Metal Cutting: Explore 6 Vital Techniques

In the metal cutting process, there are various types of workpiece materials. These materials have distinct cutting formation and removal characteristics, and it is important to understand these properties in order to effectively cut the materials.

According to the ISO standard, metal materials are classified into six groups, each with its own specific processing properties. In this article, we will provide a comprehensive overview of each group.

The six categories of metal materials are:

(1) P-Steel
(2) M-Stainless Steel
(3) K-Cast Iron
(4) N-Non-Ferrous Metals
(5) S-Heat Resistant Alloy
(6) H-Hardened Steel

Cutting characteristics of different materials

01-P Steel

What is steel?

Steel is a widely used material in the metal cutting industry. It is an alloy primarily composed of iron (Fe) and is produced through a smelting process. Steel can be either non-hardened or hardened through a quenching and tempering process, resulting in a hardness level of up to 400HB.

Non-alloy steel has a low carbon content of less than 0.8% and contains no other alloying elements besides iron. On the other hand, alloy steel contains a carbon content of less than 1.7% and additional alloying elements such as nickel (Ni), chromium (Cr), molybdenum (Mo), vanadium (V), and tungsten (W) are added to enhance its properties.

Within the metal cutting field, the P group is the largest material group, encompassing a broad range of industrial applications. The material in this group is typically characterized by the formation of long chips and the ability to produce continuous, uniform chips. The specific chip form is largely dependent on the carbon content of the material.

Materials with a low carbon content are typically tough and viscous, while those with a high carbon content are brittle.

Processing characteristics:

• Long chip material.
• Chip control is relatively easy and smooth.
• Mild steel is sticky and requires a sharp cutting edge.
• Unit cutting force kc: 1500–3100 N/mm²
• The cutting force and power required to process ISO P materials are within a limited range.

02-M Stainless Steel

What is stainless steel?

• Stainless steel is an alloy with a minimum of 11–12% chromium.
• The carbon content is usually very low (down to a maximum of 0.01%).
• The alloys are mainly Ni (nickel), Mo (molybdenum) and Ti (titanium).
• Form a layer of dense Cr2O3 on the steel surface to make it resistant to corrosion.

The Group M materials are widely used in industries such as oil and gas, pipe fittings, flanges, pharmaceuticals, and processing.

These materials have a tendency to produce an irregular, flaky chip and have a higher cutting force compared to normal steel. There are several different types of stainless steel, and their chip breaking performance (which can range from easy to almost impossible to break) depends on the properties of the alloy and heat treatment.

Processing characteristics:

• Long chip material.
• Chip control is relatively smooth in ferrite and difficult in austenite and duplex.
• Unit cutting force: 1800-2850 N/mm²
• High cutting forces, built-up edges, heat and work hardening during machining.

03-K Cast Iron

What is cast iron?

• There are three main types of cast iron: grey cast iron (GCI), ductile iron (NCI) and compacted graphite cast iron (CGI).
• Cast iron is based on Fe-C with a relatively high silicon content (1–3%).
• The carbon content exceeds 2%, which is the maximum solubility of C in the austenite phase.
• Cr (chromium), Mo (molybdenum) and V (vanadium) are added to form carbides, which increase strength and hardness but reduce machinability.

The K group materials are primarily used in the automotive, machine manufacturing, and iron-making industries.

The chip formation of these materials can vary from fine powdery chips to long chips. The power required to process this group of materials is typically low. It is important to note that grey cast iron, which typically produces fine powdery chips, differs significantly from ductile cast iron, which often resembles steel in terms of chip formation.

Processing characteristics:

• Short chip material.
• Good chip control under all conditions.
• Unit cutting force: 790-1350 N/mm²
• Processing at higher speeds can result in abrasive wear.
• Medium cutting force.

04-N Non-ferrous Metals

What is non-ferrous metals?

• This class contains non-ferrous metals and soft metals with a hardness of less than 130 HB.
• The largest part of the non-ferrous (Al) alloy containing nearly 22% silicon (Si).
• Copper, bronze, brass.

Related reading: Ferrous vs Non-ferrous Metals

The N group is dominated by the aircraft manufacturing and aluminum alloy wheel industries.

While the power required per cubic millimeter is low, it is important to accurately calculate the maximum power needed in order to achieve high metal removal rates.

Processing characteristics:

• Long chip material.
• If it is an alloy, chip control is relatively easy.
• Non-ferrous metal (Al) is sticky and requires a sharp cutting edge.
• Unit cutting force: 350-700 N/mm²
• The cutting force and power required to process ISO N materials are within a limited range.

05-S Heat Resistant Alloy

What is heat resistant alloy?

Heat Resistant Alloys (HRSA) consist of a range of high alloyed materials based on iron, nickel, cobalt, or titanium.

These materials are classified into three groups: iron-based, nickel-based, and cobalt-based. They are used in working conditions such as annealing, solution heat treatment, aging treatment, rolling, forging, and casting.

HRSA materials are characterized by their high alloy content, with cobalt providing better heat resistance and a higher tensile strength than nickel. Additionally, higher cobalt content also results in increased corrosion resistance.

Materials that are difficult to process, commonly referred to as S-materials, are primarily used in industries such as aerospace, gas turbines, and generators. These materials have a wide range of applications, but they typically require a high cutting force.

Processing characteristics:

• Long chip material.
• Difficult chip control (serrated chips).
• A negative rake angle is required for ceramics and a positive rake angle is required for cemented carbide.
• Unit cutting force:
• For heat resistant alloys: 2400–3100 N/mm²
• For titanium alloy: 1300-1400 N/mm²
• High cutting force and power required.

06-H Hardened Steel

What is hardened steel?

• From a processing point of view, hardened steel is the smallest grouping.
• This group contains quenched and tempered steel with a hardness > 45–65 HRC.
• Typically, the hardness of a hard part being turned is typically between 55–68 HRC.

Hardened steel in Group H finds application in various industries, including automotive, machine manufacturing, and mold business. During the cutting process, the steel typically produces continuous red hot chips due to the high temperatures involved. This elevated temperature helps to reduce the kc1 value and is essential in addressing application challenges.

Processing characteristics:

• Long chip material.
• Relatively good chip control.
• Require a negative rake angle.
• Unit cutting force: 2550-4870 N/mm²
• High cutting force and power required.