What Is Broaching？
Broaching refers to the use of a special broach to cut a very thin metal layer from the workpiece in turn, so that the surface can achieve high dimensional accuracy and low roughness. It is a high-efficiency processing method. Broaching can process parts that need a combination of gear hobbing machine, gear shaping machine, and milling machine in a short time. During broaching, the broach makes the surface to be machined at one time, so the broaching machine only has the main movement, no feed movement. . The chips of broaching are thin and the cutting motion is smooth, so it has higher machining accuracy and smaller surface structure value. This processing method is widely used in automobile manufacturing. In addition, broaching is also used for high-precision machining of difficult-to-cut materials such as turbine disks used in generator manufacturing and aircraft engine manufacturing.
There is only one main movement in broaching, that is, the broach moves linearly at the cutting speed under the action of the hydraulic pressure of the broaching machine. The feeding movement is realized by the structure of the broach itself, that is, the feeding movement is replaced by the breaking amount of each tooth in turn. Broaching can be seen as a planning process performed by multiple planning knives arranged in a row in order of height. Broaching can process through holes of various shapes, as well as various planes and forming surfaces. Due to the limitation of the broaching tool manufacturing process and the power of the broaching machine, too small or too large holes are not suitable for broaching (broaching hole diameter is generally 10-100mm, and the depth-diameter ratio of the hole is generally not more than 5mm), blind holes, Step holes and thin-walled holes are also not suitable for broaching. Broaching is mainly used in batch and mass production occasions.
Features of broaching
The main features of broaching are as follows:
(1) High production efficiency. A broach is a multi-tooth tool, and the number of teeth involved in the work is large at the same time, which increases the total cutting width, and the roughing, semi-finishing, and finishing processes can be completed in one stroke of the broach, so that the basic process time and auxiliary The time is greatly shortened, so the production efficiency of broaching is higher.
(2) The processing range is wide. Broaching can not only process flat surfaces and unobstructed outer surfaces but also process through holes of various shapes, so the processing range of broaching is wide.
(3) High machining accuracy and low surface roughness. The broach has a calibration part, whose function is to calibrate the size and trim the amount of light. In addition, the cutting speed of broaching is generally low (<18 m/min), and the cutting thickness of each cutting tooth is small, so the cutting process is relatively stable, and the adverse effect of the pseudo-chip edge can be avoided. Therefore, broaching can achieve high precision and low surface roughness. Generally, the dimensional accuracy tolerance level of broaching is IT7~IT8, and the surface roughness Ra is 0.4~0.8μm.
(4) The structure of the broaching machine is simple. Broaching is only the main movement. That is, the linear movement of the broach, and the feed movement is realized by the fact that the last tooth of the broach is higher than the previous one. The height of the tooth is called the tooth lift, so the broach has a simple structure and convenient operation.
(5) The service life of the broach is long. During broaching, the cutting speed is low, the tool wear is slow, and once the sharpening is performed, several workpieces can be processed, and a broach can be reground many times, so the service life of the broach is long.
Although broaching has the above advantages, because the structure of the broaching tool is more complicated than that of the general hole machining tool, the manufacturing is difficult and the cost is high, so the instrument is suitable for batch and mass production. In single-piece and small-batch production, broaching is also used for some forming surfaces with high precision requirements and special shapes that are difficult to process by other methods. Blind holes, deep holes, stepped holes, and obstructed outer surfaces cannot be processed by broaching.
The broaching method refers to the order and method of cutting the broaching allowance from the workpiece, which determines the cross-sectional shape of the cutting layer cut by each tooth, so it is also called the broaching pattern. Whether the broaching method is reasonable or not directly affects the distribution of the tooth load, the length of the broach, the size of the broaching force, the wear and durability of the broach, and the surface quality of the T-piece, productivity, and manufacturing costs. Different broaching methods and different broach design methods are important links that should be determined first in the broach design. Broaching methods can be divided into three types: layered type, block type, and comprehensive type.
1. Layered Type
Layered broaching is when the teeth of the broach cut the broaching allowance layer by layer sequentially, and each tooth cuts off a layer of allowance according to the amount of tooth lift. The cutting thickness of layered broaching is small, so the broaching process is relatively stable and the broaching surface quality is high. However, the unit cutting force is large, the number of cutting teeth required is large, and the broach is long. The cost of the tool is high and the productivity is low. According to the different forming processes of the machined surface, the layered broaching can be divided into the same profile type and the gradual type.
