Broaching
Broaching is the process of removing a layer of material of the desired width and depth, generally in a single pass, using a thin bar-type cutting tool with a series of progressively larger cutting edges. It is also called “tığ çekme” in Turkish.
In broaching, the broaching tool is generally a profiled die for the internal or external surfaces of the workpiece. Broaching is used in mass production wherever profiles with high surface quality and great dimensional accuracy need to be produced. Because the broach (cutting) tool is very expensive, it is only economical for large quantities of parts.
The broaching method is divided into two groups: internal broaching and external broaching.
Internal Broaching
When internal surfaces are to be broached, the part must already have a starting hole for the broach.
In internal broaching, profiles are broached outward from a hole. The workpiece is not clamped; it is simply slid along the tool’s guide bar and placed flat against the machine’s workpiece support die. The broaching tool is pulled through the hole by its pull-end shank, producing the desired profile.
In screw broaching (cutting a helical groove), the teeth of the broaching tool are made in a screw shape. The workpiece is mounted so it can rotate, placed inside a holding jaw.
During broaching, the workpiece is rotated by the sides of the teeth so as to produce the lead angle of the groove being broached.
External Broaching
In external broaching, the tool is guided along the outside of the workpiece. Because of the large cutting and pull-out forces involved, the tool must be rigidly clamped and supported.
In circumferential (surround) broaching (tube and shell broaching), the tool completely surrounds the workpiece, making it possible, for example, to broach an external gear tooth form in a single pull. The tool is made in sections.
Advantages of broaching
Provides high surface quality
Completed in a single pass.
Cutting speed is high.
Provides dimensional accuracy.
Allows complex profiles to be produced.
Provides easy and cheap labor.
Machined parts are identical
Weaknesses of the broaching process
Broach tool cost is high for large and complex-shaped parts.
It is not possible to machine very small parts.
Parts must be firmly clamped.
Not much stock can be removed
Broaching Tools
Broaching tools are generally made of tool steel (such as M2 HSS, PM-M4 HSS, PM-T15 HSS). There are also carbide-tipped versions used on CNC machines.
Broaching tools consist of a shank, a guide bar, the tooth section, a rear guide section, and a rear end section. In the cutting portion of the tool, the chip thickness (h) is produced by progressively stepping up the cutting-edge teeth.
The roughing teeth take on the largest share of material removal (h=0.2 mm), while the finishing section consists of at least 3 teeth (h<0.2 mm). The last finishing tooth and the first burnishing tooth are made to the manufacturing size. The tool has at least 3 finishing (burnishing) teeth of the same profile and dimensions, which burnish (calibrate) the surfaces of the workpiece. These also serve as a reference during resharpening.
The cutting-edge angle and chip clearance are determined based on the length of the surfaces to be broached and the machinability of the workpiece material. A pilot hole is drilled before broaching. The relief angle is very small (1.5°~2°), so that this value stays the same through resharpening of the tooth face. Chip-breaker grooves on the relief surfaces prevent chips from jamming between the side relief surfaces and the broached surfaces and cracking them. The broach used during broaching should be selected so that 2 to 6 teeth are cutting at the same time. The more teeth in contact with the material at once, the smoother the broaching operation proceeds.
Broaching tool terminology
Where Is the Broaching Technique Used
Any material that can be machined using cutting techniques can also be machined by broaching. Widely used metal alloys, some plastics, hard rubbers, wood, composite materials, and graphite can all be machined by broaching. Steel materials with a hardness between 18-32 HRC can be machined with this technique. Cast iron, brass, bronze, aluminum, and magnesium can be machined at high cutting speeds.
Broaching is used to produce various grooves, keyways, flat or complex-shaped surfaces, rack gears, and internal gears. The broaching technique is used in the automotive, aerospace, and hand-tool manufacturing industries, in parts made by the milling technique, in white-goods manufacturing, in firearms parts production, in lock manufacturing, and in electric motor production. It is also used in the firearms industry to cut rifling in gun barrels.
Broach Tooth Rise
Each tooth on a broaching tool grows progressively, removing a little more material than the tooth before it.
Single-Pass Finishing
Broaching is an efficient method that can machine complex internal profiles to their final dimension, usually in a single pass.
High Investment Cost
Because broach tools are complex and expensive to produce, the method is generally preferred for high-volume mass production.
Related Questions
The tool is a precisely and specially manufactured part with a large number of progressively increasing cutting teeth. Because this manufacturing cost can only be brought down per part by producing a large number of identical parts, it is not economical for small production runs.
The broaching tool is pulled through a pre-drilled hole, and this pulling motion already holds the part firmly against the support die. The operation can also be completed without needing any clamping mechanism.
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