Distinguish counter milling when feeding the workpiece towards the rotation of the cutter (Fig. 84, V) And incidental when the directions of rotation of the cutter and feed coincide (Fig. 84, G).
When counter milling, the tooth gradually cuts into the metal,
and the load increases from zero to the maximum value. This method is used when roughing parts that have a hard surface crust, since the tooth works from under the crust. In this case, cutting forces tend to tear the workpiece away from the table surface, which, with large cross-sections of chips, leads to vibrations and deterioration in the quality of processing.
During down milling, the cutter tooth is immediately subjected to maximum load, the workpiece is pressed against the table surface, which ensures a higher quality of the machined surface and increases the durability of the cutting tool.
The main work performed on milling machines is
And the tool used
Horizontal planes processed with cylindrical cutters (Fig. 85, A) on horizontal milling machines or with end mills (Fig. 85, b, V) on vertical milling and longitudinal milling machines.
Vertical planes processed on horizontal milling machines with end or disk cutters, on longitudinal milling machines with end mills and on vertical milling machines with end mills (Fig. 85, V, G, d).
Inclined planes And bevels processed on horizontal milling machines with corner cutters (Fig. 85, e) or on vertical milling machines with a rotary head - end-face (Fig. 85, and). In this case, the spindle head with the cutter fixed in it is rotated to the required angle.
Rectangular grooves And ledges milled with disk cutters on horizontal milling machines or end cutters on vertical milling machines (Fig. 85, h, And).
T-slots and type "dovetail" milled on a vertical milling machine in two passes. Previously, a rectangular groove is cut with a cylindrical end mill, and then the groove is finally machined with a cutter of the appropriate profile (Fig. 85, To, l).
| a B C D E |
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| V |
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Open keyways processed on horizontal milling machines with disk cutters (Fig. 85, O), A closed -
on vertical milling machines with end (Fig. 85, m) or special key cutters.
Shaped surfaces processed with shaped cutters of the appropriate profile (Fig. 85, P, R), mainly on horizontal milling machines, and complex spatial shaped surfaces - on special copy-milling machines. Complex surfaces, which are combinations of horizontal, vertical and inclined planes, and sometimes curved surfaces, are often milled with a set of cutters 1 , 2 , 3 , 4 on horizontal and longitudinal milling machines (Fig. 85, With).
Milling machines
There are many types of milling machines: 1) cantilever milling machines; 2) longitudinal milling; 3) continuous milling machines; 4) key-milling; 5) thread milling; 6) copy-milling; 7) special, etc.
Cantilever milling machines have a table on which a fixture with a workpiece is installed, placed on a cantilever beam (console). The console can move along the vertical guides of the frame. These machines can perform a variety of milling work.
Cantilever milling machines are divided into horizontal milling, universal milling, vertical milling, and universal. In a horizontal milling machine, the spindle axis is horizontal, so only disk or cylindrical cutters can be mounted on it.
Vertical milling machine is designed similarly to a horizontal milling machine, but its spindle axis is located vertically. Milling on these machines is carried out using face and end mills.
Practical lesson 1,2. Metalworking Basics
Lesson plan:
1. Features of the milling process
There are various types of machining: turning, milling, drilling, planing, etc. Despite the structural differences between the machines and the features of the technologies, control programs for milling, turning, electroerosive, woodworking and other CNC machines are created according to the same principle.
Milling process consists of cutting off an excess layer of material from a workpiece to obtain a part of the required shape, size and roughness of the machined surfaces. In this case, the machine moves the tool (cutter) relative to the workpiece or moves the workpiece relative to the tool.
To carry out the cutting process, it is necessary to have two movements - main and feed movement. When milling, the main movement is the rotation of the tool, and the movement submissions is the forward movement of the workpiece. During the cutting process, new surfaces are formed by deformation and separation of surface layers with the formation of chips.
When processing, a distinction is made between up and down milling. Down milling or feed milling - a method in which the directions of movement of the workpiece and the cutting speed vector coincide. In this case, the chip thickness at the tooth entry into cutting is maximum and decreases to zero at the exit. During down milling, the conditions for insert entry into cutting are more favorable. It is possible to avoid high temperatures in the cutting zone and minimize the tendency of the workpiece material to harden. The large chip thickness is an advantage in this case. Cutting forces press the workpiece to the machine table, and the plates press into the housing sockets, facilitating their reliable fastening. Climb milling is preferable provided that the rigidity of the equipment, fixtures and the material being processed allows this method to be used.
