One of the methods of finishing materials is milling. It is used for processing metal and non-metal workpieces. The workflow is controlled using cutting modes.
The essence of the process
Milling is carried out with the aim of deep roughing and finishing, forming a specific surface profile (grooves, grooves), cutting teeth on gears, adjusting the shape, art turning of patterns and inscriptions.
The working tool - a mill - makes the main rotational movement. Auxiliary is the progressive feed of the workpiece relative to its stroke. This process is intermittent. Its most important feature that distinguishes from turning and drilling is the fact that each tooth works separately. In this regard, it is characterized by the presence of shock loads. It is possible to reduce their influence taking into account a rational assessment of the situation and selection of modes.
Basic concepts of milling machines
Depending on the way the spindle is mounted and the cutter is fixed in it, on the types of actions taken and on the control methods, the main types of milling equipment are distinguished:
- horizontal
- vertical
- universal;
- CNC milling machines.
The main components of the vertical milling machine:
- The bed, which houses the gearbox, which regulates the rotation of a vertically mounted spindle and a milling cutter mounted on it.
- A table that includes a console with transverse runners for attaching and moving the workpiece and a feed box that regulates feed movements.
In horizontal milling machines, the tool is fixed horizontally. And universal ones have several varieties.
There is universal horizontal equipment, which is characterized by the presence of a table revolution and, thereby, expanding the range of possible work performed. In addition, there is a universal, having both spindles in its structure and allowing to carry out all types of milling.
CNC milling machines are distinguished by the availability of software and computer control. They are intended for the artistic processing of workpieces, including in 3D-format.
Cutter classification
Milling cutters are tools for cutting. The main physical parameters with which they are evaluated: height, diameter, chamfer and backing, circumferential pitch. There is a huge variety of them, distributed according to various signs:
- by the type of surfaces that are processed (for wood, plastic, steel, non-ferrous metals, etc.);
- in the direction of rotation - right-handed and left-handed;
- depending on structural features - solid, brazed, folding (have inserted knives), welded;
- in shape: conical, cylindrical, disk;
- Depending on the working conditions and requirements for the cutting part, they can be made of various materials. These include: carbon tool and high-speed steel (alloyed, with a high content of tungsten), hard alloy (durable - for roughing, wear-resistant - for finishing). Variants are common when the case is made of carbon or high-speed steel, and the knives are hard-alloy inserted;
- depending on the purpose: cylindrical, end, end, slotted, detachable, shaped.
The most informative features: material of the cutting edge and purpose.
Types of mills for flat surfaces
In order to remove layers of material on horizontal, vertical or inclined planes, cylindrical and face mills are used.
The first type of tool can be solid or with mounted knives. Large one-piece milling nozzles are designed for roughing, and small - for finishing. Insert knives for folding cutting heads can be made of high-speed steel or equipped with carbide inserts. Tungsten carbide milling cutters have greater productivity than those made of alloy steel alloy.
The end face is used for elongated planes, its teeth are distributed on the end surface. Large folding are used for wide planes. Incidentally, carbide knives are required to remove chips from difficult to process refractory metals. To use these groups of milling devices, a significant width and length of the product is needed.
Types of tools for artistic milling
To give the material a certain profile, apply a pattern, form narrow recesses, end and disk milling nozzles are used.
An end or groove cutter is common for cutting grooves, narrow and curved planes. All of them are solid or welded, the cutting part is made of high-speed alloy steel, carbide can be deposited, and the body is made of carbon steel. There are low-start (1-3 spirals) and multi-start (4 and more). Used for CNC machines.
Disk is also a slot mill. It is applicable for grooves, grooves, cutting teeth on gears.
Artistic milling is carried out on wood, metal, PVC.
Types of edge milling cutters
Chip removal from corners, giving them a rational shape, modeling, dividing the workpiece into parts can be implemented using slotted, angled and shaped milling nozzles:
- Cutting and splined has the same purpose as a disk, but more often used for incisions and separation of excess parts of the material.
- Corner is needed for part edges and corners. There are single-angle (only one cutting part) and two-angle (both conical surfaces are cutting).
- Shaped is used for complex designs. It can be semicircular or concave. Often used for cutting profile taps, countersinks, twist drills.
For almost all types, one-piece steel construction is possible or folding, with the presence of inserted carbide knives. Carbide milling cutters have a qualitatively higher performance and its duration for the tool as a whole.
Classification of milling types
There are several classification features by which the types of milling are divided:
- by the method of arranging the spindle and cutter, respectively, on horizontal and vertical;
- in the direction of movement, oncoming and passing;
- depending on the tool used, for cylindrical, end, shaped, end.
Cylindrical machining is applicable for horizontal planes, carried out using appropriate milling machines on horizontal machines.
Face milling can be considered universal. It is applicable for all types of horizontal, vertical and inclined planes.
The end trim provides the formation of the necessary profile for curved grooves, drills and instruments.
Shaping is carried out for surfaces with a complex configuration: angles, edges, grooves, cutting teeth for gears.
