The main elements of the cutting mode include depth, feed and cutting speed. Let's consider the cutting scheme for turning using the example of turning a cylindrical surface on a lathe.
Depth of cut
t – cutting depth, the amount of metal layer removed, measured perpendicular to the machined surface and removed in one pass of the cutting tool:
where D zag – diameter of the treated surface, mm;
d – diameter of the treated surface, mm;
The depth of cut t is usually assumed to be equal to the allowance. During the finishing pass, t should be no more than 1…2 mm.
Figure 4.1 – Cutting elements and geometry of the cut layer
Innings
Feed S is the amount (path) of movement of the cutting edge per revolution of the workpiece being processed, or per stroke of the workpiece or tool in the direction of the feed movement, mm/rev, mm/double stroke.
The feed is determined based on the condition of ensuring the required roughness of the machined surface. Usually they work at S pr = (0.20…0.25) mm/rev. High purity is obtained when working at S pr = 0.03...0.05 mm/rev.
These parameters, the cutting mode elements t and S, directly affect the size of the chips removed, as follows:
a is the thickness of the cut layer, the distance between two successive positions of the main cutting edge per one revolution of the workpiece is determined by a = S sinφ;
c – width of the cut layer, distance between the machined and machined surfaces, measured along the cutting surface: b=t/sinφ.
The shaded area is called the cross-sectional area of the cut layer F:
F = t · S = a · b, mm 2.
Cutting speed
V – cutting speed, the path of movement of the processed surface of the workpiece relative to the cutting edge of the cutter per unit of time, m/min.
M/min, m/s,
n – number of workpiece revolutions/min.
If the main movement is reciprocating (for example, planing), and the working and idle speeds are different, then the cutting speed in m/min is found from the following relationship
V = Lm(K=1)/1000,
where L is the design stroke length of the tool;
m – number of double tool strokes per minute;
K – coefficient showing the ratio of working and idle speeds.
To increase the productivity of the machining process, cutting V should be greatest. However, the cutting speed is limited by the durability of the cutting edge of the tool, i.e.
where T is the tool life, i.e. the ability to maintain cutting edges in working condition (until the critical blunting criterion h zkr is reached);
C v is a coefficient that takes into account specific processing conditions: physical and mechanical properties of the material being processed, quality of the workpiece surface, cutter angles, cooling conditions, etc.;
x y and y v are exponents at cutting depth t and feed S, just like C v are indicated in standard cutting reference books. To determine the optimal cutting speed, an economic analysis is needed to find out what is more profitable - increasing the cutting speed or increasing tool life. For example, calculations or experiments have revealed that at cutting speeds
| V, m/s | 1,2 | 1,5 | 1,7 | 2,0 |
| T, sec | 425 | 166 | 100 | 33 |
Analyzing these results, it can be noted that an increase in cutting speed by 25% leads to a decrease in tool life by almost three times. Therefore, you need to consider what is more profitable in terms of time - increasing speed or maintaining stamina? The reference books contain recommended cutting speeds V for given specific processing conditions. When assigning V, its effect on surface roughness is taken into account, which has a significant impact on the wear resistance of the working surfaces of the part, its fatigue and corrosion resistance, as well as on the efficiency of machines.
Roughness– one of the indicators of surface quality is assessed by the height, shape, direction of irregularities, including protrusions and depressions on the surface of parts, characterized by small steps, i.e.
It is characterized by three height parameters R a, R r, R max, two step parameters S m, S and a relative reference length t r.
Roughness is affected by the cutting mode, tool geometry, vibration, and physical and mechanical properties of the workpiece material.
According to modern concepts, the friction force F t includes the force of molecular interaction of contacting surfaces and the force of resistance to their movement due to the engagement of irregularities.
With a favorable profile, the wear resistance of the part is higher due to the lower contact stresses. It must be borne in mind that fatigue failures are caused by alternating loads and cracks develop from the surface, and in places of the most stress, i.e. in depressions where there is a high degree of plastic deformation.
Therefore, the cutting speed is set in such a way that after a certain time (life period T) the cutter wears out to the criterion value h 3 . So T = 30...60 min for cutters made of high-speed steel and T max = 90 min for cutters with brazed hard alloys.
Example
For certain processing conditions on a screw-cutting lathe model IK62, we determine the values of the theoretical cutting speed V t:
The values C v = 5640 and 1500, m = 0.8, X v = 0.55 and Y v = 0.55 were taken from reference normative materials on cutting.
It should be noted that the cutting speed does not have a significant effect on the roughness as does the feed value.
Using the passport data of the IK62 machine, we determine the actual cutting speed V d.
Estimated spindle speed, pr (for V t = 120 m/min):
On the machine, V t is the theoretical cutting speed for given processing conditions, m/min; D з – workpiece diameter, mm.
Machine processing time determined by the formula
where l is the length of the workpiece, mm;
l 2 – overtravel length, according to standard tables: for depth of cut
mm, l2 = 2 mm,
where d is the diameter of the treated surface;
l 1 – plunge length
where φ is the main angle in the plan of the cutter, we will take it equal to 60°.
