Exploring the Application of Carbide Drill Bit Reconditioning and Coating Technologies
Summary: The bridge gearbox workshop previously used a radial drilling machine with standard high-speed steel drill bits to process the passive bevel gear hub holes.
Category: Company News
Release date: 2021-05-19
I. Brief Overview of the Project Background
The bridge gearbox workshop previously used a radial drilling machine with standard high-speed steel drill bits to process the passive bevel gear hub bores. However, as the company’s production scale continued to expand, this traditional method could no longer keep up with the required production pace. To address this issue, the workshop has now introduced advanced CNC machining centers. These CNC machines offer high processing efficiency and consistently reliable product quality, making them particularly well-suited for large-scale manufacturing.
Currently, the product being processed in the workshop is the HOWO passive bevel gear, which features 16 through-holes measuring φ17 each. The material used is SAE8822H. The raw material for this product has a hardness ranging from HB165 to 190. Due to the presence of a certain amount of Ni element, the material exhibits significant stickiness. Given these characteristics of the workpiece, selecting the appropriate drill bit and optimizing cutting parameters to enhance machining efficiency have become top priorities.
2. Selection of Drill Bit Materials
Based on the material's processing characteristics, the drill bit is required to have the following features:
1. Sufficient hardness: The hardness of the drill bit must exceed that of the workpiece being machined.
2. Sufficient strength and toughness. Since the drill bit experiences significant torsional and axial forces during workpiece machining, it must possess adequate strength and toughness.
3. Sufficient wear resistance. Since the workpiece material is highly ductile, the cutting edge must be exceptionally sharp during machining. Therefore, the tool material needs to exhibit robust wear resistance to minimize work-hardening effects.
4. The tool material should have poor affinity with the workpiece. Given the workpiece's highly reactive chemical properties, the tool material must exhibit low affinity toward titanium alloys to prevent diffusion during machining, which could lead to galling or drill breakage.
Currently, there is an extremely wide variety of materials suitable for making cutting tools, including tool steels, cemented carbides, and superhard materials, among others.
Initially, we tried experimenting with coated standard high-speed steel drill bits, which are inexpensive. After applying the coating technology, we observed a significant reduction in the friction coefficient between the tool and the workpiece, leading to improved tool life. However, due to the relatively high spindle speed of the machine tool, the drill bits used weren’t strong enough, causing them to break during the machining process—resulting in less-than-ideal outcomes.
Hard alloy is a newly developed cutting tool material suitable for both rough and finish machining of most materials, including steel, cast iron, special alloys, and plastics. Therefore, it has been decided to use solid carbide drill bits.
3. Determination of Parameters for Drill Bit Grinding
1. The main auxiliary planes of a twist drill
(1) Base Plane: If auxiliary motion is disregarded, the base plane at any point on the main cutting edge of a twist drill is the plane that passes through that point and includes the drill’s centerline. Due to the unique structural characteristics of the twist drill, the base plane varies depending on the specific location along the main cutting edge.
(2) Cutting Plane: The cutting plane at any point on the main cutting edge is the plane that contains the cutting velocity direction at that point and is tangent to the machined surface at that same point. The cutting plane is perpendicular to the reference plane, and the cutting planes at different points along the main cutting edge vary from one another.
(3) Mid-section: The plane passing through the centerline of the drill bit and parallel to the two main cutting edges is called the mid-section; there is only one mid-section.
(4) Drill Bit Cross-Section: At any point along the main cutting edge, the drill bit's cross-section is a cylindrical surface passing through that point and centered on the drill’s axis. Different points on the main cutting edge correspond to cylindrical surfaces with varying radii.
2. Selection of Main Geometric Parameters for Twist Drill Bits
(1) Selection of the drill tip angle.
The point angle is the angle formed by the projections of the two main cutting edges onto the center plane. A standard twist drill typically has a point angle of 118°. When the drill's point angle is smaller, the cutting edges become longer, producing wider chips—resulting in higher drilling torque and greater axial resistance. At the same time, the chips tend to curl into tighter spirals, occupying more space and making chip removal less efficient, which can also hinder cooling during the process.
The drill point angle determines both the width of the chip and the magnitude of the drill's rake angle. When the drill diameter and feed rate remain constant, increasing the point angle narrows the chip, thereby reducing the load on each cutting edge. At the same time, the tip angle at the outer circumference of the drill decreases, slowing down the wear rate of the cutting edge while also enhancing heat dissipation, which ultimately improves the tool's durability. Moreover, the point angle significantly influences the rake angle—increasing it accordingly helps optimize the cutting conditions near the drill center. Additionally, the point angle affects the direction in which chips are discharged: a larger point angle results in chips that curl less tightly into a spiral shape, remaining relatively straight and easier to evacuate, thus improving chip-removal performance.
According to relevant cutting tool design data, and considering that the hardness of the processed product is not high but its stickiness is relatively strong, the drill bit with a vertex angle of 140° has been selected.
(2) Selection of the rake angle and relief angle for the drill cutting edge.
Keywords: Exploring the Application of Carbide Drill Bit Reconditioning and Coating Technologies
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