Bal-tec spindle authenticator, AUT-4000
Spindle Authenticator, AUT-4000

The precise axis of rotation of a machine tool spindle can be determined with microinch accuracy using a Bal-tec Authenticator. There are two accepted ways to measure the axis of rotation of a precision spindle. The first is to machine or grind a test part while it is mounted on the rotating spindle, then to remove the test part from the spindle and measure it very accurately on a roundness measuring machine. This is a time-consuming process and requires access to a high-quality roundness measuring machine. In addition, distortion of the test part caused by the clamping forces used to hold it on the spindle and other errors in the machine tool may reflect as roundness errors, thus somewhat clouding this evaluation.

A similar method commonly used that is absolutely useless, is to machine a test part while it is mounted on the spindle and then measure its run out with an indicator while the test part is still attached to the spindle. If you visualize a systematic spindle error that is modulating the distance between the spindle and a tool or grinding wheel and then imagine trying to measure the same error by replacing the tool with an indicator, it will be obvious that the indicator will read almost none of the true error.

The second valid method is to fix a very accurately round test part so that its axis of rotation is nearly coincident with the axis of rotation of the spindle. The best test part geometry would be a perfect sphere because its axis can be defined by a single point in space. This greatly simplifies alignment problems thus giving a fast, inexpensive method for determining the accuracy of a spindle's axis of rotation. After the sphere is aligned to the spindle axis, it is just a matter of carefully measuring the point to point error.

In practice, there is no such thing as a perfect sphere, so we have to settle for the best quality that is available. There are three factors that generally limit the spherical perfection of a small diameter ball:

  1. The limited ability to actually measure sphericity. The commercial limit of this ability is generally accepted to be about 2.5 microinches (63.5 NM).
  2. The lack of homogeneity or physical anisotropy of the sphere material itself. This is on both a micro and macro level. The original macro structure is imparted to the ball blank by the rolling and/or forging processes it has undergone. The micro structural variations are caused by variations in the Young's Modulus of Elasticity due to the many different attitudes of crystallographic orientation. The macro structure tends to influence the waviness and sphericity and the micro or crystallographic structure influences surface finish.
  3. The Hertzian or elastic deformation of the sphere and of the generating tools. This deformation is due to the pressure applied to both the sphere and the tools during the abrading process.

We have reduced the effects of #2 and #3 to a level beyond our ability to measure them. The Ultra-Precise Test Spheres supplied on the Authenticators are spherical to less than 2.5 microinches (63.5 NM) and have a surface finish that is below 0.5 micro inches (12.7 NM) Ra. The required characteristics of the test sphere somewhat limit our choices of materials. It must be a good electrical conductor. It must be hard and wear-resistant. It must have good long term dimensional stability. It must be corrosive-resistant. It must not be expensive and it must be commercially available.

The standard material used for the Ultra-Precise Test Spheres meets all of these criteria. It is one inch (25.4 mm) in diameter, made of very fine grain, high chrome, high carbon, stainless steel that is hardened to 58 Rockwell C for wear resistance and ultra cold-cycled for long term dimensional stability. We will custom manufacture spheres of any size or any material to meet special customer requirements.

There are two effective methods for precisely adjusting the point axis of the sphere coincident with the axis of rotation of the spindle. One is to rigidly mount the ultra-precise sphere to a post supported by a powerful magnet. The magnet holds this assembly to a flat face plate that is perpendicular to the spindle's axis of rotation. The magnetic assembly is simply tapped with a small hammer to reduce the plus readings until the sphere is rotating nearly concentric to the spindle axis.

The other method is to rigidly mount the sphere to a four point adjustable device. By loosening one of the four adjusting screws and tightening the one opposite it, the axis of the ultra-precise sphere can be moved to coincide with the axis of rotation of the spindle. The data can be taken by a sensitive contacting probe type electronic gage at very low RPM or a capacitance gage at high speed.

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