Broach is a kind of high-precision and high-efficiency multi tooth tool used in mass production.
Different machined surfaces according to tool position
According to the different surface machined by the tool position, it can be divided into internal broach and external broach.
(1) Internal broach is used to process internal surfaces of various shapes, and its broach name is generally determined by the shape of the machined hole, such as round hole broach, four / six square broach, keyway broach and spline broach.
The internal broach can also process spiral internal splines and internal gears.
The hole diameter that the internal broach can process is usually 10 ~ 120mm, which can be processed to 5 ~ 400mm in special cases.
The broached groove width is generally 3 ~ 100mm, and the length of the hole is generally not more than 3 times the diameter, which can reach 2m in special cases.
Common broaches include round hole broach, spline broach, square hole broach and keyway broach (see Fig. 6-13).
Fig. 6-13 internal broach
a) Round hole broach
b) Square hole broach
c) Spline broach
d) Involute broach
(2) The external broach is used to process the external surfaces of various shapes (see Fig. 6-14), such as plane, formed surface, groove, complex tenon and tenon in steam turbine, to replace the milling, planing, grinding and other processing of these parts.
It is especially suitable for the surface of some parts in mass production of automobiles, motorcycles, tractors and so on.
In production, the internal broach is more commonly used than the external broach.
Fig. 6-14 external broach
1 – cutter tooth 2 – cutter body
According to different stress directions of broach during operation
The broach can be divided into broach and push knife according to the different stress directions when the broach is working.
The former is under tension and the latter is under pressure.
Considering the stability of the pressure bar, the length diameter ratio of the pusher should be less than 12.
According to the structure of broach
According to the different structure of broach, it can be divided into combined type, integral type and assembled type.
1. Combined broach
The use of combined broach can not only save tool materials, but also simplify the manufacture of broach, and can be easily adjusted or replaced when the teeth of broach are worn or damaged.
The combined spline broach is shown in Fig. 6-15.
Fig. 6-15 combined spline broach
1 – cutter body
2. 9 – round nut
3 – pressure ring
4 – pressure block
5. 7 – screw
6 – blade
8 – retaining ring
10 – tail
2. Integral broach
The integral type is mainly used for high-speed steel integral broach of medium and small sizes.
3. assembly broach
The assembly type is mainly used for large-size and cemented carbide combination broaches.
Broach can be used to process various cross-section shapes of through holes, straight or curved outer surfaces.
Fig. 6-16 shows the typical workpiece section shape for broaching.
Fig. 6-16 various internal and external surfaces of broaching
Chain continuous broaching
In normal broaching, the workpiece does not move, and the broach does the main movement.
In order to improve productivity and realize automatic production, a chain continuous broaching method has emerged.
As shown in Fig. 6-17, the broach is fixed, and the workpiece to be processed is installed on the traveling fixture of the continuous moving chain conveyor belt to make the main movement, so as to realize the continuous broaching mode.
This broaching method has been applied in automobile manufacturing industry.
Fig. 6-17 continuous broaching of chain conveyor belt
1 – broach
2 – workpiece
3 – chain conveyor
Broaching is carried out with broach. The latter (or group) tooth of broach is higher than the former (or group) tooth, and the allowance is cut layer by layer to obtain the required machining surface.
During broaching, the broach moves in a straight line at a constant speed.
Since the latter (or a group) tooth of the broach is higher than the previous (or a group) tooth, it can cut the excess metal from the workpiece layer by layer, as shown in Fig. 6-18.
Fig. 6-18 schematic diagram of broaching
Due to the low broaching speed and small cutting thickness, higher precision and better surface quality can be obtained.
Compared with other machining methods, broaching has the following characteristics:
(1) The broaching machine has simple structure.
Broaching usually has only one main motion (broach linear motion), and the feed motion is completed by the tooth lift of the broach teeth.
Therefore, the broaching machine has simple structure and convenient operation.
(2) High machining accuracy and surface quality.
General broaching machine adopts hydraulic system, with stable transmission;
The broaching speed is low, generally 0.04 ~ 0.2m/s (2.5 ~ 12m/min), which is not easy to produce chip buildup.
The cutting thickness is very small.
