<br>Cooling with a cheap pump and water is fine, so I'll say $100.<br>I have no clue about optics though, but the optics can't cost a lot more than the laser so I'm making that $1000. <br><br>laser: $6500 - $1375<br>CNC table: $ 500 - $ 350<br>Optics: $2500 - $1000<br>Cooling: $ 500 - $ 100<br>electronics:$5000 - $ 0 (driving electronics included in table price) <br>total: $15000 - $2825<br><br>Savings: $12 175<br><br>Or, if I am wrong about the optics and it's indeed $2500, the total is still only $4 325. Savings $10 675.<br>I hope this is of use to someone =)<br><br>I just realised that the $350 table might be a bit unrealistic for a laser cutter cutting steel sheets - but even a $1000 table still means significant savings.<br><br>Just my $10k ;)<br><br>--Nathan
Hi,<br><br>Do you have any sources to buy Laser Head, Optics..?<br><br>Thanks<br>Yatish
Hi,I am Praful here.I am setting up Laser cutting with Coherent 400W especially for sheet metal.Can you please let me know who can support me for laser head,optics etc.I found 1 laser head frm precitech but its too expensive (20k-Euro).Can you pl give me some inputs rgd this.
dear sir <br>I want to build a cnc laser for cutting stanless steel 6 mm <br>so I need electrical parts . like power suply400w- laser gun-and ets <br>if it is possible for you send me comlete lists of parts and price I.ll need to <br>building these machine. <br> <br>best regards
Really neat and creavtive. Another options, a plasma cutter to save much more money.
I think mr owhite made a mistake the energy density while making the calculations <br>the correct correct is 0.1643 watts/mm2 .
You might update your 'ible with the embedded pics. Apparently the links are dead.
really very nice, great job
dude that is sick awesome!! but way to expensive. its gonna take me forever to gather that much money.
I have a 5KW CO2 laser- when cutting stainless use Nitrogen (99.5% pure) and you'll have clean shiney edges on your steel :), also do you where any eye protection?! I really hope you do, as CO2 lasers emit radition that burns away your eyes lens without you feeling it.
what kind of eye protection do you recommend, would regular safety glasses/sunglasses work or would you need smoething stronger
Regular goggles won't do anything for the laser frequency... you need goggles that are designed just for that purpose... start here...<br/><br/><a rel="nofollow" href="http://cascadelaser.com/safetyeyewear.html">http://cascadelaser.com/safetyeyewear.html</a><br/><br/>Jerry<br/>
CO2 lasers emit a 10.6 micron wave. Regular safety glasses will work fine.
Most safety glasses filter UV. This is IR, 808nm (or 10.6micron, whatever you like).<br>And either way, I wouldn't bet my eyes on a cheap pair of safety glasses.<br>A $100 pair of laser goggles might seem expensive - until you lose your sight.<br>The $50 you saved won't get you back your eye.
808 nm doesn't equal 10.6 microns. That would be 10,600nm which is in the far IR band, and regular lexan blocks it effectively. A few sheets of plexglass would make an adequate enclosure. <br> <br>808 nm, on the other hand is in the visible range and will pass right through anything that visible light passes through.
It is ingenuous to think protection glasses are the definitive solving panacea in protecting your eyes from hundredsW/mm&sup2; and up, reflection fluences.<br><br>It is really important to have the laser cutter inside an enclosure or to precisely study the angles of possible reflections coming out to the observer to create and set protecting panels in the correct paths.<br><br>Most people isn't aware of this and handle lasers with absolutely insufficient care.<br><br>Be aware! You have only two eyes, no spare!
is your laser a diode or a miror laser?
CO2 based laser
Thus, a &quot;mirror&quot; laser.<br>Known to most as a gas laser.
Nice I'ble, but waaaay too expensive for me... Good write-up. But how do you get from 106 watts/in&sup2; to 1550 watts/mm&sup2;? The conversion of 1in&sup2; = 645mm&sup2; is correct, but that would give 164 mW/mm2. Real world check: 100W on a square inch, that's the heat of a strong (old school, non-green ;-) ) light bulb on the area of a bigger stamp. That would be warm, even hot, but not enough to melt steel. 1550W on a square milli-metre? That would be the energy of a bathroom heater on the surface of a ball point tip (give-or-take...). Yes, that might be enough to melt metal. By the way that would mean 1,55kW*645 or about 1MW per square inch!
There's this magical thing called lenses too..
You don't have to melt the steel in order to cut it. The laser ( or torch of say oxy-acetylene or propane-acetylene) only has to heat the surface to the point where the oxygen will start burning the steel which is well below the melting point.<br>The oxidized steel has a melting point of ~ 1450 C and runs off the steel which melts at a little over 1500 C.
the last one is my favorite :D<br /> <br />
What IS the last one anyway?
I believe he/she is referring to the &quot;Items I've Made&quot; section with the last photo being his finger cut by the laser...
Yes, I would have to agree with others. Very, very nice job on the device, but put a metal box around it when it's in use. Cheap insurance against instant blindness...
I think most of the media links on the intro page are broken...
Shame, the video is missing.<br><br>Great blog though, many thanks, Andy
I loved this concept. Would you just clarify the ownership cost?<br>Approximate would do it too.
This really isn't an instructable, but thanks for sharing. This is more like sharing a really complicated project, where no one was meant to follow without extra attention. :)

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