In casting and fabrication shops the measurements made are of more precise nature as compared to those made in machine shop and tool room
                         
                    
                    
                                                                                
                            
                                
                            
                            
                                
                            
                            
                                
                            
                            Periodical calibration is made for optical measuring instruments
                         
                    
                    
                                                                                
                            
                                
                            
                            
                                
                            
                            
                                
                            
                            There is no need of periodic calibration in universal microscope
                         
                    
                    
                                                                                
                            
                                
                            
                            
                                
                            
                            
                                
                            
                            Preventive maintenance is not necessary former optical measuring instruments