Milling is a surface treatment method based on the successive operation of the cutter teeth. There is a huge variety of tools, depending on their functional purpose, materials being processed, and characteristics of manufactured parts.
Process features
The milling process, like all existing methods of processing materials by cutting, is based on the main and auxiliary movements. The first is the rotation of the tool, and the second is its supply to the working stroke.
Surface milling is usually done in several successive steps:
- Draft - the initial removal of bulk chips in order to design the necessary general profile, has a low accuracy class. The machining allowance (thickness of the layer to be removed, taking into account all additional factors) can be from 3 to 7 mm, depending on the material of the workpiece.
- Semi-finishing - the second stage of cleaning the intended milling object, the shavings are smaller, the accuracy of the work increases and reaches 4-6th grades.
- Finishing - a thorough finish ensures high quality surfaces and contours, high accuracy (grades 6-8). The allowance should be 0.5-1 mm.
The implementation of each of the processing steps has its own distinctive requirements for working tools according to the nature of their design, material, quantity and quality of cutting edges. For example, a milling fixture having a roughing purpose is characterized by large teeth, while a finishing mill has a fine multi-tooth structure.
Types of Milling
A wide range of existing cutters allows the processing of materials of varying complexity and configuration, from any angle. All types of processes can be divided into several groups:
- Work with flat surfaces. Draft and finish cleaning of non-voluminous planes having a horizontal, vertical or inclined position is performed.
- Processing volumetric shaped workpieces and parts. Volumetric stripping is carried out, giving the objects a certain shape.
- Separation. The parts are divided into several parts, cutting off excess material.
- Modular finish. It is based on the formation of the necessary profile of the existing workpiece, the design of grooves, grooves, teeth, shaped recesses.
For each individual method, a separate milling attachment is most often used. Workpieces of special complexity are processed using a set of cutters. So, milling of wide surfaces is carried out using a set of tools that have multidirectional helical teeth in order to reduce axial forces.
Varieties of cutters, depending on the purpose
Several classification features are known according to which all known milling devices are distributed: by material, by type of knives, by shape, depending on the direction of the working stroke. The main parameter is still the appointment.
- Cylindrical - machining by milling all horizontal and vertical planes.
- End - finishing all planes in any position.
- End - work of varying complexity, the ability to carry out flat, shaped, modular, artistic milling.
- Corner and shaped - removal of chips from the side surfaces of workpieces, profile objects, cleaning of cone-shaped recesses.
- Cutting, cutting, splined - separation, cutting teeth on the workpieces, the formation of grooves.
The same type of tools may have differences in diameter, number of knives and their features.
Structural differences of cutters
The characteristics of the knives and methods of their fastening are important parameters that determine the purpose of the cutter, in particular, by the quality of the processing.
- Whole. They are made of tool alloyed and high-speed steels. Most often - cylindrical, disk, slotted, cutting mills.
- Compound. There are two options. In the first, a structural steel shank is welded to the cutting head - from tool, high-speed steel, less often - from hard alloy. In the second, high-speed or carbide knives are soldered to the device body. They are used in face and end mills.
- National teams. Knives, most often carbide, are mechanically connected to the main body.
Solid milling cutters have a larger number of teeth, which allows for more accurate processing. Composite tools consisting of a carbide head and a structural shank have the same opportunity. Their disadvantage is a high degree of wear. Most often, this equipment is involved in the semi-finishing and finishing stages of chip removal.
Prefabricated cutters are characterized by a high degree of resistance to wear, strength, hardness and sharpness of knives, ease of turning and dismantling. However, quantitatively, in the ratio of one head, they significantly lose. These are mainly involved in roughing.
Machine tools
Milling operations to be performed determine the necessary equipment, including the type of machine on which they will be produced.
Horizontal milling machines are designed for machining horizontal planes and shaped surfaces, manufacturing gears, and designing some profile objects. Their device determines the horizontal fastening of the tool, most often a cylindrical, disk or end mill.
The same types of work, but with distinctive features, allows you to perform a vertical milling machine. A feature is the vertical fastening of the tool and, therefore, the predominant use of end, end and modular cutters.
Universal milling machines have additional devices for turning the table in 3 planes, which allows you to work with horizontal, vertical and shaped surfaces.
In serial production of parts having the same profile, copy milling installations are used, which allow repeating patterns or recesses on the plane with increased accuracy.
The equipment of the future is CNC machines. They ensure the fulfillment of a programmed set of actions, mainly for artistic milling or non-serial production of parts. End, face and modular mills with various number of cutting edges are used.
Milling is work on a special cutting machine, which provides the working stroke of the tool and the supply of the workpiece.
The influence of cutting conditions on the results of work
Results are determined not only by rationally selected equipment. Their quality depends on how correctly the milling modes are selected.
- It is necessary to precisely determine the required diameter of the cutter, its design, material, number of teeth, to establish the relationship between the dimensions of the tool and the thickness of the layer to be removed. It is important for a professional to strive to ensure that the required metal thickness is removed in one pass.
- The size of the tool determines the set speed of its rotation and, accordingly, the speed of work. They are set on the machine by setting the spindle speed - the fundamental axis for fixing the cutter. Too slow or too fast main working movements of the cutting head result in poor machining quality.
- An important cutting mode is feed. There is a separation in this whole concept. Initially, the cutter feed per tooth is determined. It is selected from the reference books in accordance with the tool used and the type of work surface. After the feed is determined for one revolution and per minute, respectively.
The calculation of milling is based on information on the permissible equipment capacities, type of surface to be machined and the selected tools. There are nominal tables filled with the required and control values. Rational selection and calculation of the main parameters of work determines its quality.
Accompanying Phenomena
Milling is the process of chip removal, which is characterized by increased thermal effects and mechanical stresses, which can negatively affect the abilities of tools and features of the finish. Some phenomena that affect the results of milling operations:
- Chipping and shrinkage. Adhesion of metal on the cutting surface, pressing it spoils the finishing process and the knives themselves. This is more relevant for soft materials.
- Riveted. An increase in hardness, a decrease in the strength and ductility of the surface layer of a part is a side effect of plastic deformation removed by subsequent heat treatment.
- Friction, increased heat in the working area, vibration - factors that reduce the performance of the cutter.
To prevent side effects, you must use additional technologies and tools.
Protection of workpieces and tools
To avoid or minimize the negative effects of cutting processes on the tool and the processed material, the following methods are used:
- The use of cooling and lubricating substances and liquids, feeding them directly to the milling zone reduces friction, hardening, chip buildup, and maintains a long knife service life.
- The provided chip removal system eliminates the effect of shrinkage, and a rational selection of cutting conditions for especially soft metals prevents its sticking.
- Vibrations can be reduced by selecting the front and rear corners of the cutting edges, the desired speeds and the use of vibration dampers.
Milling with minimal side processes requires high professionalism and experience.
Milling is a complex complex process of finishing a variety of surfaces, the success of which is determined by the rational choice of equipment, tools, cutting conditions, cutting agents and additional devices that improve the quality of work.