I Just Ordered A Lot More Stuff: I just placed my second order with LightObject.com, now that I've had time over the last two days to consider a lot of the laser design and the features I want on my laser.
SKU Description Quantity Price
ECNC-2M542 2 Phase 4.5A 1-axis Stepping Motor Driver. Support to Nema 16~34. Leetro replacement 3 $135.00
ESW-5MLMT-C1 A pack of 5 Micro Limit Sensor Switch C1 1 $5.00
EPM-3D100MAG 3 digit Mini Green LED 100mA Current Meter for CO2 laser machine 1 $11.50
LSR-HVCBL High Voltage HV Cable for CO2 Laser Power Supply 5 $4.25
EWP-SNR104 Water-flow On/Off Sensor (G1/2) 1 $12.50
EW-W01 Water Level Sensor 1 $8.50
LSR-R5MW 5mW 650nm Red Laser with adjustable focus 2 $11.50
EWP-D50C2440T High volume 35L/m (565GPH) DC24V Brushless Water/ Oil/ Gasoline pump 1 $42.50
WC-WAT240 240mm Water Evaporator/ Cooler for spindle or CO2 Laser Cooling 1 $32.50
EFAN-FFB1212EHE Ultra Strong 12cm DC12V Cooling fan. 200CFM 2 $33.00
The second order cost $317.06. This pretty much completes my LightObject.com shopping list.
The LinuxCNC Control PC: I picked up two 19" LCD monitors from a guy on Craig's List yesterday for $33 each. Tonight, I bought two Compaq EVO tower PCs from a different Craig's List seller, for $60 each. He let me test them with the LinuxCNC live CD, which verified that the CD works and is bootable, the video hardware autodetects and won't be a problem, and I ran the Latency Test from the CD (Menu-CNC-Latency Test) to make sure the PC can run the realtime Linux kernel fast enough for CNC control. A Max Jitter under 20,000 ns is good. These PCs clocked in at about 11,000, which should be very good. Don't assume that a fast PC will work. LinuxCNC doesn't need a fast PC, but it needs a PC that allows LinuxCNC to be in full control. Many PCs with fast processors have hardware interrupts that put the main processor on hold while something else takes precedence, such as a video card interrupting to access system RAM. You need to do the latency test. Sometimes, various hardware can be disabled to restore realtime control to the processor, but it's usually easier to pick a PC that "just works".
One PC and LCD from the recent Craig's List purchases is going to be used on my CNC laser, and the other is for a future CNC milling machine conversion. I installed Ubuntu and LinuxCNC together from the live CD (easy three click install) and made sure the PCs boot and seem OK. I'm currently running a thorough memory test to make sure there are no mysterious intermittent problems reading and writing all of those bits that could send a CNC machine into lala land. Three hours and five complete passes of the memory test with zero errors. So far, so good. I'll leave the memory test running all night for a pseudo burn-in.
Here's a list of known good PCs if you'd like to take the guesswork out of shopping for a LinuxCNC PC:
http://wiki.linuxcnc.org/cgi-bin/wiki.pl?Latency-TestCooling System Calculations: I spent a little time today designing the laser cooling system. Real calculations! (iPod Touch calculator app while on the throne) In rough numbers, the waste heat from an 80W CO2 laser tube is about 800 watts, which is about 200 cal/second. You can type "800 watts in cal/s" into Google and it'll do the unit conversions for you. Knowing that a calorie is a degree C increase for a gram of liquid water, I calculated that If I used ten gallons of water, I'd probably get two hours of continuous lasing before it overheated at 75 degrees C. I'm planning on using pet safe water/antifreeze mix, aka RV antifreeze, so I assumed it'd have a specific heat less than the 1 cal/C of water. That two hour number was tempting, but the 2'X2' laser cutting area will allow me to run some continuous 4-5 hour full power laser jobs, and even for jobs that are easier to interrupt, I tend to work for 4-5 hours at a time. I could have used 20-30 gallons of water, but I decided to use less water (2-5 gallons) and add a radiator and fans for active forced convection cooling. I even entertained the idea of plumbing the aluminum extrusion frame and running the coolant through the inner cavity, and using the frame as a big passive radiator, but that seemed "too clever by half" and the $70 for a radiator and a couple of professional grade fans smelled like the right way to do that job. I'm cheap, but I don't want to spend a lot of money and have an extra cheese result because I tried to skimp another 10% on cost. I want a nice laser that's a good value, and not the cheapest thing I can build and still call it a laser.
