What is broaching operation?
Fig. 1 (a) Parts created from internal broaching, (b) Parts made from surface broaching & (c) Broaching machine
Broaching process is similar to shaping process and is used to machine internal and external surfaces with a long multi point tooth cutter.
In a single stroke a bigger broach can remove material up to 38 mm.
Broaching operation is an important production process and can produce parts with good surface finish and dimensional accuracy.
Other manufacturing process (such as boring, milling, shaping, and reaming) directly compete with broaching process to produce intricate shapes. Broaching are quite costly but they are still used for applications having higher production.
What are the terminologies associated with a broach?
Fig. 2 Terminologies of Broach
The rake (hook) angle depends on the material cut (as it does in turning and other cutting operations) and usually ranges from 0° to 20°.
The clearance angle is typically 1° to 4°,finishing teeth have smaller angles.
If the clearance angle is lesser there is a chance of friction between the teeth and the broached surface. The length of cut, tooth strength, and size and shape of chips helps in identifying the pitch of the teeth.
In broaching process atleast two teeths should always be in contact of the workpiece.
The pitch of a broach for a length l can be calculated as follows:
where k is a constant, equal to 1.76 when l is in mm.
Small broach tool have a pitch in the range of 3.2 to 6.4 mm while for large ones it is in the range from 12.7 to 25 mm.
The depth of cut per tooth depends on the workpiece material and the surface finish required.
The depth of cut ranges from 0.025 to 0.075 mm for medium sized broaches and can be greater than 0.25 mm for larger sized broaches.
What is turn broaching?
Fig. 3 Terminology for internal broach
This process is typically used for broaching the surfaces where high finish is required.
The broaching tool which contains the multiple carbide inserts is used to remove the material by rotating the crankshaft between centers.
Straight as well as circular broaches are used successfully in turn broaching. In latest trends, machines are capable of broaching a number of crankshafts simultaneously.
What is a broaching machine?
The broaching machines are actuated hydraulically which moves only in linear motion. Due to this the construction of these machines are quite simple.
Broaching machines are available in sizes to make needle-like parts and also for broaching gun barrels, including rifled (with internal spiral grooves) gun barrels.
Broaching machines are of two types:
Horizontal broaching machine.
Vertical broaching machine.
Push broaches are in the range of 150 to 350 mm which are considered to be shorter.
Longer strokes are achieved with the help of Horizontal machines.
The needed force to pull or push the broach depends on the
(a) strength of the workpiece material,
(b) total depth and Width of cut,
(c) cutting speed,
(d) tooth profile, and
(e) use of cutting fluids.
The pulling force capabilities of broaching machines are as high as 0.9 MN.
What are the process parameters in the broaching process?
Fig. 4 Features of chip breaker on (a) Flat broach & (b) Round broach
For high-strength alloys the cutting speed of broaching is around 1.5 m/min to as much as 15 m/min for aluminum and magnesium alloys.
M2 and M7 high-speed steels, as well as carbide inserts are mostly used as broach materials.
The majority of broaches are now coated with titanium nitride for improved tool life and surface finish.
Ceramic inserts also are used for finishing operations in some applications.
Although carbide or ceramic inserts can be indexed after they are worn, high-speed steel broach teeth have to be resharpened by grinding, which reduces the size of the broach.
For internal broaching cutting fluids are mostly used.
What are the design considerations in broaching process?
Clamping in broaching machines should be easily possible for the parts.
The designed parts should be capable to withstand the deflections the cutting forces during broaching.
Keyways, splines, gear teeth, etc., all have standard sizes, and these dimensions allow the use of common broaches.
Balanced cross sections are preferable to keep the broach from drifting, thus maintaining close tolerances.
In broaching, radii are difficult to manufacture, while chamfers can be easily done.
Inverted or dovetail splines should be avoided.