Machine Tool Metrology

Alignment (geometrical) tests check the accuracy of the machine tool itself, without cutting: that the slideways are straight, the spindle runs true and the moving parts are parallel or square to each other. The results are compared with the permissible values in standards (for example the ISO 230 series). The quality of workpieces depends on the accuracy of machine.

Test for level of installation

The machine is installed on a firm foundation; a precision spirit level (0.02 mm/m) is placed on the bed, once at the head and then at the tail, along the bed and then across it, with the carriage at various places. The readings are noted and the levelling screws or wedges adjusted until the bed is level in both directions to within 0.02 mm per 1000 mm. This makes sure that the bed is not twisted or sagging by its weight and that the other tests are valid.

Spindle tests

1. Spindle true running (runout of the spindle nose): the dial indicator is held on the carriage with its stylus on the nose of the spindle (or on the locating cone), and the spindle is rotated by hand; the movement of the pointer is the runout (typical permissible value about 0.01 mm).
2. Run-out of the spindle taper (axis): a test mandrel is fitted in the taper bore; the dial indicator reading is taken near the nose and at 300 mm from it. The reading gives the eccentricity and the axis alignment (permissible 0.01 mm near the nose and 0.02 mm at 300 mm).
3. Axial slip (end float) of the spindle: a dial gauge touches the centre of the spindle end face (or a steel ball in the centre hole) while the spindle is rotated and pushed along its axis; the pointer movement is the axial slip (about 0.01 mm).
4. Parallelism of spindle axis with the bed: the dial indicator is traversed with the carriage along the test mandrel in the vertical and horizontal planes; the readings should be within 0.02 mm over 300 mm, with the free end of the mandrel up only.

Rise per division

Sensitivity $= 0.02$ mm per 1000 mm, so for the 200 mm base:

$$1\ \text{div} = 0.02 \times \frac{200}{1000} = 0.004\ \text{mm}$$

Heights

Each rise is $\text{reading} \times 0.004$ mm; the height at a position is the sum of the rises up to it.

The line joining the end points rises $0.028$ mm over $1200$ mm, so at the $k$th station it is $0.028k/6$.

Profile

 height
 0.028 |                          *
 0.024 |             *
 0.020 |       *           *   *
 0.008 | *
 0.000 *
       +---+---+---+---+---+---> mm
       0  200 400 600 800 1000 1200

The bed is highest above the end line at 400 mm ($+0.0107$ mm) and lowest at 1000 mm ($-0.0033$ mm). The straightness error is the width of the zone between two parallel lines enclosing the profile:

$$0.0107 - (-0.0033) = 0.014\ \text{mm over 1200 mm}$$

Answer: straightness error = 0.014 mm over 1200 mm (maximum hump of 0.0107 mm at 400 mm).