CMM Coordinate-Measuring Machine By Prof. A.U.Awate Department of Mechanical Engineering PRMIT & R Badnera-Amravati

CMM is a 3D device for measuring the physical geometrical characteristics of an object. CMM is a machine which takes readings in six degrees of freedom and displays these readings in mathematical form. CMM is a specialized form of  industrial robot

Parts CMM include three main components: 1.Main Structure which include three axes of motion 2. Probing system 3. Data collection and reduction system Application software Machine controller Desktop computer

1.Main Structure Gantry type Advantage: Measurement of large size parts  Disadvantage: Geometric changes caused by non-uniform temperature distribution owing to their large size. Application: Heavy machine construction, car body and mold making sectors of the automotive industry, measuring wind tunnel models.

Maximum accessibility Disadvantage: 1.Main Structure Cantilever type Advantage: Large measuring range Maximum accessibility Disadvantage: Bending of the cantilever above the measuring area Application: For checking sheet metal, cast iron and steel parts in the automotive industry, aircraft construction and shipbuilding.  

High accelerations and speeds owing to the large supporting base 1.Main Structure Column type (horizontal arm type) Advantage: High accelerations and speeds owing to the large supporting base of the column and its low weight  Disadvantage: Suitable for small measuring ranges only since the projecting part of the column must have short length due to its rigidity. Application: In precision measurements on gages and master parts.

1.Main Structure Bridge type    Advantage: Most widely used High rigidity  owing to compact bridge design and thus small measuring deviations. Disadvantage: Limited accessibility caused by the bridge. Application: For medium to large measuring range

Portable CMM Have six rotary axes with rotary encoders, instead of linear axes. Less accurate than a bridge type CMM Use angular measurements taken at the joints of the arm to calculate the position of the stylus tip. Can reach the insides of complex parts Applications: Reverse engineering, rapid prototyping, and large-scale inspection of low-volume parts are ideally suited for portable CMMs

CMM Features Granite Table Structurally and thermally stable material Low porosity Low moisture absorption Low coeff. of thermal expansion Superior strength Uniformity of texture Non-glaring surface M8 threaded table inserts

CMM Features Air bearings Provided for ensuring friction free travel to all axes Compressed air is forced through a series of very small holes in a flat bearing surface to provide a smooth but controlled air cushion on which the CMM can move in a frictionless manner

CMM Features Each axis is fitted with a reference system and a linear measurement transducer for positional feedback. (0.5 micron resolution) Passive vibration damping system isolates external vibrations FEA designed bridge beam extrusion provides optimum M.I. for minimum deflection when operating at high accelerations

2. Probing system The touch probe forms the sensing device on the end of the quill Mechanical probe Contact type Soldering a hard ball to the end of a shaft No control on measuring pressure Electronic touch trigger probe Stylus is spring-loaded with ruby ball at the tip As the probe touched the surface of the component the stylus deflected and simultaneously sent the XYZ coordinate information to the computer Optical probes and Laser probes Non-Contact type White light/ Laser beam is projected against the surface of the part Better for scanning

Pre-travel variation (PTV) Pre-travel is the amount of stylus bending 2. Probing system Pre-travel variation (PTV) Pre-travel is the amount of stylus bending at the point that a trigger occurs. Pre-Travel depends on Contact Force FC needed to overcome Spring Force FS Length L and stiffness of the stylus Direction of measurement R Pre-travel in the Z direction tends to be small despite the higher forces Pre-travel can be compensated by probe calibration. A datum feature (of known size and position) is measured to establish the average pre-travel

2. Probing system Stylus modules

Correction for Tip Radius 2. Probing system Qualifying Probe Tips Correction for Tip Radius CMM detects dimension from and to the center of the tip (ruby ball) which calls for correction for tip radius. Each recorded value is corrected for tip radius.

Application software PC-DMIS CMM PC-DMIS PRO PC-DMIS CAD PC-DMIS CAD++ 3. Data collection and reduction system Application software PC-DMIS CMM Dimensional Measuring Interface Standard is the world’s leading metrology software PC-DMIS PRO Full-featured metrology software system without CAD capabilities. PC-DMIS CAD It adds CAD capabilities to PC-DMIS PRO. It allows import and export CAD modules Possible to develop, test and debug part programs directly on CAD models Compares the measured data directly to the CAD model Lets graphically test and debug inspection routines PC-DMIS CAD++ Handles difficult scanning tasks (contact & non contact type) and supports sophisticated scanning devices 

Application software Modules 3. Data collection and reduction system Application software Modules PC-DMIS CMM PC-DMIS Planner PC-DMIS Gear PC-DMIS Blade PC-DMIS NC PC-DMIS Portable PC-DMIS Vision

3. Data collection and reduction system Application software Alignment PC-DEMIS mathematically relates the Part Coordinate System and Machine Coordinate System.

Measured and Constructed Features 3. Data collection and reduction system Application software Measured and Constructed Features

Controller is the heart of CMM Provides precise control of the 3. Data collection and reduction system CMM controller Controller is the heart of CMM Provides precise control of the motion of the machine Interfacing to sensors and with the metrology software.

CMM Applications Metrology Linear Measurement (Measured and Constructed Features) Angular Measurement (Measured and Constructed Features) Geometrical Features Profile Checking Stages of Inspection Receiving In-proses Final Mode of Inspection Operation Manual Automated (DCC) Purpose of Inspection First-piece approval Process Control Pre-assembly qualification of parts Reverse Engineering Location of Inspection Standard Room Machine Shop on Site