Co-Ordinate Measuring Machines

CO-ORDINATE MEASURING MACHINES

Measuring machines are used for measurement of length over the outer surfaces of a length bar or any other long member. The member may be either rounded or flat and parallel. It is more useful and advantageous than vernier calipers, micrometer, screw gauges etc. the measuring machines are generally universal character and can be used for works of varied nature. The co-ordinate measuring machine is used for contact inspection of parts. When used for computer-integrated manufacturing these machines are controlled by computer numerical control. General software is provided for reverse engineering complex shaped objects. The component is digitized using CNC, CMM and it is then converted into a computer model which gives the two surface of the component. These advances include for automatic work part alignment on the table. Savings in inspection 5 to 10 percent of the time is required on a CMM compared to manual inspection methods.

Types of Measuring Machines

1. Length bar measuring machine.

2. Newall measuring machine.

3. Universal measuring machine.

4. Co-ordinate measuring machine.

5. Computer controlled co-ordinate measuring machine.

Constructions of CMM

Co-ordinate measuring machines are very useful for three dimensional measurements. These machines have movements in X-Y-Z co-ordinate, controlled and measured easily by using touch probes. These measurements can be made by positioning the probe by hand, or automatically in more expensive machines. Reasonable accuracies are 5 micro in. or 1 micrometer. The method these machines work on is measurement of the position of the probe using linear position sensors. These are based on moiré fringe patterns (also used in other systems). Transducer is provided in tilt directions for giving digital display and senses positive and negative direction.

Types of CMM

(i) Cantilever type

The cantilever type is very easy to load and unload, but mechanical error takes place because of sag or deflection in Y-axis.

(ii) Bridge type

Bridge type is more difficult to load but less sensitive to mechanical errors.

(iii) Horizontal boring Mill type

This is best suited for large heavy work pieces.

Fig 4.12 Types of CMM

Working Principle

CMM is used for measuring the distance between two holes. The work piece is clamped to the worktable and aligned for three measuring slides x, y and z. The measuring head provides a taper probe tip which is seated in first datum hole and the position of probe digital read out is set to zero. The probe is then moved to successive holes, the read out represent the co-ordinate part print hole location with respect to the datum hole. Automatic recording and data processing units are provided to carry out complex geometric and statistical analysis. Special co-ordinate measuring machines are provided both linear and rotary axes. This can measure various features of parts like cone, cylinder and hemisphere. The prime advantage of co-ordinate measuring machine is the quicker inspection and accurate measurements.

Fig 4.13 Schematic Diagram

Causes of Errors in CMM

1)                The table and probes are in imperfect alignment. The probes may have a degree of run out and move up and down in the Z-axis may cause perpendicularity errors. So CMM should be calibrated with master plates before using the machine.

2) Dimensional errors of a CMM is influenced by

·        Straightness and perpendicularity of the guide ways.

·        Scale division and adjustment.

·        Probe length.

·        Probe system calibration, repeatability, zero point setting and reversal error.

·        Error due to digitization.

·        Environment

3)                Other errors can be controlled by the manufacture and minimized by the measuring software. The length of the probe should be minimum to reduce deflection.

4)                The weight of the work piece may change the geometry of the guide ways and therefore, the work piece must not exceed maximum weight.

5)                Variation in temperature of CMM, specimen and measuring lab influence the uncertainly of measurements.

6)                Translation errors occur from error in the scale division and error in straightness perpendicular to the corresponding axis direction.

7) Perpendicularity error occurs if three axes are not orthogonal.

5  Calibration of Three Co-Ordinate Measuring Machine

The optical set up for the V calibration is shown in figure

The laser head is mounted on the tripod stand and its height is adjusted corresponding to the working table of CMM. The interferometer contains a polarized beam splitter which reflects F1 component of the laser beam and the F2 Component parts through. The retro reflector is a polished trihedral glass prism. It reflects the laser beam back along a line parallel to the original beam by twice the distance. For distance measurement the F1 and F2 beams that leave the laser head are aimed at the interferometer which splits F1 and F2 via polarizing beaming splitter. Component F1 becomes the fixed distance path and F2 is sent to a target which reflects it back to the interferometer. Relative motion between the interferometer and the remote retro reflector causes a Dopper shift in the returned frequency. Therefore the laser head sees a frequency difference given by F1-F2 ± ΔF2. The-F2 ±F1ΔF2 signal that is external interferometer is compared in the measurement display unit to the reference signal. The difference ΔF2 is related to microscope of CMM is set at zero and the laser display unit is also set at zero. The CMM microscope is then set at the following points and the display units are noted.1 to 10mm, every mm and 10 to 200mm, in steps of 10mm. The accuracy of linear measurements is affected by changes in air temperature, pressure and humidity.

Performance of CMM

· Geometrical accuracies such as positioning accuracy, Straightness and Squareness.