History and Process
– Broaching originated in the early 1850s for cutting keyways in pulleys and gears.
– After World War I, broaching was used for rifling gun barrels.
– Advances in form grinding and broaching machines in the 1920s and 30s improved tolerances and reduced costs.
– Broaching became useful for machining other surfaces and shapes for high volume workpieces.
– Customized broaches are viable for high volume workpieces, but can be expensive to produce.
– Surface broaching involves moving the workpiece against a stationary surface broach.
– Internal broaching requires clamping the workpiece into a special holding fixture and lowering the broach through the workpiece.
– Cutting fluids are used to cool, lubricate, and flush the chips from the teeth.
– Broaching speeds vary from 20 to 120 surface feet per minute (SFPM).
– Broaching can involve linear or rotational motion to create different shapes.
Usage and Limitations
– Broaching is used for precision machining of odd shapes.
– Commonly machined surfaces include holes, splines, keyways, and flat surfaces.
– Broaching is favored for high-quantity production runs.
– Broaching speeds vary, resulting in a cycle time of 5 to 30 seconds.
– Broaching has limitations, such as no obstructions over the length of the surface to be machined and no curves in multiple planes.
Types of Broaches
– Broaches can be categorized by use: internal or surface.
– Broaches can serve single or combination purposes.
– Broaches can have push, pull, or stationary motion.
– Broaches can be solid, built-up, hollow, or shell construction.
– Broaches can function for roughing, sizing, or burnishing.
– Slab broaches are general purpose tools for cutting flat surfaces.
– Slot broaches are used for cutting slots of various dimensions at high production rates.
– Contour broaches are designed to cut concave, convex, cam, contoured, and irregular shaped surfaces.
– Pot broaches cut the outside diameter of a cylindrical workpiece.
– Pot broaches are named after the pot-looking fixture in which they are mounted.
Design and Broaching Machines
– The rise per tooth (RPT) is the amount of material removed by each tooth.
– The RPT varies for each section of the broach: roughing, semi-finishing, and finishing.
– Chip breakers are notches in the teeth that break the chip and decrease material removal.
– Workpieces should have 0.020 to 0.025in more material than the final dimension of the cut.
– The hook angle and back-off angle provide clearance and vary for different materials.
– Broaching machines can be hydraulic or mechanically driven.
– Machines can have horizontal or vertical motion, depending on the stroke required.
– Vertical machines can be designed for push, pull-down, pull-up, or surface broaching.
– Horizontal machines are used for pull, surface, continuous, and rotary broaching.
– Surface broaching is commonly done on vertical machines.
Rotary Broaching and Comparison with Push/Pull Broaching
– Schematic of a rotary broach starting a cut.
– Off-axis (wobble) angle.
– Rake.
– Front relief.
– Pilot diameter.
– Advantages of rotary broaching: does not require a broaching machine, can be used on various machine tools, two tooling components (tool holder and broach), misalignment produces a rotating edge for cutting.
– Internal broaching: sides of the broach are drafted inward to become thinner.
– External broaching: sides of the broach are drafted outward to make the pocket bigger.
– Draft prevents broach from jamming.
– Rate of cut and spiraling: ideal rate of cut, choking, interrupted or zig-zag cuts, releasing load on the non-cutting edge, spiraling issues, reversing rotation or interrupting the cut.
– Comparison with push/pull broaching: accuracy, usability in blind holes, advantages of rotary broaching on common machine tools. Source: https://en.wikipedia.org/wiki/Broaching_(metalworking)
Broaching is a machining process that uses a toothed tool, called a broach, to remove material. There are two main types of broaching: linear and rotary. In linear broaching, which is the more common process, the broach is run linearly against a surface of the workpiece to produce the cut. Linear broaches are used in a broaching machine, which is also sometimes shortened to broach. In rotary broaching, the broach is rotated and pressed into the workpiece to cut an axisymmetric shape. A rotary broach is used in a lathe or screw machine. In both processes the cut is performed in one pass of the broach, which makes it very efficient.
Broaching is used when precision machining is required, especially for odd shapes. Commonly machined surfaces include circular and non-circular holes, splines, keyways, and flat surfaces. Typical workpieces include small to medium-sized castings, forgings, screw machine parts, and stampings. Even though broaches can be expensive, broaching is usually favored over other processes when used for high-quantity production runs.
Broaches are shaped similar to a saw, except the height of the teeth increases over the length of the tool. Moreover, the broach contains three distinct sections: one for roughing, another for semi-finishing, and the final one for finishing. Broaching is an unusual machining process because it has the feed built into the tool. The profile of the machined surface is always the inverse of the profile of the broach. The rise per tooth (RPT), also known as the step or feed per tooth, determines the amount of material removed and the size of the chip. The broach can be moved relative to the workpiece or vice versa. Because all of the features are built into the broach, no complex motion or skilled labor is required to use it. A broach is effectively a collection of single-point cutting tools arrayed in sequence, cutting one after the other; its cut is analogous to multiple passes of a shaper.
