Stepped Twist Drill Testing

Stepped twist drill inspection mainly conducts scientific testing and analysis on the key performance indicators, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Stepped Twist Drill Testing Items

Geometric dimension and tolerance testing: This is the core testing item, including the precise measurement of the diameters of the various steps, step lengths, total length, drill point angle and helix angle. High-precision measuring tools must be used to ensure that the dimensions meet the requirements of the design drawings. The tolerances are usually controlled at the micron level, to ensure the positioning precision and dimensional consistency of the drilled holes.

Surface roughness testing: Mainly detects the roughness of the key working surfaces such as the margin, chip flute and flank of the drill. The roughness value directly affects the cutting force, chip evacuation performance and the quality of the machined hole wall. A surface roughness profilometer is usually used for the measurement, ensuring that the Ra value is within the specified range, to avoid the cutting heat accumulation and premature wear caused by the excessively rough surface.

Cutting edge quality and sharpness testing: Checks whether the cutting edge is straight, free of the chipping and burrs, as well as the micro-sharpness of the cutting edge. Usually, a tool microscope or high-power projector is used for the observation. The sharp cutting edge can reduce the cutting resistance and improve the machining efficiency and hole wall finish. The passivation or edge chipping will directly lead to the decline of the drilling quality or even the drill damage.

Material hardness and metallographic structure testing: The hardness of the working part of the drill is detected with a Rockwell or Vickers hardness tester, ensuring that it reaches the specified value (such as the hardness of the high-speed steel HSS usually at 63-66 HRC). The metallographic testing analyses the microstructure of the material, such as the carbide distribution and grain size, to verify whether the heat treatment process is qualified. This is the fundamental guarantee of the wear resistance and toughness of the drill.

Concentricity and radial run-out testing: A precision run-out tester or coordinate measuring machine is used to detect the concentricity between the shank of the drill and the various stepped cutting parts. The excessive radial run-out will cause the hole diameter enlargement, position deviation and abnormal wear of the drill. For the stepped twist drills, the concentricity requirements between the various steps are particularly strict.

Coating quality testing (if applicable): For the coated stepped drills (such as the TiN and TiAlN coated), the coating thickness, uniformity, adhesion and surface hardness need to be detected. The X-ray fluorescence thickness gauge, scratch method adhesion tester and micro hardness tester can be adopted. The good coating can significantly reduce the friction coefficient and improve the wear resistance and service life.

Stepped Twist Drill Testing Scope

Ex-factory inspection of the new drills: The comprehensive quality inspection carried out by the manufacturers before the ex-factory of the drills, ensuring that each batch of products meets the enterprise standards or the relevant industry standards. It covers all the key dimensions, material performances, appearance and packaging marks, and is the first gate of the quality control.

Incoming inspection: The verification testing carried out by the users (such as the machining plants) after the procurement of the stepped twist drills and before the warehousing. Usually, the sampling inspection is adopted, emphatically checking the key dimensions, material certificates and appearance, to prevent the unqualified products from entering the production process.

Pre-use state confirmation: The quick inspection carried out by the operators or quality inspectors before the drill is installed on the machine tool for the key process machining. Mainly including the visual inspection of whether the cutting edge is intact, measuring the key step diameters, and confirming that the drill has no bending deformation, to prevent the machining accidents.

Wear monitoring during use: During the batch production, the wear state of the drill is regularly spot-checked and monitored. Mainly checking the cutting edge wear band width (VB value), whether the chipping occurs and whether the coating peels off, to judge the remaining life of the drill, implement the predictive tool change and avoid the batch defective products.

Quality testing after the re-grinding: After the re-grinding of the stepped twist drill, the strict testing must be carried out to ensure that the geometric parameters after the re-grinding (such as the drill point angle, web thinning state and the lengths of the various steps) are restored to the usable state. The re-ground drills failing the testing will seriously affect the quality of the subsequent machining.

Failure analysis and quality arbitration: The in-depth testing and analysis carried out when the early failure (such as the fracture and abnormal wear) of the drill occurs or the machining quality disputes arise. Through the comprehensive analysis of the macroscopic morphology, microstructure and hardness gradient, the root causes of the failure are found and the responsibility attribution is clarified.

Stepped Twist Drill Testing Methods and Techniques

Contact measurement method: The traditional measuring tools such as the outer diameter micrometers, lever micrometers and depth gauges are used for the direct measurement. The method is intuitive with the low cost, and is suitable for the rapid detection of the macroscopic dimensions such as the diameters and lengths. However, it has the high requirements for the operator skills, the relatively low measurement efficiency, and is difficult to measure the complex geometric shapes.