Generally, the cutting thickness of the precision cutting teeth is 0.005 ~ 0.015mm, so the machining accuracy can reach IT7, and the surface roughness value Ra=0.8 ~ 2.5 μ m.
(3) High productivity.
As the broach is a multi tooth tool, there are many teeth participating in the work at the same time, the total length of the cutting edge is large, and one stroke can complete rough, semi precision and finish machining, so the productivity is very high.
(4) Long service life of broach.
Due to the low broaching speed and slow broaching wear, the broach has high durability.
At the same time, the teeth of the broach can be reground several times after being blunt, which has a long service life.
Because the structure of broach is more complex than that of ordinary tools and the manufacturing cost is high, broach is generally used for mass or batch production.
However, for some internal and external formed surfaces with high precision and special shape, it is difficult to process them by other methods.
Although they are single piece and small batch production, they are also processed by broach.
Main components of broach
（1） Structure of broach
The structure of round hole broach is shown in Fig. 6-19:
Fig. 6-19 structure of round hole broach
1 – head
2 – neck
3 – transition cone
4 – leading part
5 – cutting part
6 – calibration section
7 – rear guide part
8 – tail
(1) Head – connected to the machine tool to transmit motion and tension.
(2) Neck – the connecting part between the head and the transition cone, which is also the place for marking.
(3) Transition cone part – it plays a guiding role and makes the broach easy to enter the preformed hole of the workpiece.
(4) Leading part – guide the broach to transition to the cutting part smoothly and without skew.
(5) Cutting part – cutting off all machining allowances. It consists of rough cutting teeth, transition teeth and fine cutting teeth.
(6) Calibration part – the last few cutter teeth without tooth lift and chip separation groove play the role of polishing and calibration, so as to improve the machining accuracy and surface quality of the hole, and can be used as backup teeth for fine cutting teeth.
(7) Back guide part is used to keep the correct position of the last few tool teeth of the broach and prevent the machined surface quality and tool teeth from being damaged due to the workpiece sag when the broach is about to leave the workpiece.
(8) Tail – when the broach is long and heavy, a bracket can be used to support the tail of the broach to prevent the tension from sagging due to its own weight. Generally, the broach with light weight does not need a tail.
（2） Geometric parameters of cutter teeth
The main geometric parameters of the broach cutting part are shown in Fig. 6-20.
Fig. 6-20 main geometric parameters of broach cutting part
(1) Tooth lift af – the difference between the radius or height of the front and rear cutter teeth (or tooth groups).
Rough cutting af=0.02 ~ 0.20mm, fine cutting af=0.005 ~ 0.015mm.
(2) Tooth pitch P – axial distance between two adjacent cutter teeth.
The tooth pitch is calculated according to the broaching length of the hole, and the number of simultaneous working teeth of the broach can be 3 ~ 8.
(3) Anterior horn γO – front angle is selected according to workpiece material.
Rake angle of cutting teeth of general high speed steel broach γo=5 °～ 20 °, front angle of carbide broach γo=0 °～ 1.0 °, calibrate the front tooth angle γog is the same as the rake angle of the cutting tooth.
(4) Edge band ba1 – in order to increase the regrinding times of broach, improve the stability of cutting process and facilitate the control of the diameter of cutter teeth during manufacturing, an edge with a back angle of 0 ° is reserved on the back face of cutter teeth.
Seection of anterior horn γo and posterior horn α
Due to the low broaching speed and small cutting thickness, higher precision and better surface quality can be obtained.
Broach has more complex structure and higher manufacturing cost than ordinary tools, so it is generally used for mass or batch production.
（1） Selection of front angle γo and rear angle α
(1) Anterior horn γo of broach is selected according to the properties of the material to be broached.
When broaching ductile materials, a larger rake angle shall be selected, and when broaching brittle materials, a smaller rake angle shall be selected.
(2) Back angle of broach α is determined according to the type of broach and the machining accuracy required by the workpiece.
Increasing the back angle can reduce the friction between the back face of the broach and the workpiece, but it will quickly reduce the size of the broach during regrinding and make it lose accuracy.
