Limit Column Testing

Limit post inspection mainly conducts scientific testing and analysis on the dimension precision testing, geometric tolerance testing, surface hardness testing, surface roughness testing, material composition and metallographic structure analysis, and appearance and flaw detection testing, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Dimension precision testing: One of the core testing items, aiming to verify whether the actual machined dimensions of the limit posts meet the design drawing requirements. The testing contents include the tolerance band conformity of the key dimensions such as the overall heights, post body diameters, mounting hole diameters and positioning step heights and diameters. For the precision molds or high-precision assemblies, the dimension precision directly affects the positioning accuracy and repeatability of the equipment.

Geometric tolerance testing: Evaluating the shape and position errors of the limit posts, ensuring that they can correctly and stably perform the limit functions after the assembly. The main testing items include the cylindricity, straightness (for the post body axes), roundness, coaxiality (for the cylindrical surfaces of the different diameters), as well as the perpendicularity between the end faces and axes. The deviations of these parameters may cause the equipment jamming or aggravated wear.

Surface hardness testing: Through the measurement of the hardness of the limit post working surfaces, their wear resistance and plastic deformation resistance are evaluated. The commonly used testing methods include the Rockwell hardness (HRC) or Brinell hardness (HB). According to the application scenarios (such as the stamping molds and injection molds), the hardness requirements are usually between HRC 45-60, to ensure that they maintain the shape stability under the repeated impacts.

Surface roughness testing: Testing the surface micro-geometric characteristics of the limit post matching surfaces and working surfaces. The lower roughness (such as Ra 0.4-0.8μm) can reduce the friction resistance, decrease the wear and improve the positioning precision. The surface roughness profilometers or comparison specimens are usually used for the measurement, with the special attention to the key areas in contact with the plates or slides.

Material composition and metallographic structure analysis: Used to verify whether the raw materials of the limit posts meet the specified designations (such as the SUJ2 bearing steel and SKD61 die steel), and through the metallographic microscopes, observe their micro-structures after the heat treatment (such as the martensite grades and carbide distributions). This is related to the overall strengths, toughness and service lives of the limit posts.

Appearance and flaw detection testing: Visually checking whether the limit post surfaces have the defects such as the cracks, rust, scratches and nicks. For the limit posts bearing the high loads, the non-destructive testing methods such as the magnetic particle flaw detection or ultrasonic flaw detection need to be adopted, detecting whether there are the micro cracks or inclusions on the surfaces and near the surfaces, to prevent the fractures during the work.

Testing Scope

Stamping die limit posts: Applied to the various punch presses and stamping dies, limiting the die closing strokes between the upper and lower dies and preventing the die overload. The testing emphases are the extremely high compressive strengths, excellent impact fatigue resistance and the precision of the installation matching surfaces, ensuring the stability and reliability during the high-speed and high-pressure stamping processes.

Injection mold limit posts: Installed between the moving and fixed molds of the injection molds, controlling the mold opening strokes and ejection spaces. The testing needs to pay attention to their dimension stability, surface hardness and corrosion resistance (especially when using the corrosive coolants) in the long-term thermal cycling environments.

Die casting mold limit posts: The working environments are extreme, bearing the impacts and thermal stresses of the high-temperature molten metals. In addition to the routine items, the testing scope needs to specially strengthen the evaluation of the high-temperature hardness, thermal fatigue properties and thermal erosion resistance. The materials mostly adopt the hot-work die steels.

Automation equipment positioning limit posts: Used in the precision equipment such as the manipulators, conveying lines and assembly tables, realizing the precise positioning of the components or the end-of-stroke limiting. The testing core is the extremely high dimension precision and geometric tolerances, as well as the long-term dimension stability, usually requiring the micron-level precision.

Standard parts and customized parts: The testing scope covers the common standard limit posts in the market (such as the brand series products of the Punches and Misumi), as well as the non-standard limit posts specially designed according to the customer drawings. For the non-standard parts, the testing plans need to be completely customized according to their unique design functions and working conditions.

New product incoming and in-service parts periodic testing: The testing scope includes not only the incoming inspection of the newly purchased or machined limit posts, but also the periodic testing of the in-service limit posts during the equipment maintenance and overhaul, to evaluate their wear conditions, predict the remaining lives and ensure the continuous safe operation of the equipment.

