Selecting or Building CNC Plasma Tables
CNC plasma tables come in a variety of sizes and configurations. The first step in choosing one is to identify what you intend to do with it. If you plan to use your table extensively, or produce large quantities of parts, you will want a larger set-up than you would for a home shop or hobby work.
Steel is the least expensive when supplied in full size sheets, such as 4′ x 8 or 5′ x 10′. However, you must have sufficient shop space and a forklift to handle the material, as well as a table with enough cutting surface to accommodate it.
CNC plasma tables are like pool tables, in that they need substantial space to be able to easily access them from all sides. Put yours up against a wall, and you will not be able to reach a part that tips up unexpectedly, or single parts cut on the far side.
Our focus here is going to be on smaller, home sized machines that can be purchased at reasonable cost or built yourself. Plans for a small, precision cutting table can be found in the downloads section.
Torch Height Controls
Full size sheets of steel are seldom completely flat. In order to assure the proper torch tip to work distance your system will need a torch height control (THC). These are of two basic types.
The simplest type uses proximity sensors to detect the surface of the work and maintain a fixed distance. The other type, which is almost universally used today, is the arc voltage THC.
As the torch to work surface distance increases, the amount voltage necessary to maintain the arc increases. The arc voltage THC reads the amount of voltage being used and makes adjustments in the torch height to maintain a constant distance.
Torch height controls are expensive and generally cost $2,000 or more for a good one. One of the benefits of a smaller table, say a 2′ x 2′ capacity unit, is that a THC is not required. Since a two-foot square of steel is relatively flat compared to a full sheet, a tip to work distance can be set that is a compromise between being too close or too far away.
This will reduce consumables life, but for occasional use or hobby work that matters little. Another benefit of a small table is that you can easily handle the smaller size material without assistance.
Stepper Motors or Servo Motors
A subject of constant debate among CNC plasma users is their preference for stepper or servo motors. Large, commercial CNC machines generally use servo motors since they are available in sizes large enough to handle the movement of heavy gantrys holding multiple cutting and marking heads. The fact that large, expensive machines have servo motors is quite naturally mis-interpreted to mean that servos are better or more accurate than steppers.
Stepper motors are sophisticated bean counters that divide each revolution into a large number of small increments. These increments can range from a couple hundred to over a thousand, depending on the motors and electronics. The machine’s software tells the motors to move a certain number of steps in sync with each other.
The combination of the two movements in perpendicular directions provide the cut direction. The movements can be either absolute or relative. In other words, they can be an instruction to go to a certain set of coordinates, or instructions for each motor to take a certain number of steps.
Servo motors use encoders that provide two-way communication between the controller and the motors, providing constant feedback to the controller. Servo motors develop maximum torque at high RPMs, whereas steppers provide maximum torque at low speeds.
High speeds are not necessary in plasma cutting except perhaps in rapid traversing between cuts. Since it is desirable to minimize the distance between cuts, and the fact that acceleration and deceleration is required between cuts, high traverse speeds are seldom an absolute necessity.
Types of CNC Plasma Tables
There are two main types of CNC plasma tables, with the gantry style being by far the most widely used. A gantry machine has a beam that moves back and forth down the length of the table. This is referred to as the X axis.
There is also a carriage that moves back and forth across the beam perpendicular to the X axis. This is known as the Y axis. The torch is mounted on the Y axis carriage. As the two axes move in concert with each other, the torch produces the desired shape. A small gantry machine can be seen in the top right-hand photo.
Note that the X axis uses cam followers riding on and guided by a flat cold roll steel bar. The Y axis, on the other hand, uses a precision roller bearing linear rail and cassette. Although somewhat more expensive, this is a substantially more precise alignment method.
The other type of table is the cantilevered arm style. This also uses an X axis that rides back and forth along the length of the table. However, the Y axis has no carriage, is supported only on one end, and itself moves side to side through a stationary housing mounted on the X axis. The material being cut is located off to the side of the machine. This makes it easier to load but requires twice the space.
A photo of a small table-top gantry machine is shown at bottom right. This home-made machine uses cam followers supporting and riding against the edges of a steel workbench.
The gear rack for the X axis can be seen screwed to the benchtop in the foreground. While relatively cheap to build, this type of guidance system is probably the least precise approach possible.
It is important to bear in mind that even the most primitive axis guidance methods may well be more accurate than the plasma arc process itself. With a kerf width of approximately .050″ (for 1/8″ material), some slop in the mechanical components is overshadowed.
Below: Examples of large (10' x 40') and small (18" x 18") CNC plasma tables
