Face- and Body-Diagonal Length Tests using a Double Ball-Bar for Squareness Errors of Machine Tools

This study realizes face- and body-diagonal length tests using a double ball-bar (DBB) to measure squareness errors at the full working volume of machine tools (MTs). Each test consists of two steps: 1) length measurement at diagonal paths formed using machine commands, and 2) squareness error calculations. For each diagonal path, the machine receives sequential commands according to two nominal coordinates, and tool-tip positions are fixed on the workpiece table. Then, the distance between the two positions is measured using a DBB, which is not calibrated for initial length. The actual tool-tip coordinates are modeled using nominal coordinates and squareness errors. The relationship between the measured lengths and squareness errors is derived using the actual tool-tip coordinates to calculate the squareness errors. The two realized methods are applied to a MT, and verified by comparing the measurement lengths without and with compensation for the measured squareness error. The main advantages of the methods are 1) robust results, unaffected by set-up errors, 2) flexible measurements for various machine tools having different working volumes using a DBB with extension bars only, and 3) cost-effective measurements using the DBB, and the need for only a short time (~5 min) to complete measurements.