Testing Methods

Coordinate measuring method: As the main method of the dimension and geometric tolerance testing, the probes of the coordinate measuring machines (CMM) are used to contact the multiple points on the limit post surfaces. Through the software to construct the three-dimensional models, the various geometric parameters are precisely calculated. This method has the high precision and good repeatability, especially suitable for the evaluation of the complex profiles and position relationships.

Precision measuring tool testing method: The traditional precision measuring tools such as the outer diameter micrometers, inner diameter dial gauges, height gauges, lever dial indicators, marble surface plates and gauge block combinations are used for the testing. Suitable for the fast on-site inspection or sampling inspection in the batch production, with the high requirements for the operation skills of the testing personnel.

Hardness testing method: According to the sizes and heat treatment states of the limit posts, the Rockwell hardness testers (HRC scales) or Brinell hardness testers (HB scales) are selected for the testing. Before the testing, the tested surfaces need to be slightly ground to remove the decarburized layers, and the measurements are taken at least at the three different positions to take the averages, ensuring the representativeness of the results.

Surface roughness profilometer method: The stylus type surface roughness measuring instruments are used. The diamond styli are driven to move across the limit post surfaces, converting the vertical displacements of the styli into the electrical signals. After the processing, the key roughness parameters such as the Ra, Rz and Rmax are obtained. During the testing, the representative and flat axial or radial sections need to be selected.

Spectroscopic analysis and metallographic inspection method: The direct-reading spectrometers or X-ray fluorescence spectrometers are used for the fast quantitative analysis of the material compositions. For the metallographic inspection, the samples need to be cut, and after the mounting, grinding, polishing and etching, the micro-structures are observed under the metallographic microscopes, evaluating the heat treatment qualities and whether there are the abnormal structures.

Non-destructive flaw detection method: The magnetic particle flaw detection is suitable for the surface and near-surface defect testing of the ferromagnetic materials; the ultrasonic flaw detection can detect the internal defects (such as the inclusions and shrinkage cavities). The operations need to follow the corresponding standards (such as ASTM E1444 and ASTM E587), recording and evaluating the suspicious signals.

Testing Standards

Domestic Standards:

1. GB/T 230.1-2018, Metallic materials — Rockwell hardness test — Part 1: Test method

2. GB/T 231.1-2018, Metallic materials — Brinell hardness test — Part 1: Test method

International Standards:

1. ASTM E1444, Standard practice for magnetic particle testing

2. ASTM E587, Standard practice for ultrasonic angle-beam contact testing

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Coordinate measuring machine: The core equipment of the high-precision geometric quantity testing, consisting of the main machines (including the motion guideways and probes), control systems and measurement software. When testing the limit posts, their spatial length measurement uncertainties are usually required to be within (2.0+3L/1000)μm, which can efficiently complete the evaluation of the complex dimension chains and geometric tolerances.

Digital hardness tester: Including the Rockwell hardness testers and Brinell hardness testers. The modern models are usually equipped with the digital displays, automatic loading systems and result storage functions, reducing the human reading errors. Need to be regularly calibrated with the standard hardness blocks, ensuring the accuracy of the value transfer.

Surface roughness measuring instrument: Consisting of the drivers, transducers (styli), pickup units and data processing units. The high-end models have the multi-parameter evaluation, graphic display and profile filtering functions. When testing the limit posts of the different materials, the appropriate stylus radii and measuring forces need to be selected according to the surface characteristics.

Precision optical instruments: Such as the tool microscopes and projectors, suitable for observing the micro-morphologies of the limit posts, measuring the small dimension features (such as the chamfers and small holes) or carrying out the fast comparative measurements. For the limit posts with the graduation lines or special marks, this type of equipment is particularly effective.

Material analysis instruments: The direct-reading spectrometers are used for the fast composition analysis; the metallographic microscopes (including the image analysis systems) are used for the structure observation and grading; the scanning electron microscopes (SEM) combined with the energy dispersive spectrometers (EDS) can be used for the in-depth analysis of the defect causes and material micro-area compositions.

Non-destructive testing equipment: The magnetic particle flaw detectors (including the magnetic yokes, coils, fluorescent magnetic particles and UV lamps) are used for the surface defect testing. The ultrasonic flaw detectors (with the probes of the different frequencies and angles) are used for the internal defect testing. The operators of these equipment need to hold the corresponding qualification certificates.

Routine measuring tools and verification equipment: Including the grade 0 marble surface plates, high-precision gauge blocks, electronic levels, and the micrometers and dial indicators of the various specifications. All the measuring tools must be included in the metrological management system, regularly sent to the qualified institutions for the verification or calibration, and the complete traceability records must be kept.