Search

EP-4737059-A1 - METHOD FOR MEASURING WORK-AFFECTED LAYER OF WORKPIECE, METHOD FOR PROCESSING WORKPIECE, DEVICE FOR MEASURING WORK-AFFECTED LAYER, AND MACHINE TOOL

EP4737059A1EP 4737059 A1EP4737059 A1EP 4737059A1EP-4737059-A1

Abstract

A method includes installing an indenter (42) for pressing into a workpiece (W) on a spindle (20), pressing the indenter (42) into the workpiece (W) using a feed device (22, 26, 34) of a machine tool (10), continuously measuring an indentation load (L) and an indentation depth (h), continuously calculating an indentation load coefficient (C), which is a coefficient when the indentation load (L) is functionally approximated by the indentation depth (h), suspending pressing of the indenter (42) into the workpiece (W) when a rate of change of the indentation load coefficient (C) with respect to the indentation depth (h) becomes a value sufficiently close to zero from negative, and calculating a work-affected layer depth (Z) by multiplying the indentation depth (h) when pressing is suspended by a depth conversion constant.

Inventors

  • TEZUKA, RYO
  • YOSHIMURA, Taishi

Assignees

  • Makino Milling Machine Co., Ltd.

Dates

Publication Date
20260506
Application Date
20240731

Claims (7)

  1. A method for measuring a work-affected layer of a surface of a workpiece, on a machine tool for machining the workpiece attached to a table with a tool installed on a spindle, the method comprising the steps of: installing, in place of the tool, an indenter for indenting the workpiece on the spindle, pressing the indenter into the workpiece using a feed device of the machine tool, continuously measuring an indentation load for pressing the indenter and an indentation depth of the indenter into the workpiece, continuously calculating an indentation load coefficient, which is a coefficient when the indentation load is functionally approximated by the indentation depth, suspending pressing of the indenter into the workpiece when a rate of change of the indentation load coefficient with respect to the indentation depth becomes a value sufficiently close to zero from negative, and calculating a work-affected layer depth by multiplying the indentation depth of the indenter when pressing is suspended by a depth conversion constant.
  2. The method for measuring a work-affected layer of a workpiece according to claim 1, wherein the indenter is pressed along a direction normal to a surface of the workpiece.
  3. The method for measuring a work-affected layer of a workpiece according to claim 1, wherein a load start point of the indentation load measured while pressing the indenter is defined as a time point when a moving average of a momentary change of the indentation load exceeds a predetermined threshold.
  4. The method for measuring a work-affected layer of a workpiece according to any one of claims 1 to 3, wherein the indenter is a pyramid indenter having a bottom surface which is square or a rhombic pyramid indenter having bottom surface having a major axis and a minor axis, the method further comprising the steps of: measuring, prior to machining the workpiece, an indentation load and an indentation depth by pressing the pyramid indenter having a bottom surface which is square and the pyramid indenter having a bottom surface which is rhombic into a reference test sample in an unstressed state, which is composed of the same material as the workpiece, using a feed device of the machine tool, and calculating an indentation load ratio of an indentation load of the pyramid indenter having a bottom surface which is rhombic relative to an indentation load of the pyramid indenter having a bottom surface which is square at the same indentation depth, measuring an indentation depth of the pyramid indenter having a bottom surface which is rhombic into the workpiece and a first indentation load for pressing the pyramid indenter having a bottom surface which is rhombic by pressing the pyramid indenter having a bottom surface which is rhombic in a state in which the long axis of the bottom surface is oriented in a first direction into the workpiece using a feed device of the machine tool, measuring an indentation depth of the pyramid indenter having a bottom surface which is rhombic into the workpiece and a second indentation load for pressing the pyramid indenter having a bottom surface which is rhombic by pressing the pyramid indenter having a bottom surface which is rhombic in a state in which the long axis of the bottom surface is oriented in a second direction orthogonal to the first direction into the workpiece using a feed device of the machine tool, measuring an indentation depth of the pyramid indenter having a bottom surface which is square into the workpiece and a third indentation load for pressing the pyramid indenter having a bottom surface which is square by pressing the pyramid indenter having a bottom surface which is square into the workpiece using a feed device of the machine tool, and calculating a first residual stress in the first direction and a second residual stress in the second direction at each indentation depth from the indentation load ratio, the first indentation load, the second indentation load, and the third indentation load.
  5. A method for machining a workpiece, for performing, after rough machining of a workpiece attached to a table of a machine tool with a tool installed on a spindle, measurement of a work-affected layer of a surface of the workpiece on the machine tool, and performing finishing after measurement of the work-affected layer, the method comprising the steps of: installing, after the rough machining, an indenter for indenting the workpiece on the spindle in place of the tool, pressing the indenter into the workpiece using a feed device of the machine tool, continuously measuring an indentation load for pressing the indenter and an indentation depth of the indenter into the workpiece, continuously calculating an indentation load coefficient, which is a coefficient when the indentation load is functionally approximated by the indentation depth, suspending pressing of the indenter into the workpiece when a rate of change of the indentation load coefficient with respect to the indentation depth becomes a value sufficiently close to zero from negative, calculating a work-affected layer depth by multiplying the indentation depth of the indenter when pressing is suspended by a depth conversion coefficient, and performing finishing when the calculated work-affected layer depth is less than a finishing allowance of the workpiece.
  6. A machine tool for performing measurement of a work-affected layer of a surface of a workpiece, on a machine tool for machining the workpiece attached to a table with a tool installed on a spindle, the machine tool comprising: an indenter which can be installed on the spindle in place of the tool and which forms an indentation by pressing into the workpiece using a feed device of the machine tool, an indentation load detector for continuously measuring an indentation load of the indenter on the workpiece, an indentation depth detector for continuously measuring an indentation depth of the indenter on the workpiece, an indenter pressing suspension control means for continuously calculating an indentation load coefficient, which is a coefficient when the indentation load is functionally approximated by the indentation depth, and suspending pressing of the indenter into the workpiece when a rate of change of the indentation load coefficient with respect to the indentation depth becomes a value sufficiently close to zero from negative, and a work-affected layer depth calculation means for calculating a work-affected layer depth by multiplying the indentation depth of the indenter when pressing is suspended by a depth conversion constant.
  7. A device for measuring work-affected layer which is included in a machine tool for machining a workpiece attached to a table with a tool installed on a spindle, and which is for performing measurement of a work-affected layer of a surface of the workpiece, the device for measuring work-affected layer comprising: an indenter which can be installed on the spindle in place of the tool and which forms an indentation by pressing into the workpiece using a feed device of the machine tool, an indenter holder to which the indenter is detachably attached and which has a sensor for detecting an indentation load of the indenter against the workpiece, and an arithmetic device which can be connected to a controller of the machine tool, for continuously calculating an indentation load coefficient, which is a coefficient when the indentation load is functionally approximated by the indentation depth, suspending pressing of the indenter into the workpiece when a rate of change of the indentation load coefficient with respect to the indentation depth becomes a value sufficiently close to zero from negative, and calculating a work-affected layer depth by multiplying the indentation depth of the indenter when pressing is suspended by a depth conversion constant.