When turning a cylindrical surface, the main (machine) time and elements of the cutting mode are related by the relationship
where L i = l + l 1 + l 2 – path of the cutting tool relative to the workpiece in the feed direction (l – length of the machined surface, mm; l 1 = t·ctgφ – cutter penetration value, mm; l 2 = 1–3 mm output cutter (overtravel)), i =H/t number of working strokes of the cutter required to remove the material left for processing (H – thickness of the removed metal layer, mm).
In general, piece time consists
T pcs = T o + T v + T ob + T p,
where T in is the auxiliary time necessary to perform actions related to preparation for the cutting process (approach and removal of tools, installation and removal of the workpiece, etc.);
T ob – time for servicing the workplace, equipment and tools in working condition;
T p – time for rest and natural needs, allocated to one detail.
Parfenyeva I.E. TECHNOLOGY OF CONSTRUCTION MATERIALS. M.: Textbook, 2009
3. Classification and characteristics of cutting motion. Cutting modes. Quality of the machined surface Cutting process parameters. General characteristics of the turning method.
3.1. Classification and characteristics of cutting motion
In order to cut a layer of metal from a workpiece, it is necessary to impart relative movements to the cutting tool and the workpiece. These relative movements are provided by the working parts of the machines, in which the workpiece and tool are installed and secured.
The movements of the working parts of machine tools are divided into working or cutting movements, installation and auxiliary movements.
Workers or cutting movements- these are movements that ensure cutting off a layer of metal from the workpiece. These include the main cutting movement and the feed movement.
Behind main cutting movement take the movement that determines the rate of metal deformation and chip separation. Behind feed movement adopt a movement that ensures continuity of cutting of the cutting edge of the tool into the workpiece material. These movements can be continuous or intermittent, in nature - rotational, translational, reciprocating. The speed of the main movement is indicated by the letter V, feed movement speed (feed amount) - S.
Installation movements– movements that ensure the relative position of the tool and the workpiece for cutting a certain layer of material from it.
Auxiliary movements– movements of the working parts of machine tools that are not directly related to the cutting process. Examples are: rapid movements of working bodies, switching cutting speeds and feeds, etc.
For any cutting process you can create processing scheme. The diagram conventionally indicates the workpiece being processed, its installation and fastening on the machine, the fastening and position of the tool relative to the workpiece, as well as the cutting movements. The tool is shown in the position corresponding to the end of the surface treatment of the workpiece. The treated surface is highlighted in the diagram with thick lines. Show the nature of cutting movements.
The workpiece is distinguished: processed surface 1, from which the metal layer is cut off; treated surface 3, from which the metal has already been cut; cutting surface 2, formed during processing by the main cutting edge of the tool.
Fig.1. Schemes for processing workpieces by turning and drilling
3.2. Cutting modes
The main elements of the cutting mode are: cutting speed V, feed S and cutting depth t. Let's consider the elements of the cutting mode using the example of turning.
Fig.2. Elements of cutting mode and geometry of the cut layer
Cutting speed V is the distance traveled by the point of the cutting edge of the tool relative to the workpiece in the direction of the main movement per unit time. The cutting speed has the dimension m/min or m/sec.
When turning, the cutting speed is equal to:
M/min
Where D zag– largest diameter of the workpiece surface being machined, mm; n– workpiece rotation speed per minute.
By filing S call the path of the point of the cutting edge of the tool relative to the workpiece in the direction of the feed movement in one revolution or one stroke of the workpiece or tool.
Depending on the technological processing method, the feed has the following dimensions:
mm/rev – for turning and drilling;
mm/rev, mm/min, mm/tooth – for milling;
mm/two stroke – for grinding and planing.
According to the direction of movement, feeds are distinguished: longitudinal S pr, transverse S p, vertical S in, inclined S n, circular S cr, tangential S t and etc.
Depth of cut t called the distance between the processed and machined surfaces of the workpiece, measured perpendicular to the latter. The depth of cut is referred to one working stroke of the tool relative to the machined surface. The depth of cut has the dimension mm. When turning a cylindrical surface, the cutting depth is determined by the formula:
Where d– diameter of the processed cylindrical surface of the workpiece, mm.
Depth of cut always perpendicular direction of feed movement. When cutting an end, the depth of cut is the amount of the cut layer measured perpendicular to the machined end. When slicing and cutting, the depth of cut is equal to the width of the groove created by the cutter.
Depth of cut and feed are technological quantities that are operated under production conditions (with standardization). For theoretical studies, the geometric dimensions of the cut layer are important: width, thickness and area of the cut layer.
Width of cut layer I " b" is the distance in mm between the machined and machined surfaces, measured along the cutting surface.
where is the principal plan angle.
Thickness of the cut layer « a" is the distance in mm between two successive positions of the cutting surface per revolution of the workpiece, measured perpendicular to the width of the cut layer
Square cut layer " f"is equal to
mm2.
This cross-sectional area of the cut layer is called nominal. The actual area of the cut layer will be less than the nominal one due to the ridges left by the cutter on the treated surface. The height and shape of the remaining ridges affects the roughness of the machined surface.