Generally, the back angle of cutting teeth of round hole broaches, spline broaches, quadrangular broaches, hexagonal broaches and other types of broaches with broaching IT7 ~ IT8 accuracy is 2 ° ~ 3 ° 30′, the back angle of cutting teeth of the above broaches with broaching IT9 accuracy and lower accuracy is 3 ° ~ 4 °, the back angle of cutting teeth of groove broaches, plane broaches and external broaches with single side teeth is 4 ° ~ 7 °, and the back angle of calibration teeth is 0 ° 30′ ~ 1 ° 30′.
（2） Selection of tooth lift af
The tooth lift af is the size difference between two adjacent cutter teeth in the radial direction of the broach, which is called the tooth lift of the broach.
The tooth lift is also the cutting thickness and feed rate of each tooth of the broach.
The tooth lift of the broach directly affects the quality of the broaching surface, broaching force and the structure of the broach.
It is one of the most important parameters of the broach.
The value of tooth lift is usually determined according to the nature of the material to be machined, the type of broach, the quality and accuracy requirements of the surface of the workpiece to be broached and other factors.
Generally, the tooth lift of the broach does not exceed 0.15mm.
If there are broaches with small requirements for the roughness of the surface to be broached, poor processing performance of the material to be machined, broaches for high-speed broaching, parts with poor processing rigidity (thin-wall and soft metal), broaches with small cross-section and low strength, etc, the tooth lift should be smaller.
（3） Selection of pitch p
The tooth pitch is the axial distance between two adjacent tool teeth of a broach, and its size is generally determined by the broached length of the workpiece.
When broaching common materials, the tooth pitch p= (1.25 ~ 1.5) L × 1/2 (L is the broached length of the workpiece) select the small value for short workpiece and brittle material, and the large value for long workpiece or ductile material.
When broaching superalloy materials, the tooth pitch p= (1.9 ~ 2.0) l × 1/2, number of simultaneous working teeth zi= (L/p) +1 (take an integer).
It can be seen that the smaller the tooth pitch of the broach, the more the number of working teeth of the broach, the more stable the working process, and the smaller the broaching surface roughness value, but it will increase the broaching force and reduce the chip holding space accordingly;
If the tooth pitch of broach is too large, the number of simultaneous working teeth will be reduced and the broaching process will be unstable.
Therefore, in the design, the number of simultaneous working teeth of the broach is generally 3 ≤ Zi ≤ 8.
The tooth pitch and the number of simultaneous working teeth are interrelated.
The broaching stability, broaching force, broaching length, chip space and broach strength must be comprehensively considered.
（4） Selection of chip holding coefficient K and chip holding groove depth h
In broaching, the chips cut by each tooth of the broach must be completely contained in the chip holding groove.
The chip holding coefficient shall meet the following conditions:
Where Ac — effective area of chip holding tank, Ac= πh ²/ 4
Aj — cutting area, Aj=afL
（5） Shape and size of chip holding groove
The chip holding groove of the broach shall be able to hold all the cut chips and the free curl of the chips, and ensure that the cutter teeth have sufficient strength and a certain number of regrinding times.
The commonly used chip holding groove shapes are linear tooth back type and curved tooth back type.
The linear tooth back type is easy to manufacture and is mostly used for broaching brittle materials.
The curved tooth back type is conducive to the curling of chips and the falling of chips in the chip groove after broaching, which is suitable for broaching ductile materials.
The dimensions of the broach chip holding groove are shown in Fig. 6-21.
The relationship between the dimensions of each part of the broach chip holding groove and the tooth pitch p is as follows:
r = 0.5h
Fig. 6-21 dimension diagram of broach chip holding groove
The design and production shall follow the principle of minimizing the size of chip tank and meeting the production needs.
（6） Selection of broaching speed
Broaching speed is generally 1 ~ 18m/min.
When the increase of broach teeth is large, the broaching speed should be smaller. When broaching hard materials (280 ~ 320HBW) or soft materials (hardness <170HBW), the broaching speed should be reduced accordingly.
When broaching hardness >320HBW, broaching speed should be reduced.
Broaching surface roughness Ra<1.6 μ m, the broaching speed shall be controlled below 2m/min.
Precautions for use of broach
The broaching machine has simple structure and convenient operation.
