Frequently Employed Processing Techniques

Broaching

Broaching is a type of machining operation, which is a cutting process that uses a broaching machine (broach) to process various internal and external forming surfaces. Broaching tools are very complex components, made from a single piece. The material is mainly high-speed steel, and cemented carbide is used only as the tool material for the machining of gray cast iron. Unlike many other cutting operations, the main consideration is tool wear or tool life.

In the case of broaching, several teeth mesh simultaneously and the chip width is often very large. The removal of chips can be very problematic, so low viscosity oils are also usually required. From the perspective of cutting fluid supply, internal broaching is more problematic than external broaching, and horizontal broaching is more difficult than vertical broaching.

Classification of broaching methods:

Use broaches as a cutting tool. When the broach moves linearly concerning the workpiece, the machining allowance of the workpiece is sequentially cut off by the teeth of the broach increasing in size one by one. Usually, it can be processed and formed in one working stroke, which is an efficient finishing method. However, broaching is mainly used for mass production because of its complicated structure, high manufacturing cost, and certain specificity. According to the characteristics of the processed surface, broaching is divided into internal broaching and external broaching.

• Internal broaching: used to process through holes and holes in various cross-sectional shapes, such as round holes, square holes, polygon holes, spline holes, key slot holes, internal gears, etc. There must be a machined hole before broaching, so that the broach can be inserted through it. The hole diameter of broaching is 8 to 125 mm, and the hole depth does not exceed 5 times the hole diameter. Under special circumstances, the aperture range can be as small as 3 mm, as large as 400 mm, and the hole depth can be up to 10 meters.
• Outer broaching: used to process non-closed surfaces, such as planes, forming surfaces, grooves, tongues and grooves, blade tenons and external gears, etc. It is especially suitable for processing relatively large planes and composite profiles in mass production, such as Cylinder blocks, bearing blocks and connecting rods for automobiles and tractors. The dimensional accuracy of the broaching profile can reach IT8～5, the surface roughness is Ra2.5～0.04 microns, and the accuracy of broaching gear can reach 6～8 grade (JB179-83).

When broaching, the order and method of cutting the machining allowance from the workpiece are a formal form, a progressive form, a wheel cut type, and a comprehensive wheel cut type.

• Into a form. The processing precision is high, the surface roughness is small, but the efficiency is low; the broach length is long, mainly used for processing small and medium-sized round holes and forming surfaces with high precision requirements.
• The progressive type is suitable for rough broaching complex machining surfaces, such as square holes, polygonal holes and spline holes. The broaches used in this way are easier to manufacture, but the quality of the processed surface is poor.
• Wheel-cutting cutting efficiency is high, which can reduce the length of the broach, but the quality of the processed surface is poor. It is mainly used for processing round holes with large size, large machining allowance, and low precision requirements.
• Integrated wheel cutting type is rough broaching by the wheel cutting method and fine broaching by the forming method, which has the advantages of both, and is widely used in round hole broaching.

Boring

Boring is an internal diameter cutting process that uses tools to enlarge holes or other round contours. Its application range is generally from semi-roughing to finishing. The tool used is usually a single-edge boring tool (called a boring bar).

Drilling through holes or blind holes on solid materials with twist drills, flat drills or center drills is called drilling. In addition to drilling, the drilling process also includes reaming and hole drilling. So drilling is a type of drilling.

The method of processing a reverse boring hole with a reverse boring tool is called reverse boring.

On CNC machine tools, we often use non-standard tools (eccentric boring cutters, rotating blades, special reverse boring cutters) to use CNC machining programs for reverse boring.

The rotating single-edge boring tool is used to enlarge the pre-made holes on the workpiece to a certain size, to achieve the required accuracy and surface roughness cutting. Boring is generally carried out on boring machines, machining centers and combination tool machines. It is mainly used for processing cylindrical holes (see figure), threaded holes, grooves and end faces in workpieces such as boxes, brackets and machine bases; when special for accessories, it can also process inner and outer spherical surfaces, tapered holes, etc. The boring precision of steel materials can generally reach IT9～7, and the surface roughness is Ra2.5～0.16 microns.

