The main features of CNC – CMM are shown in fig. The stationary granite measuring table, length measuring system, air bearings, control unit and software are the important parts of CNC and CMM.

Stationary granite measuring table

Granite table provides a stable reference plane for locating parts to be measured. It is provided with a grid of threaded holes defining clamping locations and facilitating part mounting. As the table has a high local carrying capacity and is accessible from three sides. It can be easily integrated into the material flow systems of CIM.

Length measuring systems

The three axes CMM are provided with digital incremental length measuring system for each axis.

Air bearing

The bridge cross beam and spindle of the CMM are supported on air bearings.

Control unit

The control unit allows manual measurement and programme. It is microprocessor control.

Software

In the CM, the computer and the software represents one system, the efficiency and cost effectiveness depend on the software.

Features of CMM Software

1. Measurement of diameter, centre distance, length.
2. Measurement of plane and spatial curves.
3. Minimum CNC programme.
4. Data communications.
5. Digital input and output command.
6. Programme for the measurement of spur, helical, bevel and hypoid gears.
7. Interface to CAD software.

New software is for reverse engineering complex shaped objects. The component is digitized using CNC CMM. The digitized data is converted into a computer model, which is the true surface of the component.

Recent advances include the automatic work part augment and to orient the co-ordinate system. Savings in inspection time by using CMM is 5 to 10% compared to manual inspection method.

Digital Devices

Digital indication is better by far when an exact initiative value is desired. The important elements of any electronic signal readout system are the scale unit or transducer and the counter of digital readout unit.

Most of the digital readout systems compromise a glass scale and optical detector head. These are however liable to be affected under workshop conditions by contaminants such as oil, swart and dirt. To avoid this, manufacturers use the magnetic principle based instruments. These instruments also provide Binary Code Digit (BCD) output which is based on the principle of magnetic recording, the linear scale comprises a magnetic strip which is unaffected by workshop contaminants.

Readout is provided by a range of digital display and digital position readout units. It varies in complexity from a simple unit with one or two axis display and thumb wheel rest to comprehensive three six-display multifunction capabilities.

The advantages of digital systems are,

1. Measuring element is free from errors.
2. Learning time is short.
3. More accurate measurement.
4. Excessive reading errors can be eliminated.
5. Clear readability of digital readout is advantageous for persons with impaired vision.
6. The display can be zero wherever it is desired.
7. BCD output makes the instrument computer compatible.

For example, digital devices are digital height gauge with statistical production control. The modern techniques reflect a growing need for statistical process control (SPC). The improvement in quality, the reduction of scarp and increase of efficiency in production capacity are all closely related.

The digital height gauge with capability to provide SPC is best suited in this direction and having the following features.

1. Easy identification of component features out of tolerance.
2. Storage of up to 400 dimensions.
3. Entry of data via custom touch keypad.
4. Calculation of mean standard deviation and range.
5. Assessment of 2-sigma, 3-sigma, and 4-sigma confidence intervals.
6. Calculation of capability indices for quick and easy assessments of machine.
7. Automatic entry of data as component is measured.

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