To ensure optimal tool application performance, it is essential for the professional grinding center to adjust the grinding strategy in a timely manner based on the failure mode of the tool being polished and monitor tool usage.
To further improve its efficiency and experience, the professional tool grinding center should regularly analyze its experiences and apply the knowledge gained to its grinding processes.
This article will highlight six critical factors that must be taken into consideration during tool grinding, including the hardware components such as tool materials, grinding equipment, and testing equipment, as well as other factors such as grinding standards, grinding technicians and much more.
Tool material
In tool grinding, common materials include high-speed steel, powder metallurgy high-speed steel, carbide, and some super-hard materials such as polycrystalline diamond (PCD), cubic boron nitride (CBN), and cermet.
High speed steel tools are known for their sharpness and durability, while carbide tools have high hardness but low toughness.
It is important to note that the density of carbide tools is much higher than that of high-speed steel tools.
These two materials are mainly used for the production of drills, reamers, milling cutters and taps.
The performance of high-speed steel produced through powder metallurgy falls between two materials commonly used for manufacturing roughing cutters and taps.
High-speed steel tools are known for their good material toughness, making them less susceptible to breakage in the event of collisions.
On the other hand, carbide tools have high hardness but are brittle and prone to breaking if subjected to collisions. Therefore, great care must be taken during the grinding process to avoid collisions between tools or accidental dropping of tools.
Given the relatively low accuracy of high-speed steel tools, they require less stringent grinding specifications and are therefore more affordable. This is why many manufacturers have established their own in-house tool grinding workshops.
On the other hand, carbide tools often need to be sent to professional grinding centers for proper maintenance.
Statistics from several national tool repair centers indicate that more than 80% of repaired tools are made of carbide.
Tool Grinder
Due to the hardness of the tool material, it can normally only be reshaped by grinding. There are several tool grinders common in tool manufacturing and grinding, including:
- Slot grinder: Used to grind grooves or backs of tools such as drills or end mills.
- Vertex angle grinder: Used to grind the conical apex angle (also known as eccentric relief angle) of drill bits.
- Trimming machine: Used to correct the chisel edge of drill bits.
- Hand Universal Tool Grinder: It can be used to grind the outer circle, groove, back, apex angle, chisel edge, plane, rake face, etc.
- CNC grinding machine: Typically it is a software-controlled five-axis linkage machine. It is mainly used for grinding simple tools with high precision and large quantity requirements, such as drills, end mills and reamers. The main suppliers of these grinding machines are located in Germany, Switzerland, the United States, Australia and Japan.
Grinding wheel
3.1. Abrasive particle
Different materials require different abrasive particles for your grinding tools. The size of the abrasive grains must be appropriate for the specific part of the tool to achieve the ideal balance between edge protection and machining efficiency.
Alumina: This material is used to grind high-speed steel (HSS) tools. Grinding wheels made from alumina are economical and can be easily shaped to grind complex tools such as corundum.
Silicon Carbide: This material is used to modify cubic boron nitride (CBN) and diamond grinding wheels.
CBN (Cubic Boron Nitride): CBN is used to grind HSS tools. Although it is more expensive, it is also more durable.
Globally, grinding wheels are designated by the letter “B”, such as B107, where 107 represents the diameter of the abrasive particle.
Diamond: This material is used to grind high-speed steel tools and is known for its durability despite its high cost. Diamond grinding wheels are identified by the letter “D”, such as D64, where 64 represents the diameter of the abrasive particle.
3.2. Form
For efficient grinding of various tool components, a grinding wheel with different shapes is required. The two most commonly used forms are:
- Straight grinding wheel (1A1): Used for grinding angles, external and rear diameters, among others.
- Saucer disc (12V9, 11V9): Ideal for grinding spiral channels, main and auxiliary cutting edges of milling cutters, grinding edges, among others.
It is important to note that after a period of use of the wheel, its shape, including plane, angle and rounded corners (R), may need to be modified. To maintain the grinding wheel's grinding ability, it is common to use cleaning stones to remove chips that have accumulated between the abrasive grains.
Grinding Patterns
Having a comprehensive set of tool grinding standards is a criterion for evaluating the professionalism of a grinding center.
Typically, grinding standards specify the technical parameters of the cutting edge for different tools when cutting various materials, such as blade inclination angle, apex angle, inclination angle, back angle, inverted blade and chamfer.
In carbide drills, the cutting edge passivation process is known as “inverted blade”, the width of which depends on the material to be cut and normally varies from 0.03 mm to 0.25 mm.
The process of creating a chamfer on the tip of the tool is known as “chamfering”.
Each professional company has its own set of grinding standards that have been developed and refined over many years.
Difference between HM drill and HSS drill:
HSS Drills: The apex angle is normally 118°, but may be greater than 130° in some cases. The blade is sharp and the precision requirements such as blade height difference, symmetry and circumferential deviation are relatively low. There are several methods to repair the blade.
HM Drills: The top angle is usually 140°, while straight slot drills usually have an angle of 130°. Three-edge drills typically have an angle of 150°. The blade and tip (at the edges) are not sharp and are often passivated through processes known as inverted blade and bevel, which require high precision. The edge of the chisel is usually S-shaped to help with chip breaking.
Back angle:
The angle at the back of the cutting edge is crucial to the effectiveness of the tool. If the angle is too steep, the blade will be more likely to break and get stuck, and if the angle is too shallow, the friction will be excessive and impede the cutting process.
The back angle of a tool can vary based on the material being cut, the type of tool, and the diameter of the tool.
As a general rule, the back angle decreases as the tool diameter increases.
Additionally, the back angle is smaller for cutting harder materials and larger for softer materials.
Tool Detection Equipment
Tool inspection equipment can be classified into three categories: tool setting gauges, projectors, and universal tool measuring instruments.
Tool setting gauges are mainly used to prepare tool setting length in CNC machines such as machining centers, and can also be used to detect parameters such as angle, radius and step length.
Projectors also have the ability to detect parameters such as angle, radius and step length.
However, neither of these two options can typically measure the back angle of a tool. On the other hand, a universal tool measuring instrument can measure most of the geometric parameters of the tool, including the back angle.
As a result, professional tool grinding centers must have a universal tool measuring instrument.
Unfortunately, suppliers of this type of equipment are limited and German and French products are available on the market.
Grinding Technician
Even the best equipment needs to be operated by qualified personnel. Therefore, the training of grinding technicians is a crucial aspect.
Unfortunately, the Chinese tool manufacturing industry is relatively underdeveloped and there is a serious shortage of vocational and technical training. As a result, training of tool grinding technicians must be done by individual companies.