1. Block Type
The broach designed according to the block broaching graphic is composed of several tooth groups. There are 2 to 3 cutter teeth in each tooth group, they have the same diameter, and together they cut off a layer of metal in the machining allowance. Each cutter tooth only cuts a part of the layer. The most commonly used is wheel cutting. The truncation of the wheel-cut broach and its broaching pattern, that is, the diameter of the first tooth and the second tooth are the same, but the positions of the cutting edges are staggered. Metal was removed by the third cutting edge of the same group. The tooth is no longer made with a circular chip splitter. In order to avoid friction between the cutting edge and the surface of the workpiece cut by the first two cutting edges and to cut off a whole circle of chips, its diameter should be 0.02-0.04mm smaller than the blind diameter of the other two teeth in the same group.
3. Comprehensive Type
Comprehensive broaching combines the advantages of layered broaching and block broaching, that is, the rough cutting teeth and transition teeth are made into a wheel-cutting structure, and the fine cutting teeth are made of a layered structure. The length of the broach is shortened, high productivity is maintained, and better surface quality of the workpiece can be obtained.
When the broach moves linearly relative to the workpiece, the machining allowance of the workpiece is sequentially cut off by the teeth of the broach with increasing sizes. Usually, it can be formed in one working stroke, which is a high-efficiency finishing method. However, due to the complex structure of the broach, the high manufacturing cost, and the certain specialty, broaching is mainly used for mass production. According to different machined surface features, broaching is divided into internal broaching and external broaching.
1. Internal broaching: It is used to process through-holes and through-holes in various cross-sectional shapes, such as round holes, square holes, polygonal holes, spline holes, keyway holes, internal gears, etc. Before broaching, there must be a machined hole through which the broach can be inserted. The hole diameter of broaching ranges from 8 to 125 mm, and the hole depth does not exceed 5 times the hole diameter. In special cases, 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.
2. External broaching: It is used to process non-closed surfaces, such as planes, forming surfaces, grooves, tongue grooves, blade tenons, and external gears, etc. It is especially suitable for processing relatively large planes and composite surfaces in mass production, such as Cylinder blocks, bearing seats and connecting rods of automobiles and tractors, etc. The dimensional accuracy of the broaching profile can reach IT8-5, the surface roughness is Ra2.5-0.04 microns, and the broaching gear accuracy can reach grades 6-8 (JB179-83).
When broaching, the order and method of removing the machining allowance from the workpiece include forming type, progressive type, round cutting type, and comprehensive round cutting type.
1. Forming type. The machining accuracy is high, the surface roughness is small, but the efficiency is low; the length of the broach is long, and it is mainly used for machining small and medium-sized round holes and forming surfaces with high precision requirements.
2. The progressive type is suitable for rough broaching of complex machined surfaces, such as square holes, polygon holes, and spline holes, etc. The broach used in this method is easier to manufacture, but the quality of the machined surface is poor.
3. The round cutting type has high cutting efficiency and can reduce the length of the broach, but the quality of the machined surface is poor.
4. Comprehensive round cutting method is used for rough broaching by wheel cutting method and fine broaching by forming method. It has the advantages of both and is widely used in round hole broaching.
Notes on Broaching
When broaching ordinary structural steel and cast iron, the rough drawing speed is generally 3 to 7 m/min, and the fine drawing speed is less than 3 m/min. For difficult-to-machine metal materials such as superalloys or titanium alloys, only the use of carbide or new high-speed steel broaching tools on a high-speed broaching machine with good rigidity can be compared at a speed of 16-30 m/min or higher. Satisfying result. The helical broaching device is used to make the helical tooth broach and the workpiece perform relative linear motion and rotary motion, and can also broach internal threads, helical spline holes, and helical internal gears. Broaching generally uses cutting fluids with better lubricating properties, such as cutting oils and extreme pressure emulsions. In high-speed broaching, the cutting temperature is high, and chemical cutting fluids and emulsions with good cooling properties are often used. If the internal cooling broach is used to inject the cutting fluid into each chip groove of the broach at high pressure, it will have a good effect on improving the surface quality, reducing the wear of the tool, and improving the production efficiency.