Test code for machine tools - Part 2: Determination of accuracy and repeatability of positioning of numerically controlled axes
1 Scope
This document specifies methods for testing and evaluating the accuracy and repeatability of positioning of numerically controlled machine tool axes by direct measurement of individual axes on the machine. These methods apply equally to linear and rotary axes.
This document can be used for type testing, acceptance tests, comparison testing, periodic verification, machine compensation, etc.
When several axes are simultaneously under test, the methods do not apply.
The methods involve repeated measurements at each position. The related parameters of the test are defined and calculated. Their uncertainties are estimated as described in ISO/TR 230-9:2005, Annex C.
Annex A presents the estimation of the measurement uncertainty.
Annex B describes the application of an optional test cycle: the step cycle. The results from this cycle are not to be used either in the technical literature with reference to this document, nor for acceptance purposes, except under special written agreements between manufacturer/supplier and user. Correct reference to this document for machine acceptance always refers to the standard test cycle.
Annex C contains considerations related to periodic positioning error.
Annex D describes tests using ball array and step gauge.
Annex E provides information on the minimum incremental step size.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are indispensable for its application. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
GB/T 17421.1-2023 Test code for machine tools - Part 1: Geometric accuracy of machines operating under no-load or quasi-static conditions (ISO 230-1:2012, IDT)
ISO/TR 230-9:2005 Test code for machine tools - Part 9: Estimation of measurement uncertainty for machine tool tests according to series ISO 230, basic equations
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
axis travel
maximum travel, linear or rotary, over which the moving component can move under numerical control
Note: For rotary axes exceeding 360°, there might not be a clearly defined maximum travel.
3.2
measurement travel
part of the axis travel, used for data capture, selected so that the first and the last target positions can be approached bi-directionally
Note: See Figure 1.
3.3
functional point
cutting tool centre point or point associated with a component on the machine tool where cutting tool would contact the part for the purposes of material removal
Note: In this document, tests address errors in the relative motion between the component of the machine that carries the cutting tool and the component that carries the workpiece. These errors are defined and measured at the position or trajectory of the functional point.
[SOURCE: GB/T 17421.1-2023, 3.4.2, modified]
3.4
target position
Pi (i = 1 to m)
position to which the moving component is programmed to move
Note: The subscript i identifies the particular position among other selected target positions along or around the axis.
3.5
actual position
Pij (i = 1 to m; j = 1 to n)
measured position reached by the functional point on the j th approach to the i th target position
3.6
positioning deviation
deviation of position
Xij
actual position reached by the functional point minus the target position
Note 1: Positioning deviations are determined as the relative motion between the component of the machine that carries the cutting tool and the component that carries the workpiece in the direction of motion of the axis under test.
Note 2: Positioning deviations constitute a limited representation of positioning error motion, sampled at discrete intervals.
[SOURCE: GB/T 17421.1-2023, 3.4.6, modified]
3.7
unidirectional
refers to a series of measurements in which the approach to a target position is always made in the same direction along or around the axis
Note: The symbol ↑ signifies a parameter derived from a measurement made after an approach in the positive direction, and ↓ one in the negative direction, e.g. Xij ↑ or Xij ↓.
3.8
bi-directional
refers to a parameter derived from a series of measurements in which the approach to a target position is made in either direction along or around the axis
3.9
standard uncertainty
uncertainty of the result of a measurement expressed as a standard deviation
[SOURCE: ISO/IEC Guide 98-3:2008, 2.3.1]
3.10
combined standard uncertainty
standard uncertainty of the result of a measurement when that result is obtained from the values of a number of other quantities, equal to the positive square root of a sum of terms, the terms being the variances or covariances of these other quantities weighted according to how the measurement result varies with changes in these quantities
[SOURCE: ISO/IEC Guide 98-3:2008, 2.3.4]
3.11
expanded uncertainty
quantity defining an interval about the result of a measurement that can be expected to encompass a large fraction of the distribution of values that could reasonably be attributed to the measurand
[SOURCE: ISO/IEC Guide 98-3:2008, 2.3.5]
3.12
coverage factor
numerical factor used as a multiplier of the combined standard uncertainty in order to obtain an expanded uncertainty
[SOURCE: ISO/IEC Guide 98-3:2008, 2.3.6]
3.13
mean unidirectional positioning deviation at a position
Xi ↑ or Xi ↓
arithmetic mean of the positioning deviations obtained by a series of n unidirectional approaches to a position Pi
3.14
mean bi-directional positioning deviation at a position
Xi
arithmetic mean of the mean unidirectional positioning deviations Xi ↑ and Xi ↓ obtained from the two directions of approach at a position Pi
3.15
reversal error at a position
reversal value at a position
Bi
difference between the mean unidirectional positioning deviations obtained from the two directions of approach at a position Pi
3.16
reversal error of an axis
reversal value of an axis
B
maximum of the absolute reversal errors |Bi| at all target positions along or around the axis
3.17
mean reversal error of an axis
mean reversal value of an axis
B
arithmetic mean of the reversal errors Bi at all target positions along or around the axis