Counter milling, which is sometimes called traditional, is observed when the cutting speeds and the feed movement of the workpiece are directed in opposite directions. When cutting in, the chip thickness is zero, at the exit it is maximum. In the case of up milling, when the insert starts working with chips of zero thickness, high friction forces arise, pushing the cutter and the workpiece away from each other. At the initial moment of cutting into a tooth, the cutting process is more reminiscent of smoothing, with the accompanying high temperatures and increased friction. This often results in unwanted hardening of the surface layer of the part. At the exit, due to the large thickness of the chips as a result of sudden unloading, the cutter teeth experience a dynamic impact, leading to chipping and a significant decrease in durability.
The thickness of the cut layer during milling is affected by main plan angle, which is measured between the main cutting edge of the insert and the machined surface. Reducing the entering angle results in thinner chips for a given feed range. The reduction in chip thickness occurs due to the distribution of the same volume of metal removed over a larger length of the cutting edge. At a smaller approach angle, the cutting edge gradually enters and exits the work. This reduces the radial component of the cutting force and protects the cutting edge from possible breakage. On the other hand, an unfavorable factor is an increase in the axial component of the cutting force, which causes a deterioration in the surface roughness of thin-walled parts.
At coal 90° in plan The cutting force is directed radially according to the feed direction. The main area of application of such cutters is the processing of rectangular shoulders.
When working cutter with a 45° lead angle The axial and radial cutting forces are almost equal and the power consumption is low. These are cutters for universal use. They are especially recommended for processing materials that produce elemental chips and are prone to chipping under significant radial forces at the tool exit. When plunging the tool, there is less load on the cutting edge and less tendency to vibration when secured in fixtures with low clamping forces. The smaller thickness of the cut layer at a cutting angle of 45° allows you to increase the minute feed of the table, i.e. increase processing productivity.
10° lead angle cutters recommended for longitudinal milling with high feeds and plunger milling, when small chip thicknesses and high speed parameters are characteristic. The advantage of processing with such cutters is low radial cutting forces. And also the predominance of the axial component of the cutting force, both in the radial and axial feed directions, which reduces the tendency to vibration and provides great opportunities for increasing material removal rates.
For cutters with round inserts The leading angle varies from 0 to 90° depending on the cutting depth. These cutters have a very strong cutting edge and can operate at high feed rates because they produce fairly thin chips over a long cutting edge length. Round insert cutters are recommended for machining difficult-to-cut materials such as titanium and heat-resistant alloys. The direction of cutting forces varies along the radius of the insert, so the direction of the total load depends on the depth of cut. The modern geometry of round inserts makes them more versatile, providing stability of the cutting process, lower power consumption and, accordingly, lower requirements for equipment rigidity. Currently, these cutters are widely used for removing large volumes of metal.
milling process
Milling is the most common method of processing planes, grooves, shaped surfaces, and threads. The method provides surfaces of 3-4 cells. accuracy (8-10 sq.) with a cleanliness of 4-7 cl. The cutting tool is a milling cutter - a multi-toothed tool made in the form of a body of rotation, on the generatrix or end of which cutting teeth with cutting edges are located. The main movement during milling is the rotation of the cutter, and the feed movement is the translational movement of the workpiece fixed on the machine table.
There are two main types of milling: cylindrical and end milling (Fig. 55)
Cutter geometry(Fig.56)
Typically, the cutter teeth are made along a helical line under the angle of inclination of the teeth to the cutter axis ω. In a cylindrical cutter with a helical tooth, the direction of the main cutting edge coincides with the direction of the helix.
Rake angle γ is considered in a plane perpendicular to the main cutting edge (section A-A) and is located between the tangent to the front surface and the plane perpendicular to the cutting plane.
Relief angle α viewed in a plane perpendicular to the axis of the cutter (section B-B) and located between the tangent to the rear surface and the tangent to the cutting surface (cutting plane)
3. Elements of cutting modes and cut layer during cylindrical milling(Fig.44)
A) cutting depth t(mm) – the size of the cut layer in the direction perpendicular to the treated surface;
b) feed S– when milling, there are 3 types of feed:
- minute feed S m – the amount of movement of the workpiece relative to the cutter in 1 minute
S m = S z z n (mm/min), where
- feed per 1 cutter tooth S z (mm/tooth) – the amount of movement of the workpiece relative to the cutter during its rotation by 1 tooth;
z – number of cutter teeth; n – number of revolutions (rotation frequency) of the cutter.
- feed per 1 revolution of the cutter S o = S z z (mm/rev) – the amount of movement of the workpiece relative to the cutter for 1 revolution.
V) milling width B– the width of the machined surface in a direction parallel to the axis of the cutter.