The machining precision and surface quality are high.
Generally, the broaching machine adopts the hydraulic system, with stable transmission.
Moreover, one stroke can complete rough, semi precision and finish machining, so the productivity is very high.
（1） Rational use of broach
(1) For broaching high-performance and difficult to machine materials, appropriate heat treatment can be selected to improve the machinability of the materials.
(2) Before the broach is used, the antirust oil must be cleaned with gasoline to remove the cuttings or other dirt in the teeth and chip holding groove, and check whether the teeth are sharp and whether the teeth are bruised, broken edges or other damage;
Check the tooth lift, chip holding space and broach strength, and never broach with a broach with damaged tool teeth.
(3) The broaching length of hole processing broach is strictly regulated.
It cannot exceed the specified range of broach at will, otherwise the broach will be broken or broken due to insufficient strength.
However, the broaching length should not be too short.
It is necessary to ensure that the number of teeth working at the same time during broaching is not less than 3, so as to ensure that the broaching process is stable.
(4) When using a long broach, adjust the bracket to be coaxial with the workpiece.
When the gap between the workpiece hole and the leading part of the broach is large, the workpiece shall be moved to the broach tooth consistent with the hole diameter to start broaching, otherwise the position of the hole will shift or the broach will be broken.
(5) The positioning end face of broach must be perpendicular to the feeding axis of broach, and the perpendicularity of the positioning flange tray of broach to the feeding axis shall be regularly checked.
(6) The stable and powerful broaching of broaching machine is closely related to the heating of oil tank before broaching machine operation, the discharge of stored air in hydraulic cylinder, and the stable rise and fall of oil pressure.
It is particularly important to eliminate the vibration or crawling during broaching.
(7) When the broach is inserted into the collet, pay attention to whether the inserting plate is inserted correctly;
After the broach is inserted, the automatically locked chuck should be pulled back by hand to see if it has been locked;
After the broach is inserted, the claw chuck shall be observed and manually pulled to see if it is properly installed and stuck.
Then push the workpiece to be broached onto the positioning surface of broaching before broaching.
（2） Eliminate broaching surface defects
(1) Improve the grinding quality of the cutter teeth, prevent the edge from chipping, keep the edge sharp, and keep the front angle of each tooth consistent with the width of the blade belt.
(2) Keep the broaching process stable, increase the number of working teeth, reduce the tooth pitch of precision cutting teeth and calibration teeth, and improve the rigidity of the broaching system.
(3) Reasonably select broaching speed.
(4) Use carbide broach and coated broach.
(5) Reasonably select and fully pour cutting fluid.
（3） Requirements for broached workpiece
The machining surface quality and dimensional accuracy during broaching are closely related to the process preparation of the workpiece before broaching.
For broaching of inner hole, the preformed hole of the workpiece is not only the surface to be machined but also the positioning reference.
Its dimensional and geometric accuracy, the perpendicularity with the reference end of the workpiece, and the cutting performance of the workpiece material play an important role in the broaching quality and the normal broaching process.
The workpiece before broaching shall meet the following requirements:
(1) Before broaching, the prefabricated hole must ensure certain geometric accuracy, such as axial degree, perpendicularity, size accuracy of H8 ~ H11, and surface roughness value Ra=3.2 ~ 6.3 μ m.
The leading part of the broach shall be able to completely penetrate into the hole.
Broaching cannot be carried out if it cannot penetrate or only passes through half of the hole.
Before broaching, both ends of the prefabricated hole shall be chamfered to avoid burrs affecting the passage of the broach and the positioning of the workpiece.
(2) The base surface during broaching must be flat and smooth. If the precision of prefabricated hole and positioning base surface is poor, spherical supporting fixture shall be used.
(3) When the length of a short workpiece is less than two tooth pitches of a broach, a fixture can be used to fasten several workpieces together for broaching.
(4) The steel parts shall be normalized, annealed and modulated.
When the hardness is 180 ~ 240HNW, the broaching performance is the best, the machined surface quality is the easiest to achieve, and the broach life is also long.
Therefore, the hardness of the workpiece to be processed should be pre heat treated to this range as far as possible.