Today’s Machining World Archive: July/August 2009, Vol. 5, Issue 06

Examples of small and large internal broaches (Photo courtesy of American Broach & Machine Co.)

Making accurate and complex cuts easy and economical for high-volume parts

A broach is a cutting tool with many rows of teeth, each slightly larger than its predecessor. They are designed to produce simple or complex forms quickly, usually in one pass, with repeatable and reliable accuracy. As the broach moves past the workpiece (or the workpiece past the broach), each tooth takes a shallow cut along the whole length of the part, carrying the chip to the end of the part, said Dave West, general manager at V-W Broaching Service, Inc., Chicago, Ill., which provides broaching and broach sharpening services and manufactures broaching tools. In many cases, a single pass of the broach completes the machining of the surface. For some workpieces, multiple passes with multiple broaches may be required, depending on the geometry of the part and the amount of stock to be removed.

A typical broach consists of many rows of teeth that do roughing, then a few rows of teeth for semi-finishing and another few rows that finish-machine the surface. The tool design is based on the shape being cut, the properties of the workpiece material and related factors. You can broach internal or external surfaces to almost any shape imaginable, from simple flats and slots to gears to turbine blade hubs for aircraft engines.

Broaching can be quite simple in geometry—cutting a keyway in a gear or other component, for example—or quite complex. Broaching is often used to cut precise diameters or to produce non-round holes in shapes such as a hex, square, or “double D.” You can also use broaching to cut splines, gear teeth and other shapes. West spoke about a surface-broaching job at V-W Broaching that cut 50-plus different dimensions in one pass. “All the dimensions are built into the tool,” he said

A very simple manual broaching job, such as cutting a keyway in a single part, requires only a broach, an arbor press and the appropriate fixturing. Production broaching requires specialized machines and is best for a very large number of parts.

Broaching can reduce the cost of machining certain features to pennies per part. In addition, broaching can sometimes perform cuts that would be impossible to make any other way. In use for more than 100 years, broaching is still widely recognized as the best process for many applications.

Broaching machines come in different configurations; horizontal or vertical, and are designed for internal, external, spiral or surface broaching. In a typical internal broaching machine the part is fixtured and the broach is pulled through it. For broaching outside diameters, typically the broaches are fixtured in the machine and the part is pushed past them. Spiral broaching is often done on a horizontal machine that drives the broach to spiral through an inside diameter and create helical grooves, such as those in a rifle barrel.

Applications

Many materials can be broached. “Almost anything you can cut by machining,”said West, including ferrous and nonferrous metals and even some plastics. V-W Broaching runs dozens of broaching machines, producing parts large and small, for just about any use or industry you can think of—hand tools, appliances, automotive, farm implements, turbines, plumbing, military and many others.

Broaching works best in materials with hardness in the range of 26 to 28 Rockwell C, said Ken Nemec, president of American Broach & Machine Co., Ypsilanti, Mich., manufacturer of broaching machines, broaches and CNC sharpening machines. It is commonly done in the range of 10 to 32 Rockwell C. Chip formation is critical in making good broaching cuts, however, and soft materials

Example of a chip resulting from a broach operation

“are like bubblegum,” Nemec said, but in the ideal range of 26 to 28 Rockwell C, you get clean chips and good tool life. The tool designer needs to take the workpiece material properties into account and needs to run the tool at the appropriate cutting speed to achieve the best results for a given part.

For precision parts that need to be heat treated, such as gears, a part can be broached to near net shape before heat treating. Then a finish broaching operation is performed, removing just a small amount of the hardened material, Nemec said. In this case, the very expensive, specialized machines can cut material as hard as 58 to 60 Rockwell C.

Broach it yourself

The machines and the broaches tend to be quite expensive, but if quantities justify the investment you could bring this capability into your shop. “Most people have sticker shock when they get into broaching,” said Nemec. “It is pretty expensive, especially if you want to get into high production.” You may have a high-volume part on which it costs 20 cents to machine a particular surface that is suitable for broaching. “We can show you how to do it in a lot less time, but you have to invest in capital equipment,” he said. In addition to high-volume machines, American Broach & Machine also offers lower cost broaching machines suitable for shops that want to broach smaller quantities of parts.

