5 Basics You Should Know About Metal Cutting

Metal materials will be extruded by cutting tools and deform in the cutting process.

This physical phenomenon directly affects cutting force, cutting temperature, tool wear, machined surface quality and production efficiency.

Therefore, it is necessary to study it and understand its basic laws.

(1) Three deformation zones during cutting

Taking the cutting of plastic metal as an example, the essence of the metal in the cutting layer changing into chips and separating from the matrix is that the surface material of the workpiece is strongly squeezed by the cutting edge and the rake face of the tool during the machining process, resulting in continuous elastic deformation plastic deformation fracture failure, so that the cutting metal is continuously transformed into chips and flows out of the rake face, as shown in Fig. 1-9.

Fig. 1-10 is a schematic diagram of three deformation zones in the cutting layer during low-speed cutting.

cutting deformation

Fig. 1-9 cutting deformation

  • a) Schematic diagram of cutting deformation
  • b) Metallographic diagram of cutting deformation
three deformation zones formed during cutting

Fig. 1-10 three deformation zones formed during cutting

1. First deformation zone

When the rake face of the tool squeezes the cutting layer at the cutting speed vc, a point in the cutting layer begins to produce shear slip along the OA surface until its flow direction begins to be parallel to the rake face of the tool, and there is no slip along the OM surface.

The chip formed in the cutting layer flows out along the rake face of the tool.

The plastic deformation starts from the OA surface and the shear slip from the OM surface is basically completed.

This area is called the first deformation area.

The main characteristics of the first deformation zone are shear slip deformation along the slip surface and subsequent work hardening.

2. Second deformation zone

When the chip formed by shear slip flows out of the tool rake face, the chip bottom layer is further squeezed and rubbed by the tool, causing the metal near the tool rake face to produce shear deformation again, which is called the second deformation area.

3. Third deformation zone

It is the contact area between the workpiece and the tool flank, which is squeezed and rubbed by the tool edge and the tool flank, resulting in the deformation of the machined surface, which is called the third deformation area.

This is because in the actual cutting, the cutting edge of the tool inevitably has a blunt circle radius rn, which makes the extruded layer subject to the tensile and friction of the tool back surface again, further produce plastic deformation, and aggravate the deformation of the machined surface.

(2) Chip morphology

Due to the different properties of processing materials and cutting conditions, the degree of deformation in the cutting process is different.

According to the micro deformation degree in the cutting process, as shown in Fig. 11-1.

types of chips

Fig. 1-11 types of chips

  • a) Banded chip
  • b) Nodal chip
  • c) Granular chip
  • d) Chipping chip

1. Strip chip

The chips are continuously banded, the inner surface is smooth, and the outer surface has no obvious cracks, which is in micro sawtooth shape.

Generally, plastic metal materials (such as low carbon steel, copper and aluminum) are processed with large tool rake angle γ o.

When the nominal thickness of the cutting layer is smaller, hD, and the cutting speed is higher, vc, this kind of chip is most likely to form.

When forming strip chips, the cutting force fluctuation is small, the cutting process is relatively stable, and the machined surface roughness value is small, but chip breaking measures need to be taken to ensure normal production, especially the production of automatic production line and automatic machine tool.

2. Nodal chip

There are deep cracks on the outer surface of the chip, showing a large sawtooth shape, and sometimes cracks on the inner surface.

Generally, metal materials with low plasticity (such as brass) are processed at the front corner of the tool γ O small, the nominal thickness of the cutting layer hD is large, the cutting speed vc is low, or the processing of carbon steel materials is not rigid enough in the process system, it is easy to form this kind of chip.

When nodal chips are formed, the cutting force fluctuates greatly, the cutting process is not stable, and the machined surface roughness value is large.

3. Granular chip

Also known as chip unit.

When cutting plastic materials, if the shear stress on the whole shear plane exceeds the breaking strength of the material, the resulting crack runs through the chip section, and the extrusion crack is granular chip.

This kind of chip is easy to form when using small rake angle or negative rake angle to cut at very low cutting speed and large nominal thickness of cutting layer.

When granular chips are formed, the cutting force fluctuates greatly, the cutting process is unstable, and the machined surface roughness value is large.

4. Chipping

When machining brittle and hard materials, the cutting layer is usually extruded and cracked without plastic deformation after elastic deformation, forming irregular broken chips.

The more brittle and hard the workpiece material, the smaller the rake angle of the tool and the larger the nominal thickness of the cutting layer, the easier it is to produce chipping chips.

When chipping chips are formed, the cutting force fluctuates greatly, the cutting process is unstable, and the cutting layer metal is concentrated at the cutting edge, which is easy to damage the tool, and the machined surface roughness value is large.

(3) Chip shape

In actual production, the treatment and transportation of chips is an important problem to be solved.