3.3. Surface quality
The quality of the processed surface is determined by the geometric and physical characteristics of the surface layer. The geometric characteristics of the surface give an idea of the machining errors. These errors include:
· macrogeometry of the surface, characterized by shape errors, such as convexity or concavity of flat surfaces and taper, barrel-shaped, saddle-shaped, ovality and faceting of cylindrical surfaces;
- surface microgeometry (roughness);
- waviness.
The physical properties of the surface layer differ from the physical properties of the base material. This is explained by the fact that during cutting, the surface layer is exposed to high temperatures and significant forces, which cause elastic and plastic deformations. The thickness of the deformed layer is about 50,000 Ao during grinding, and 15,000 Ao during polishing (Ao = 10-7 mm). Thus, even with finishing processing such as grinding, the surface layer with a thickness of more than 5 microns differs from the base metal.
Surface roughness determines the duration of normal operation of parts and machines. The degree of surface roughness determines the wear resistance of the surfaces of rubbing pairs, the anti-corrosion resistance of machine parts, and the stability of fits.
The rougher the part is processed, the less wear resistance it has. The presence of microroughness causes stress concentration in the depressions of the ridges, which leads to the appearance of cracks and reduces the strength of parts (especially those operating under alternating loads).
The roughness of the parts after processing has a significant effect on corrosion resistance. Foci of corrosion form primarily in depressions. The cleaner the surface is treated, the higher its corrosion resistance.
Roughness affects the stability of movable and fixed landings. Significant roughness changes the calculated value of the gap or interference.
The height of the irregularities on the machined surface depends on the feed rate, cutter geometry (cutter radius at the tip, main and auxiliary angles in the lead and ). In addition, the height of the asperities depends on the material being processed, cutting speed, built-up edge, cutter wear, vibration, etc.
The total height of irregularities consists of the calculated (theoretical) part of the roughness and roughness arising from technological factors.
When processing with a cutter for which the apex radius = 0, the theoretical height of the irregularities is equal to
Where S– feed, mm/rev; , - main and auxiliary plan angles, degrees.
At :
The dependence is approximate, since it does not take into account the influence of technological factors. The height of the asperities increases with increasing feed, as well as angles and decreases with increasing radius.
Influence of technological factors on surface roughness:
1. Cutting speed. In the range of cutting speeds, where the build-up has a maximum value, the highest roughness is obtained. Thus, for steel of medium hardness, the highest surface roughness is obtained in the range of 15-30 m/min.
2. The depth of cut does not directly affect the height of microroughnesses.
3. The higher the viscosity of the material being processed, the greater the height of the roughness.
4.The use of coolant reduces the size of irregularities.
The roughness of the machined surface is affected by the roughness on the cutting edge of the tool. It is copied and directly transferred to the treated surface.
3.4. Cutting process parameters
Cutting process parameters are the variables used to describe and analyze the cutting process. These include many sizes of the processed surface (linear, angular), many roughness parameters; the main time directly spent on cutting That, tool life T, effective cutting power, cutting speed, geometric parameters of cutters, etc.
Basic technological processing time That– this is the time spent directly on the process of changing the shape, size and roughness of the workpiece surface being processed.
For turning
where is the path of the cutting tool relative to the workpiece in the feed direction; l– length of the treated surface, mm; – the amount of infeed () and overrun of the cutter (1–2), mm;
i– the number of working strokes of the cutter required to remove the material left for processing;
n– workpiece rotation speed, rpm;
S– feed, mm/rev.to – main (technological) time spent on cutting;
t V - auxiliary time required for installing and removing a part, measuring it, controlling the machine, etc.;
t about- maintenance time of the machine and workplace, related to one part;
t P- time of breaks for rest and natural needs, also classified as one detail.
Individual components of piece time are determined based on normative and reference data.
Cutting mode elements are assigned as follows:
1. First select the cutting depth. In this case, they strive to remove the entire processing allowance in one pass of the cutting tool. If for technological reasons it is necessary to make two passes, then on the first pass 80% of the allowance is removed, on the second 20%;
2. select the feed amount. It is recommended to assign the highest permissible feed rate, taking into account the requirements for accuracy and roughness of the machined surface, as well as the cutting properties of the tool material, machine power and other factors;
3. determine the cutting speed using empirical formulas. For example, for turning
Where CV- coefficient depending on the processed and tool materials and cutting conditions;
T– cutter life in minutes;
m- indicator of relative resistance;
XV,YV– degree indicators.
4. Based on the found speed, the number of revolutions of the machine spindle is determined and the nearest smaller one is selected according to the machine passport
Cutting mode refers to the combination of depth of cut, feed, cutting speed and tool life.
Elements of the cutting mode are set in the following sequence: first, the maximum possible depth of cut (allowed by the processing technology) is determined; based on the selected depth, the maximum feed rate (allowed by the processing technology) is determined; Based on the selected depth and feed, given a certain period of tool life, the permissible cutting speed is found. Then the selected elements of the cutting mode are checked. The feed is controlled by the strength of the machine mechanisms, the speed - by the correspondence between the cutting power and the power of the machine.
The depth of cut is determined mainly by the allowance left for processing. If there are no restrictions on the accuracy and roughness of processing, then the entire allowance is cut off in one working stroke. If technical conditions do not allow processing in one working stroke, the allowance is divided into roughing and finishing working strokes. Rough working strokes are performed with a maximum depth of cut, and a minimum allowance is left for finishing strokes, ensuring the production of a part with a given roughness and tolerance.
