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A push style 5/16" keyway broach. Note how the teeth are larger on the left end.

Broaching is a machining operation which uses a toothed tool, called a broach, to remove material. The broach is used in a broaching machine, which is also sometimes shortened to broach. It is used when precision machining is required, especially for odd shapes. Broaching finishes a surface in a single pass, which makes it very efficient. Commonly machined surfaces include circular and non-circular holes, splines, 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 favorable to other processes when used for high-quantity production runs.[1]

Broaches are shaped similar to a saw, except the teeth height 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 a unique machining process because it is the only one to have 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.[2]

Contents

Process

The process depends on the type of broaching being performed. Surface broaching is very simple as either the workpiece is moved against a stationary surface broach, or the workpiece is held stationary while the broach is moved against it.

Internal broaching is more involved. The process begins by either clamping the workpiece into workholder of the broaching machine or the workpiece is placed on a spherical workholder designed to automatically align the workpiece to the broach. The elevator of the broaching machine then lowers the pilot of the broach through the workpiece where the puller engages the broach pilot. The elevator then releases the top of the pilot and the puller pulls the broach through the workpiece completely. The workpiece is then removed from the machine and the broach is raised back up to reengage with the elevator.[3] The broach usually only moves linearly, but sometimes it is also rotated to created a spiral spline or gun-barrel rifling.[4]

Cutting fluids are used for three reasons. First, to cool the workpiece and broach. Second, to lubricate cutting surfaces. Third, to flush the chips from the teeth. Fortified petroleum cutting fluids are the most common, however heavy duty water soluble cutting fluids are becoming more popular.[5]

Usage

An example of a broached workpiece. Here the broaching profile is a spline.

Broaching was originally developed for machining internal keyways. However, it was soon discovered that broaching is very useful for machining other surfaces and shapes for high volume workpieces. Because each broach is specialized to cut just one shape either the broach must be specially designed for the geometry of the workpiece or the workpiece must be designed around a standard broach geometry. A customized broach is usually only viable with high volume workpieces, because the broach can easily cost $15,000 to $30,000 to produce.[6]

Broaching speeds vary from 20 surface feet per minute (SFPM) to 120 SFPM. This results in a complete cycle time of 5 to 30 seconds. Most of the time is consumed by the return stroke, broach handling, and workpiece loading and unloading.[7]

The only limitations on broaching are that there are no obstructions over the length of the surface to be machined, the geometry to be cut does not have curves in multiple planes,[8] and that the workpiece is strong enough to withstand the forces involved. Specifically for internal broaching a hole must first exist in the workpiece so the broach can enter.[9] Also, there are limits on the size of internal cuts. Common internal holes can range from 0.125 to 6 in (3.2 to 152.4 mm) in diameter but it is possible to achieve a range of 0.05 to 13 in (1.3 to 330 mm). Surface broaches' range is usually 0.075 to 10 in (1.9 to 254.0 mm), although the feasible range is 0.02 to 20 in (0.51 to 508 mm).[10]

Tolerances are usually ±0.002 in (±0.05 mm), but in precise applications a tolerance of ±0.0005 in (±0.01 mm) can be held. Surface finishes are usually between 16 and 63 microinches (μin), but can range from 8 to 125 μin.[10] There may be minimal burrs on the exit side of the cut.[7]

Broaching works best on softer materials, such as brass, bronze, copper alloys, aluminium, graphite, hard rubbers, wood, composites, and plastic. However, it still has a good machinability rating on mild steels and free machining steels. When broaching the machinability rating is closely related to the hardness of the material. For steels the ideal hardness range is between 16 and 24 Rockwell C (HRC); a hardness greater than HRC 35 will dull the broach quickly. Broaching can also be used on harder materials, like stainless steel and titanium,[11] but it is tougher.[8][12]

Types

Broaches can be categorized by many means. The table below outlines various methods.[4]

Use[8] Purpose Motion Construction Function
Internal
Surface
Single
Combination
Push
Pull
Stationary
Solid
Built-up
Hollow or shell
Roughing
Sizing
Burnishing

If the broach is large enough the costs can also be reduced by using a built-up or modular construction. This involves producing the broach in pieces and assembling it. If any portion wears out only that section has to be replaced, instead of the entire broach.[13]

Most broaches are made from high speed steel (HSS) or an alloy steel; TiN coatings are common on HSS to prolong life. Except when broaching cast iron, tungsten carbide is rarely used as a tooth material because the cutting edge will crack on the first pass.[13]

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Surface broaches

The slab broach is the simplest surface broach. It is a general purpose tool for cutting flat surfaces.[8]

Slot broaches are cut slots of various dimensions at high production rates. Slot broaching is much quicker than milling when more than one slot needs to be machined, because the broach can produce both slots at the same time.[8]

Contour broaches are designed to cut concave, convex, cam-, contoured, and irregular shaped surfaces.[8]