The main factor affecting the handling and transportation of chips is the shape of chips. Therefore, it is also necessary to classify according to the macro shape of chips.

Different workpiece materials, tool geometric parameters and cutting parameters will lead to different chip shapes.

From the perspective of chip treatment, the shapes of chips generally include banded chips, C-shaped chips, collapse chips, spiral curls, long and tight curls, wound curls, pagoda chips and random chips, as shown in Table 1-1.

Cutting shapeShape picture  Formation conditions Shape featureInfluence on machining process
Strip cutting Strip cuttingWhen high-speed cutting plastic metal materials, if appropriate chip breaking measures are not taken, it is easy to form banded chips.The strip chips are continuous and often wound on the workpiece or tool.Pulling the workpiece surface or breaking the chip edge may even hurt people. Therefore, the formation of banded chips is usually avoided as far as possible.
C-shaped chip C-shaped chipWhen turning general carbon steel and alloy steel workpieces, the turning tool with chip curling groove is adopted, which is easy to form C-shaped chips when the parameters are reasonable.The shape is similar to the “C” shape. The C-shaped chip will not be wound on the workpiece or tool. It has a moderate length and is not easy to hurt people. It is a better chip shape.Most of the C-shaped chips collide and break on the back face of the turning tool or the surface of the workpiece. The high-frequency collision and breaking of chips will affect the stability of the cutting process and have a certain impact on the surface roughness. This chip shape shall be avoided during fine turning.
Chipping chip Chipping chipThis kind of chip is easy to form when turning brittle materials such as cast iron and brittle brass.Cut into needles or fragmentsIt is easy to splash, which may hurt people and damage the sliding surface of the machine tool, such as the guide mechanism of the machine tool.
Spiral chip Spiral chipThis chip shape can be obtained when the depth, width and angle of the tool chip breaking groove are appropriate.Spiral curl, no gap, flowing out along the straight direction, easy to clean.It is desirable to form chips of this shape during finish turning.
Long and tight curls Long and tight curlsWhen it is required to form long and tight curls, the geometric parameters and cutting parameters of the tool must be strictly controlled.Spiral curl, no gap, flowing out along the straight direction, easy to clean.The formation process of long and tight curls is relatively stable, which is convenient for management. It is a better chip shape on horizontal lathe.
Winded curl Winded curlTurning steel parts with large back draft and large feed on a heavy-duty lathe increases the arc radius of the groove bottom of the chip curling groove, so that the chip curls into a hair strip.It is in the shape of “spring”. It breaks on the processing surface of the workpiece and falls by its own weight.It is necessary to protect the cutting edge from damage and the safety of the operator.
Pagoda shaped curl Pagoda shaped curlWhen the depth, width and angle of the chip cutting groove are suitable, the chip shape can be obtained.The shape is similar to the “pagoda” shape, which is easy to clean.It will not wrap around the workpiece or tool, and it is more convenient to clean. It is a better chip shape. Suitable for cutting on automatic machine tools or automatic lines.
Chaotic crumbChaotic crumbWhen the cutting speed increases, the machining allowance is small, and the chip breaking groove does not work, the chip breaking is bad and tends to be disorderedIrregular shape, chips stretch but not rollIt is often wound on workpieces, cutting tools and machine tools, causing difficulties in processing and easy to scratch the tools and workpieces

It can be seen from Table 1-1 that the specific conditions of cutting are different, and the shape of chips is also required to be changed.

It is of no practical significance to evaluate the quality of a chip shape in isolation from specific conditions.

Table 1-2 shows the influence of cutting conditions on chip shape.

Table 1-2 effect of cutting conditions on chip breaking

 Influence of cutting conditions on chip shape
Feed rateWith the increase of feed rate, chip breaking is improved, but the surface quality becomes worse, which is suitable for rough machining
Back biteThe back feed increases and the chip breaking becomes worse
Anterior hornNegative rake angle is conducive to chip breaking, but the surface quality becomes worse, which is suitable for rough machining
Principal deflection angleThe main deflection angle increases and the chip breaking becomes better
Ground chip breaking grooveThe ground chip breaking groove has good chip breaking

In the cutting process, the metal deformation caused by tool extrusion will directly affect the important parameters such as tool wear and surface quality.

In the following part, we will continue to learn two important problems – the degree of deformation and chip buildup.

(4) Expression method of deformation degree

The metal deformation of cutting layer is mainly shear slip deformation, so relative slip is used to represent the deformation degree of cutting layer.

1. Relative slip

As shown in fig. 1-12a, it is assumed that the quadrilateral ohnm becomes ogpm after shear deformation, and its relative slip is:

Relative slip

The deformation of the tool during cutting is shown in figure 1-12b.

When the workpiece moves towards the tool at the cutting speed vc, if there is no tool obstruction, point M will move to point N.

