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Broaching flats leads to productivity and quality improvements.

Broaching flats leads to productivity and quality improvements

Broaching may be one of the most misunderstood and often underestimated metal-removal processes. Broaching is similar to planing and is almost a study in self-contradiction: It is best applied when machining simple surfaces or very complex contours. When intelligently applied to either of these two demands, broaching benefits become readily apparent. These benefits include increased productivity, improved surface finish, reduced tool cost, and lower overall cost per part.

This was the experience of the Weldon Tool Co when it installed a broach to machine flats onto cylindrical surfaces of end mill shanks. Weldon was able to increase productivity while reducing labor expense and improving quality. Before describing Weldon's experience in detail, it would be helpful to discuss broaching in general and broaching flats in particular.

Broaching can be used for the external or internal machining of flat, round, or contoured surfaces. This is done by pulling (or pushing) a multi-tooth broach tool over the workpiece, or vice versa. The broach tool is somewhat related to a wood rasp. It is usually a tapered bar with rows of teeth located along its axis. The teeth increase in size (usually height) or change shape from the starting end to finishing end. Each tooth takes a chip of fixed thickness from the workpiece. This permits a complete operation in a single pass as the depth of the cut increases while the broach moves along (or across) the workpiece.

Broaching differs from other metal-removal systems in that the rough, semi-finish, and finish cutting teeth are combined in a single tool. Each tool is specific to the application and the workpiece; a different application and workpiece would require a different broach. Also, while broaching usually is not the preferred method for heavy stock removal, there are certain surface broaching applications in which as much as 0.5" can be cut away in a single pass.

Some common applications of internal broaching are cutting irregular or special forms such as projections, splines, channels, grooves, keyways, or gear teeth. Round, square, or rectangular holes are extremely common. While internal broaching is rather versatile, it does require a starting hole in the workpiece for tool insertion. In general, internal surfaces are parallel to the direction of tool travel.

For through-hole broaching, no obstructions can be present to block passage of the broach. When broaching blind holes, the operation must first provide a recess larger than the diameter of the hole to be broached at the bottom of the blind hole for chip removal. Blind holes can be broached by limiting the travel to a series of short push-pull type broaches.

Of course, there are exceptions to every rule. For cutting helical gear teeth, it's not mandatory to have a parallel surface, because the tool or workpiece can rotate in relation to each other.

Surface broaching, which is what this article is about, is almost unlimited in its applications: multiple surfaces, squares and hexes, radii, angles, slots, teeth, flats, and contours. The list is extensive. Almost any surface form can be produced as long as surfaces are in a straight line and unobstructed.

When to (not to) broach

Given that broaching is a versatile metal-removal technique for simple or complex patterns, you still must consider other factors when weighing its application. Advantages include high productivity, close tolerances, good finishes, economical operation, and versatility.

High-volume production puts broaching in a competitive position. Its ability to rough and finish in a single stroke of the broach tool is particularly attractive.

Because broaching can hold very close tolerances and the relationship of several surfaces can be built into a single tool, broaching is extremely accurate.

While the initial tooling costs can be high, unless standard tooling is used, the cost per workpiece is generally low because of long tool life. Relatively low cutting speeds and the small amount of stock removed by each tooth distribute wear over a greater area compared to other cutting tools.

Broaching generates high work forces, so broaching machines must be extremely rigid. Durable workholding devices can add to capital equipment costs. Production requirements must be high enough to justify the additional expense.

Broaching flats

Broaching flats on cylindrical surfaces is a common application. Generally, the closest you can broach to the centerline is 0.060". Anything closer requires special fixturing. Ideally, you would not go that deep, because tooling and fixturing costs can rise rapidly. The closer you cut to the centerline, the more difficult it becomes to clamp the part securely. Forces exerted on any type of broach fixture probably exceed those encountered in any other machining process because so many more cutting teeth are in contact with the workpiece at one time. That's why fixturing is so crucial.

Often flats are broached on motor shafts or tools requiring a flange or set-screw flat to lock a hub in place. A fan blade on a motor shaft is one example. End mills have flats on their shanks to securely retain the cutting tool in a holder and to drive the tool during milling.

Broaching flats depends on several factors. The first is the configuration of the part. A straight shaft allows easier loading and unloading - and lends itself to automatic handling because it can be easily pushed out of a magazine holder or fixture.

Another factor is material type. It's much easier to cut flats on a soft metal as opposed to a hard material such as stainless steel. The hardness and inherent characteristics of the material may adversely affect cutting speed.

Another consideration is the corner angle, especially if the part is heat treated after broaching. A 90-deg angle with a sharp corner will be susceptible to cracking during heat treating. Broaching a slight radius in the corner can help prevent this but may add to tooling costs. A compromise 45-deg angle is less costly than a radius, but not quite as stress resistant.

