Today’s Machining World Archive: April 2008, Vol. 4, Issue 04

For thousands of years, humans have made decorative and useful objects by melting metal and pouring it into molds. Now, industrial casting processes are available to produce a variety of complex parts economically in different metals.

Sometimes cast parts can be used in the form they leave the mold. Often, they need to be machined to provide sealing surfaces or threaded holes. In the machine shop, cast parts can present challenges. To machine them, it helps to know about the casting process and the physical characteristics of castings, so you can adapt your machining practices accordingly.

In any casting process, molten metal fills a cavity the shape of the part you want. When the metal solidifies, it has taken on the shape of the cavity. The various casting processes create the cavity in different ways and use different methods to introduce the metal into the mold.

As an introduction to casting and machining cast parts, this “How It Works” covers aspects of aluminum castings, including four common casting methods, and information about machining cast aluminum parts. Many of the principles and processes apply to other metals, as well.

Sand casting

In sand casting, special casting sand is compacted around a three-dimensional pattern, which forms the part cavity and the channels through which the metal will flow. The pattern is removed and the cavity remains. The mold is made in two halves, called the cope (top half) and the drag (bottom half). Where they come together is called the parting line. On the cast part, you can usually see where the parting line is.

Sand cores are used in a variety of metalcasting processes to produce hollows in the metal. (Photo courtesy of American Foundry Society)

If the casting needs to have hollow sections or holes, one or more sand cores are set in place, usually in the drag, fitted into grooves left by the pattern. The cope and drag are married together and the metal is poured. It cools, sand is removed from the outside of the part, and the cores are broken up and removed. The surface of the part picks up the texture of the mold, which may be fine or coarse, depending on the sand used. Cost is relatively low, and, depending on the part, sand casting may be used for any production quantity.

Die casting

Die casting is done in a reusable steel mold, or die. It is the fastest of these casting processes. Molten metal is forced at high pressure into the mold. The mold is cooled by air or water until the part is solid and can be removed. Cycle times of a minute or less are common. Die casting is suitable for quantities that are high enough to make the cost of tooling worthwhile. It can produce thin wall sections and yields parts with a good surface finish. Porosity can be an issue.

Permanent and semi-permanent mold casting

Permanent mold casting also uses a steel mold, but the metal enters the cavity by gravity, or at low pressure and/or under vacuum. The cycle time is slower than for die castings, often three to five minutes. Movable metal cores can provide holes or internal passageways. “Semi-permanent mold” casting uses sand cores to provide holes and hollows in the parts. Parts have a good surface finish, and often have better mechanical properties than die cast parts.

Investment casting

Investment casting, known as the lost-wax process, uses patterns that duplicate the final part, made from wax or a similar meltable material. Patterns are attached to a gating system made of wax, then the whole assembly is coated with a ceramic slurry (fine particles suspended in a liquid) in several layers and dried to create a shell. The wax is then melted out of the shell. Metal is poured in. After it solidifies, the mold is broken and removed. Investment casting can produce netshape or near-net-shape parts with excellent surface finish.

As with any manufacturing process, there are trade offs – production quantities, part size and complexity – in selecting which casting process to use, said Alfred Spada, director of marketing, public relations and communications at American Foundry Society, Schaumberg, Il.

Not the same as bar

Castings bring a degree of difficulty to the machining process. If you are accustomed to machining nice, clean, precise bar or other dimensioned stock, castings may surprise you.

“The main difference is, if you start out with bar stock, you know what you’re getting – tolerances of maybe a couple of thousandths of an inch,” said Lanning Brandel, president of AMT, Inc., Sharon Springs, N.Y., a producer of ferrous and non-ferrous precision investment castings. Even on investment castings, which tend to hold tight tolerances, they are likely to be in the neighborhood of a few thousandths per inch.

On a six-inch aluminum sand casting, the foundry will likely be able to hold tolerances of +/– 0.010” to 0.020”, and there may be a mismatch across the parting line, where the two halves of the mold come together, of up to 0.020”.

Two-part molds will produce a visible parting line on the casting. Often, the part is designed so this parting line is on a surface that will be machined. If it will not be machined, and the parting line is intrusive, it will be removed with a belt sander.

Metal may leak out between the mold halves, producing flash on a part. For die casting processes, an automated trim press may be used for cleanup. Sometimes this may cut into the part, or sometimes fold over the flash rather than remove it.

