Mechanical Structure

To achieve the motion of the probe, several physical components are utilized. Mechanical configurations of CMM are categorized into six types. Each of has advantages & disadvantages associated with them. These are following:

Cantilever

Such type of construction uses three movable components. These components move along mutually perpendicular guide ways. The workpiece is supported on the worktable along the help of CMM holding up fixture. Three- dimensional measurements are accomplished through having the probe connected to vertical quill that moves vertically in the z direction relative to the horizontal arm. To achieve y axis motion, the quill might also move horizontally in the y direction along the length of the arm. The arm is supported at one end only in cantilever fashion and moves horizontally in the x direction relative to machine base. The advantages of this construction are following:

1. Small size, low cost & minimum floor space requirement.

2. Convenient access to the work table.

3. Ability to measure large work parts.

4. High rate of mounting and measuring on CMM.

Its disadvantages are restricted accuracy because of cantilever nature of system and lower rigidity than most other CMM construction.

Moving Bridge

It provides a more rigid structure after that the cantilever design and give increase to more accuracy. In this case, the probe is mounted on the bridge structure. The part, that is to be measured, is positioned on the table and the bridge structure is moved associative to the stationary table. The common difficulty connected with moving bridge design is yawing (walking). It happens because two legs of the bridge move at slightly different speeds & resulted in the twisting of the bridge. This phenomenon decreased the accuracy of the measurement. These types of designs are most widely utilized in industry.

Fixed Bridge

The bridge is connected to the CMM bed and worktable is moved in the x direction beneath the bridge. This structure removes the phenomena of yawing and leads to more accuracy & more rigidity. Though, in this design throughput is somewhat affected because of involvement of additional mass.

Horizontal Arm

Horizontal arm CMMs come in a variety of configuration such like moving table, moving ram, and fixed table. These CMMs are mainly utilized to measure the dimensional and geometric accuracy of the machined or fabricated workpieces.

These machines are quite same in their operation to horizontal machine tools. Because of cantilever design of the horizontal arm, it becomes less rigid and therefore less accurate. However, it allows good accessibility to the work area.

Gantry

This type of construction is utilized for the inspection of large objects. This is gantry crane type of construction in which x axis motion is attained by moving the cross beam along two elevated rails. The probe quill moves vertically relative to a cross beam to attain its axis motion.

Column

Its construction is quite similar to that of the machine tool. The work table is utilized to obtain the x axis & y axis motion. The probe quill is moved vertically to achieve z axis motion. These machines are also referred as universal measuring machine (UMMs).

In all of these constructions, special design features are utilized to build high accuracy & precision into the frame. These features involve precision rolling-contact bearings & hydrostatic-bearings, installations mountings to isolate the CMM and decrease vibrations in the factory from being transmitted through the floor, and several schemes to counter balance the overhanging arm in the case of the cantilever construction.