Broaching especially lends itself to cutting flats when other operations can be performed at the same time. You can broach different configurations on the ends of flats. The step-like surface of timing-gear shafts is a good example. You also can make dovetail cuts at the same time - chamfer the end while broaching the shaft. This is particularly useful, because it allows easier loading of the finished part during assembly. A screwdriver slot or timing slot can be cut in one pass of the broach as long as all surfaces to be cut at one time are in the same direction.

Weldon's situation

The Weldon Tool Co was founded in 1918 in a two-man loft-space machine shop in downtown Cleveland, OH. It has grown during the years to more than 225,000 sq ft of manufacturing, engineering, and office facilities at its present location. Weldon engineering has been responsible for many innovations in cutting tool technology. From the beginning, the company's founder, Carl A Bergstrom, believed that the straight-fluted end mill was not the best method of metal removal, and he developed the first end mill with helical flutes. To compensate for the "pull-out" forces and to drive this new end mill, the Weldon straight shank with drive flats was developed. Known universally today as the "Weldon Shank," it is accepted as the industry standard.

Weldon pioneered precision holders for the Weldon Shank. Some of the other developments that followed Weldon's successful efforts to gain industry acceptance of the straight-shank and high-helix end mill were double-end end mills, cupped-end mills, and hollow-ground tooth faces, controlled-peripheral, primary-relief, and secondary-clearance angles.

In the past, Weldon had developed a system for cutting flats on end-mill shanks using milling machines that production engineering had adapted for automatic and semi-automatic feeding. The automatic machines cut flats on shanks up to 0.75" dia. Larger shanks, 0.875" dia and above, were cut on the semiautomatic machines.

Although these modified machines served Weldon's needs, they were multiple-machine, multiple-person, multiple-shift operations. Broaching was one of the ways studied to improve production as well as quality.

Weldon installed a 5-ton; 36" stroke "Hothead" vertical surface-broach machine, supplied by Ohio Broach & Machine Co, for cutting flats on 0.375" through 1.750" dia shanks. The broach is equipped with a tip-back table and solid-state programmable controls. It has three modes of operation to fit varying production requirements. The design includes a dual-magazine feed and dual-cavity fixture that allow broaching two shanks at one time on end mills up to 0.750" dia. There is a single-magazine feed, single-cavity setup for up to 1" dia tools. The third mode is semiautomatic with manual loading, used for low volumes and for end mills with different shank and cutting diameters. When using either dual- or single-magazine loading, the broaching machine is in the automatic mode. In the semiautomatic mode, the tipback table is a valued feature. It allows safe manual loading of shanks, and prevents broach contact on return strokes, thus avoiding unnecessary wear on cutting teeth.

The results

Weldon has been able to improve production capability from a multiple-machine, multiple-person, multiple-shift operation to a one-person, one-machine, one-shift operation. The broach can be loaded with magazines to run two pieces at a time at rates up to 1200 pieces/hr. Single-magazine loading and semiautomatic loading of larger shanks (and those with offsetting diameters) have boosted production capabilities to 600 pieces/hr. Larger shanks require a longer stroke length because more metal needs to be removed.

Compared with the previous system, production efficiency has greatly improved, and the resulting benefits are in relation to lot sizes: the larger the lot, the more efficient. The broach is a much simpler mechanical system, and maintenance downtime has been greatly reduced. Setup time varies with the degree of changeover. Simple changes can be accomplished in less than one-half hour. Complete changeovers from semiautomatic to fully automatic magazine feed can take from one to two hours. Magazine loading, of course, is the most efficient method for broaching larger lot sizes.

Tool maintenance philosophy

Weldon, being a metal-cutting-tool manufacturer, understands the subtleties of tool resharpening and has what can be considered an unusual policy regarding its broaching tools. Weldon uses the strategy that gaining the maximum number of pieces per total broach life is better than the traditional attitude of getting the greatest number of pieces per resharpening.

The size of the wear land on broach teeth determines the amount of material that has to be removed during the regrinding operation. Rather than allowing the maximum wear land to develop before the broach is resharpened, the operator monitors the wear to a predetermined point. At that time, a lesser amount of material needs to be removed during regrinding, resulting in more available regrinds over the life of the broach.

Weldon has found that this broaching operation is a process that benefits from TiN coating of the cutting tool. Both new and reground broaches are TiN coated to extend tool life.

PHOTO : End mills have flats on their shanks to securely retain the cutting tool in a holder and to drive the tool during milling.

PHOTO : The broach tool usually is a tapered bar with rows of teeth located along its axis. Teeth increase in size or change shape from the starting end to the finishing end. Each tooth takes a chip of uniform thickness.

PHOTO : Broaching can be for external or internal machining of flat, round, or contour surfaces. It can hold very close tolerances and the relationship of several surfaces can be built into a single tool.
COPYRIGHT 1989 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1989 Gale, Cengage Learning. All rights reserved.

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Author:Van DeMotter, Chris; Haupthoff, Wilfred
Publication:Tooling & Production
Date:Dec 1, 1989
Words:1916
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