I've got an Airco burning machine with a Westinghouse optical tracer and want to convert it to CNC too. Here's what I've found out so far.
The motors on these machines usually have tach feedback instead of the encoders needed to retrofit a diy-cnc control. Depending on the motor configuration, the tachs can be replaced with encoders, the motors can be replaced with servos and encoders, or the motors can be replaced with steppers. Some may have servos with encoders, and if so, thats the ideal starting point, but I think thats a rare thing. The motors on mine have the tachs sorta permanently attached to the back end of the motors, so I'm probably going to use some small steppers in their place.
You wouldn't be using the PC based control to feed signals to the existing "box" but rather the PC would replace the box, and, in conjunction with some inexpensive drives and a power supply, the PC now controls all the motion.
Here's most of what you need to do this, along with approximate prices.
PC running Windows 2000 or XP $500 new
Mach2 control software. $150, and a fully functional trial version is available for download free from
http://www.artofcnc.ca This software converts the pc to an all in software cnc control using step and direction signals to control up to 4 simultaneous axes of motion. If using plasma, mach2 has the ability to operate with a torch height control, and has plasma specific algorithms to control the motion thru corners where the lag of the cut causes beveling under normal conditions. The limits on the free trial version are in max operating speed and limit to about 1000 lines of code. The speed limit would prevent fast rapids but wouldnt affect normal cutting speeds at all. With the $150 you get free lifetime support and upgrades, downloadable from the above website.
breakout board about $125 this device optically isolates the outputs from the computer to the drives to prevent damage from back currents, etc and also has on-board solid state relays for activation of auxiliary functions such as spindle, coolant, plasma, oxygen valves, or whatever needs to be turned off and on.
2 geckodrive solid state drives. about $115 each for stepper drives and about $145 each for servo drives.
Power supply for the drives/motors probably $100 max since these are small motors.
Adding for incidentals like wiring, terminal boards, enclosure, etc the total will run less than $1500 even if both drive motors are replaced with new ones. As Ries pointed out previously, this amount would barely buy the box a commercial burning machine control come in.
One last thing.....several guys across a wide area (california to the uk) have worked together to produce this stuff, with the idea of it all integrating together easily. One guy in Michigan (i think) Tom Caudle, is an electronic engineer who makes a living running a CNC plasma table he built. He designed an electronic torch height control that works on arc voltage for plasma cutters. Tested it with a bunch of people for several months and put it on the market at about $500. When it was abvious Tom's control was going to be a real thing and not just talk, Art Fenerty, the developer of Mach2, added the ability to Mach2 so it can interface with and operate with the THC. The guy in the UK, Les Newell, is developing a super easy and functional cam program for sheet goods cutting, regardless of whether for router, flame, plasma, or whatever. Its in beta right now and he's encouraging folks to download and use it and provide feedback on any problems encountered. Its at
http://www.sheetcam.com If a person is familiar with ANY cam program, they'll be able to produce code with sheetcam in about 15 minutes after its installed. He's got provisions for automatic lead ins, kerf comp (ever notice the direction of travel on a tracer shifts the kerf offset from one side of the line to the other?), dwell times, and so on. Best of all, its free to everyone who gets in on the beta. If anyone downloads and uses it, you need to check back to the site once a week or so for updates, since he's continually tweaking the features and performance.
All of this stuff I've mentioned comes with one very important thing.......access to the very people who develop it. They're available on the Yahoo forums, as are several hundred users, to answer any question about anything that pops up. And, unlike a lot of the control and software types we're used to dealing with, if there's a problem these guys all want to know so they can fix it. None of the typical "no one else has had that problem" or "our equipment is perfect so it must be your fault" stuff that we've all heard before. They just ask what you were doing when it happened so they can try to replicate and fix the problem.
I know things like this aren't for everyone, but its to the point where you don't have to be any electronic whiz to put one of these systems together. I'm in the zero to minus area of knowledge when it comes to electronics, but I've done one of these conversions on a stepper Bridgeport. Used the original motors and power supply but new microstep drives. Spent around $1300 including a new PC. Probably had less than 40 hours actual working time in the whole thing. It will now do helical interpolation and continuous contouring where before it was exact stop at the end of each block and only 2 axis circular interp. I plan to add a VFD, which Mach2 will also control, and then it will do thread milling and rigid tapping. As of right now, I figure I've saved in the neighborhood of $6K versus a package retrofit, and it's been a long time since I made 6 thou in 40 hours any other way. It wasn't difficult to do at all.