However, due to the obstruction of the tool, the cutting layer flows from Mn to MP (OH to OG). At this time, the relative slip ε should be:

relative slip ε
schematic diagram of shear deformation

Fig. 1-12 schematic diagram of shear deformation

  • a) Relative slip in cutting process
  • b) Quadrilateral OHNM shear deformation diagram

ε can accurately represent the deformation degree of the cutting layer, but it is calculated according to pure shear.

In addition to shear, the actual cutting process also has extrusion effect.

Therefore, using ε to represent the deformation of the cutting layer has certain approximation and the calculation is complex.

2. Deformation coefficient

In the actual cutting process, after the metal of the cutting layer is extruded and deformed, the chip thickness becomes thicker than the cutting layer and the length is shorter than the cutting layer (see Fig. 1-13), so it can be expressed by the deformation coefficient.

cutting deformation coefficient

Fig. 1-13 cutting deformation coefficient

The ratio of chip thickness hch to cutting layer thickness hD is called thickness deformation coefficient Λh.

The ratio of chip thickness hch to cutting layer thickness hD is called thickness deformation coefficient Λh.

The ratio of cutting length LD to chip length LCH is called length deformation coefficient Λl.

The ratio of cutting length LD to chip length LCH is called length deformation coefficient Λl.

In general, the change in the width direction of the cutting layer is very small.

According to the principle of constant volume, it is obvious that:

This method is intuitive and simple, so it is often used in qualitative analysis.

In foreign literature, the cutting ratio rc is commonly used to represent deformation, and rc=1/Λh.

3. Shear angle

The shear angle is the angle between the shear slip surface and the cutting speed.

It represents the position of the shear slip surface, expressed by φ.

(5) Build-up edge

Chip buildup is an important machining phenomenon that is easy to occur in machining.

Its existence has multiple effects on the machining process.

Therefore, it is necessary to understand the formation process, conditions, effects and preventive measures of chip buildup.

1. Definition of built-up edge

In the process of metal cutting, there are often some metal cold welded (bonded) from the chip and the workpiece and stacked on the rake face to form a very hard metal deposit, which is called built-up edge (see Fig. 1-14).

built-up edge

Fig. 1-14 built-up edge

  • a) Schematic diagram
  • b) Micrograph

2. Formation of built-up edge

The friction of the chip on the contact of the rake face makes the latter very clean.

When the contact surface of the two reaches a certain temperature and the pressure is high, bonding will occur.

At this time, the chip flows through the bottom metal adhered to the cutting surface.

If the temperature and pressure are appropriate, the metal on the bottom layer will deform and harden due to internal friction, be blocked in the bottom layer and stick together.

In this way, the adhesive layer will grow gradually until the temperature and pressure are insufficient to cause adhesion.

3. Factors affecting built-up edge

(1) The influence of cutting speed is shown in Fig. 1-15:

When cutting plastic metal, there is no chip buildup in the low-speed range;

When the cutting speed is less than 20m / min, the height of chip accretion reaches the maximum with the increase of cutting speed;

When the cutting speed is 20 ~ 60m / min, the height of chip accretion decreases with the increase of cutting speed;

When the speed is more than 60m / min, the built-up edge will no longer form.

effect of cutting speed on chip buildup

Fig. 1-15 effect of cutting speed on chip buildup

Processing conditions: the material is 45 steel, the back draft is 4.5mm, and the feed rate is 0.67mm/r

(2) The influence of feed rate, rake angle and other parameters is shown in Fig. 1-16.

relationship between cutting speed, feed rate and rake angle

Fig. 1-16 relationship between cutting speed, feed rate and rake angle

Processing conditions: Material: alloy steel, front angle 10 °, back draft 2mm, tool tip radius 0.5mm

4. Influence of chip buildup on cutting process

(1) Increase the actual rake angle of the tool.

(2) The accumulation of built-up edge increases the back knife consumption △ac.

The generation, growth and falling off of built-up edge is a periodic dynamic process, and the△ac value changes, so it is easy to cause vibration.

(3) The top of the chip built-up edge is very unstable and easy to crack, leaving it on the machining surface, making the machining surface very rough.

(4) When using cemented carbide cutting tools, the fracture of chip built-up edge may peel off the tool particles and aggravate the wear.

5. Main prevention methods of built-up edge

(1) Adjust the cutting speed so that the bonding phenomenon is not easy to occur.

(2) High speed cutting is adopted.

(3) Use cutting fluid with good lubrication performance to reduce friction.

(4) Change the geometric parameters of the tool, such as increasing the rake angle of the tool to reduce the pressure in the chip contact area;

Silver white chip cutting method (see Fig. 1-17) is adopted.

cutting principle of silver chip cutter

Fig. 1-17 cutting principle of silver chip cutter

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