A Micron of Difference, Measuring the Accuracy of Circular Interpolation

March 16, 2016  

Let’s say you’ve built your very own CNC horizontal boring machine and would like to take it for a spin, tear into some iron or steel, cut metal like there’s no tomorrow.  That’s fine, especially if you’re not particular about the outcome.  But, if you’re getting paid big bucks to machine a valuable component in an industrial work of art, something with a tolerance for error a fraction of the width of a human hair, you need to make sure your machine is not only powerful but precise—and it’s calibrated properly.

To do so, you run a ballbar test, which measures how the spindle holds to a programmed circular path, comparing its result to that of a an imaginary perfect circle.  Does the actual outcome deviate from the values programmed into the machine’s CNC system, that of an absolutely round circle, no eccentricity?  Is the machine interpolating and executing the proper trajectory from the coordinates fed into its computer system and creating an arc of movement matching what’s desired, or is it drifting off course, and if so, by how much?  This is especially important for horizontal boring machines which must reach deep into the center of the workpiece, working against the resistance of the metal and the pull of gravity.  A well-built machine must adjust for these variables and keep the spindle on target, executing perfect machining techniques.

Ballbar Test

The way the ballbar test works is to mount one side of a type of contraption composed of (as the name implies) a ball and a bar onto the spindle and the other side onto the table.  Actually, it is a very intricate piece of equipment: it consists of a small telescopic bar with a precision sphere and a kinematic magnetic cup joint located at both ends. One end is fixed to a point on the table and the other to the machine’s spindle.  When the machine is told to “draw” an arc with its spindle along certain points (like a child connecting the dots with his pencil), you see how advanced of an artist it is, just how skilled it can be.  Then, even more information is derived from conducting a dome test drawing XZ and YZ arcs.  The results of the three tests are then integrated to give an overall picture of the performance in 3 planes (X, Y, Z), establishing a theoretical volumetric reading. This is pretty advanced drawing.

Deviations from the nominal interpolation, i.e., a perfect arc or circle, is measured by a high-accuracy linear sensor within the ballbar device.   If the length of the bar changes when it’s rotating like a spoke of a wheel around the hub (and this cannot happen in a circle; its radius doesn’t change in rotation), then it indicates that the machine is off target.  It must be recalibrated… or worse, the machine must be fixed or rebuilt.

Ballbar Traceability Chart of Calibrations and Measurements Standards

The ballbar test is extremely precise, capable of measuring deviations under 1 micron.  For comparison, that’s about the width of your standard, rod-like, single cell bacteria.  So, it’s very, very small.  Check out this chart of particle sizes to give you a clue as to just how big (or small) the world can be.

FERMAT’s CNC horizontal boring machines are built for high performance, power and precision.  Before each machine is handed over to its customer, it undergoes a series of tests and measurements, including a ballbar test.

A recent ballbar test measuring the accuracy of a FERMAT CNC horizontal boring machine, conducted at different test parameters, showed deviations in a range as low as 3 microns to at most 5.1 microns.  That is impressive!

Please visit, http://www.fermatmachinery.com/products, for information on the different FERMAT machine tools and accessories that can improve your manufacturing business. Also, try to Build Your Own machine using our on-line configurator found on our homepage, http://www.fermatmachinery.com/. Should you have any enquiries, please send us an e-mail: export@fermatmachinery.com, or for the U.S. market, e-mail Lucas Precision, a FERMAT Group company: info@lucasprecision.com, or call toll-free: 1-800-336-1262, or telephone in the U.S.: (216) 451-5588, or send us a fax (216) 451-5174.