Regardless of the type of work being carried out and the materials being processed, the result should be distinguished by a high smoothness of the finish layer, the absence of notches, and the accuracy of the finish. In order to obtain a clean machined surface, it is important to control the feed rates of the workpiece with respect to the tool.
Counter and passing milling
When counter-type metal milling is performed, the workpiece is fed towards the rotational movements of the nozzle. At the same time, the teeth gradually cut into the processed metal, the load increases in direct proportion and evenly. However, before cutting the tooth into the part, it slides for a while, forming a rivet. This phenomenon accelerates the exit of the cutter from the working state. Used for roughing.
When performing the associated type - the workpiece is fed along the rotational movements of the tool. The teeth work shockingly under heavy loads. Power 10% lower than with counter milling. It is carried out during the finishing of parts.
The basic concept of milling work on CNC machines
They are characterized by a high degree of automation, precision workflow, high productivity. Milling on a CNC machine is most often carried out using end or end mills.
The latter are the most widely used. In this case, depending on the processed material, the corresponding type of forming chips, the specified software parameters, different end mills are used. They are classified by the number of spirals that provide cutting edges and a ditch chip discharge.
It is advisable to mill materials with wide shavings using tools with a small number of visits. For hard metals with a characteristic chip, it is necessary to choose milling tools with a large number of spirals.
Use of milling cutters for CNC machines
Low-cut CNC milling cutters can have from one to three cutting edges. They are used for wood, plastics, composites and soft ductile metals, requiring quick removal of wide chips. They are used for roughing workpieces to which high demands are not placed. This tool is characterized by low productivity, low rigidity.
Using one-way art milling of aluminum is carried out.
Widely used are two- and three-way end. They provide rigidity of higher values, high-quality chip removal, allow working with metals of medium hardness (for example, steel).
Multi-cutters for CNC have more than 4 cutting edges. They are used for metals of medium and high hardness, which are characterized by fine chips and high resistance. They are characterized by significant productivity, they are relevant for finishing and semi-finishing and are not designed to work with soft materials.
In order to choose the right tool for CNC machines, it is important to consider the cutting mode during milling, as well as all the characteristics of the machined surface.
Cutting modes
To ensure the desired quality of the milled layer, it is important to correctly determine and maintain the necessary technical parameters. The main indicators that describe and regulate the milling process are operating modes.
Calculation of cutting conditions during milling is carried out taking into account the main elements:
- Depth (t, mm) - the thickness of the metal ball, which is removed in one working stroke. Select it with allowance for processing. Draft work is done in one go. If the allowance is more than 5 mm, then the milling is carried out in several passes, while about 1 mm is left on the last.
- Width (B, mm) - the width of the surface to be machined in the direction perpendicular to the feed motion.
- Feed (S) - the length of the workpiece relative to the axis of the tool.
There are several interrelated concepts:
- One tooth feed (S z , mm / tooth) - changing the position of the part when the cutter rotates a distance from one working tooth to the next.
- Feed per revolution (S rev , mm / rev) - movement of the structure with one full revolution of the milling nozzle.
- Feeding in one minute (S min , mm / min) - an important cutting mode during milling.
Their relationship is established mathematically:
S min = S rev * n = S z * z * n,
where z is the number of teeth;
n is the spindle speed, min -1 .
The feed rate is also affected by the physical and technological properties of the treated area, tool strength and performance of the feed mechanism.
Calculation of cutting speed
As a nominal design parameter, the degree of quick spindle revolution is taken. Actual speed V, m / min depends on the diameter of the cutter and the frequency of its rotating movements:
V = (Ο * D * n) / 1000
The frequency of rotation of the milling tool is determined by:
n = (1000 * V) / (Ο * D)
Having information about the minute feed, you can determine the necessary time for the workpiece with a length L:
T 0 = L / S min
Calculation of cutting conditions during milling and their installation is relevant to carry out before setting up the machine. The establishment of rational preset parameters, taking into account the characteristics of the tool and the material of the part, ensures high productivity.
Tips for Defining Modes
It is impossible to perfectly select the cutting mode during milling, but you can be guided by the basic principles:
- It is desirable that the diameter of the cutter corresponds to the depth of processing. This will ensure surface cleansing in one pass. Here the main factor is the material. For too soft, this principle does not work - there is a risk of removing chips that are thicker than necessary.
- Shock processes and vibrations are inevitable. In this regard, an increase in feed values ββleads to a decrease in speed. It is optimal to start work with a feed per tooth equal to 0.15 mm / tooth, and in the process - adjust.
- The tool speed should not be as high as possible. Otherwise, there is a risk of lower cutting speeds. Its increase is possible with an increase in the diameter of the cutter.
- Increasing the length of the working part of the cutter, the preference for a large number of teeth reduce the productivity and quality of processing.
- Estimated speeds for various materials:
- aluminum - 200-400 m / min;
- bronze - 90-150 m / min;
- stainless steel - 50-100 m / min;
- plastics - 100-200 m / min.
It is better to start at an average speed, and in the process to adjust it to a smaller or greater side.
The cutting mode during milling is important to determine not only mathematically or using special tables. To correctly select and set the optimal parameters for the machine and the tool you need to operate with some features and personal experience.