Due to the complexity of the tool application, the professional grinding center must timely modify the grinding scheme according to the failure mode of the tool being polished, and also track the use of tools.
A professional tool grinding center must also continuously summarize its experience in order to grind tools better and more professionally.
The following will introduce the six problems that must not be ignored in tool grinding from the perspective of the hardware, like tool materials, grinding equipment, testing equipment and other perspectives like grinding standards, grinding technicians and so on.
Common tool materials in tool grinding:
High speed steel tools are sharp and tough while cemented carbide tools have high hardness but poor toughness.
The density of cemented carbide tools is significantly higher than that of high speed steel tools.
These two materials are the main materials for drills, reamers, milling cutters and taps.
The performance of powder metallurgy high speed steel lies between these two materials, which are mainly used for manufacturing rough milling cutters and taps.
High-speed steel tools are less sensitive to collisions due to their good material toughness.
However, carbide tools are high in hardness and brittle, sensitive to collisions, and easy to break the cutting edge.
Therefore, during the grinding process, care must be taken in the handling and placement of carbide tools to prevent collisions between the tools or the falling of the tools.
Because the precision of high-speed steel tools is relatively low, their grinding requirements are not high, so are the prices.
Therefore, many manufacturers set up their own tool workshops to grind them.
However, carbide tools often need to be sent to a professional grinding center for grinding.
According to statistics from some domestic tool repair centers, more than 80% of the repair tools are made of carbide.
Because the tool material is very hard, so it’s generally only possible to change its shape by grinding.
There are several common tool grinders in tool manufacturing and grinding:
Notch grinder: Grind the groove or back of the tools such as grind drill or end mill.
Vertex angle grinder: Grind the conical vertex angle (or be called eccentric relief angle) of the drill bit.
Trimming machine: Correct the chisel edge of the drill.
Manual universal tool grinder: Grinding outer circle, groove, back, vertex angle, chisel edge, plane, rake face, etc. Often used for grinding tools with small numbers and complex shapes.
CNC grinding machine: Generally it is a five-axis linkage, the function is determined by software. Generally used for grinding those uncomplicated tools with a large number and high precision requirements, such as drills, end mills, reamers, etc. The main suppliers of these grinding machines are from Germany, Switzerland, the United States, Australia and Japan.
3.1. Abrasive particle
Abrasive particles of different materials are suitable for grinding tools of different materials.
Different sizes of abrasive grains are required for different parts of the tool to ensure the best combination of edge protection and machining efficiency.
Alumina: Used for grinding HSS tools. Grinding wheels are cheap and easy to modify into different shapes for grinding complex tools (corundum).
Silicon Carbide: Used to modify CBN and diamond wheels.
CBN (Cubic Boron Carbide): Used for grinding HSS tools. High price but durable.
Internationally, grinding wheels are represented by B, such as B107, where 107 represents the size of the abrasive particle diameter.
Diamond: Used for grinding HM tools, high price, but durable. Be represented by D on the grinding wheel, such as D64, where 64 represents the size of the abrasive particle diameter.
In order to grind the different parts of the tools expediently, the grinding wheel should have different shapes.
The most commonly used are:
Straight wheel(1A1): Grinding angle, outer diameter, back, etc.
Saucer wheel (12V9,11V9): Grinding spiral grooves, main and auxiliary cutting edges of milling cutters, trimming edges, etc.
After using the wheel for a period of time, its shape (including plane, angle and rounded corner R) needs to be modified. Grinding wheels must often use cleaning stones to remove the chips filled between the abrasive grains to improve the grinding ability of the wheel.
Whether you have a good set of tool grinding standards is a standard to judge whether a grinding center is professional.
In the grinding standard, the technical parameters of the cutting edge of different tools when cutting different materials are generally specified.
Including blade inclination angle, apex angle, rake angle, back angle, inverted blade, chamfer, etc.
In cemented carbide drills, the process of passivating the cutting edge is called an “Inverted blade”.
The width of the inverted blade is related to the material being cut, and is generally between 0.03 to 0.25mm.
The process of chamfer on edges(tool tip) is called “Chamfering”.
Every professional company has its own grinding standards that have been summarized over many years.
Difference between HM drill and HSS drill:
HSS drill bits: The vertex angle is generally 118°, sometimes greater than 130°. The blade is sharp and the requirements for accuracy (blade height difference, symmetry and circumferential run-out) are relatively low. There are many ways to repair the blade.
HM drill bit: The top angle is generally 140°, the straight groove drills are often 130°. Three-edged drills are generally 150°. The blade and the tip (on the edges) are not sharp and are often passivated, or called inverted blade and chamfer, which require high accuracy. The chisel edge is often trimmed into an S-shape to facilitate chip breaking.
The back angle of the cutting edge is very important for the tool. If the back angle is too large, the blade is easy to snap and “stick”; if the back angle is too small, the friction will be too large and it is not good for cutting.
The back angle of the tool varies with the material being cut, the type of tool, and the diameter of the tool.
Generally speaking, the back angle decreases as the diameter of the tool becomes larger.
In addition, the back angle is smaller if the material being cut is hard, otherwise, is larger.
Tool detection equipment
Tool inspection equipment is generally divided into three categories: tool setting gauge, projector and universal tool measuring instrument.
The tool setting gauge is mainly used for tool setting preparation (length) of CNC equipment like machining centers, and also used to detect parameters such as angle, radius and step length.
The function of the projector is also used to detect parameters such as angle, radius, and step length.
However, the above two cannot generally measure the back angle of the tool while the universal tool measuring instrument can measure most of the geometric parameters of the tool, including the back angle.
Therefore, the professional tool grinding center must be equipped with a universal tool measuring instrument.
However, there are not many suppliers of this equipment. German and French products are available on the market.
The best equipment also needs to be operated by personnel. The training of grinding technicians is naturally one of the most critical links.
Due to the relative backwardness of China’s tool manufacturing industry and the serious lack of vocational and technical training, the training of tool grinding technicians can only be solved by enterprises themselves.