What Is Metal Cutting Process?

Cutting tool is an essential part of NC machining process.

The selection of appropriate tool is the key factor affecting cutting efficiency and quality.

Practice has proved that the correct selection and use of tools to improve productivity is more reasonable than the method of increasing equipment and workers, which can greatly reduce the risk of idle production capacity in the economic downturn.

What is metal cutting process?

Metal cutting is a machining method that uses cutting tools higher than the hardness of the workpiece to remove the excess metal of the workpiece.

It is one of the basic machining methods in the machinery manufacturing industry.

Its research began in the middle of the 19th century.

In 1851, Frenchman M. cochkira first measured the torque when the drill bit cut cast iron and other materials, and listed the table of power required to cut unit volume of materials;

In 1864, the Frenchman josel first studied the influence of tool geometric parameters on cutting force;

In 1870, Russian Jimei first explained the process of chip formation and put forward the view that the metal material is not only extruded but also sheared in front of the tool;

In 1896, the Russian Brix began to introduce the concept of plastic deformation into metal cutting.

So far, there is a more complete explanation of chip formation in the cutting process.

With the rapid development of science and technology, some advanced machining technologies have emerged one after another, such as precision casting, cold rolling technology, EDM and ECM technology, which can partially replace cutting.

However, because metal cutting has the advantages of high machining precision, high production efficiency and low machining cost, most parts must be realized by machining, especially the machining of high-precision metal parts.

Therefore, at present, metal cutting is still the main method of machining, accounting for about 40% ~ 60% of the total manufacturing workload in general production.

The process of metal cutting is that the tool cuts off the excess metal from the workpiece to make the workpiece obtain the specified machining accuracy and surface quality.

Taking turning as an example (see Fig. 1-1), cutting must meet three conditions:

(1) There is relative motion between the tool and the workpiece.

(2) The cutter has appropriate geometric parameters – geometric angle.

(3) The cutting tool material has certain cutting performance.

turning motion and surface on workpiece

Fig. 1-1 turning motion and surface on workpiece

(1) Machined surface on workpiece

1. Surface to be processed

The workpiece surface to be cut off when the surface to be processed.

2. Machined surfaces

The machined surface is the surface of the workpiece with excess metal removed after machining.

3. Transition surface (also known as machining surface)

The surface between the surface to be machined and the machined surface being cut off by the cutting edge.

(2) Cutting motion

During cutting, in order to obtain the required part shape, the tool and the workpiece must have a certain relative motion, that is, cutting motion.

According to its role, cutting motion can be divided into main motion and feed motion, as shown in Fig. 1-2.

planing motion and surface on workpiece

Fig. 1-2 planing motion and surface on workpiece

1. Main movement

The movement provided by the machine tool or manpower, which makes the main relative movement between the tool and the workpiece.

The main motion is characterized by the highest speed and the largest power consumption.

2. Feed movement

The movement provided by the machine tool or manpower, which makes the additional relative movement between the tool and the workpiece.

The feed movement will continuously put the cut metal layer into cutting to process the required machined surface.

Feed movement is characterized by low speed and low power.

3. Synthetic motion

Synthesis of main motion and feed motion.

(3) Cutting parameters

Cutting parameter is the number of main motion and feed motion parameters.

It is the general term of cutting speed VC, feed F and back draft ap.

It is an important parameter for adjusting the machine tool and calculating cutting force, cutting power and man hour quota, as shown in Fig. 1-3.

schematic diagram of cutting parameters

Fig. 1-3 schematic diagram of cutting parameters

1. Cutting speed VC

The instantaneous speed of the cutting edge relative to the main motion of the workpiece, in m / min.

When the main motion is rotary motion, its calculation formula is

Cutting speed VC

Where

  • d — diameter of the workpiece or tool corresponding to the selected point on the cutting edge, unit: mm;
  • n — rotational speed of main motion, unit: r / s or r / min.

In the current production, the unit of grinding speed is m / s, and the unit of cutting speed for other machining is used to m / min.

When the speed n is constant, the cutting speed is different with different selected points.

Even if the speed n is constant, the cutting speed will be different due to the different working diameters of each point on the cutting edge.

Considering the influence of cutting speed and machined quality, the maximum cutting speed should be taken in the calculation.

2. Feed rate f

The displacement of the tool relative to the workpiece in the feed movement direction.

The relative displacement of the tool or workpiece along the feed movement direction can be used for each rotation of the tool or workpiece, and the unit is mm / r or mm / stroke.

The feed rate per unit time is called the feed speed vf, and the unit is mm / min.

The relationship between them is

Feed rate f

When cutting with multi edge cutting tools such as milling cutter, reamer, broach and gear hob, the feed rate fz of each cutter tooth shall also be specified, that is, the feed rate of the latter cutter tooth relative to the previous cutter tooth, and the unit is mm / z.

3. Back cutting amount ap

The distance between the surface to be machined and the vertical surface is mm.

The formula for calculating the back draft during cylindrical turning is

formula for calculating the back draft during cylindrical turning

For drilling

drilling formula

Where

  • dw – diameter of surface to be machined, unit: mm;
  • dm — diameter of machined surface, unit: mm.

(4) Structure of cutting part of tool

The cutting part of the tool is composed of rake face, rake face, cutting edge and tool tip, as shown in Fig. 1-4.

tool structure

Fig. 1-4 tool structure

1. Rake face

The surface through which chips flow out of the tool.