Location, location, location

An as-cast part will not have a nice, flat surface to seat on the machining center table, or a close-to-perfect diameter to hold on to. You have to figure out how to hold the piece, and you will have to gage off of locating surfaces incorporated into the design. These are surfaces or features built into the casting, left as-cast, and indicated as target or datum locations on the drawing. You will use them for initial setup in the machining center.

Locators should be on the same half of a two-part mold, usually the drag. “The worst is taking your machining locators across the parting line,” said Tom Prucha, vice president of technical services at the American Foundry Society. That way, any cope/drag mismatch would add to the other tolerances.

Once you mount the part in the machining center, you can also use a touch probe to pick up selected features and to give the correct offsets to the machine control.

“In general, you would like to start by locating at a surface that won’t be machined,” Brandel said. Pick up three locating points and make sure the print indicates where they are and notes that they should be left as-cast and not be damaged, ground or machined.

Hold on

Workholding for castings can also pose a challenge. Without the right fixtures, a part can easily be misaligned. However, you don’t want your fixturing so tight that it requires the locators to be pristine, said Prucha. With cast parts you need to take the tolerances into account, and can’t depend on the features being perfectly flat or perfectly smooth.

“I can’t emphasize enough having quality-built fixtures” to hold castings during machining, said Mike Stahl, sales manager, Olson Aluminum Castings, Rockford, Il, manufacturer of high-end commercial and industrial grade sand castings. “It’s well worth the few dollars up front to have dedicated fixtures.”

Machined surfaces

Creating a quality machined surface on a cast part isn’t just between you and the metal. It’s a team effort that includes you, and the foundry, and your customer.

The areas you finish machine need to have enough metal for you to remove the entire cast surface, allowing for the tolerances of the casting. The foundry needs to make the part selectively oversized to provide enough material so you can machine over the whole area. This additional material is called the “finish allowance” or the “machine stock.” If there isn’t enough machine stock, after you’ve run the cutter over the surface, some as-cast surface may remain, and you will have to scrap the part.

Porosity inside heavier sections of die cast parts can be an issue, so you won’t want to cut into them too deeply.

Fortunately, die casting holds tight tolerances, in the range of a couple of thousandths, so with a properly designed casting, you don’t need to remove much material, and can stay clear of the porosity.

Other types of castings may also have porosity problems, so when you cut into the piece, pits show in the surface. You can work with the foundry to remedy the situation, or, if necessary, you can convince your customer to find another foundry that can produce better-quality castings.

Hard to tell

If you look at the Aluminum Association specs for 356 aluminum, a common sand-casting alloy, said Stahl, you’ll see a wide range in the Brinell hardness spec. Particularly with aluminum, the hardness affects machining qualities. Maybe last month’s lot of aluminum castings worked fine, but you now are having trouble machining this month’s shipment. The aluminum tears and smears and won’t make clean chips. Check the hardness, he suggested. You will probably find it is softer than last month’s shipment. A casting supplier that carefully controls the alloy formulation and does its own heat treating will be able to provide you with parts that will machine the same from one lot to the next.

In the middle

“There seems to be a lot of ‘matrix buying’ out there,” said Stahl. The customer will source a low-cost casting supplier and a low-cost machining supplier, and put them together. The machine shop then may have a problem.

“You don’t know what will come in. Where are the datums? Where is the parting line?” said Prucha. “Usually none of that information is conveyed.” Porosity and inconsistent hardness may degrade the finish of the machined areas.

As a result, your customer may blame your shop for problems that originate in the casting. You may be able to work with the foundry to improve the situation. If not, you may need to push back on your customer to source better castings.

Start at the very beginning

When you are going to machine a casting, it’s a good idea to begin communicating with both your customer and the foundry as early as possible.

Melting aluminum ingots in a crucible. (Photo courtesy of Olson Aluminum Castings.)

Try to get as much information as you can up front, said Stahl. “Even an hour talking about [the part] will save time down the road.”

“The most important things are the surface finish [of the] castings, and the tolerances,” said Brandel. “And [you] need to look at the parts and see how much of the part can be used as-cast and how much needs to be machined.”

The design of the casting is critical for providing good flow and fill characteristics for the foundry, and providing you with useful locating features and sufficient machine stock. If there’s not enough machine stock designed in, you’ll be scrapping parts after you’ve invested machining time into them.

