Milling Cutters and Tools -Introduction and Selection Guide

A milling tool is used on a cnc machine to remove material from the surface of a plastic or metal workpiece. These tools are of different shapes and sizes due to various purposes to achieve different types of designs. In this article, we provide you with the different types of milling cutter tools, materials, and guides to choosing the right milling tool for your CNC machining projects.

What Are Milling Tools?

Milling tools, as one kind of CNC machining tools, are critical components of milling machines, these tools consist of one or more rotating blades designed for the milling process. During operation, each blade intermittently cuts away the rest of the workpiece. Milling tools are primarily used for tasks such as milling flat surfaces, steps, grooves, shaping surfaces, and cutting workpieces. CNC milling machines and machining centers demand more capable and versatile milling tools. In recent years, the development of advanced milling tools has been driven by this demand.

In the past, especially for large-scale production, specialized tools were often the only effective solution. However, today’s production methods require more flexibility, and new milling tool concepts provide tools with broader machining capabilities. Modern tool design and manufacturing can realize more complex tool concepts, optimizing machine output, and making CNC machining processes more reliable.

Milling tools play a crucial role in processing bearings, and their use in conjunction with CNC machining centers ensures the precision of bearings.

CNC Milling Tools Materials

Tool Material property

Milling tool materials need to possess fundamental properties that significantly impact tool life, machining efficiency, machining quality, and machining costs. Milling tools must withstand high pressure, high temperatures, friction, impact, and vibrations during cutting. Therefore, milling tool materials should have the following basic properties:

Hardness and Wear Resistance: Milling tool materials must be harder than the workpiece material, typically requiring a hardness of 60 HRC or higher. The higher the hardness of the tool material, the better its wear resistance.
Strength and Toughness: Milling tool materials should have high strength and toughness to withstand cutting forces, impacts, and vibrations, preventing tool brittleness and chipping.
Heat Resistance: Milling tool materials should exhibit good heat resistance to endure high cutting temperatures and possess excellent oxidation resistance.
Processing Performance and Cost-Effectiveness: Milling tool materials should have good forging, heat treatment, and welding capabilities, as well as grinding machining performance. Additionally, they should strive for a high performance-to-price ratio.

Coatings Tool

Coating tools are one of the important methods to enhance tool performance. The introduction of coated tools has led to significant breakthroughs in cutting tool performance. Coated tools involve applying one or multiple layers of wear-resistant, refractory compounds onto a base tool with good toughness. This combination bonds the tool body with the hard coating, resulting in a substantial improvement in tool performance. Coated tools can enhance machining efficiency, improve machining accuracy, extend tool lifespan, and reduce machining costs. Approximately 80% of cutting tools used in modern CNC machine shops are coated tools. Coated tools will be the most essential tool variety in the future CNC machining field.

Characteristics of Common CNC Milling Tool Materials

Currently, widely used CNC tool materials mainly include diamond tools, cubic boron nitride tools, ceramic tools, coated tools, hard alloy tools, and high-speed steel tools, among others. There are many different grades of tool materials, and their performance varies significantly. The table below presents the main performance indicators for various tool materials:

Category		Density (g/cm3)	Thermal Resistance (°C)	Hardness	Bending strength (Mpa)	Thermal conductivity [W/(m.k)]	Coefficient of thermal expansion (x10^-6/°C)
Polycrystalline Diamond		3.47~ 3.56	700~800	>9000HV	600~1100	210	3.1
Polycrystalline Cubic Boron Nitride		3.44~ 3.49	1300~1500	4500HV	500~800	130	4.7
Ceramic Cutting Tools		3.1~5.0	>1200	91~95HRA	700~1500	15.0~38.0	7.0~9.0
Carbide	Tungsten and Cobalt	14.0~15.5	800	89~91.5HRA	1000~2350	74.5~87.9	3~7.5
	Tungsten-cobalt-titanium	9.0~14.0	900	89~92.5HRA	800~1800	20.9~62.8
	General purpose alloys	12.0~14.0	1000~1100	~92.5HRA
	TiC-based alloys	5.0~7.0	1100	92~93.5HRA	1150~1350		8.2
High speed steel		8.0~8.8	600~700	62~70HRC	2000~4500	15.0~30.0	8.0~12.0