If you have a high-volume part or family of parts that look as if they may lend themselves to broaching, the machine manufacturer would start from the print. “First we design a broach for you, then design a machine for the broach,” said Nemec. “We have about 10 different types of broaching machines. We stretch them bigger, and use more or less pressure,” depending on the specific application. Since each machine is different, it is designed and built to order. Delivery can be 26 weeks or longer, he said. Options such as automated parts handling or pressure- monitoring can add capability. “Buying a broaching machine is like buying a car,” Nemec said. There’s a base price and then you add on the options. In his experience, some customers want relatively bare-bones machines. But adding options can save money in the long run—and sometimes, in the not-so-long run.

TLC for tools

An increase in force during a cut indicates that the broach needs sharpening. American Broach & Machine offers a pressure monitoring option that allows you to track the condition of the broach. This capability will add about $16,000 or $17,000 to the cost of the machine, Nemec said, but it can quickly pay for itself. Considering that a broaching tool may cost $2,000, you can easily scrap enough tools in one year to pay for the monitor, he said.

To make good use of the machines you’ll need to understand a few basics of broaching and how to keep your process in order. The machine manufacturer should provide training—just two or three hours with your staff “can save a lot of time, trouble and money,” said Nemec.

An example of a broach part.

Care of broaches, both on and off the machine, is critical to keeping a broaching process profi table. You can run a new $2,000 broach until it fails, get maybe 8,000 parts and then throw it away. Or, you could run 3,000 parts and then sharpen the tool, as many as 20 times, said Nemec. This makes your cost come way down—8,000 parts versus 60,000 parts with the same tool. If sharpening costs $80, this works out to a tooling cost of 25 cents per part versus 6 cents per part.

Off the machine a user must take pains not to damage the cutting edges. Don’t leave broaches lying around on the bench, Nemec said. Store them in wood, plastic or cardboard containers or sleeves, which allow the teeth to dig in but won’t damage them.

Timely and correct sharpening extends tool life and helps keep tooling costs down. You can send tools out to a shop that specializes in sharpening broaches. However, by the time you have three or four broaching machines, you would save a lot of money by sharpening them yourself on a CNC sharpening machine instead, said Nemec. “The guy who is putting that broach in a box could sharpen it.”

Rotary broaching

Rotary broaching is a completely different process. It can cut the same forms as conventional broaching, but you can use it on your screw machine or lathe. A special rotary broaching tool holder mounts on the machine turret, and rotary broaching becomes just another step in your process. This eliminates the need

Internal and external rotary broaching tools, with sample parts.

for secondary operations to form square holes, hex holes, splines or gear teeth, or almost any other internal or external shape you want. Rotary broaching easily works in blind holes, which is not possible with conventional broaching.

A rotary broaching tool has cutting edges the shape of the hole or form you want. It mounts in a toolholder that holds the tool at a 1-degree axial tilt in relation to the center line of the workpiece. Bearings in the toolholder allow the tool to rotate freely. The workpiece is turning, and when the tool comes in contact, it rotates right along with the workpiece. Because of the 1-degree axial tilt, the tool appears to wobble as it rotates. Because of this, rotary broaching is sometimes called “wobble broaching.” It is also known as “Swiss broaching.”

Rotary broaching in action

Before the rotary broaching operation, the workpiece needs to be drilled or turned to the correct diameter for use with the rotary broaching tool. This minimizes the amount of material that the tool will cut. Then, the area where the tool will contact the workpiece is countersunk or chamfered, to allow smooth engagement of the tool. If the chamfer or countersink is not acceptable in the final part, you can design your process to remove it afterward. Then the part is ready for broaching. The following describes internal rotary broaching; external is similar.

As the prepared workpiece is turning, the rotary broaching tool/toolholder advances toward it. Because of the 1-degree axial tilt, only one corner of the tool engages the workpiece at first. When the tool makes contact, the workpiece drives the tool to rotate in unison with it. During rotation, first one corner of the tool contacts the workpiece, then the next, and so on, around and around.