Clear communication with your customer and the foundry will help provide you with the good castings you need to start with. “You can’t machine quality into a casting,” said Stahl.

Ready to cut

The foundry should have thoroughly inspected the castings`before shipping them. However, you would be wise to do your own inspections, too, before putting castings into your machining center. You may want to check the hardness, especially for aluminum parts. Make sure the locating features are intact. They should not have been ground, knocked off, dinged, dented, or otherwise damaged.

Look at the cast surface. The overall roughness should be uniform. Note any rougher patches or protrusions.

If the part is from a metal mold process, look for indentations from the ejector pins used to push the part out of the mold, and see if there are any areas that were scraped during removal from the mold.

Working with castings is definitely different from cutting bar stock. To save yourself headaches and keep your customers happy, work with the foundry to get good castings to start with, and keep in mind the special requirements for machining cast parts.

Today’s Machining World Archive: January 2008, Vol. 4, Issue 01

Parts without chips

“Cold heading is basically putting material in a die and hammering on it” according to one industry expert. Unlike hot forging, where the metal is heated before forming, or casting, where the metal is melted and solidified, cold heading and other methods of cold forming cause the metal to deform at room temperature. Though there was a U.S. patent issued in 1794 for a “cold header” (really a rivet machine), cold forming became a practical fabrication technology after World War II.

The typical one-die, two-punch method is common in producing headed parts, especially fasteners. The first blow combines partial head upset (coning) with shank extrusion. Then the second blow finishes the head shape. (Illustrations courtesy of Capenter Technology Corporation.)

Die and Punch

Modern cold-forming machines, also called headers or parts formers, provide from one to seven die stations, opposite a number of punches mounted on a slide, which usually moves horizontally. A common type used for making bolts or screws is the one-die, two-blow header, similar to the example shown. This has one die, which is the diameter of the shank of the bolt or screw being formed. The material in the die is struck with two different punches, one after the other.

Closeup of punches in a multi-die header. (Reed & Prince Manufacturing Corporation. Photo by Jean Butler.)

There is a limit to how much you can deform the material with a single blow, so two hits are often necessary to create the correct geometry of the head, said Steve Copeland, vice president of sales and marketing, Reed Machinery, Inc., Worcester, Mass. The first blow makes a shape like a tulip. Then a shifting mechanism moves the first punch over and places the second one in position. The second punch comes in to produce the finished shape of the head. After the second blow, a knockout pin pushes the piece out of the die. On a multiple-die header, there is a transfer rack with fingers that grab the part and move it from one die station to the next. If needed, the transfer mechanism can turn the part around 180 degrees so the next punch hits the other end of the part.

Die and Punch

The form of metal stock most often used in cold forming is “wire,” which may be a half-inch or more in diameter. It comes in large coils, convenient for feeding into machines making many parts per minute. When metal is compressed within a die, it is important to introduce just the right amount of material into the die, often within plus or minus one percent, or even less. Too little material won’t fill the die, making a bad part. Too much material can result in a malformed part, or produce flash that needs to be removed. The excess material can cause the die to split when the punch hits. Cold-forming machines are designed to cut off a precise length of wire, but the diameter of the wire must also be precise. To provide this, many shops will do a final draw of three to 10 percent on the wire before it is cut and formed to get a nice, tight diameter tolerance, said Kevin Hughes, specialist in wire products, Carpenter Technology Corporation, Reading, Pa.

Not Just Fasteners Anymore

Originally, cold heading was used to create heads for fasteners. A piece of wire was held in place and an impact to one end of it caused some of the material to spread or “mushroom,” creating a head for the fastener without having to machine away a lot of material to form the shaft. Once the head was formed, then the threads could be either cut or rolled. This process not only formed the fastener quickly, but it made efficient use of the material, producing little or no scrap.

Example of a part converted from screw machining to cold forming. Originally, the flat on the right side of the shank was machined and the hole was drilled. On the cold-formed part, the flat is formed and the hole is created by backward extrusion. A multi-die parts former produces these at 140 pieces per minute. (Photo courtesy of Reed & Prince Manufacturing Corporation.)

In addition to fasteners, many kinds of parts with complex shapes can be made by cold forming. The machines may be called “headers,” but they do much more than heading.

Tooling can be designed to extrude the material. When the punch hits at each station, it presses the material forward into the die to create a narrowed section, taper or shank. Or the punch can press on the material within the confines of the die so the material extrudes backward over the punch, creating the walls of a hole. In addition, shapes and contours can be built into the tooling to create splines, gear teeth and other features.