Pot broaches are cut the inverse of an internal broach; they cut the outside diameter of a cylindrical workpiece. They are named after the pot looking fixture in which the broaches are mounted; the fixture is often referred to as a "pot". The pot is designed to hold multiple broaching tools concentrically over its entire length. The broach is held stationary while the workpiece is pushed or pulled through it.[14] This has replaced hobbing for some involute gears and cutting external splines and slots.[8]

Straddle broaches use two slab broaches to cut parallel surfaces on opposite sides of a workpiece in one pass. This type of broaching holds closer tolerances than if the two cuts were done independently.[8] It is named after the fact that the broaches "straddle" the workpiece on multiple sides.[14]

Internal broaches

Hollow or shell broaches are internal cutting broaches for cutting large diameters. They are designed to mount on an arbor. This is cheaper to produce than a solid broach, especially if it will need to be replaced after wearing out.[13]

A common type of internal broach is the keyway broach. It uses a special fixture called a horn to support the broach and properly locate the part with relations to the broach.[8]

A concentricity broach is a special type of spline cutting broach which cuts both the minor diameter and the spline form to ensure precise concentricity.[8]

The cut-and-recut broach is used to cut thin-walled workpieces. Thin-walled workpieces have a tendency to expand during cutting and then shrink afterward. This broach overcomes that problem by first broaching with the standard roughing teeth, followed by a "breathing" section, which serves as a pilot as the workpiece shrinks. The teeth after the "breathing" section then include roughing, semi-finishing, and finishing teeth.[15]

Design

For defining the geometry of a broach an internal type is shown below. Note that the geometry of other broaches is similar.

Broach geometry.png Broach tooth geometry.svg

where:

  • P = pitch
  • RPT = rise per tooth
  • nr = number of roughing teeth
  • ns = number of semi-finishing teeth
  • nf = number of finishing teeth
  • tr = RPT for the roughing teeth
  • ts = RPT for the semi-finishing teeth
  • tf = RPT for the finishing teeth
  • Ls = Shank length
  • LRP = Rear pilot length
  • D1 = Diameter of the tooth tip
  • D2 = Diameter of the tooth root
  • D = Depth of a tooth (0.4P)
  • L = Land (behind the cutting edge) (0.25P)
  • R = Radius of the gullet (0.25P)
  • α = Hook angle or rake angle
  • γ = Back-off angle or clearance angle
  • Lw = Length of the workpiece (not shown)
A progressive surface broach

The most important characteristic of a broach is its RPT, which changes for various parts of the broach. Here they are defined as: roughing (tr), semi-finishing (ts), and finishing (tf). The roughing teeth remove most of the material so the number of roughing teeth required dictates how long the broach is.[16] The semi-finishing teeth provide surface finish and the finishing teeth provide the final finishing. tf is zero so that as the first finishing teeth wear the later ones continue the sizing function. The maximum RPT is about 0.006 in (0.15 mm) for free-machining steels and a minimum of 0.001 in (0.025 mm) for finishing teeth. For surface broaching the RPT is usually between 0.003 to 0.006 in (0.076 to 0.15 mm) and for diameter broaching is usually between 0.0012 to 0.0025 in (0.030 to 0.063 mm). The exact depth depends on many factors, however if the cut is too big it will impart too much stress into the teeth and the workpiece; if the cut is too small the teeth will not cut but rub the workpiece.[4] The starting material condition should be 0.020 to 0.025 in (0.51 to 0.63 mm) greater than the final dimension for broaching to be effective.[7]

The hook (α) determines the primary rake angle and is a parameter of the material being cut. For steel is between 15 and 20° and for cast iron it is between 6 and 8°. The back-off (γ) provides clearance for the teeth so that they don't rub on the workpiece; it is usually between 1 and 3°.[4]

When radially broaching a workpieces that require a deep cut per tooth, such as forgings or castings, a rotor-cut or jump-cut design can be used; these broaches are also known as free egress or nibbling broaches.[8] In this design two or three rows of teeth have the same RPT, but each tooth is notched in a different area; the teeth are notched so that the entire circumference or area is cut. This allows for a deep cut while keeping stresses, forces, and power requirements low. Chip breakers are similar in design, but there are not multiple teeth with the same RPT. Chip breaker notches are designed to break chips on circular broaches.[4]

There are two different options for achieving the same goal when broaching a flat surface. The first is similar to the rotor-cut design, which is known as a double-cut design. Here four teeth in a row have the same RPT, but each progressive tooth takes only a portion of the cut due to notches in the teeth. The other option is known as a progressive broach, which completely machines the center of the workpiece and then the rest of the broach machines outward from there. All of these designs require a broach that is longer than if a standard design were used.[4]

For some circular broaches, burnishing teeth are provided instead of finishing teeth. They are not really teeth as they are just rounded discs that are 0.001 to 0.003 in (0.025 to 0.076 mm) over-sized. This results in burnishing the hole to the proper size. This is primarily used on non-ferrous and cast iron workpieces.[7]

