aramok

aramok

Wednesday, May 21, 2014

Desktop CNC



I currently work at a place called Arrgh!! manufacturing. A local electronics shop that makes specialty devices for businesses, mostly a hydrogen gas detector and a few devices for maintaining lead acid battery's. These days, I help with the engineering side of things, making and fixing machines to facilitate the mass production of the products we sell.

My recent task has been the setup and operate a desktop CNC machine for making small quantity's of prototype parts. These small CNCs have become quite popular with the DIYers. They are very high precision and have a small but very effective bed size. They go for about 600 bucks, but they are not plug and play, easy to use machines. The CNC came into the shop, a ragtag lot of components, each part made by a different no-name Chinese company with only a photo copy of a picture describing what plugged into what.

The package came with:
  1.  A 3 axis machine with drill motor attached 
    1. Drill Motor, 800W, 400Hz/12000RPM
    2. A few engraving bits, end-mill, tools
    3. clamps (extremely under-engineered)
  2. Stepper motor controller.
    1. The PC parallel port communicates direction/step to the controller which then powers the stepper motors
  3. 3 phase motor driver. 400Hz. manually controlled.
  4. Pump & tubes, NO radiator
    1. The motor is designed to be water cooled, but it is necessary
    2. My manager wanted me to build a water cooling system from a PC cooler before turning it on to test how hot it got under load. As it turns out, the water cooling is not needed, The large aluminum clamps holding the motor to the large aluminum CNC table seams to be sufficient to cool the motor, even under a heavy work load.
A lot of software is needed to operate the CNC. Cad software, Gcode generation software and then Gcode executing software. Gcode is the world standard CNC script. It contains a list of simple instructions in text format and can contain thousands of lines. I use Aspire to generate my Gcode, and I also use it to draw 2D vectors. Real CAD software would be required for any precision 3D work. The CNC itself is controlled by a PC program called MACH3. mach3 interprets Gcode instructions in real time and pulses out direction/step signals through the parallel port. It is up to the settings in MACH3 to apply the amount of stepper motor steps per inch. Without further adieu, here's my list of my biggest headaches, in order of occurrence:


  1. The Parallel port must be in a certain mode to work. In the BIOS, the parallel port must be ECP or EEP depending on your controller. proper addresses must be set in MACH3 too.
  2. Although the "manual" had the correct parallel port addresses in the screenshots, all the other information was incorrect. Most notably the Steps Per Inch numbers were way off. In order for mach3 to move the CNC to the correct location (say 6 inches), it must be configured to know that say, 4184 steps is equal to one inch. Even though most CNCs use 1.8 degree per step steppers, the design of the worm gear will determine the distance per revolution. On top of the all the reverse engineering involved to get the thing running right, I needed to set up the machine for inches, instead of millimeters.
  3. MACH 3 also has configurable speed and acceleration curves. The CNC can jam for no particular reason if the speed and acceleration are in correct amounts for what I call a "jamming resonation". Jamming resonation can occur when the stepper motor stepping happens at a frequency that the whole machine resonates to. The vibrating machine has the effect of kicking the stepper motor back wards enough that on the next step, the motor turns backwards, then forward again on the next step, back and forth and back and forth. The machine usually oscillates like this for 20 steps until it breaks out of its loop and continues on its way. Of course the problem is Mach3 thinks nothing has happened and the CNC loses track of its true position.
  4. For cutting metal like aluminum and copper, the rate in which you design your Gcode to "plunge" and "feed" greatly effects cutting performance. It usually has to be done off the cuff and it takes many attempts to achieve a good quality cut. Too fast and the cutting motor will screech to a halt while the CNC continues ramming into the piece. To slow and... well you get bored waiting. The speeds in between will give you different amounts of unwanted burrs left over that have to be cut off, ground off, and sanded off. There are standards for choosing feed rates based upon number of "flutes" (cutting edges per rev) on a bit and what type of metal is being cut. However the motor on this CNC can not provide enough power to cut at normal speeds and needs to go much slower then the recommended feed rate.
While the Desktop CNC does "SEE EN SEE", getting good results takes a lot of practice. I can say it takes less skill then it would to operate a milling machine by hand. I can't recommend this CNC package unless you are willing to make IT your project rather than use it FOR your project. The next post about the CNC machine will surely be adventures into engraving copper clad PCB boards for prototype circuit boards. I have heard it's finicky business, but I hope it will prove invaluable to my work and my own projects. If you have any questions about this CNC, I am eager to answer them.
Wuba-luba-dub-dub and a good day to you!

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