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Beginner's Guide to CNC Conversions

  If you're thinking of converting your tired lathe or mill to CNC, I recommend you do so as it can be a fun and intensely rewarding project.  

  Before you do, it's probably a good idea to consider what you typically use it for and whether a CNC conversion will help you do it better or faster.  Generally the further along the spectrum you are toward the production side, the better off you are converting.  

  For one-off hobbying around or oddball maintenance I wouldn't recommend it, as it tends to get in the way of getting anything done that requires "feel" on the part of the operator.  Check out the backlash and the generally serviceability of the machine beforehand, as CNC's tend to get run pretty hard under unforgiving conditions.

  I should also mention that one needs to be comfortable with computers in order to use CNC effectively. You don't need to be a whiz, but you should be comfortable navigating around with files and using multiple programs for editing.

System Overview

You can envision a typical hobby CNC system broken down as a column of eight elements as shown below:

Highest Level

• CAD/CAM
• Gcode Interpreter
• Computer
• Parallel Port
• Drive(s)
• Motor(s)
• Machine

  Probably the simplest piece to understand is the machine, all the way at the bottom rung. This is the same type of machine used in everyday operations. It can be a lathe, mill, gear cutter, injection mold machine, or anything that has moving parts you want to control. Most hobbyists work with small three axis milling machines like the Sherline.

  A common question is whether to use the factory lead-screws or upgrade to ballscrews.  The best answer is that it depends on the condition of the existing hardware, whether there is physical space or some rigging issue with the conversion, and whether you can afford it.  Ballscrews are most useful on a mill where cutting is often done in both directions.

Motors

  Moving forward, we need to have some way of driving this machine automatically. A human would turn the hand wheels in order to make parts. A CNC machine will use motors to accomplish the same thing.

  Motors come in two basic varieties, stepper motors and servo motors. 

     Steppers are a special type of multiple pole motor.  Each pulse of current turns the shaft some small angle, typically 1.8 degrees.  They have high torque at low speeds, and can command and hold a particular shaft orientation very precisely.

     Servos are the best thing to have for CNC. "Servo" is Latin for slave, which is exactly what the motors do. In a nutshell, they're regular DC synchronous motors that are optimized for low inertia, high acceleration, and continuously variable speed through their range and have an integral encoder. Generally these take a lot of computing horsepower to direct accurately since they have to be monitored and controlled continuously, usually using a PID filter or other dedicated driver and an encoder.  These days, such things are for the most part invisible to the user, and so there isn't a large conceptual gap between servo driven systems and stepper driven systems.

  I would recommend that a beginner to CNC use stepper motors. They're much easier to use and far cheaper. You can connect to motor to your machine using a rigid coupling made of aluminum. Simply bore a hole in each end to the size shaft you need to mate, and hold it all together with setscrews.

     Some standard kits to do this are available for the Bridgeport clones and the popular Sherline 5400 bench top mill..

Drives

  The type of motor you buy influences the type of drive you should buy. Think of the drive is an amplifier that allows a computer to control a motor through an interface of low voltage digital signals.

  Stepper motors and servos cannot use the same drives. If you're using a stepper motor I can recommend the Geckodrive G210 or 201. Servo users should use the G320 or G340. 

  For low powered machines like the Sherline lathe and mills I can recommend the Xylotex drives as they're much cheaper.

  You will need to obtain a power supply for the drives that can supply the proper voltages and current for your drive/motor combination.  The best thing to do is to use a large linear power supply with a capacitor filter.

  Switching power supplies such as those found in computers do not work well with the current regulation in most modern drives.

Parallel Port

  Next you need to connect the drives to your computer's parallel port. The parallel port is the 25 pin female connector somewhere in the back of your computer that looks like this:

  This is relatively easy to do but in recent days it has gotten easier still with the introduction of parallel port breakout boards from several manufacturers. Typically these have several relays and high current outputs to control coolant and other goodies.

  Fortunately the complexity of controlling the motors is abstracted from you, from the drives level upward. All the computers sends the motors are step and direction signals, which require only two wires and a ground. On the computer parallel port pins 18 through 25 are ground.

Computer

  This brings us to the topic of the computer itself. What kind of computer should you use? For TurboCNC you can use anything from an old 486 PC to the latest Pentium. Most other CNC software has similar basic requirements (EMC, Mach2, CNCPro).  

  I'm not currently aware of any CNC interpreters that run on a Mac, to answer an uncommon but perfectly reasonable question.

  Since TurboCNC is DOS based, you may want to install a version of Windows in order to use your favorite CAD/CAM system on top of DOS. TurboCNC works best with Windows 3.1, 95 and 98. Naturally you want to make sure that your computer has a parallel port.

G-code Interpreter

The G-code interpreter (sometimes called just the interpreter) is the program that actually does the work of sending signals out to drive your motors and thus your machine as well. 

G-code is a script language for defining the motion involved in making a part.  Here is an example of g-code:

G00 X1.00 Y0.75
G01 Z-0.250 F3.0
G02 X0.50 R1.345

Good luck with your projects.

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This page last updated on January 11, 2015 . 

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