As the tool advances into the workpiece, each corner, in turn, cuts into the metal. This way, bit by bit, the tool cuts a shape that matches the shape of the tool.

How large a form you can rotary broach depends on the material. In aluminum, you can usually rotary broach up to 2”, in steel to 1″. You can rotary broach harder materials, but in smaller sizes. For example, you could broach a quarter-inch hole in Inconel, said Peter Bagwell, engineer at Slater Tools Inc., Clinton Township, Mich.

Toolholders and setup

The technology was developed decades ago, but rotary broaching companies continue to improve tool holders and tools to increase tool life and make the technique easier to use. Because of the precision alignment and offset required, rotary broaching tool holders traditionally required many adjustments and painstaking setup, which could take considerable time, depending on the employee’s experience. A standard rotary broach setup might include six set screws, two bolts with nuts and a sliding plane between the toolholder body and the machine adapter.

Rotary broaching engineers have developed innovations to streamline setup procedures. A specially designed tapered-centering-pin gage can allow you to set

How rotary broaching works. (Illustration provided by Barbara Donahoe & Somma Tool Company, Inc.)

up a fully adjustable tool holder in a minimum of time, said Dick Noti, sales engineer at Somma Tool Company, Inc., Waterbury, Conn. Some toolholders require only an X-axis adjustment, and, in recent years, no-adjustment rotary broaching toolholders for Swiss-type machines have become available.

If you are sending out parts for broaching, or have a job that might take advantage of rotary broaching, contact a rotary broaching tool manufacturer. An application engineer can look at the part and advise you. If you’re not sure you want to make the investment, many suppliers will let you try out a toolholder and broaches on your own machine, without obligation.

Cutting fluids also influence the tool life and part finish, of course. Generally, you should use a good water-soluble oil, Nemec said. In more challenging applications a heavy cutting oil may be needed. Many cutting fluid suppliers offer specially formulated coolants for broaching applications. Your machine manufacturer and tool supplier can recommend appropriate coolants.

With brass parts you’ll want to use a water-soluble oil that won’t discolor the material. You’ll also need a watersoluble fluid with certain thin-walled parts. “Especially for internal [broaching] with thin wall sections, sometimes coolant can make a difference in what your final tolerance is,” said West. “If you broach a round hole [with] a thin wall, it gets hotter than blazes and expands.” In this case, a water-soluble oil will help dissipate the heat.

Whether you broach high-volume parts yourself or send them out for broaching, you can take advantage of the capabilities of this time-honored process: precision, low per-part cost and the ability to cut complex forms with accuracy and repeatability not found with many machining processes.

Today’s Machining World Archive: January 2009, Vol. 5, Issue 01

Cutting a demonstration part on an Atometric G4-ULTRA machining center. (Photo courtesy of Atometric, Inc.)

It’s still cutting metal, but micromachining presents some unique challenges.

Strictly speaking, “micro” machining could be defined as work on the scale of a micron. However, many in the machining business will say that micromachining is making any very small part with very small features.

No matter how you define it, micromachining means small features and tight tolerances.

Micromachined parts can be made from metal or plastic. They’re used in many high-tech applications. For example, in a medical instrument, a tiny micromachined gripper can fit inside a blood vessel and remove a sample of tissue for lab analysis.

“Micromachining is characterized by very small features and tight tolerances,” said Andrew Honegger, vice president, Microlution, Inc., Chicago, Ill. “If you’re making a feature 100 microns, or 0.004″ in size, the tolerance is going to be tight — in single-digits of microns.”

Other very small mechanical systems you’ve probably heard of include MEMS (microelectromechanical systems) and nanotechnology devices. MEMS are typically made up of components in the one-to-100-micron size range, and a MEMS device or system might be up to 1 mm (0.039″) in size. MEMS components are usually made by thin-film operations, such as deposition processes, photolithography and etching, rather than machining. Nanotechnology works in the neighborhood of a nanometer, a billionth of a meter (0.000000039″); some nanotech materials, mechanical devices or features are built up one atom or molecule at a time.