Innings. To increase labor productivity, it is advisable to work with the highest possible feed. The feed amount is generally limited by machine torque, weak link strength of the feed mechanism, workpiece rigidity, tool strength, and workpiece surface roughness requirements. Feed values in practice are usually taken from reference books.
Cutting speed. After determining the depth of cut and feed, the cutting speed is determined.
Spindle speed P(in rpm) of the machine is determined by the formula
The calculated rotation speed is adjusted taking into account the actual rotation speed of the machine. Based on the actual rotation speed, the actual cutting speed is calculated. The actual rotation speed of the machine should not differ from the calculated one by more than 5%.
Checking selected cutting mode elements
Speed check. The speed is checked based on the power of the machine. It may turn out that the power of this machine will not be enough to process the selected basic elements of the cutting mode. Estimated power of the machine motor N res must be less than or at least equal to the power of the machine's electric motor N st, i.e. N res 
N st .
If it turns out that the machine’s power is not enough, then the accepted speed must be reduced.
Feed check. During roughing, the assigned feed must be checked by the strength of the machine feed mechanism parts. The axial component of the cutting force is determined R x upon accepted serve. It must be less than or at least equal to the greatest force allowed by the strength of the machine mechanism P st, which is indicated in the manufacturer’s machine passport, i.e. R x 
R st. If R x 
R st ,
it is necessary to reduce the feed.
§ 14. Information about instrumental materials. Requirements for them
At the end of the past. and at the beginning of this century, chip removal processes in the metalworking industry were at a very low level of development. The main tool material was carbon steel, which has low wear resistance and insufficient ability to withstand thermal loads. During the cutting process, the cutting edge of the tool, made of tool steel with a carbon content of 1.2% and hardened to a hardness of 66 HRC, could withstand temperatures of 200-250 ° C and allow processing at cutting speeds of 10-15 m/min.
Somewhat later, tool steels alloyed with additives of chromium, tungsten, molybdenum, vanadium, etc. appeared, which made it possible to work at speeds of 20-25 m/min. Cutters made of carbon and alloy steels are made in one piece, from one piece of metal.
In the first two decades of the twentieth century, high-speed steel was discovered (1906), which, with a tungsten content of about 19%, could operate at temperatures up to 650 °C. High-speed steels allow cutting speeds 2-3 times higher than those possible when using tools made from carbon tool steels.
Further experiments with materials having a high content of cobalt (Co), chromium (Cr) and tungsten (W) led to the production of an alloy of these metals - stellite (1915) with a temperature limit of 800 ° C.
These two new materials represented great advances in the field of machining. Turning a steel roller with a diameter of 100 mm and a length of 500 mm with a tool steel cutter required 100 minutes of machine time. High-speed steel made it possible to reduce this time to 26 minutes, and stellite cutters brought it to 15 minutes.
In 1920, a metal-ceramic hard alloy was produced for the first time. This discovery was destined to play the most important role in the development of cutting tools. In the 1930s, metal-ceramic hard alloys found widespread use in metalworking. Already the first tools made of hard alloys made it possible to reduce the processing time of a standard roller to 6 minutes. Nowadays this tool material occupies a dominant position in the field of metal cutting.
Hard alloys retain relatively high hardness when heated to a temperature of 800-900 ° C and allow processing at high cutting speeds. With appropriate geometric parameters of the tool, the cutting speed reaches 500 m/min when processing steel grade 45 and 2700 m/min when processing aluminum. Carbide tools can be used to process parts made of hardened (HRC up to 67) and difficult-to-cut steels.
Hard alloys are produced in the form of plates, standardized in shape and size, and solid or hollow columns. An important event in the tool industry was the creation, based on the principle of “non-sharpening” in the mid-50s, of instruments with rotary non-sharpening plates.
When one cutting edge wears out, the plate is not removed for regrinding, but is rotated, and the new cutting edge continues cutting. In the 50s, mineral ceramic material appeared. Its production is very similar to the manufacturing process of metal-ceramic hard alloys. The basis of mineral-ceramic materials is very often corundum (aluminum oxide Al 2 O 3). Mineral ceramics, however, have not found widespread use. The main reason for this is insufficient strength.
In 1969-1973 rotating plates with a coating appeared, the essence of which is that a layer of wear-resistant carbide is applied to a durable carbide base. The first carbide inserts had a layer of titanium carbide 4-5 microns thick. The use of the coating increased the service life of the records by approximately 300%. This significant improvement is explained by the fact that the applied layer acts as a diffusion barrier, which has high chemical stability at elevated temperatures.
In 1976, double-coated records (type GG015) were created using aluminum oxide. The outer layer, 1 micron thick, is made of aluminum oxide, and the intermediate layer, 6 microns thick, is made of titanium carbide.
Carbide inserts with a two-layer coating of this type have excellent cutting properties at high, medium and low cutting conditions when processing steel and cast iron at temperatures up to 1300 °C.
Diamonds occupy a special place among tool materials, being the hardest, most wear-resistant materials, but fragile and the most expensive of all materials.