G) cutting width b– length of contact between the cutting edge of the tooth and the workpiece being processed. For a spur cutter b = B. When milling with a cylindrical cutter with a helical tooth, the width of the cut layer is variable.
d) slice thickness a– variable value; at the moment of tooth entry into contact with the machined surface a = min, and at the moment of exit a = max (during down-milling), and vice versa for counter milling.
e) Cutting speed V cut– peripheral speed of rotation of the cutter. Initially, the speed allowed by the cutting properties of the cutter is determined using the analytical formula:
V cut = (m/min);
Then, using the found peripheral speed, the number of revolutions (rotation frequency) of the cutter is determined using the formula:
(rpm), (min -1)
There are various types of machining: turning, milling, drilling, planing, etc. Despite the structural differences between the machines and the technology features, control programs for milling, turning, electrical erosion, woodworking and other CNC machines are created according to the same principle. This book will focus on milling programming. Once you have mastered this versatile technology, you will most likely be able to figure out how to program other types of processing on your own. Let's remember some elements of milling theory that will definitely come in handy when creating control programs and working on the machine.
The milling process consists of cutting off an excess layer of material from a workpiece to obtain a part of the required shape, size and roughness of the machined surfaces. In this case, the machine moves the tool (cutter) relative to the workpiece or, as in our case (for the machine in Fig. 1.4–1.5), moves the workpiece relative to the tool.
To carry out the cutting process, it is necessary to have two movements - the main movement and the feed movement. In milling, the main movement is the rotation of the tool, and the feed movement is the translational movement of the workpiece. During the cutting process, new surfaces are formed by deformation and separation of surface layers with the formation of chips.
When processing, a distinction is made between up and down milling. Climb milling, or feed milling, is a method in which the directions of movement of the workpiece and the cutting speed vector coincide. In this case, the chip thickness at the tooth entry into cutting is maximum and decreases to zero at the exit. During down milling, the conditions for insert entry into cutting are more favorable. It is possible to avoid high temperatures in the cutting zone and minimize the tendency of the workpiece material to harden. The large chip thickness is an advantage in this case. Cutting forces press the workpiece to the machine table, and the plates press into the housing sockets, facilitating their reliable fastening. Climb milling is preferable provided that the rigidity of the equipment, fixtures and the material being processed allows this method to be used.
Up milling, sometimes called conventional milling, occurs when cutting speeds and feed movements of the workpiece are directed in opposite directions. During plunge-in, the chip thickness is zero, at exit it is maximum. In the case of up milling, when the insert starts working with chips of zero thickness, high friction forces arise, pushing the cutter and the workpiece away from each other. At the initial moment of cutting into a tooth, the cutting process is more reminiscent of smoothing, with the accompanying high temperatures and increased friction. This often results in unwanted hardening of the surface layer of the part. At the exit, due to the large thickness of the chips as a result of sudden unloading, the cutter teeth experience a dynamic impact, leading to chipping and a significant decrease in durability.
During the milling process, chips adhere to the cutting edge and interfere with its operation at the next moment of cutting. During up milling, this can lead to chip jamming between the insert and the workpiece and, consequently, damage to the insert. Climb milling allows you to avoid such situations. On modern CNC machines, which have high rigidity, vibration resistance and which have no backlash in the lead screw-nut interface, down milling is mainly used.
Allowance is a layer of workpiece material that must be removed during processing. The allowance can be removed, depending on its size, in one or several passes of the cutter.
It is customary to distinguish between rough and finishing milling. When rough milling, processing is carried out with the maximum permissible cutting conditions to remove the largest volume of material in the minimum time. In this case, as a rule, a small allowance is left for subsequent finishing. Finish milling is used to produce parts with final dimensions and high quality surfaces.
The essence of the milling process. Milling is a metal cutting process carried out by a rotating cutting tool with simultaneous linear feeding of the workpiece. Material is removed from the workpiece to a certain depth using a milling cutter, working either on the end side or on the periphery. The main movement during milling is the rotation of the cutter v(Fig. 33). The speed of the main movement determines the rotation speed of the cutter. Feeding movement s when milling there is a translational movement of the workpiece in the longitudinal direction,
Rice. 33. Milling schemes:
a - cylindrical, b and c-face milling; 1 - processed surface, 2 - axis of rotation of the cutter, 3 - processed surface, 4- chips, 5 - workpiece, 6 - cutter knife.
transverse or vertical directions. The milling process is a discontinuous process. Each tooth of the cutter removes a friend of variable thickness. Milling operations can be divided into two types: a) cylindrical milling (Fig. 33, a); b) face milling (Ois. 33, b and V).
In cylindrical milling, cutting is carried out by teeth located on the periphery of the cutter, and the machined surface 1 is a plane parallel to the axis of rotation of the cutter 2.
In Fig. 33, and a cutter with a straight tooth is shown. Along with straight teeth, cutters with helical teeth are used (Fig. 34).
Rice. 34. Milling with a cylindrical screw cutter: IN- milling width, t- milling depth, s - maximum cut thickness
When face milling (see Fig. 33), cutting is carried out by the peripheral and end cutting edges of the teeth. The thickness of the cut increases towards the center of the cut and decreases at the point where the cutter leaves contact with the workpiece. The initial and final cut thickness depends on the ratio of the workpiece width to the cutter diameter. The change in cut thickness also depends on the symmetry of the location of the cutter relative to the workpiece. Most other milling processes are a combination of cylindrical and face milling methods.
