Ballbar vs. Circle Diamond Square Test

Date: March 15, 2001

Scope: This document will attempt to outline a variety of reasons why the Renishaw Ballbar has become the universally accepted method for measuring a CNC machine tool's circularity/interpolation performance, in lieu of the Circle–Diamond–Square cutting test. This document will also identify many of the additional advantages the Ballbar delivers, outside of those required by the Standards.

Circle Diamond Square Test [NAS 979; Composite cutting test 4.3.3.5]

Prior to the development of the Ballbar, the Circle – Diamond - Square test [NAS cutting test] was widely used throughout the manufacturing engineering industry to test and measure a CNC machine tool's accuracy performance. The standard was first published by the National Standards Association for the Aerospace Industries Association of America in April 1966 and revised in January 1969. It formed part of a series of tests developed by NAS to "Provide a standard for the selection of cutting tests required to evaluate the performance of conventional and Numerically Controlled machine tools, excluding drilling and turning machines. To provide a standard format for recording and reporting actual performance results."

The NAS Composite Cutting test was designed to measure:

1. 5 Deg ramp and .005" taper cuts:
   Uniformity of servo response and slide way stiction by visual inspection of the surface finish.
2. Outside Square surface for:
   Dimensional accuracy, flatness, squareness, parallelism, and Surface Finish.
3. 5 Deg Ramp for: Angular deviation
4. The circle:
   Dimensional accuracy, roundness, diameter variation, and finish
5. The center 45Deg Canted square:
   Dimensional accuracy, squareness, parallelism and surface finish

Problems associated with the NAS Cutting test:

1. Time consuming, an estimate of the cutting time required: 30 min at 25ins/min.
2. Consumes material, (14x14x3in Aluminum blank); milling cutters (2 ins dia. 2 flute end mill) and manpower.
3. After each test, the part has to be measured, in earlier times this meant the use of a surface plate, sign bar and indicators, precision rotary table or roundness measuring machine. Today, a CMM and roundness/surface testers would be used as the measuring device. This process is obviously time consuming and expensive, and in some facilities even requires the test parts to be sent away for measurement.
4. Once the measurements have been made and the results obtained, it is necessary to interpret and decipher the results of the test in order to make corrections, should the machine fail to meet specification. This takes a high degree of experience and knowledge.
5. Should the machine fail any of the required specifications, the procedure is then repeated, adding to the initial time and cost.
6. It is clear this test was designed as a final inspection cutting Test and as such involves the interaction of that process with the machines fundamental mechanical design, stiffness, damping characteristics and other dynamic features. Thus the end results are not a true reflection of the machines un-loaded performance.

While some companies still use the NAS test, it's primarily used for evaluating cutting performance vs. accuracy measurement. Today however, the overwhelming majority of both manufacturers and machine tool builders throughout the world now consider the Renishaw Ballbar as the true standard for machine tool circularity/interpolation and performance testing.

The BallBar Instrument Test

The Ballbar was first developed in the mid-1980's by Jim Bryan, Lawrence Livermore Laboratories, as a means of testing high accuracy diamond turning lathes. The ASME "B5 Committee" began work on drafting the B5 standard in 1985 and was adopted in 1989 by both the AMSE B5.54 and ISO 230 Committees. Most recently, the Ballbar was included in the ASME B5.57 Standard for checking CNC lathes. Ford Motor Company has a representative who actively participates in the ASME B5 Committee.

The Ballbar is essentially a displacement transducer, held between two very accurate spheres. This simple, "instrumented test," is used to evaluate CNC machine circularity performance and is a direct replacement for the NAS Circle test described above. With the acceptance of the Ballbar in both the National and International Standards (AMSE B5.54, B5.57 and ISO230), the Ballbar is now recognized around the world as the chosen method for measuring a CNC machine tool's circularity performance.

Advantages of the Renishaw QC10 Ballbar system

The Renishaw QC10 Ballbar is a very simple and highly accurate (±.5 micron) measuring system for all types and sizes of CNC machine tools. It's easy to use diagnostic software, instantly provides machine condition and performance data in both graphical and data table formats. The average test usually takes less than 15 minutes!

