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Home Built 40W CNC Laser

Basis Research on Engraver/Cutters

I learned all about how lasers work from Sam's Laser FAQ. It confirms my hunch that CO2 lasers are the way to go. The Chinese laser tubes on eBay seem to be the cheapest solution. I will start with a 35W tube and matched power supply. There appears to be very little documentation on these things, but a few success stories on some forums. Here are some good links.

Laser Tubes

General Specifications (for reference only ... compiled from multiple sources)

Specification

20W 25W 35W

40W

50W

60W

80W

100W

Triggering voltage(KV)

13
15-19
20

22

22

24

28

35

Operating voltage(KV)

6.5
9-11
15

15

16

16

18

20

Operating  current(mA)

16
15-22
18

18

20

22

25

30

Outer  diameter(mm)

50
50
50

50

52

55

60

60

Length ( mm)

600
730
700-780

700

1000

1200

1600

1800

Beam Dia(mm)
5.5
7
Divergence (mrad)

 

 

Power Supply. The power supplies appear to be controlled with PWM (20-50kHz) or a potentiometer. I found a PDF that has some details. I will start out with pot control then move to something electronically controllable. Mach 3 can do PWM but probably too slow for the power supply. A custom PWM source or an electronic pot will be investigated.

laser power supply connector schematic

Optics. For this build I am going to need mirrors, mirror holders, a collimating lens and a lens holder. I have seen two basic types of the holders. There are the cheaper silver ones and the more expensive but better looking black ones from ColeTECH. There are a lot of options on the mirror materials. I will probably go with the cheapest one. The lens holder from ColeTECH appears to have a air nozzle on it. I was unable to get a better drawing from them. It cost a lot so I think I can make a decent one myself. The lenses are all made out of ZnSe (Zinc Selenide) It seems like a pretty nasty material. Use gloves.

I have been researching the optical calculations and made this a javascript page to calculate the laser optics. There are a lot of unknowns that will be measured later, these are just place holders for now.

 

Aperture Dia. 5.5 mm
Divergence 2.5 mrad
Beam length (after tube) 2 m
Beam dia at lens 10.5 mm
Focal length 55 mm
Perfect Spot dia 0.07 mm
Depth of Field 1.25 mm

 

laser mirror mountlaser lens holderlaser mirror mountlaser lens holder

Thorlabs

Sources

Linear Slides. As opposed to a router or mill this needs to be fast and light if decent engraving speed is to be achieved. I will target about 1000/min in the x-axis (horizontal) and at least 200 in the y-axis. I have some v-wheels from VXB bearings that look appropriate. They are a little expensive, but I have them already. McMaster-Carr sells some v track fairly cheaply. There are plastic versions that would work if you are starting from scratch.

 

Frame. The extruded frame from Misumi looks like it will work. It is a lot cheaper than I ever imagined. I am sure there are cheaper methods, but this will allow a lot of flexibility to tweak the system later. I looked at 80/20, but didn't think they would work well with the v track because they have a larger radius on the corners and this does not locate the v track consistently.

 

Motion Control. I will built my own using the A3977 chip from Allegro Microsystems. Any cheap 2 axis controller would work though. I have some 269oz nema 34 sized steppers I got from Xylotex. They are way overkill, but I had them. I will use Mach3 to control it. They have a plugin for a laser engraving. It looks a little rough. I may rewrite it later. I will use a belt drive system from Stock Drive. I did some basic calculations on this Google spreadsheet.

 

Z-Axis/Table. There are two options here. I could move the lens up and down or move that table up and down. Most engravers move the table up and down. I think this will also make the moving parts a lot lighter. I want the lightness for engraving speed so I plan to go that route. I will motorize the table for easy moving. I will put a stepper on it to allow CNC control of it. I have some aluminum honeycomb material I will try as the bed.

 

Table Height Setting: I think I will start with a gauge rod. This is a rod that is the correct distance off the work piece to a fixed point in relation to the lens. This may be able to be build into the laser lens car assembly for easy usage. A limit switch could help automate it. I also have a 2 red laser idea described below in the Aiming Laser section.

 

Mach3 Control. Control of the laser via Mach will be research and implemented when the system is ready. Here are some ideas I will look at.

Linux EMC2 Control. This will be investigated and tested.

 

Costs. Costs are being added to the drawings and are being tracked on this google spreadsheet.

 

Design. The design is being done in Pro/Engineer Wildfire 4. I can post Pro/E, DXF, PDF and STEP files when they are done.

 

Aiming Laser. The ideal solution is a coaxial red laser. This could be done by rotating the red laser or a mirror into the beam path. This needs to be very precise so some sort of fine tunable stop points need to be used. Another idea is to use two red lasers aimed from different locations that cross at the aiming point. this would also help with the Z level setting. you see two points until you get to the correct height and then the single spot is aim point.

 

Safety Window: First, I plan to wear safety glasses at all times. I have heard some cheap chinese laser engravers just use colored plastic. The better systems have a window with the a rating. I think I will experiment with some acrylic and glass to see how IR opaque they are and may go that route. According to Wikipedia and several other sites with basically the same text, Acrylic (PMMA) essentially blocks IR from 2800 to 25000 nm. CO2 laser work at 10600nm so we should be OK. The Synrad safety sheet also suggests Acrylic is acceptable.

The other issue is how bright of a light the cutting/engraving gives off. Depending on what you cut, it can be brighter than the tip of a welding torch. With something like paper or cardboard, you a get reasonably bright yellow light, but engraving something like marble or glass gives off an intense pure white light in a tiny spot. It appears that it could hurt your eyes. This may be the reason some commercial machines have tinted windows.

Cooling Pump / Sensor

Assist Air / Nozzle

 

 

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