Optical projection comparison method: A tool microscope or projector is used to enlarge and project the drill profile onto the screen for the comparison with the standard profile drawings or CAD drawings. This method is very suitable for detecting the complex profiles such as the drill point angle, helix angle and cutting edge shape, with the strong intuitiveness, and is the common means of the geometric shape testing.

Coordinate measuring machine (CMM) testing method: This is a high-precision and comprehensive testing method. The probe contacts the multiple points on the drill surface, and the software reconstructs the three-dimensional model, which can precisely obtain the data such as the dimensions, geometric tolerances (such as the concentricity and position tolerance) and profile tolerances. It is suitable for the comprehensive testing and reverse engineering of the high-precision stepped drills.

Non-contact optical scanning method: A laser scanner or structured light three-dimensional scanner is used to obtain the complete point cloud data of the drill surface. It is fast without the contact stress, and can completely reproduce the three-dimensional morphology of the drill. It is particularly suitable for detecting the surface micro-morphology, coating uniformity and carrying out the full-size comparison analysis.

Physical and chemical property analysis method: The material hardness is tested at the specific positions near the cutting edge with a Rockwell hardness tester or micro Vickers hardness tester; the metallographic structure needs to be sampled, mounted, polished and etched, and then observed and rated under a metallographic microscope; the coating performance needs to be analysed with the large instruments such as the XRD and SEM/EDS for the composition and structure analysis.

Functional trial cutting testing method: The most direct testing method, that is, the trial drilling is carried out under the standard working conditions (the specified workpiece material, cutting parameters and cooling conditions). Through the detection of the actual dimensional precision, position tolerance and surface roughness of the machined holes, as well as the observation of the chip morphology and the listening to the cutting sound, the comprehensive performance of the drill is comprehensively evaluated.

Testing Instruments and Equipment

High-precision digital measuring tools: Including the digital outer diameter micrometers, digital depth gauges and dial callipers with the tables. These equipment have the data output functions, with the resolution up to 1 μm, and can quickly and accurately read the diameters and length dimensions of the various steps, reducing the human reading errors and improving the testing efficiency and record traceability.

Tool microscope and image measuring instrument: The tool microscope has the eyepiece and objective lens magnification systems, used for the micro-observation and measurement; the image measuring instrument uses the high-resolution CCD camera and image processing software to automatically measure the angles, radii and distances. Both are the key equipment for detecting the cutting edge conditions and geometric angles.

Surface roughness measuring instrument: Usually a stylus-type profilometer. The diamond probe moves on the workpiece surface, the vertical displacements are recorded through the sensor, and the roughness parameters such as the Ra and Rz are output after the processing by the processor. It is specially used to quantitatively evaluate the smoothness of the surfaces such as the flutes and margins of the drill.

Coordinate measuring machine: A precision equipment in the testing field, composed of the mechanical main body, probe system, control system and computer software. It has the high measurement precision and good repeatability, can complete almost all the geometric measurements of the stepped twist drill, and generate the detailed testing reports. It is the core equipment of the high-end quality control.

Hardness testers: The Rockwell hardness tester (such as the HRC scale) is used for the rapid detection of the overall hardness of the drill; the micro Vickers hardness tester is used for the hardness testing of the small areas or specific points (such as the cutting edge, coating surface and substrate), and can analyse the material characteristics through the hardness indentations.

Run-out tester and balancing machine: The run-out tester, combined with the dial indicators or lever indicators, is used to detect the radial run-out and face run-out after the drill clamping. For the drills for the high-speed applications, a balancing machine also needs to be used to detect and correct the unbalance, to prevent the vibration during the high-speed rotation, which affects the machining precision and the spindle life of the machine tool.

Dedicated profile tester and optical scanning equipment: For the complex curved surfaces such as the helical flutes, a dedicated profile tester can be used to scan along the cross-sections. The more advanced ones adopt the laser three-dimensional scanners to rapidly obtain the full-size three-dimensional data, and carry out the comparison with the CAD models, generating the colour-coded deviation maps, intuitively displaying the out-of-tolerance areas.

Testing Standards

Domestic Standards:

1. GB/T 228.1-2021, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

2. GB/T 229-2020, Metallic materials — Charpy pendulum impact test method

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

4. GB/T 232-2010, Metallic materials — Bend test method

5. GB/T 10125-2021, Corrosion tests in artificial atmospheres — Salt spray tests

International Standards:

1. ISO 235, Parallel shank twist drills

2. ISO 6892-1, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

3. ISO 148-1, Metallic materials — Charpy pendulum impact test — Part 1: Test method

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.