How to choose the material for CNC milling tool

Generally, PCBN, ceramic tools, coated hard alloy tools, and TiCN-based hard alloy tools are suitable for CNC machining of ferrous metals such as steel. On the other hand, PCD tools are suitable for processing non-ferrous metals such as Al, Mg, and Cu, as well as their alloys and non-metallic materials. The table below lists some workpiece materials that are suitable for the aforementioned tool materials.

Cutting Tools	High Hardness Steel	Heat-resistant alloys	Titanium alloys	Nickel-based high-temperature alloys	Cast iron	Pure steel	High Silicon Aluminum Alloy	FRP compound material
PCD	x	x	◎	x	x	x	◎	◎
PCBN	◎	◎	o	◎	◎	o	●	●
Ceramic Cutting Tools	◎	◎	x	◎	◎	●	x	x
Coated carbide	o	◎	◎	●	◎	◎	●	●
TiC-based alloys	●	x	x	x	◎	●	x	x

Note: Symbol meanings are: ◎ excellent, o – good, ● – fair, x – unsuitable

Types of Milling Tools

In order to make milling a versatile machining process, there are various milling cutters available in the market. These milling cutters are manufactured in various sizes, shapes, and materials. Some milling cutters are made from high-speed steel (HSS), while others feature carbide alloy tool heads.

End Mills

End mills have cutting edges on both sides, making them highly versatile for various milling operations. “End mill” is a common term for flat-bottomed cutters. The primary difference between drills and end mills is that drills can only cut axially, whereas end mills can cut in multiple directions. End mills come with one or more flutes, ultimately used for various milling operations.

They are made from high-speed steel or hardened materials. This type of cutter usually has two variations, one being center-cutting, where cutting edges are on both sides of the tool, and the other being non-center-cutting, where cutting edges are only on one side.

Roughing End Mills

Roughing end mills, also commonly known as “Pippa” cutters, excel in the most demanding machining conditions. As the name suggests, they are used to remove a large volume of unnecessary material from the workpiece. Typically, these cutters have multiple serrated teeth. Roughing end mills generate very small chips, resulting in a rough surface finish.

Peripheral Milling Cutters

Milling cutters that have cutting teeth positioned around the circumference or periphery of the cutter are referred to as peripheral milling cutters. These cutters are exclusively used on horizontal milling machines.

Side Milling Cutters

Side milling cutters are another type of milling cutter that have cutting teeth positioned both around the periphery and on the face or end. Side milling cutters are typically used for slot milling operations and face milling operations. They can also be employed for cutting grooves and creating deep but narrow slots.

Face Milling Cutters

Face milling cutters consist of a large-diameter cutting body with multiple mechanically secured insertable cutting blades on top. Through the cutting stroke of a face milling cutter, a substantial amount of unwanted material can be removed by radial depth and axial narrow cutting. The diameter of the face milling cutter body is typically determined by the length of the workpiece and the available clearance on both sides of the workpiece. These face milling cutters can also be used for contour milling operations. Face milling cutters are highly rigid cutting tools, and the surface finish they provide depends on the feed rate and the number of teeth on the cutter.

Contour Milling Cutters

Contour milling cutters fall within the category of form tools. Form tools are typically designed to create specific shapes on the workpiece. This type of milling cutter is a specially manufactured tool designed to correspond with the convex profile of a circular contour. The circular contour is usually equal to or smaller than a semicircle.

Woodruff Cutters

“Woodruff” cutters are typically used for cutting keyways in wood materials. The edge of the woodruff cutter is slightly concave, and its teeth are not used for side cutting. There are two types of tooth shapes for woodruff cutters: straight teeth and staggered teeth.