Features of chip formation during milling. The process of chip formation during milling is accompanied by the same phenomena as during turning. These are deformations, heat generation, build-up, vibration, tool wear, etc. But milling has its own characteristics. When turning, the cutter is under constant action of chips along the entire length of the cut. When milling, a tooth is exposed to chips for only a short time during one revolution of the cutter. For most of the revolution, the tooth is not involved in cutting; during this time it cools, which has a positive effect on its durability. The entry of a tooth into contact with the workpiece is accompanied by an impact on its cutting edge; Impact load reduces tooth resistance; cutters.
Milling against feed and along feed. When milling with cylindrical and disk cutters, a distinction is made between up-milling - against the feed and down-milling along the feed. When the peripheral speed of the cutter is opposite to the feed direction (Fig. 35, a), the process
Rice. 35. Milling against feed (o) and along feed (b)
milling is called counter milling. The slice thickness varies from zero (at the point A) to the maximum value when the tooth leaves contact with the workpiece (at the point IN). When the direction of the peripheral speed of the cutter and the feed speed coincide (Fig. 35.6), the milling process is called “downhill” milling. With this milling method, the cut thickness varies from the maximum value at the point IN at the beginning of the tooth entering contact with the workpiece to zero at the point A(when the tooth leaves contact with the workpiece).
Up milling is characterized by the fact that the load on the tooth increases gradually, as the cut thickness changes from zero at entry to maximum at the exit of the tooth from the workpiece. The cutter tooth works from under the crust, breaking out the crust from below, the cutter “tears” the workpiece from the table, lifting the machine table with it, increasing the gaps between the table and bed guides, which, under significant loads, leads to trembling and an increase in the roughness of the machined surface.
During down milling, the workpiece is pressed against the table, selecting the existing gaps in the guides of the table and bed. The cutter tooth begins to work at its greatest thickness and is immediately subjected to maximum load.
Uniformity of milling. During the milling process with a straight cutter, the cutter tooth comes into contact with the workpiece being processed and leaves it immediately along the entire milling width. It may turn out that only one tooth of a spur cutter will be in operation, that is, when the tooth in front has already left contact with the workpiece being processed, and the tooth following it has not come into contact. In this case, the cross-sectional area of the cut will vary from zero value to a maximum value and then drop to zero or from a maximum value to zero. The cutting force will also change unevenly, and therefore there will be an uneven periodic load on the machine, tool and workpiece. This phenomenon is called uneven milling. In Fig. 36
Rice. 36. Scheme of operation of a single-tooth (conventional) cutter
A simplified diagram of the operation of a spur cutter is shown. The cutter conventionally shows one tooth. The tooth cuts into the workpiece immediately across the entire milling width. The cutter experiences a shock. With further rotation of the cutter, the chip thickness will gradually increase (position 2, 3, 4), The cutting force will also increase. On the site 4-5 The cutter tooth simultaneously leaves the metal being processed, and the cutting force quickly decreases to zero.
As you can see, the load on the cutter tooth changes dramatically during the cutting process. The greater the number of teeth involved in the work at the same time, the more uniform the milling will be. In Fig. Figure 37 shows a diagram of the operation of a cylindrical cutter with helical teeth. The tooth of such a cutter cuts into
Rice. 37. Diagram of operation of a cutter with a helical tooth
the workpiece not immediately along its entire length, but gradually. In section 1- 3 The cross-sectional area of the cut layer (shaded) increases, which means the cutting force also increases. Location on 3 -4 The cross-sectional area of the cut layer and the cutting forces are constant. With further movement of the tooth (section 4-6) The cross-sectional area of the cut layer and the cutting force gradually decrease. Thus, the change in cutting force during operation of a screw tooth occurs more smoothly, and in some areas the cutting force is constant.
To ensure uniform milling, at least two cutter teeth must be involved in the work simultaneously. Each subsequent tooth should begin to work at the moment when the previous one begins to come out of the metal. To fulfill this condition, it is necessary that at the moment when one of the two teeth gets into position 6, the second tooth was in position 1. This is possible if the distance between two adjacent cutter teeth, measured along its axis (axial pitch), should be equal to the milling width B (see Fig. 34) . If more than two teeth are simultaneously involved in the work, then the axial step must fit across the milling width an integer number of times. A necessary condition for uniform milling is equality or multiple (in whole numbers) of the milling width IN axial pitch of the cutter.
When face milling, uneven milling always occurs. The greater the number of simultaneously working teeth of an end mill and the greater the ratio of the milling width to the diameter of the cutter, the greater the uniformity of milling.