Machines for micro

Though you could cut very small parts on a regular CNC machine, you might have trouble maintaining the necessary tolerances, which may be tens of microns or less. A number of manufacturers produce machining centers specifically designed with the rigidity, and vibration and thermal control needed to meet the dimensional and tolerance requirements of micro or miniature parts. Many of these machines offer capabilities and options that tailor them to micromachining applications: Tool changing, tool-tip measurement and offset, optional high-speed spindles, different coolant methods, pallets and built-in measurement systems.

Demonstration piece made on an Atometric G4-ULTRA machining center. For scale, part of a key is shown. (Photo courtesy of Atometric, Inc.)

Atometric, Inc., Rockford, Ill., offers the G4-ULTRA, a general-purpose horizontal machining center that can be configured for 3-, 4-, or 5-axis operation. It machines within a work space 100 mm (4 inches) on a side. The machine positions to within a fraction of a micron, and the tool tip is positioned to within a micron. The G4-ULTRA comes with a 100,000 RPM servo spindle as standard equipment and a 200,000 RPM spindle is available. The machine provides automatic tool-changing from a 14-position holder. It places the tools directly into the spindle collet to minimize runout, rather than use tool holders. Conductive probing through the tool is available, and this capability can be used for broken tool sensing, as well. An optional confocal laser measuring system can measure conductive or nonconductive workpieces on the machine.

Microlution, Inc., Chicago, Ill., produces a 363-S horizontal 3-axis milling machine, suitable for parts up to two inches on a side that require tools 1/8″ diameter or less, said Honegger. A 50,000 RPM spindle is standard, and options include 80,000 and 160,000 RPM spindles. The 363-S also has a 36-pocket automatic tool changer. Positioning is within two microns, and repeatability is within 0.2 microns. The unit’s standard software uses G- and M-codes; Microlution can also create custom software for specialized applications. For example, one customer requested to have a camera integrated into the unit for measuring fixtures and parts.

Kern Precision, Inc., Webster, Mass., offers three models of 5-axis-capable machining centers for micromachining. The entry-level Micro system has a 10″ x 8.5″ x 8″ workspace. It provides one-micron positioning accuracy, and +_ 2.5 microns achievable part accuracy. The Kern Evo’s work space is 12″ x 11″ x 10″; it has a fully integrated pallet system and a tool changer expandable from 32 up to 95 tools. The Kern Evo provides 500 nm (nanometer) positioning accuracy and +_ 2 microns achievable part accuracy. The highest precision unit, the Pyramid Nano, has a work envelope of 20″ x 20″ x 16″, and provides 300 nm positioning and an achievable part accuracy of +_ 1 micron. The Micro and Evo models are available with spindles up to 160,000 RPM, and the Pyramid Nano up to 50,000 RPM.

Electrical discharge machining (EDM) has its own micromachines. SmalTec, Lisle, Ill., produces two micro EDM units. The EM203 and GM703 provide three dimensional machining, similar to that done by a CNC machine, but where a very small spark does the material removal. In addition, these units can also machine with conventional cutting tools. The EM203 has a positioning range of 200 mm x 200 mm x 95 mm (7.8″ x 7.8″ x 3.7″), and provides machining accuracy of 1 micron on 10 mm, and 5 microns on 100 mm. The GM703 has a positioning range of 50 mm x 50 mm x 65 mm (2″ x 2″ x 2.6″), and provides machining accuracy of 30 m on 10 mm, and 170 nm on 50 mm.

Tools the diameter of a human hair

“We often use 100-micron or four-thousandths-diameter tools,” said Lindem, president at Atometric, Inc. That’s about the diameter of a human hair. It’s a challenge making a new part without breaking tools, he said, but then a tool will cut for hours.