Thread Milling Cutters:

Thread milling cutters are used to cut the teeth shapes of external and internal threads on workpieces. The cutting process using thread milling cutters can produce single-pitch threads ranging from M2 to a nominal diameter of 1 millimeter or variable-pitch threads.

Ball End Mill

Ball end mills are commonly referred to as ball nose end mills. These milling cutters are named for their cutting end, which is hemispherical in shape. These cutters are typically used to reduce stress concentration during machining operations. They are often suitable for machining three-dimensional curved surface shapes on workpieces.

Fly Cutter

A fly cutter consists of a body into which one or two cutting heads are inserted. As the cutting heads rotate, they perform either narrow or wide cuts. Face mills are more versatile in various situations but tend to be more expensive. These fly cutters can perform similar machining tasks as face mills, but they are more budget-friendly, although less efficient in terms of cutting speed compared to face mills.

How to choose a milling tool?

Milling Cutter Diameter

The choice of milling cutter diameter varies significantly depending on the product and production batch. The selection of tool diameter primarily depends on the equipment specifications and the workpiece’s machining dimensions.

Flat Milling Cutter: When selecting the diameter of a face milling cutter, it is essential to consider that the power required by the tool should fall within the machine tool’s power range. You can also choose the diameter based on the spindle diameter. The face milling cutter diameter can be selected as D=1.5d (where d is the spindle diameter). For large-scale production, the tool diameter can be selected as 1.6 times the width of the workpiece.
End Mill: The selection of end mill diameter should primarily meet the requirements of the workpiece’s machining dimensions, ensuring that the power required by the tool falls within the rated power range of the machine tool. For smaller diameter end mills, the critical consideration should be whether the machine tool’s maximum speed can reach the minimum cutting speed of the tool (60m/min).
Slotting Cutter: The diameter and width of the slotting cutter should be chosen based on the dimensions of the workpiece being machined, and its cutting power should fall within the allowable power range of the machine tool.

Milling Cutter Inserts

For precision machining, it is advisable to use ground inserts. These inserts offer excellent dimensional accuracy, which results in high positioning accuracy of the cutting edges during milling. This leads to improved machining precision and surface roughness.
For rough machining, it is preferable to use pressed inserts, which can help reduce machining costs. While the dimensional accuracy and sharpness of pressed inserts may be lower than ground inserts, they possess superior edge strength and can withstand high cutting depths and feeds during rough machining.
Sharp inserts with a large lead angle can be used for milling sticky materials like stainless steel. The sharp edge of these inserts reduces friction between the insert and the workpiece material, allowing chips to be removed more efficiently from the cutting edge.

Milling Cutter Bodies

First and foremost, when selecting a milling cutter, the number of teeth must be considered. The size of the tooth pitch determines the number of teeth engaged in cutting simultaneously, affecting the smoothness of cutting and the requirements for cutting speed on the machine tool. Coarse-toothed milling cutters are commonly used in rough machining because they feature larger chip flutes. At the same feed rate, coarse-toothed milling cutters subject each tooth to a higher cutting load than fine-toothed cutters.
For precision milling with shallow cutting depths (typically 0.25-0.64mm), it is recommended to choose fine-toothed milling cutters.
In heavy-duty rough milling processes, excessive cutting forces can lead to vibrations in low-rigidity machine tools. These vibrations can cause carbide inserts to chip prematurely, thereby reducing tool life. Using coarse-toothed milling cutters can lower the power requirements for the machine tool. Hence, when the spindle hole size is small (e.g., R8, 30#, 40# taper holes), coarse-toothed milling cutters can effectively perform milling operations.

Conclusion

Whether choosing a milling cutter for a CNC machine shop or conventional milling machines, it is essential to consider factors such as the material and hardness of the workpiece, as well as the specifications of the milling cutter, including parameters like flute length, overall length, cutter diameter, shank diameter, and more. High-speed steel milling cutters are typically suitable for conventional milling machines, while CNC milling machines generally prioritize the use of carbide tooling.

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