Description

FIELD The present invention relates to a method for measuring a work-affected layer of a workpiece, a method for processing workpiece, a device for measuring work-affected layer, and a machine tool. BACKGROUND Workpiece materials are created through several steps including rolling, drawing, extrusion, forging, and surface treatment, resulting in a work-affected layer on the surface thereof. The workpiece material is then machined to achieve the desired shape and dimensions. Rough machining is performed during this initial stage, resulting in a work-affected layer on the workpiece surface. Residual strain in this work-affected layer can easily lead to residual stress, which can cause stress corrosion cracking, fatigue failure, and creep failure. Thus, finishing is used to remove the work-affected layer during the final product stage. In recent years, for example, difficult-to-machine materials such as nickel-based superalloys have become increasingly popular for lightweight and rigid aircraft components and for achieving both high performance and fuel efficiency in jet engines, and managing the work-affected layer of such materials has become increasingly important. Conventionally known methods for evaluating work-affected layers include, for example, observation using an optical microscope or electron microscope, X-ray diffraction (XRD), electron backscatter diffraction (EBSD), micro-Vickers testing, etc. However, these evaluation methods require cutting the workpiece and observing the cross section, as well as dedicated measurement devices, management qualifications for handling them, and measurement know-how, making them difficult to implement on a regular basis at machining sites. For example, Patent Literature 1 discloses a three-dimensional hardness distribution measurement method and system utilizing indentation testing. Indentation testing is a quasi-nondestructive testing method for evaluating the residual stress distribution in the sample depth direction from the load-displacement curve when a pyramid indenter or ball indenter indents the sample surface. FIG. 11 shows a schematic longitudinal cross-section of a sample surface indented with a pyramid indenter. FIG. 12 shows the relationship between indentation load L (vertical axis) and indentation depth h (horizontal axis) from the start of loading to unloading in an indentation test. It can be understood that the indentation load L can be approximated by a quadratic function of the indentation depth h. For example, as described in Non-Patent Literature 1, the indentation load coefficient (i.e., L/h2), which is a coefficient when approximating the indentation load L as the quadratic function, decreases as the indentation depth h increases because it is affected by the hardened or softened work-affected layer in shallow indentation depth regions. Furthermore, it is known that when the pyramid indenter is pressed deeply, the influence of the work-affected layer decreases beyond a certain depth, and the indentation load coefficient becomes roughly constant. Thus, the depth of the work-affected layer can be determined from the change in the indentation load coefficient. However, in common indentation testing methods, such as the Instrumented Indentation Technique (IIT) disclosed in Non-Patent Literature 1 and Non-Patent Literature 2, a load is applied to the workpiece until a preset indentation depth or indentation load is reached, which can leave an indentation that is deeper than necessary on the workpiece surface. For example, as shown in FIG. 11, this can occur when a pyramid indenter is indented to an indentation depth D exceeding the thickness M of the work-affected layer WL. Thus, though an indentation testing method is performed after the rough machining step to evaluate the depth of the work-affected layer and determine whether finishing is necessary in accordance with the depth, if an indentation is deeper than the amount to be removed in the finish machining step, finishing is no longer possible, and the workpiece may be deemed defective. Furthermore, in general indentation testing methods (or IIT), it is difficult to adjust the position and phase of the indenter used for indentation, making it difficult to perform an indentation test during the machining step of a machined part. [CITATION LIST] [PATENT LITERATURE] [PTL 1] Japanese Patent Application Publication No. 2021-85804 [NON-PATENT LITERATURE] [NPL 1] HIKAWA Shuhei, OKANO Shigetaka, MOCHIZUKI Masahito, HASHIMOTO Tadafumi, TEZUKA Ryo, "Depth evaluation of affected layer by instrumented indentation technique and examination of its applicability to curved surface shape", Abstract of the Japan Welding Society National Convention, Vol. 112, Japan Welding Society, April 2023, pp. 166-167 [NPL 2] OKANO Shigetaka and MOCHIZUKI Masahito, "Proposing a new semi-nondestructive measurement of non-equiaxial residual stress field using indentation technique without reference value of hardness under