Calculations

The pitch defines the tooth construction, strength, and number of teeth in contact with the workpiece. There should be at least two teeth in contact with the work piece at any time. The pitch is usually defined by the workpiece length using the following equation:[16]

P \cong 0.25 \sqrt{L_w}

The number of roughing teeth required is calculated as:[17]

n_r = \frac{DOC - n_ft_f - n_st_s}{t_r}

where DOC is the total amount of material to be removed. The length of the broach is defined as:[17]

L_B = \left( n_r + n_s + n_f \right) P + L_s + L_{RP}

The total length of the stroke can then be calculated as:[17]

L = LBLw when workpiece is held stationary and the broach is moved
L = LB + Lw when broach is held stationary and the workpiece is moved

Finally the cutting time can be calculated as:[17]

T_m = \frac{L}{12V}

where V is the cutting speed in surface feet per minute (SFPM). The force required to push or pull the broach through the workpiece can be estimated by the following equation:[6]

F \cong 5 \tau_s n t_r W

where τs is a derived valued from the Brinell hardness of the material, n is the number of teeth in contact with the material (n \cong L_w/P), and W is the width of the broach.

Broaching machines

The hydraulic cylinder of a horizontal broaching machine.

Broaching machines are relatively simple as they only have to move the broach in a linear motion at a predetermined speed and provide a means for handling the broach automatically. Most machines are hydraulic, but a few specialty machines are mechanically driven. The machines are distinguished by whether their motion is horizontal or vertical. The choice of machine is primarily dictated by the stroke required. Vertical broaching machines rarely have a stroke longer than 60 in (1.5 m).[18]

Vertical broaching machines can be designed for push broaching, pull-down broaching, pull-up broaching, or surface broaching. Push broaching machines are similar to an arbor press with a guided ram; typical capacities are 5 to 50 tons. The two ram pull-down machine is the most common type of broaching machine. This style machine has the rams under the table. Pull-up machines have the ram above the table; they usually have more than one ram.[19] Most surface broaching is done on a vertical machine.[8]

Horizontal broaching machines are designed for pull broaching, surface broaching, continuous broaching, and rotary broaching. Pull style machines are basically vertical machines laid on the side with a longer stroke. Surface style machines hold the broach stationary while the workpieces are clamped into fixtures that are mounted on a conveyor system. Continuous style machines are similar to the surface style machines except adapted for internal broaching.[19]

Horizontal machines used to be much more common than vertical machines, however today they represent just 10% of all broaching machines purchased. Vertical machines are more popular because they take up less space.[8]

Rotary broaching

A somewhat different design of cutting tool that can achieve the irregular hole or outer profile of a broach is called a rotary broach or wobble broach. This type of tool is often used on rotating machines such as lathes, screw machine or Swiss lathe.[20]

Rotary broaching requires two tooling components: a tool holder and a broach. The leading (cutting) edge of the broach has a contour matching the desired final shape and this leading edge of the tool is wider than the body. The broach is free to rotate within the tool holder, but the axis of rotation is inclined slightly to the axis of rotation of the work. A typical value for this misalignment is 1 degree. If the work piece rotates, the broach is pressed against it, is driven by it, and rotates synchronously with it. If the tool holder rotates, the broach is pressed against the work piece, but is driven by tool holder rotation. Since the axis of rotation is different, the tool holder appears to "wobble" with respect to the work. This is the reason for the original term wobble broach.[20]

If the tool is inclined at an angle of 1 degree to the work, the sides of the tool must have a 1 degree or greater draft.

Ideally the tool advances at the same rate that it cuts. The rate of cut is defined as:

\text{Rate of cut} = \text{diameter of tool}\cdot\sin(1^\circ)

If it advances any faster than that, then the tool becomes choked; if it advances any more slowly, then you get an interrupted or zig-zag cut. Since all work material is elastic, you would actually cut a little less than the ideal rate, just to release the load on the non-cutting edge of the tool.

There is some spiraling of the tool as it cuts, so the form at the bottom of the work piece may be rotated with respect to the form at the top of the hole or profile. Spiraling may be undesirable because it binds the body of the tool and prevents it from cutting sharply. One solution to this is to reverse the rotation in mid cut, causing the tool to spiral in the opposite direction. If reversing the machine is not practical, then interrupting the cut is another possible solution.

In general, a rotary broach will not cut as accurately as a push or pull broach. However, the ability to use this type of cutting tool on high-production machinery such as a screw machine, and eliminate secondary operations, makes this a desirable manufacturing method.

History

The concept of broaching can be traced back to the early 1850s, with the first applications used for cutting keyways in pulleys and gears. After World War 1, broaching was used to rifle gun barrels. In the 1920s and 30s the tolerances were tightened and the cost reduced thanks to advances in form grinding and broaching machines.[21]

References

Notes

Bibliography

External links


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