Next Up: I need to take another look at the control electronics. For this fairly straight forward XYZ stepper configuration with minimal I/O, I can probably get by with having LinuxCNC direct drive the optically isolated stepper drivers directly from the parallel printer port, but if I want the extras like a digital display of coolant temperature on the PC, I'd need some analog inputs, and that will require MESA Electronics (or another manufacturer, but I've pretty much standardized on MESA for these home brew CNC projects). I think I'll just buy a little stand alone digital thermometer for the front panel, next to the stand alone laser current meter, and avoid the cost and implementation time of the MESA Electronics cards. I'll have the controls hardwired to shut off the laser if the door is open, or the coolant isn't flowing, or the coolant level is too low, or the coolant temperature is too high. I'll have laser reverse engraved fault indicators on the front panel so the fault is explained in English.
The bare minimal PC I/O now looks like this:
Outputs: STEPX
DIRX
STEPY
DIRY
STEPZ
DIRZ
MOTION ENABLE
LASER ENABLE
Inputs: LASER ENABLE (Sum of coolant flowing, coolant not overheated, etc.)
X+ LIMIT SWITCH
X- LIMIT SWITCH
Y+ LIMIT SWITCH
Y- LIMIT SWITCH
Z+ LIMIT SWITCH
Z- LIMIT SWITCH
I might need to either combine the +/- limit switches on each axis and assume that motion in the positive direction activated the positive limit switch and negative direction activated the negative limit switch, or I may need to get a $79 7i43P interface board from Mesa Electronics.
The I/O count is minimal because many of the fault signals are combined into one LASER ENABLE input and the laser is disabled in hardware without LinuxCNC being in the decision loop. Philosophically, I hate hardwired control logic for anything but an E-Stop circuit, but I'm having trouble justifying the cost and time to configure the MESA Electronics board. Besides, the various fault interlocks are sort of like an E-Stop circuit. It's nice to have the fault directly disable the laser to avoid a situation that could be hazardous to people or property, without relying on the PC to be running properly to halt the motion and laser.
If I wanted to get fancy, I could run the various fault signals into LinuxCNC as separate signals and I could make a custom Axis (my preferred LinuxCNC GUI front end interface) control screen that displayed the faults on the monitor, but I think I'll just go old school and have red LEDs light up fault status indicators on the reverse laser engraved front panel, and leave the PC for designing parts in CAD or a graphics program, and monitoring & controlling the job progress in LinuxCNC... and watching YouTube videos while keeping an eye on otherwise unattended laser operation for those long and boring jobs.
Here's a list of supported hardware for LinuxCNC that can be a big help in deciding what interface hardware to buy:
http://wiki.linuxcnc.org/cgi-bin/wiki.pl?LinuxCNC_Supported_HardwareThe above I/O list is subject to change as I still need to research how LinuxCNC should modulate the laser power for cutting and engraving. I think it can use PWM, but I'm not sure if the LASER ENABLE signal is modulated, or a separate LASER PWM POWER signal. I have read enough to know that it's a bit of a trick to fool LinuxCNC into being a vector laser cutter, and it's a much bigger trick to fool LinuxCNC into being a raster laser engraver. Relatively few people have used LinuxCNC to control a desktop laser. If I have a simple and useful implementation, I'll see if it can be documented and added to the growing list of LinuxCNC canned configurations available at startup so it'll be much easier to use LinuxCNC for a stepper motor driven laser engraver. I'll make YouTube videos and a webpage, as well as this build log.
