A closer look at a small CNC machine

Let’s take a look at the main elements of this little desktop CNC machine. Even though the machine is quite small, it still operates using the same principles that larger machines costing hundreds of thousands of dollars use. In fact, even the control code, commonly referred to as g-code, is nearly identical. The differences in the control code are mainly accounted for by capabilities of the machine itself. We’ll touch more on that later.

CNC machines employ the use of motors to move the tool through the toolpath needed to cut material from the stock. The stock material is essentially the block of material you cut your part from. Materials don’t have to be made of blocks though. They could be cylinders, wedges, or even parts that have been cast or molded but just need a little touching up or to have mounting holes drilled in them.

Motors used in CNC machines are generally of 2 basic types; servo motors and stepper motors. Servo motors are normally used in more expensive and accurate machines, where stepper motors are often used in less expensive machines where less speed and accuracy are needed.

stepper-motor

Stepper motor on left attached to ball screw on right

These 2 types of motors, steppers and servos, do the same basic thing though, which is to move the tool with a specific degree of speed and precision while the cutting is taking place. Servos operate in a “closed loop” system where the system has the ability to precisely monitor the positioning of the tool at any time, whereas stepper motors operate by moving in small pre-defined steps starting from a known position, normally called the machines “home” position. This is known as an open loop setup because the machine cannot measure where the tool is, it only assumes that it is where it should be. If something were to happen, such as trying to cut material too quickly, the stepper motors ability to reliably move to the next intended position can be compromised. In other words, it can “skip” over steps, which can result in something as minor as an improperly machined part or as major as damaging the machine or perhaps physical injury. Normally though it results in a damaged work piece that will have to be scrapped.

The photo below shows switches on my CNC machine known as “limit switches” because they are meant to signal to the CNC controller where the physical limits of the machines ability to move to exist. Limit switches are used to find the machines home position and also to stop the machine if it tries to go beyond the boundaries.

limit-switch

Example of a limit switch

Back to the motors. The earlier photo of one of the stepper motors on this CNC machine shows a motor on the left that’s connected to a shaft that looks like a big screw that connects to the platforms that move when the motor turns. Since the stepper motors move in very well defined increments, the number of “steps” to make one complete revolution is known for each motor. That knowledge, combined with the pitch of the screw, let’s us determine precisely how many steps per inch or per millimeter are needed. We’ll see later how this is set up in the CNC control software.

There is one more important type of motor to talk about, and that is the “spindle” motor. This is the motor that drives the tool that does all of the actual cutting. These vary widely, ranging from makeshift drill motor conversions to dremel tools, woodworking routers, roto-zips… you name it and someone has probably used it as a spindle motor. They come in such a wide variety depending on the specific need.  The one in the photo below is fairly light duty, good for machining plastics, some metals (like aluminum), engraving, making circuit boards, and other useful things. It has a fairly small amount of “slop”, so that the precision is pretty good without too much wobbling.

spindle-motor

Spindle motor

The final thing to notice in this blog entry is the work table. This is where the work is secured while the machine does it’s thing. The work may be clamped to the table directly, held in a vise, placed in a fixture or jig, etc. It must be held down very securely for safety reasons as well as to ensure the work does not move while it’s being cut. Work tables may have channels, threaded holes, of other special features that facilitate holding the work down.

work-table

Example of work table with vise secured by clamps

Next up, we’ll take a look at a CNC controller that drives this whole thing.




2 Responses to “A closer look at a small CNC machine”

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