When boring, the workpiece is installed on the machine tool table or machine tool fixture, the boring tool is clamped on the boring bar (it can also be made integral with the boring bar), and the main shaft drives the rotation. When the boring die is used, the boring bar and the spindle are floatingly connected, and the processing accuracy depends on the accuracy of the boring die; when the boring die is not used, the boring bar and the main shaft are rigidly connected, and the processing accuracy depends on the accuracy of the machine tool. Because the overhang distance of the boring bar is large, it is easy to produce vibration, and the cutting amount used should not be very large. The boring process is divided into rough boring, semi-fine boring and fine boring. The cutting speed when boring ordinary steel with high-speed steel cutter head is generally 20-50 meters/minute; the cutting speed when using carbide head is 40-60 meters/minute for rough boring and 150 meters for fine boring /Min or more.

For precision boring, which requires high precision and surface roughness, diamond boring machines are generally used, and tools with ultra-hard materials such as cemented carbide, diamond, and cubic boron nitride are used. A small feed rate (0.02～0.08 mm/ Turn) and cutting depth (0.05 ~ 0.1 mm) higher than the cutting speed of ordinary boring. The precision of precision boring can reach IT7～6, and the surface roughness is Ra0.63～0.08 microns. Before precision boring, prefabricated holes go through rough boring, semi-precision boring and fine boring procedures, leaving a thin and uniform machining allowance for precision boring.

Grinding

Grinding refers to a method of using abrasives and abrasive tools to cut off excess material on the workpiece. Grinding is one of the more widely used cutting methods.

Grinding processing belongs to finishing processing (mechanical processing is divided into rough processing, finishing processing, heat treatment and other processing methods), and the processing volume is small and the precision is high. It is widely used in the machinery manufacturing industry. After heat treatment and quenching of carbon tool steel and carburized and quenched steel parts, a large number of regularly arranged cracks often appear on the surface that is substantially perpendicular to the grinding direction during grinding-grinding cracks , It not only affects the appearance of the part, but also directly affects the quality of the part.

The principle of grinding:

Use high-speed rotating grinding wheels and other abrasive tools to cut the workpiece surface. Grinding is used to process the inner and outer cylindrical surfaces, conical surfaces and flat surfaces of various workpieces, as well as special and complex forming surfaces such as threads, gears and splines. Due to the high hardness of the abrasive grains, the abrasive tool is self-sharpening, and grinding can be used to process various materials, including hardened steel, high-strength alloy steel, cemented carbide, glass, ceramics and marble, and other high-hardness metal and non-metal materials. Grinding speed refers to the linear speed of the grinding wheel, which is generally 30 to 35 m/s. When it exceeds 45 m/s, it is called high speed grinding. Grinding is usually used for semi-finishing and finishing, the accuracy can reach IT8 ~ 5 or even higher, the surface roughness is generally R a1.25 ~ 0.16 microns, precision grinding is R a 0.16 ~ 0.04 microns, ultra-precision grinding is R a0.04～0.01 microns, mirror grinding can reach R a0.01 microns or less. The specific power of grinding (or specific energy consumption, that is, the energy consumed to remove the unit volume of workpiece material) is larger than that of general cutting, and the metal removal rate is smaller than that of general cutting, so the workpiece is usually removed by other cutting methods before grinding For most machining allowances, only 0.1 to 1 mm or smaller grinding allowances are left. With the development of high-efficiency grinding such as slow feed grinding and high-speed grinding, it has been possible to directly grind the parts from the blank. It is also useful for grinding as a waste process, such as grinding the riser of castings, flashing of forgings and the skin of steel ingots.