Here's
Everything You Always Wanted To Know About LinuxCNC But Were Afraid To Ask:
http://wiki.linuxcnc.org/cgi-bin/wiki.plMy Clever But Not Original Targeting/Focus Laser Idea: The two 5mW red lasers that I just bought will be mounted on opposite sides of the laser head and aimed at the focal point of the 80W IR laser, so I can see where the cutting and engraving will occur when the lid is raised for a practice alignment, and it'll also serve as a quick focusing tool for the IR laser. The two red lasers converge on the IR laser focal point from 45 degree angles. When the material to be cut is at the IR laser focal point, the two red lasers will form a single dot. Above or below the IR laser focal point, the two red lasers diverge into two red dots. I thought that was clever, but saw where someone else on BuildLog.net was already doing this. Oh well, it's still clever, even if someone else thought of it before me.
To Do List, For Tomorrow and Beyond: More research about LinuxCNC control of lasers. Research DraftSight 2D CAD software for Linux. There is supposedly a one click install for Ubuntu, so I'll install it on the laser control PC along with GIMP for creating and manipulating images. Decide on how LinuxCNC will control the laser cutter (motors and I/O), draw some preliminary schematics of the various accessories (pumps, etc.) and interlocks (E-stop, door closed, low coolant level, etc.) Specify and purchase the air assist pump and the exhaust fan. Create a bill of materials, including vendor, part number, and purchase date, and post it online, and place a link to it in this thread. Start doodling on the frame ideas that are bouncing around in my brain pan.
After the order I just placed, I'll have another big order for the laser tube and power supply, and another big order for the as-yet-undesigned Misumi aluminum extrusion frame and lid, an order for the skin panels for the frame, orders for the laser bed, the air assist pump and the exhaust blower, an order or two for whatever I decide to use for the Z axis height adjustment, and several small McMaster-Carr orders for miscellaneous plumbing and wiring and hardware as those issues arise. But so far, the design and the parts ordering is progressing well, although this obsession is starting to cut into my other projects and I'm currently very sleep deprived. I'm also a bit concerned that, based on other projects, there will be weeks of finalizing the design, with daily $50 McMaster-Carr orders. Those add up, and will probably endanger my $3,500 target cost.
Preliminary Z Axis Musings: For the Z axis, I'm actually considering four inexpensive NEMA 17 stepper motors wired together, driving four acme screws in the four corners of the bed, with four zero height adjustment screws in the centers of the table edges at the absolute Z bottom position, just after the Z- limit switch is reached (disable the Z- limit switch to level the bed by bottoming out on the hard stops). I wouldn't design it that way for a commercial product, but the conceptual simplicity of four motors and four acme screws greatly reduces the design time and the fabrication needed to implement the powered Z stage. It'd be about as easy as any manual Z stage adjustment I could design for a one-off prototype, and the Z axis is only adjusted before the job starts so the Z axis motion hardware should last forever. Basically, I'm trading the cost of four stepper motors (about $60) against more clever and complicated Z axis parts that take longer to design and install.
The bed will probably consist of an outer framework of 40mm X 40mm Misumi aluminum extrusion, with a narrow inner shelf secured with T nuts in the frame slots. The shelf will support the table bed laser cutting grid (
http://www.cuttinggrid.com/lazergrids.shtml). The top of the grid will be a couple of mm below the outer frame, so it'll serve as an alignment edge. I'll almost certainly need the frame to have some 20mm X 20mm supports underneath to keep the grid from sagging. The grid will be a fairly tight fit in the outer frame, but will lift out for replacement. I need to devise some sort of crumb tray underneath that can be cleaned easily, although most of my jobs don't generate little pieces dropping through the grid. Occasionally, I cut small holes in acrylic, and those do fall through the laser bed. I probably wont make a pull out crumb drawer. If the grid is stiff enough, I'll probably just pull it up and out of the frame so I can vacuum out the debris.
A lot of the design will happen as I order the critical path parts, get them in hand, and see how they feel. That will determine how they'll be used and how the design should accommodate them.