In our country, a new superhard substance has been created based on cubic boron nitride (a substance consisting of nitrogen and boron atoms); synthetic material elbor, which has high hardness (up to 9000 kgf/mm 2) and high heat resistance (1400 C). Elbor is chemically inert towards carbon-containing materials and is stronger than diamond. Tools made from CBN have high wear resistance. CBN in powder form is used for the manufacture of grinding wheels and other abrasive tools, and CBN in the form of columns is used for the manufacture of cutters.
In Fig. 19 the development of tool materials is depicted in the form
Rice. 19. Diagram of development of instrumental materials
a graph on which the years are plotted along the abscissa axis, and the time required to turn the same roller in different years of the current century is plotted along the ordinate axis. As can be seen from the Graph, the processing time of the model roller decreased from 100 minutes in the early 1900s to 1 minute in the mid-1970s.
Requirements for instrumental materials. Cutting materials must meet the following basic requirements:
high hardness, significantly exceeding the hardness of the metal being processed;
high mechanical strength - the cutting surface of the tool must withstand high pressure, without brittle fracture and noticeable plastic deformation;
high heat resistance - the material must maintain, when heated, a hardness sufficient to carry out the cutting process;
high wear resistance - the ability of a material to work for a long time at high temperatures.
For the manufacture of tools, the following groups of materials are used, which satisfy these requirements to varying degrees (under different conditions): 1) tool carbon steels; 2) tool alloy steels; 3) high-speed steels; 4) metal-ceramic hard alloys; 5) mineral ceramic materials; 6) diamonds; 7) abrasive materials; 8) structural steels.
In table Figure 2 shows the properties of the main tool materials, and the diagram (Fig. 20) shows their hardness depending on the cutting temperature.
Tool carbon steels. For the manufacture of cutting tools, carbon steel grades are used: U7, U8, ..., U13, U7A, U8A, ..., U13A. The letter U indicates that the steel is carbon; numbers are the average percentage of carbon;
2. Properties of basic tool materials
|
Instrumental material |
material |
Hardness, HRA |
Bending strength, N/m 10 7 |
Compressive strength N/m 10 7 |
Thermal conductivity, W/m*K |
Heat resistance. hail |
Relative permissible cutting speed coefficient |
|
Carbon steel | |||||||
|
High speed steel | |||||||
|
Hard alloy | |||||||
|
Mineral ceramics |
Rice. 20. Dependence of the hardness of tool materials on temperature
the letter A indicates that the steel is of high quality with a minimal (small) content of harmful impurities. The grades and their composition are given in GOST 1435-54.
The tool, made of carbon steel, allows processing at cutting speeds of 10-15 m/min and at cutting temperatures of 200-250°C.
Carbon steels are used to make metalworking and cutting tools that operate at low speeds. Chisels are made from U9A steel, and scrapers and files are made from U13 steel. Considering that carbon steel grinds well, U12A steel is used for the manufacture of taps necessary for processing precision threads with fine pitches.
Alloyed tool steels. Alloyed tool steels differ from carbon steels by the presence of alloying elements in them - chromium, tungsten, molybdenum, vanadium, manganese, silicon. Steels with such additives are called alloyed tool steels. Alloy steels can withstand heating temperatures of 250-300°C and make it possible to work at a cutting speed of 20-25 m/min. The most widely used brands are ХВ5, ХВГ, 9ХС, ХГ. Reamers and shaped cutters are made from steel ХВ5. Large broaches are made from HVG steel. 9ХС steel is characterized by high carbide homogeneity. Tools with thin cutting elements are made from it - drills, reamers, taps, dies, end mills of small diameters. The chemical composition of alloy steel groups and grades is given in GOST 5950-63.
High speed steels. High-speed tool steels differ from alloy steels in their high content of tungsten, vanadium, chromium, and molybdenum. High speed steels have higher hardness, strength, wear resistance and heat resistance. They do not lose their cutting properties at temperatures of 550-600 °C and allow working at a cutting speed 2.5-3 times higher than tools made from carbon steels, and 1.5 times higher than tools made from alloy steels. Fast cutting steels are divided into steels of normal productivity (R18, R9, etc.) and steels of increased productivity (R18F2K5, R9F2K5, etc.). The most widely used steels are P9 and P18. The hardness of these steels is HRC 62-64. High-speed steels of normal productivity allow cutting speeds of up to 60 m/min, and high-speed steels of up to 100 m/min. Tools of many types are made from high-speed steels: cutters, drills, countersinks, reamers, cylindrical cutters, hobs, cutters, broaches, etc.
Hard alloys. Metal-ceramic hard alloys are used to make the cutting part of the tool. Metal-ceramic alloys are produced by sintering powders of carbides of refractory metals: tungsten, titanium, tantalum and cobalt that binds them. Hard alloys have high heat resistance (up to 1000°C) and wear resistance. They allow you to work at cutting speeds 3-4 times higher compared to high-speed steel tools. Hard alloys are produced in the form of plates of a certain shape and standard sizes (GOST 2209-69).
The scope of application of hard alloys is specified in GOST 3882-74. Various types of cutters, drills, countersinks, reamers, end mills, hobs, taps, etc. are made from hard alloys.
