What are the criteria for judging the accuracy of a coordinate measuring machine?

As we all know, CMM technology has developed rapidly in recent years. Amid such rapid technological advances, some unsatisfactory measuring products inevitably emerge. For high-precision measuring instruments like coordinate measuring machines, accuracy is the fundamental factor for judging their quality. How to determine whether a coordinate measuring machine meets the qualified standards? Below is a reference for the accuracy evaluation criteria of coordinate measuring machines.

Accuracy Evaluation Standard for Coordinate Measuring Machines: ISO 10360 In 1994, the international standard ISO 10360, Acceptance and Reverification Tests for Coordinate Measuring Machines, was implemented. This standard specifies the basic procedures for performance testing of coordinate measuring machines. The current national standard of China GB/T16857.2-1997, Coordinate metrology — Part 2: Performance assessment of coordinate measuring machines, is equivalent to the corresponding ISO standard.

Under the main title "Geometrical Product Specifications (GPS) — Acceptance and reverification tests for coordinate measuring machines (CMM)", ISO 10360 consists of the following six parts: — Part 1: Vocabulary — Part 2: CMMs used for measuring linear dimensions — Part 3: CMMs with the axis of a rotary table as the fourth axis — Part 4: CMMs used in scanning measuring mode — Part 5: CMMs using multiple-stylus probing systems — Part 6: Estimation of errors in computing Gaussian associated features

The ISO standard mainly includes three key parameters: Maximum Permissible Error of Indication for length measurement (MPEE), Maximum Permissible Probing Error (MPEP). For scanning measurement, Maximum Permissible Scanning Probing Error (MPETHP) is adopted.

Before purchasing a measuring machine, users need to be familiar with the relevant acceptance standards. The following is a brief introduction to ISO 10360: ISO 10360-1 (2000) "Vocabulary" Part 1 of this standard defines all terms related to coordinate measuring machines, such as "probing system" or "reference sphere". We will not elaborate further here.

ISO 10360-2 (2001) "CMMs used for measuring linear dimensions" (MPEE) and Maximum Permissible Probing Error (MPEP). Maximum Permissible Error of Indication for length measurement (MPEE): Measure a set of five gauge block sizes in seven different orientations within the measuring volume. Each gauge block length shall be measured three times. All measurement results must fall within the specified MPEE range.

Maximum Permissible Probing Error (MPEP): Take 25 measurement points evenly distributed on a precision reference sphere. The MPEP value is the maximum value of all measured radii.

ISO 10360-3 (2000) "CMMs with the axis of a rotary table as the fourth axis" This part defines the measurement errors for measuring machines equipped with rotary tables. It mainly covers three indicators: radial four-axis error (FR), tangential four-axis error (FT), and axial four-axis error (FA).

ISO 10360-4 (2003) "CMMs used in scanning measuring mode" This part applies to coordinate measuring machines with continuous scanning functions and describes measurement errors under scanning mode.

Most measuring machine manufacturers define "spatial scanning probing error under THP conditions". Besides THP, the standard also defines scanning probing errors under THN, TLP and TLN conditions.

Maximum Permissible Scanning Probing Error (MPETHP): Scan along four defined paths on a reference sphere. The MPETHP value is the maximum deviation of all scanned radii.

Note: The description of THP must include the total measurement time, for example: THP = 1.5μm (scanning time:72 seconds). THP describes the scanning characteristics at maximum point density along a known path. ISO 10360-4 further defines: TLP: scanning along a known path with low point density; THN: scanning along an unknown path with high point density; TLN: scanning along an unknown path with low point density.