A simple circular path part program is used to drive two axis of the machine tool in a continuous circle around a preset center point at which one of the precision balls is located. The transducer assembly is attached to this center ball by a magnetic cup at one end. On the other end of the Ballbar is another precision ball supported from the machine spindle with a second magnetic cup. The transducer is able to follow radially around the center of the circular path while measuring any irregularities in this motion. The PC records this information via an RS232 interface, and a data file is created.

Renishaw's Diagnostic Advantage

Renishaw's diagnostic software instantly calculates and then displays the following machine performance evaluation:

Control Loop Errors

1. Servo Mismatch
   Servo mismatch occurs when the servo loop gains of the axes are mismatched, resulting in one axis leading the other causing an oval shaped plot. The leading axis is the axis with the higher loop gain.
2. Reversal Spikes
   When an axis is being driven in one direction and then has to reverse and move in the opposite direction, instead of reversing instantaneously, it may pause momentarily at the turnaround point, causing a ‘Spike' to appear in the plot, and a flat on the work piece.
3. Back Lash or lost motion
   This is usually associated with excess clearance within the drive system, or guide way mechanism.
4. Cyclic errors
   Often associated with badly worn/manufactured, drive system elements like Ball Screws and nut, rack and pinions and their encoder devices.
5. Scaling Error
   Indicates the linear accuracy relationship between two axes within the test area. The Ballbar software provides linear accuracy values to help determine whether each of two axes are unequal and/or correct, due to either servo positioning or slides way mechanical errors.

Axis Slide Way Errors

1. Squareness
   When one axis of motion is not at 90 degrees to the other.
2. Straightness
   Measures any deviation in the axis of motion from a nominal straight line, within the test length.
3. Lateral Play (slop)
   This comes from excess clearance in the axis guide way system, allowing sideways motion of the element or table as it changes direction. A common cause of lateral play may be excessive gibb clearance.
4. Stick – Slip and Vibration
   These errors result from poor isolation and/or damping from either internally generated or externally induced disturbances, which cause an axis to move erratically. Potential surface finish problems are identified by the Ballbar through the vibration and/or stick-slip characteristics, but these represent only a small portion of the likely sources of poor surface finish on machined components.
   
   The spindle and it's bearings, along with the cutter/work piece interaction are two primary sources of vibration, in addition to work piece material, configuration, clamping, and process details such as cutter feeds and speeds. While Renishaw Ballbar plots clearly illustrate many of the potential problems affecting part surface finish, the software does not actually diagnose or provide a calculated value for vibration.
5. Positional Tolerance
   This is a calculated estimate of the likely, bi-directional, positioning capability of the two axes within the test area. This true position calculation makes use of the diagnosed values for backlash; scaling error; cyclic error; straightness; squareness and lateral play.

Are there disadvantages to using the Renishaw Ballbar vs. NAS Testing?

Unlike the NAS cutting test, material is not removed when performing the Ballbar test. The Ballbar does not quantify the surface texture finish of the machined part due to cutter work piece contact, cutter impact frequency tests, nor does it test for chip load to horse power capability. As previously stated however, the Ballbar clearly identifies machines with poor surface finish problems.

Summary

The Renishaw QC10 Ballbar is far more than a substitute for the NAS Composite cutting tests. The Ballbar measures and diagnosis over 20 machine error sources, enabling users to quickly analyze a wide range of geometric and machine tool performance characteristics, allowing adjustments to be made to control system parameters. Machine re-tests can then be quickly performed until optimum machine performance is achieved. This capability is equally important for new machine tool manufacturers during final assembly and test, as well as for service and periodic maintenance checks.

With the Renishaw Ballbar integrated into your final inspection and maintenance routines, you are assured of a simple, quick and accurate assessment of your machine's performance. One of the biggest advantages to Ballbar testing is the short amount of time needed to perform and diagnose this test, again, usually less than 15 minutes!