Mineral ceramic materials. For the manufacture of the cutting part of the tool, mineral-ceramic materials (microlite, terlicorundum) are used. Microlite, like hard alloys, is produced by sintering. Mineral ceramic plates have high hardness (HRA=91-93), high heat resistance (up to 1200 °C) and wear resistance. The disadvantages of ceramic materials are fragility and reduced strength. The TsN-332 grade material has the highest cutting properties.
Ceramic materials are used mainly for semi-finish and fine turning and for fine and fine milling with end mills with non-grindable inserts.
Diamond. Diamond is the hardest of all tool materials. The hardness of diamond is 7 times greater than the hardness of tungsten carbide and 3.5 times that of titanium carbide. Diamond has high thermal conductivity and high wear resistance. The disadvantages of diamond are fragility, low critical temperature (700-750 °C) and high cost.
Diamonds can be natural or synthetic. In nature, diamonds are found in the form of crystals and intergrown crystal grains and crystals. Artificial (synthetic) diamonds are obtained from ordinary graphite by exposing it to high temperatures and pressure. Synthetic diamonds such as “Carbonado” and “Ballas” are produced in the form of crystals and powders. Synthetic diamond grinding wheels are used for sharpening and finishing carbide cutting tools.
Cutters, end mills and feather drills are equipped with diamond. Cutting tools use crystals weighing from 931 to 0.75 carats (1 carat equals 0.2 g).
Cubic boron nitride. The domestic industry produces synthetic materials for the same purpose as artificial diamonds. These include primarily cubic boron nitride. It is a chemical compound of boron and nitrogen. The technology for its production is similar to the production of synthetic diamonds. The starting material is boron nitride, whose properties are similar to those of graphite. Industrial grades of cubic boron nitride “Elbor R”, “composite”, “cubinit” have high hardness, high heat capacity and high wear resistance.
Elbor R grades have properties that are significantly superior to mineral ceramics and hard alloys. CBN cutters are used for fine finishing turning of hardened steels (with hardness HRC45-60), chromium-nickel cast iron. CBN face mills allow finishing milling of hardened steels and obtaining surface roughness up to Ra 1.25 microns.
Recently, the production of large polycrystalline boron nitride formations with a diameter of 3-4 mm and a length of 5-6 mm, which have high strength, has been mastered. Equipping cutters and end mills with such polycrystals makes it possible to process hardened steels with a hardness of HRC up to 50 and high-strength cast irons with roughness parameters up to Ra 0.50 µm.
Structural steels. For the manufacture of holders, shank bodies and parts for marking composite tools, structural steels are used: St5" Stb, steels 40, 45, 50, etc.
One of the multifunctional methods of metal processing is turning. It is used for roughing and during the manufacturing or repair of parts. and effective high-quality work is achieved through rational selection of cutting modes.
Process Features
Turning is carried out on special machines using cutters. The main movements are performed by the spindle, which ensures the rotation of the object attached to it. The feed movements are performed by a tool that is fixed in the support.
The main types of characteristic work include: face and shaped turning, boring, processing of recesses and grooves, trimming and cutting, thread design. Each of them is accompanied by productive movements of the corresponding equipment: passing and thrust, shaped, boring, trimming, cutting and threading cutters. A variety of types of machines allow you to process small and very large objects, internal and external surfaces, flat and volumetric workpieces.
Basic elements of modes
The cutting mode during turning is a set of operating parameters of a metal-cutting machine aimed at achieving optimal results. These include the following elements: depth, feed, frequency and spindle speed.
Depth is the thickness of the metal removed by the cutter in one pass (t, mm). Depends on the specified purity indicators and the corresponding roughness. During rough turning t = 0.5-2 mm, during finishing - t = 0.1-0.5 mm.
Feed - the distance the tool moves in the longitudinal, transverse or linear direction relative to one revolution of the workpiece (S, mm/rev). Important parameters for its determination are geometric and qualitative characteristics
Spindle rotation speed is the number of revolutions of the main axis to which the workpiece is attached, carried out over a period of time (n, rev/s).
Speed - the width of the passage in one second with compliance with the given depth and quality, provided by frequency (v, m/s).
Turning force is an indicator of power consumption (P, N).
Frequency, speed and force are the most important interconnected elements of the cutting mode during turning, which set both the optimization indicators for finishing a particular object and the pace of operation of the entire machine.
Initial data
From the point of view of a systems approach, the turning process can be considered as the coordinated functioning of the elements of a complex system. These include: tool, workpiece, human factor. Thus, the effectiveness of this system is influenced by a list of factors. Each of them is taken into account when it is necessary to calculate the cutting mode during turning:
- Parametric characteristics of the equipment, its power, type of spindle rotation control (stepped or stepless).
- Method of fastening the workpiece (using a faceplate, faceplate and steady rest, two steady rests).
- Physical and mechanical properties of the processed metal. Its thermal conductivity, hardness and strength, the type of chips produced and the nature of its behavior relative to the equipment are taken into account.
- Geometric and mechanical features of the cutter: dimensions of the corners, holder, apex radius, size, type and material of the cutting edge with the corresponding thermal conductivity and heat capacity, toughness, hardness, strength.
- Specified surface parameters, including its roughness and quality.
