A Solution to Problems in Die Wear Measurement

Abnormal wear

Abnormal wear can lead to fatal damage such as operation failure of or damage to the die cavity or core. Typical types of abnormal wear include the following five types. Among these types of abnormal wear, abrasive wear and adhesive wear occur most frequently with dies, and the wear mode of these two types is called galling.

Abrasive wear:

Abrasive wear is classified into two-body wear and three-body wear. Two-body wear occurs when a harder material digs into a softer material. Three-body wear occurs when the die surface is mechanically ground away by hard wear powder (hard particles). This wear mode is called ploughing.

Adhesive wear:

Adhesive wear is a wear phenomenon in which contacting protruding parts adhere to one another, and the adhering parts then become separated from the surface. It is also called seizure.

Fatigue wear:

This type of wear occurs due to metal fatigue resulting from repeated operating starts and stops. Repeated stress causes work hardening of the surface, which creates fine cracks over time. These small cracks eventually progress to form larger cracks, resulting in separation of the surface. The shearing stress which produces the fine cracking is highest at a point slightly below the surface. The larger cracks form starting from this point. Separation of fish scale shapes (flaking) or large sheets (peeling) can occur.

Fretting wear:

This wear forms minute pitting of the surface as a result of repeated small movement (fretting) that produces friction force at the surfaces of parts that are fit together. Fine cracking occurs on a surface where fretting wear occurs. Because external load and fretting friction force are applied in combination at these locations, fatigue strength is reduced to half or less of the original level, and may result in fatigue fracture.

Corrosion wear:

This wear occurs when surface material is removed at a sliding part due to an electrical potential difference between the metals that is generated in a corrosive atmosphere. The addition of friction further accelerates the wear damage. This is also referred to as chemical wear. It occurs due to the mechanical action of friction and the chemical reaction with a gas or liquid atmosphere. When this type of wear occurs in a liquid atmosphere, it is also referred to as erosion-corrosion.

Normal wear

This wear occurs during normal use in cases when neither abrasive wear nor adhesive wear occurs. Normal wear is classified into initial wear and steady state wear. Initial wear is also called fitting wear, and it occurs when fine surface irregularities are removed from the materials after the start of operation. Steady state wear is normal wear that occurs over continued use. By replacing a part with a new part when it reaches the set dimension used for normal wear management, it is possible to prevent die failures and defects.

Countermeasures to abrasive wear

Abrasive wear countermeasures are different for two-body wear and three-body wear.

Two-body wear:

Possible countermeasures for two-body wear include increasing the material hardness and using a steel type which contains large amounts of carbides. Because abrasion resistance generally is higher when the surface is harder, performing quenching, nitriding, or similar treatment of the die is also effective.

Three-body wear:

To prevent three-body wear, completely seal the sliding surfaces of the machine in order to prevent the entry of sand or other grinding particles. One effective countermeasure is installing a filter in the lubrication system so that such particles can be removed quickly if they enter the system.

Countermeasures to adhesive wear

Possible countermeasures to adhesive wear include using a die material that has the necessary hardness and toughness. However in fact there are limits to countermeasures which can be carried out only by improving the die material. For this reason, it is necessary to enact countermeasures that prevent adhesion by reducing the coefficient of friction between the die surface and the workpiece. Optimising lubrication is particularly important, and it is necessary to apply lubricant uniformly, design dies to prevent lubricant from running out, and optimise the processing conditions. Hard surface treatment to reduce the coefficient of friction is also effective.

Problems in die wear measurement using a profile measurement system

A profile measurement system measures and records the profile of a target by tracing its surface with a stylus. In recent years, profile measurement systems have been developed that use a laser instead of a stylus to measure complex shapes by tracing the profile in a non-contact manner. Some models are even able to perform measurement of both the top and bottom surfaces.
A profile measurement system must trace an accurate measurement line for the measurement points.

This involves the following problems.

• Measurement work requires much time, including time for fastening the die to a jig and levelling it. Knowledge and skills related to using profile measurement systems are also required in order to level a target accurately.
• The stylus of a profile measurement system moves up and down in an arc centred on the fulcrum of the stylus arm, and the tip of the stylus also moves in the X-axis direction. This produces error in the X-axis data.
• Tracing the desired line with the stylus is extremely difficult work, and even slight displacement of the stylus produces error in the measured values.
• Only part of a target can be measured, and evaluation of the entire surface is not possible.

Problems in die wear measurement using a coordinate measuring machine

Typical coordinate measuring machines measure shapes by “scanning” that uses a contact probe to trace and measure the shape. This kind of scanning measures large numbers of points at a specified pitch.

However this measurement method involves the following problems.

• From straight lines along the centre of a cylinder or curved surface to lines passing through the centre of a circle, ensuring that the probe moves as intended is an extremely difficult task. In the case of a rounded corner with a small centre angle, because the entire circle is calculated from a short arc, even a small error in measurement will be magnified largely. Such deviations in the measurement points can result in slight variation in the measured values.
• When measuring a small 3D shape, the probe may be unable to contact the measurement positions. Because measurement accuracy is proportional to the number of points or lines that are measured, it is necessary to measure a large number of points or lines.

In this way, measurement using a coordinate measuring machine involves significant problems, including the fact that not all workplace operators can accurately measure shapes, the existence of parts that cannot be measured at all, and limited locations where the machine can be installed.

Advantage 1: Measure a large number of points in as little as one second

The VR Series acquires surface data (800,000 points) for the 3D target shape in as little as one second, dramatically reducing the time required for measuring large numbers of points. It accurately and instantaneously measures the maximum and minimum irregularities on the entire target surface, enabling rapid evaluation of all locations on the target based on the preset tolerances. This makes it possible to manage difficult-to-define states such as the amount of wear or chipping.

Once a workpiece has been scanned, its profile (cross-section) can also be measured at locations different from the locations used in past measurement. This eliminates the need to set and measure the same target again.

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Advantage 2: Compare measurements between parts or against CAD data

The VR Series is capable of more than just efficiently collecting data. Measurement data can be displayed in a list, and the same analysis contents can be applied to all data at the same time.
For example, it is possible to measure the shapes of products produced using both a new die and old die, and to identify the differences in the shape resulting from the dies. In addition, it is also possible to load CAD data in order to quickly identify how much a failed product differs from a good product. This enables easy quantitative analysis and evaluation of die wear.

The VR Series can quickly measure targets that have complex shapes, something which previously required much labour and time. All measurement results are digital, significantly reducing the work required for subsequent data comparison and analysis.

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