Types of grinding:

Cylindrical grinding: It is mainly carried out on an external cylindrical grinding machine, which is used to grind the outer cylinder, outer cone and shoulder end surface of shaft-type workpieces. During grinding, the workpiece rotates at a low speed. If the workpiece reciprocates in the longitudinal direction at the same time and the single-stroke or double-stroke movement in the longitudinal direction moves the grinding wheel relative to the workpiece, it is called the longitudinal grinding method. If the width of the grinding wheel is greater than the length of the surface to be ground, the workpiece does not move longitudinally during the grinding process, but the grinding wheel feeds the workpiece continuously, which is called the plunge grinding method. The efficiency of the plunge-cut grinding method is generally higher than that of the longitudinal grinding method. If the grinding wheel is trimmed into a forming surface, the plunge grinding method can be used to process the formed outer surface.

Flat grinding: It is mainly used for grinding planes and grooves on a surface grinder. There are two types of surface grinding: the grinding with the outer surface of the grinding wheel is called peripheral grinding. Generally, a horizontal axis surface grinder is used. If a shaped grinding wheel is used, various shaped surfaces can also be processed; the grinding with the grinding wheel end surface is called end grinding, generally use vertical axis surface grinder.

Inner circle grinding: It is generally carried out on a centerless grinder to grind the outer circle of the workpiece. During grinding, the workpiece is not centered and supported by the center, but is placed between the grinding wheel and the guide wheel, supported by the pallet below it, and driven by the guide wheel to rotate. When the axis of the guide wheel and the axis of the grinding wheel are adjusted to obliquely cross 1° to 6°, the workpiece can automatically make longitudinal feed motion along the axis while rotating, which is called through grinding. Through grinding can only be used to grind outer cylindrical surfaces. When using plunge less centerless grinding, the axis of the guide wheel and the axis of the grinding wheel must be adjusted to be parallel to each other, so that the workpiece is supported on the pallet without axial movement, and the grinding wheel continuously feeds transversely relative to the guide wheel. The plunge less centerless grinding can process the forming surface. Centerless grinding can also be used for internal grinding. During processing, the outer circle of the workpiece is centered on the roller or the support block, and the workpiece is rotated by the eccentric electromagnetic suction ring. The grinding wheel extends into the hole for grinding. At this time, the outer circle is used for positioning the datum ensures that the inner and outer circles are concentric. Centerless internal grinding is often used to grind the inner groove of the bearing ring on a special grinding machine for bearing rings.

Milling

The CNC lathe can process the shape of complex rotary body. Milling is to fix the blank, and use a high-speed rotating cutter to cut the blank and cut out the desired shape and features. Traditional milling is mostly used for simple contours/features such as contours and grooves. CNC milling machines can process complex shapes and features. The milling and boring machining center can carry out three-axis or multi-axis milling and boring processing for machining, molds, inspection tools, tires, thin-walled complex curved surfaces, prostheses, blades, etc. When selecting the content of CNC milling, the advantages and key roles of CNC milling machines should be fully utilized.

The milling process is a common metal cold working method. The difference from turning is that in milling, the tool rotates at high speed under the drive of the spindle, and the workpiece is relatively stationary.

The difference between turning and milling:

Turning is used to process the parts of the rotary body. The parts are clamped on the spindle of the machine tool through the three-clamp chuck and rotated at high speed. Then, the turning tool is used to follow the generatrix of the rotary body to cut out the product appearance. Lathes can also process inner holes, threads, bites, etc. The latter two are processed at low speed.

Milling process

• Curve contours on the workpiece, straight lines, arcs, threads or spiral curves, especially non-circular curves and list curves given by mathematical expressions.
• The spatial curve or surface of the mathematical model has been given.
• Although the shape is simple, it has many sizes and difficult to detect.
• The inner cavity, box interior, etc. which are difficult to observe, control and detect when machining with ordinary tools.
• There are holes or planes with strict size requirements.
• A simple surface or shape that can be processed in a single pass.
• The general processing content that adopts CNC milling processing can effectively improve productivity and reduce labor intensity.

The main processing objects suitable for CNC milling are the following types: plane contour parts, variable bevel type parts, space curved surface contour parts, holes and threads.