If all the characteristics of the system are taken into account and rationally calculated, it becomes possible to achieve maximum efficiency of its operation.
Turning efficiency criteria
Parts manufactured using turning are most often components of critical mechanisms. The requirements are met taking into account three main criteria. The most important thing is to do each of them as much as possible.
- Correspondence between the materials of the cutter and the object being turned.
- Optimization of feed, speed and depth among themselves, maximum productivity and quality of finishing: minimal roughness, precision of shape, absence of defects.
- Minimum resource costs.
The procedure for calculating the cutting mode during turning is carried out with high accuracy. There are several different systems for this.
Calculation methods
As already mentioned, the cutting mode during turning requires taking into account a large number of different factors and parameters. In the process of technology development, numerous scientific minds have developed several complexes aimed at calculating the optimal elements of cutting modes for various conditions:
- Mathematical. Implies accurate calculation using existing empirical formulas.
- Graphic-analytical. Combination of mathematical and graphic methods.
- Tabular. Selection of values corresponding to specified operating conditions in special complex tables.
- Machine. Using the software.
The most suitable one is selected by the contractor depending on the assigned tasks and the mass scale of the production process.
Mathematical method
Analytically calculated Formulas exist, more and less complex. The choice of system is determined by the features and required accuracy of the calculation results and the technology itself.
The depth is calculated as the difference in the thickness of the workpiece before (D) and after (d) processing. For longitudinal work: t = (D - d) : 2; and for transverse ones: t = D - d.
The permissible feed is determined in stages:
- figures that provide the required surface quality, S sher;
- feed taking into account the characteristics of the tool, S p;
- parameter value that takes into account the features of fastening the part, S part.
Each number is calculated using the appropriate formulas. The smallest of the obtained S is chosen as the actual feed. There is also a general formula that takes into account the geometry of the cutter, the specified requirements for the depth and quality of turning.
- S = (C s *R y *r u) : (t x *φ z2), mm/rev;
- where C s is the parametric characteristic of the material;
- R y - specified roughness, µm;
- r u - radius at the tip of the turning tool, mm;
- t x - turning depth, mm;
- φ z - angle at the tip of the cutter.
The spindle rotation speed parameters are calculated according to various dependencies. One of the fundamental ones:
v = (C v *K v) : (T m *t x *S y), m/min, where
- C v is a complex coefficient that generalizes the material of the part, cutter, and process conditions;
- K v - additional coefficient characterizing the features of turning;
- T m - tool life, min;
- t x - cutting depth, mm;
- S y - feed, mm/rev.
Under simplified conditions and for the purpose of accessibility of calculations, the speed of turning a workpiece can be determined:
V = (π*D*n) : 1000, m/min, where
- n - machine spindle rotation speed, rpm.
Equipment power used:
N = (P*v) : (60*100), kW, where
- where P is the cutting force, N;
- v - speed, m/min.
The given method is very labor-intensive. There is a wide variety of formulas of varying complexity. Most often, it is difficult to select the right ones in order to calculate the cutting conditions during turning. An example of the most universal of them is given here.
Table method
The essence of this option is that the indicators of the elements are in the normative tables in accordance with the source data. There is a list of reference books that provide feed values depending on the parametric characteristics of the tool and workpiece, cutter geometry, and specified surface quality indicators. There are separate standards that contain maximum permissible limits for various materials. The starting coefficients necessary for calculating speeds are also contained in special tables.
This technique is used separately or simultaneously with the analytical one. It is convenient and precise to use for simple mass production of parts, in individual workshops and at home. It allows you to operate with digital values using a minimum of effort and initial indicators.
Graphic-analytical and machine methods
The graphical method is auxiliary and is based on mathematical calculations. The calculated feed results are plotted on a graph, where the lines of the machine and cutter are drawn and additional elements are determined from them. This method is a very complex complex procedure, which is inconvenient for mass production.
The machine method is an accurate and affordable option for experienced and novice turners, designed to calculate cutting conditions during turning. The program provides the most accurate values in accordance with the specified initial data. They must include:
- Coefficients characterizing the material of the workpiece.
- Indicators corresponding to the characteristics of tool metal.
- Geometric parameters of turning tools.
- Numerical description of the machine and methods of securing the workpiece on it.
- Parametric properties of the processed object.
Difficulties may arise at the stage of numerical description of the source data. By setting them correctly, you can quickly obtain a comprehensive and accurate calculation of cutting conditions during turning. The program may contain inaccuracies, but they are less significant than with the manual mathematical version.
The cutting mode during turning is an important design characteristic that determines its results. Tools and cooling and lubricants are selected simultaneously with the elements. A complete rational selection of this complex is an indicator of the specialist’s experience or perseverance.
The choice of cutting mode (depth of cut, feed and cutting speed) determines labor productivity, quality and cost of manufacturing the machined parts.
A turner must be able to correctly select cutting modes based on the best use of the cutting properties of the cutter and the power of the machine while ensuring the specified accuracy and cleanliness of processing.
1. Depth of cut
The machining allowance can be removed in one or more passes; It is more profitable to work with as few passes as possible. The entire allowance should be removed in one pass if the power and strength of the machine, as well as the strength of the cutter and the rigidity of the workpiece allow this. If the machining allowance is large, and the machined surface must be precise and clean, the allowance should be distributed over two passes, leaving 0.5-1 mm per side or 1-2 mm in diameter for finishing.