Tools 1/16″ or 1/32″ in diameter, or less, can be considered “micro” tools, said Robert Savage, president of Magafor Precision Cutting Tools, Turners Falls, Mass. His company offers end mills from 0.002″ diameter in 0.002″ increments, and reamers from 0.008″ diameter. A corner-rounding tool is also available for radii of 0.004″ and up. To minimize the number of tools required, Magafor offers an eight-function Multi-V tool from 0.020″ diameter, which drills, v-grooves, chamfers and performs other operations with a single tool. All the micro tools have 3 mm (1/8″) diameter shanks. Hex broaches for cutting driver heads are available 0.051″ across the flats and larger, from Hassay-Savage, Magafor’s parent company.

RobbJack Corporation, Lincoln, Cal., offers end mills and other tools starting at 0.005″ diameter in one-thousandth increments, said Mike MacArthur, applications engineer at RobbJack. If you need an intermediate size, the company can hand-select for diameter down to the nearest 0.0002″.

For prototypes and short runs of parts, Atometric uses uncoated carbide tools. Two years ago, coated tools under 0.025″ in diameter weren’t commercially available, Lindem said. Now, he says, they’re available down to 0.010″. In production the coated tools run faster and longer, he said.

For certain specific applications, such as precision small diameters in graphite, or thin fi ns and ribs in plastic injection molds, a diamond coating may be called for, such as the CVD (chemical vapor deposition) diamond coating available from Crystallume, Santa Clara, Cal., a division of RobbJack.

Though you can’t see the geometries of a tool’s cutting edge without a microscope, they’re still important. “People complain about quality and consistency,” said MacArthur. “One common thing I see in the marketplace is small diameter tools that don’t have any ground primary relief angles.” The correct angles and ground relief are important for quality of the cut, he said.

SmalTec micro EDM equipment even allows you to make your own cutting tools. You can call up a special tool-making program and shape the tool in a section of the machine devoted to tool making, using a horizontal wire. “We can shape tools to any angle or diameter or features,” said Jerry Mraz, SmalTec’s general manager.

Making parts

“Every consideration you have — in fixturing, in coolant, in cutting a part — all the same considerations are in micromachining,” said Lindem, “but you have to be prepared that everything acts differently.” You’ll be able to apply all your knowledge, but often in new ways, he said.

“Traditional feeds and speeds go right out the door” when you’re cutting with a very small tool, said Gary Zurek, president/CEO, Kern Precision, Inc., Webster, Mass. “Go to the manufacturer of the cutters and use data from them as a starting point.”

“You can run a day’s production and have a cup of chips at the end of the day,” said Lindem. The chips “look like fairy dust, but under a microscope they look like perfectly formed chips from a big machine.” Those tiny chips pose a problem normally not encountered in larger-scale machining. Lindem told of a situation where he would normally use a synthetic coolant. However, he needed to filter the coolant down to the one-micron level to remove those chips. Some of the lubricity components of the synthetic coolant were in the one-to-five micron size range, he said, so they were filtered out, too.

“Use simpler coolant, misted coolants and oils,” said Lindem. “If you’re using a tiny tool and putting a lot of heat in a small area, you get coolant effect with misting coolant.” Also, be aware, he said, that if you’re running an end mill a couple of thousandths in diameter, if you put a stream of coolant on it, you could break it.

Work holding can be a challenge for micro parts, which may be very delicate. “Sometimes we have the drawing and it looks solid,” said Lindem. But maybe the raw material is “15-thousandths wire, and it may not have much structural integrity. Not like a cast iron engine block.” Try using small versions of standard chucks, vacuum chucks or magnetic chucks. Magnets may work. The manufacturer of your machine can help come up with work holding schemes for challenging workpieces.

Measurement of these small parts can be tricky. They’re hard to handle, they need to be fixtured, they may be fragile, and, of course, the tolerances are very, very tight. Many machining centers have contact, conductive or optical probing capability, so you can cut and measure the part without having to handle it. For measurement off the machining center, an optical coordinate measuring machine (CMM) might be a good investment if you’re moving into the micromachining business.

Machining on the micro scale can widen your market in growing industries where the parts are getting smaller, such as medical devices. What it takes is the right equipment and a willingness to learn a new way of working.