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A Brief C.N.C. Retrofit Overview
First off, what is a CNC retro-fit? It is taking a manually operated machine, and as addressed here, primarily metal cutting, or wood cutting machine tool, and making some modifications and additions to them so that they work under computer control.
Why would I want to do that? It enables you to make duplicate parts without having to hand crank the dials for each one, which are virtually the same, barring changes in temperature and tool wear. It allows you to machine complex features without having to grind custom form tools, use taper attachments or tailstock offset, or finicky tracer attachments which require a copy template anyway. It allows you to do helical interpolation, like putting threads on a boss with a milling machine, and other complex 3D contour milling and engraving operations, such as bah relief.
Isn't this a difficult and costly project? Well, a little over twenty years ago, I built my first 8080A S-100 box just for that purpose. Back then an 8K memory board cost about $120 if you scrounged parts. It had about 70 IC's and doubled as a toaster. Steppers cost like gold, and were primarily used in floppy head positioning systems. Today, P.C.'s are all over the place and very affordable, and steppers can be had for as little as 25 cents per once/inch for some of the larger motors. Many machines, especially mills, have packaged technology which is very affordable and ready to bolt on your machine.
Can't I just buy a machine already set up? You bet. If you have a 50 man shop running two shifts, springing 30K to 80K for a new cnc machine is just part of being in business. Retro-fits are most appropriate for the small shop, hobbyist, artist, engraver, or model maker. There are some very good small machines which are ready to go new out of the box for the $1,500 to $2,000 range. Training machines being hawked to the schools run about $3,500 to $5,000 or more. Several vendors have retro's for Bridgeport type mills, the most common on the planet, just a phone call away.
Do I need to have a new machine to make a retro-fit worthwhile? The answer to this question depends on what you want to use the machine for. There are fifty year old machines out there which hold tolerance better than some new machines of lesser quality. I have a Model "A" South Bend which I hold tenths on everyday (with an Ames #212.3 on the X axis). Born on September 8, 1945, she's smooth as silk. If you only need to hold a thou or two, just about any lathe may be a candidate for a retro-fit, if it is fairly sound, or can be refurbished to tighten it up. I have listed many sources for both new and used machines in the
links page. Mills, by their nature, sometimes need to hold a closer tolerance along their X (left to right) axis, to place features like holes or pockets within tolerance. Leadscrews with .0003/foot error are available, but you pay more for that accuracy. Again, it comes down to what you need the machine to do for you.
Sounds like your a lathe kinda guy, what about my 30 year old mill? Well, I do it all, but I have four lathes and two mills, and the mills are pretty well covered, as far as leadscrew replacements with new acme's, ballscrews, etc. The market is well covered, and I don't need to re-invent the wheel, so to speak. A couple of vendors listed have Bridgeport type leadscrew replacement kits ready to ship with a phone call. Lathes are a little trickier, and although they outnumber mills three to one, there is no one particular standard configuration, as with the Bridgeport type machines.
Could I build a cnc machine from scratch? You bet! I've seen them made out of angle iron, Plexiglas, even fiber board! No, they are not making rocket nose cones, but they give their closet engineers great pleasure and satisfaction, create things that are useful to the builder, and create new products and markets. They do milling, lathing, electrical discharge machining, engraving, routing, plasma arc cutting, bending tube and sheet, circuit board drilling (referred to as 2 1/2 D), wirestripping and cutoff, and on and on. Oh, did I mention grinding? My project que includes a manual surface grinder which will have an "auto dress" button! Way cool.
What about the control and software? The control of your cnc machine will be your P.C., feeding pulse and direction commands through your parallel printer port, to the stepper motors which translate into machine movement. If you don't want to relocate your new 500 horsepower P.C. to the garage, these machines will run on a $100 286, albeit slower, but certainly do-able. Software runs anywhere from free, like Forth, MS Logo, DanCad (shareware), and on up to the large shop DNC stuff which could set you back about 20K. Many suppliers bundle software with their retro-fits kits, and just like buying a scanner, bundled software deserves close examination when choosing a kit. Three axis kits start at about $300 and up.
Do I have to use my computer's printer port? No you don't. Acturally, many of the newer motherboards are coming out with a U.S.B. (Universal Serial Bus) already on the board. Most newer boards have what is called a Super I/O chip which is capable of supporting 960KB serial transfers. They generally don't advertise it because that information places great scrutiny on the secondary suppliers. Even ancient boxes can do 115,200, if not 230,400 bps.
So, what are you doing this? Well, my interest is borne of necessity. I have been making master bend check and calibration fixtures for microwave cables, and have the dubious opportunity to get back into some production work. The problem is, I can't do production on parts the size of eraser ferrules with a one thou tolerance block in 304L, without ending up like the fellow who invented the linotype machine :-) CNC is just plain not optional if I'm "back in the barrel" doing critical tolerance production work. The site is to share information with other folks with similar interests, possibly develop some new products to fill some of the void with cnc retro-fits for other legacy machines.
So what's with all this "stepper" stuff? Well, you probably have steppers all around you, but didn't know they were there! They are in your computers' drives, printers, copiers, scanners, and probably in your car! They have step resolutions ranging from 7.5 degrees per step, up to about 51,000 steps per revolution with out a gearhead; 125,000 steps/rev with a gearhead. Just like the calculator which was borne out of the space program, the steppers growth has been basically financed by applications like growing silicon crystals to make chips, to the chips themselves. As a matter of fact, some of the machines which are used in the manufacture of semi-conductors, are called "steppers", and they use ... steppers! We benefit by having an alternative to the more expensive and finicky servo type motor which heretofore was dominant in machine motion control. The stepper verses servo discussion is covered fairly well in some of the tutorials found on the links page.
Just briefly, what is the difference? A stepper does not require a feedback loop. For critical applications, and encoder can supply position feedback. I built a lathe which maintains 50 millionths resolution without feedback. If you take a hybrid stepper and attach an encoder on the back end, you basically have a Brushless D.C. Servo! It is the encoder feedback being fed to the servo drive which creates the correct tracking and commutation of a D.C. Servo.
So What Now? Goto the cnc links page and start exploring. I have placed any link that I thought might be of value found in the last two months. There is a wealth of information and products to be found there. Have fun, and please feel free to send your Suggestions for additional links to resources pertinent to these endeavors.
Thanks for visiting ... good luck with your project.
CNC Retro-Fit Links
Disclaimer: All trademarks, copyrights, tradenames, and company names are the property of their respective owners. Graphics are the property of the link above them, unless otherwise indicated. Links are generally in no specific order. Link comments are those of the author, and lack of comments in no way alludes to the value of any particular link.
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