# Measuring Machine Tool Rotary Table Accuracy

A rotary table is a machine tool accessory that has a rotating table surface for securely holding workpieces and performing rotary and indexing positioning, also known as the fourth axis of the machine.

The performance of a rotary table is determined by three factors: positioning accuracy, repetitive indexing accuracy, and origin-return accuracy.

It is crucial to determine the accuracy of the rotary table.

## I. Rotary table positioning accuracy detection

To measure the positioning accuracy of a rotary table, you will need a standard rotary table, an angular polyhedron, an angle encoder, and a collimator.

The measurement process involves rotating the table to an angle in both forward and reverse directions, stopping and locking the position, and using this as a reference for rapid rotation in the same direction. The position is locked and measured every 30 seconds.

It is recommended to measure the accuracy in both forward and reverse directions for one week, with 7 measurements in each direction.

The difference between the actual angle and the theoretical value of the measured position is taken as the indexing error, with the maximum value used as the result.

For CNC rotary tables, the average position deviation and standard deviation should be calculated using the method specified in “Evaluation Method of Position Accuracy of Digitally Controlled Machine Tools.”

The sum of the maximum and minimum values of the average position deviations and standard deviations is used to determine the positioning accuracy error of the CNC rotary table.

For dry-type transformers, it is recommended to focus on measuring the accuracy at 0 degrees, 90 degrees, 180 degrees, and 270 degrees, which are right-angle points. The accuracy at these points should be one grade higher than at other angular positions.

It should be noted that the positioning accuracy can vary with different feed speeds and may be affected by ambient temperature and the operating conditions of the axis.

## II. repeatability indexing accuracy test of the rotary table

To measure the repetitive indexing accuracy of a rotary table, three positions are arbitrarily selected for three repeated positionings in both the positive and negative directions of rotation.

The largest difference between the detected value and the corresponding theoretical position is taken as the indexing accuracy.

For CNC rotary tables, the measurement should not be limited to three positions, but instead, a measurement should be taken every 30 degrees, with five fast positionings from both positive and negative directions.

The difference between the actual position and the target position is the position deviation of the rotary table.

The standard deviation is then calculated using the method specified in “Evaluation Method of Position Accuracy of Digitally Controlled Machine Tools.”

The largest standard deviation at each measurement point is multiplied by 6 to determine the repeatability of the indexing accuracy of the CNC rotary table.

## III. origin return accuracy test of the rotary table.

The repetitive positioning accuracy of each coordinate axis is a fundamental accuracy index that reflects the stability of the axis motion accuracy. A machine tool with poor precision cannot ensure stability during production.

To measure the return-to-origin accuracy, the return-to-origin procedure should be performed at 7 arbitrarily selected positions, and the maximum difference between the measured stop position and the actual return-to-origin position is taken as the return-to-origin accuracy.