2. Feed
To obtain the greatest productivity, you should work with as large feeds as possible.
The amount of feed during roughing is limited by the rigidity of the part, the strength of the cutter and the weak links of the machine feed mechanism.
The feed amount for semi-finishing and finishing machining is determined by the requirements for the cleanliness of the machined surface and the accuracy of the part. Approximate feed rates for semi-finish turning are shown in table. 4. When working with V. Kolesov cutters (see Fig. 62) during semi-finishing, and in some cases finishing processing of steels, the feed can be very large - about 1.5-3 mm/rev. Recommended feed values when processing metals according to V. Kolesov’s method are given in table. 5.
Table 4
Average feeds for semi-finish turning of steel
Table 5
Recommended feed rates for metal processing
according to the method of V. A. Kolesov (according to Uralmashplant data)
Note: Smaller feed values are given for more durable materials, larger ones for less durable ones.
3. Cutting speed
The cutting speed depends mainly on the material being processed, the material and tool life, cutting depth, feed and cooling.
Based on the experience of high-speed turners at leading factories and laboratory research, special tables have been developed from which you can select the required cutting speed when machining with carbide cutters.
As an example in table. Table 6 shows the recommended cutting speeds for various cutting depths and feeds when longitudinal turning of structural carbon and alloy steels with tensile strength sigmab = 75 kg/mm² using T15K6 carbide cutters.
Cutting speeds indicated in table. 6, are designed for certain cutting conditions. They provide for the turning of steels σ b = 75 kg/mm² using T15K6 carbide cutters with a leading angle of φ = 45° with a cutter life T = 90 min.
Under conditions different from those indicated in table. 6, the tabular data on cutting speed should be multiplied by the corresponding coefficients given below.
Coefficients taking into account the strength of the processed material:
Coefficients taking into account the durability of the cutter:
Coefficients taking into account the grade of hard alloy:
Table 6
Cutting modes
when turning structural and alloy steels
tensile strengthσ b = 75 kg/mm²
cutters with T15K6 plates
4. Requirements for modern lathes
Lathes designed for high-performance turning are subject to higher demands than conventional lathes.
When working at high cutting speeds, there is a danger of vibrations due to insufficient rigidity of the machines, the presence of excessive clearances in the spindle bearings and in the movable joints of the support, and unbalance of individual rapidly rotating parts of the machine, chuck or workpiece.
Consequently, for quiet, vibration-free operation of the machine, its individual parts (spindle, support, tailstock) must have sufficient rigidity, and the rotating parts must be carefully balanced.
The power of a lathe for high-speed cutting must be greater, since the higher the cutting speed, the greater the electric motor power required.
These requirements are met by machines produced by the domestic machine tool industry, for example, the 1A62 screw-cutting lathe, which we examined in detail, the 1K62 machine, etc.
However, for high-performance cutting, in some cases it is possible to use old-model lathes available in factories, with some modification of their main components.
This kind of machine modification is called modernization.
Conversion of existing machines for high-performance cutting in some cases comes down mainly to increasing the spindle speed and replacing the existing electric motor with a more powerful one; in other cases, more complex alterations are required, for example, it is necessary to change the design of the friction clutch, the main drive, add devices for forced lubrication of the spindle, strengthen individual parts of the machine, etc.
Increasing the spindle speed is one of the widely used measures when converting machine tools to high-speed cutting and is achieved by changing the diameters of existing pulleys. At the same time, the electric motor is also replaced with a more powerful one. The flat-belt transmission from the electric motor to the machine is replaced by a V-belt (see Fig. 2, b). This transmission allows you to obtain the required increased power and a higher gear ratio without changing the width of the pulley.
Machines transferred to high-speed processing must be thoroughly checked and, if necessary, repaired. During repairs, you should pay attention to the headstock bearings, friction clutch, caliper, etc. The spindle bearings must be carefully adjusted, and the gaps in the moving parts of the caliper are eliminated by tightening the wedges. The friction clutch must be checked and, if necessary, strengthened accordingly. The machine should always be well lubricated, especially its gearbox.
Secure installation of the machine on the foundation is a prerequisite for avoiding vibrations, especially for machines with unbalanced rotating parts.
Control questions 1. Explain the procedure for selecting cutting depth and feed.
2. Select the cutting speed when turning structural steel σ b = 75 kg/mm² at a cutting depth t - 3 mm with a T15K6 carbide cutter, using the table. 6, taking feed s = 0.2 mm/rev.
3. Select the cutting speed when turning σ b = 50-60 kg/mm² at a depth of cut t = 2 mm with a T5K10 carbide cutter at a feed s = 0.25 mm/rev.
4. Select the cutting speed when turning alloy steel σ b = 100 kg/mm² at a depth of cut t = 1 mm with a T30K4 carbide cutter at a feed s = 0.15 mm/rev and with a cutter life of 30 minutes.
5. What are the basic requirements that a high-speed cutting lathe must satisfy?
6. What is called machine modernization?
7. List the main ways to modernize existing machines for high-speed cutting.