2. Flank

The surface on the tool opposite the transition surface of the workpiece.

3. Auxiliary flank Aα′

The surface on the tool opposite the machined surface of the workpiece.

4. Main cutting edge S

The intersection of rake face Aγ and rake face Aα on the tool is responsible for the main cutting work.

5. Auxiliary cutting edge S ′

The intersection line between the upper rake face Aγ and the auxiliary rake face Aα′ of the tool serves as the secondary cutting work.

6. Tool tip

The intersection of the main cutting edge s and the auxiliary cutting edge s’ on the tool.

(5) Tool geometry angle

The geometric angle of the tool is shown in Fig. 1-5.

Tool geometry angle

Tool geometry fig. 1-5

  • a) Tool angle dimension
  • b) Turning tool angle

1. Main deflection angle κr

The angle between the main cutting edge and the feed movement direction.

2. Secondary deflection angle κr′   

The included angle between the secondary cutting edge and the opposite direction of feed movement.

3. Knife point angle εr 

The included angle between the main cutting plane and the auxiliary cutting plane.

The size of the tool tip angle will affect the strength and heat transfer performance of the cutting part of the tool.

Its relationship with main deflection angle and auxiliary deflection angle is as follows:

Knife point angle εr 

4. Front corner γo  

Included angle between rake face and base face. When the current cutter face is parallel to the base face, the rake angle is zero.

The base plane is within the rake face and the rake angle is negative. The base surface is outside the rake face, and the rake angle is positive.

5. Rear corner αo  

The included angle between the flank and the cutting plane.

6. Wedge angle βo  

The included angle between the rake face and the rake face.

The size of the wedge angle will affect the size of the section of the cutting part and determine the strength of the cutting part.

Its relationship with the front angle γo and rear angle αo is as follows:

Wedge angle βo  

7. Blade inclination λs 

The angle between the main cutting edge and the base surface.

The provisions of positive and negative blade inclination are shown in Fig. 1-6.

When the blade tip is at the highest point, the blade inclination is positive;

When the tool tip is at the lowest point, the blade inclination is negative;

When the cutting edge is parallel to the bottom, the edge inclination is zero.

Blade inclination λs 

Fig. 1-6 positive and negative provisions of blade inclination

Cutting with λs = 0 is called right angle cutting.

At this time, the main cutting edge is perpendicular to the cutting speed direction, and the chips flow out along the normal direction of the cutting edge.

The cutting with λs ≠ 0 is called oblique cutting.

At this time, the main cutting edge is not perpendicular to the cutting speed direction, and the flow direction of chips is inclined by an angle with the normal direction of the cutting edge, as shown in Fig. 1-7.

Fig. 1-7 right angle cutting and oblique angle cutting

  • a) Right angle cutting
  • b) Oblique cutting

8. Auxiliary rear corner αo′

The included angle between the auxiliary flank and the cutting plane of the auxiliary cutting edge.

Among the above geometric angles, the most commonly used ones are front angle γo, rear angle αo′, main deflection angle κr, blade inclination λs, auxiliary deflection angle κr′ and auxiliary rear angle αo′, which are usually called basic angles.

In the geometric angle of the cutting part of the tool, the above basic angle can completely express the geometric shape of the cutting part of the turning tool and reflect the cutting characteristics of the tool.

εr, βo is the derived angle.

(6) Cutting layer parameters

The metal layer cut by the cutting edge from the surface of the workpiece to be machined in one feed is called the cutting layer.

As shown in Fig. 1-8, when turning the outer circle, the workpiece rotates, the turning tool moves from position I to position II, and advances a feed rate f.

The shaded part in the figure is the cutting layer.

The size of its cross-section is the cutting layer parameter, which determines the load borne by the tool and the thickness of chips, and will also affect the cutting force, tool wear, surface quality and production efficiency.

cutting layer parameters during cylindrical longitudinal turning

Fig. 1-8 cutting layer parameters during cylindrical longitudinal turning

  • a) Straight edge cutting
  • b) Arc edge cutting

1. Nominal thickness of cutting layer HD (hereinafter referred to as cutting thickness)

The cutting thickness is the dimension of the cutting layer measured perpendicular to the surface of the cutting layer, and the relationship between the cutting thickness and the feed rate f is as follows:

Nominal thickness of cutting layer HD

It can be seen that hD increases with the increase of f, kr.

When the cutting edge is a straight line, the hD at each point on the cutting edge is equal;

When the cutting edge is a curve, the hD of each point on the cutting edge changes.

2. Nominal width of cutting layer bD (hereinafter referred to as cutting width)

The cutting width is the size of the cutting layer measured along the transition surface, and the relationship between the cutting width and the back draft ap is as follows:

Nominal width of cutting layer bD

It can be seen that the larger ap, the wider bD.

3. Nominal cross-sectional area AD of cutting layer (hereinafter referred to as cutting area)

The cutting area is the cross-sectional area in the dimension plane of the cutting layer, and its calculation formula is:

Nominal cross-sectional area AD of cutting layer

4. Metal removal rate Q

The volume of workpiece material cut by the tool in unit time.

Equivalent to the space volume contained in the nominal cross-sectional area of the cutting layer moving one unit time along the cutting speed direction with VC value,

It is an index reflecting the cutting efficiency, and its calculation formula is:

Metal removal rate Q
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