Upsetting, another term for heading includes forming a bulge in a cylindrical part, as well as forming a head on one end.

Using sophisticated tooling producing combinations of upsetting and extrusion when the punch hits each station, a cold forming machine can produce complex parts. Dimensional tolerances and surface finishes can rival those achieved with machining, depending on the shop and precision of the tooling.

So cold forming can produce parts at near-net-shape, using the minimum of material, to close tolerances, and at the rate of dozens or hundreds per minute. In addition, cold-formed parts exhibit excellent strength, as the material flows into its final form, rather than being cut, as in machining.

All this goodness does not come for free, however. Tooling costs for cold forming can be significant, perhaps $5,000 to $25,000 for a tool set, depending on how complex the part’s geometry is. Lead time for design and setup to run cold-formed parts is often measured in weeks, much longer than it would take to program and set up the same part on a CNC lathe or machining center. For small quantities, cold forming may not be cost effective.

What kinds of quantities are economical? It depends on the cold forming shop. Reed & Prince has recurring orders of around 1,000 for special large wood screws used in Atlantic City to repair the boardwalk, and ongoing production of many millions per year of small pins with precision grooves and knurling. Other sources recommend quantities in the hundreds of thousands or millions.

Buchanan Metal Forming, Inc., Buchanan, Mich., is a cold-forging house that makes larger parts, mostly in the one- to 20-lb. range. For his process, cost effective production quantities might range from a minimum of a couple thousand to a maximum of 250,000 parts, said Chris Tapper, the company’s president and CEO.

Cold Forming vs. Machining

A machined part starts out with a chunk of material large enough to contain the desired part. Then you cut away everything that isn’t the part. This produces chips, which are wasted material. Cutting can also disturb the grain structure of the metal.

Cold forming, however, makes very efficient use of material. It also tends to produce very strong parts, as the material flows into the desired shapes, maintaining its grain structure. And cold forming produces parts rapidly – tens or hundreds of parts per minute.

Some machined parts produced in volume, especially small round work like that made on screw machines, can be made more economically by cold forming. The savings can be greater than or equal to 50 percent, said James Richardson, president of Reed & Prince Manufacturing Corporation. If a cold-formed part can replace a multiple-part assembly, the savings can be even more substantial.

So cold forming is often thought of as a direct competitor to machining. However, a machine shop can sometimes make use of cold forming’s advantages by having the blank for a part cold formed to near net shape and then doing the finish machining. This can save on material cost and processing time. Both Richardson of Reed & Prince and Tapper of Buchanan Metal Forming reported having machine shop customers who order cold-formed blanks.

At Buchanan Metal Forming, Tapper, finds that sometimes machine shops come to him with parts that are particularly challenging to machine, with an internal gear or spline, in a blind hole, for example. “One customer who had the most high-tech of shaper equipment – used to take five mins to cut an internal spline,” said Tapper. “This customer was able to cut that down to two and a half minutes. We did it in six seconds.” Buchanan cold forms the blank for the customer, who then machines it. The drawing shows a cold-forged blank with an internal spline. The broken line indicates the final machined surface.

Most cold forming companies will work with their machine shop customers to determine the correct material and geometry for blanks that will be finish machined.

Materials for cold forming

The best material for cold forming a part may not be the same material you would use to machine it. The cold-forming company or your material supplier can work with you to determine the correct alloy for your part when it is cold formed. “Material flow and ductility are very key issues,” said Hughes. Knowledge of the behavior of materials can make the difference between a successful transition to cold forming and a frustrating experiment. Your cold-forming house will advise you, and you should be open to their recommendations. Any sulfur content in the metal would be detrimental to the heading process, for example. The qualities sulfur gives to a free-machining alloy make the material more likely to fracture during the cold forming process. So a material such as Type 303 free-machining stainless would not work. However, 302 HQ (heading quality) would be ideal. The material should be as soft as possible, ordered annealed at finish.

The other thing to look for in material for cold forming is a large yield-to-tensile strength ratio. This would allow you to put in more cold work before the part fractures, Hughes said.

Cold-forged parts. (Photo courtesy of Buchanan Metal Forming, Inc..)

Same Old Principle, New Products

Today’s cold-forming technology has come a long way from the simple cold-heading process used in the fastener industry for over a hundred years. While cold forming is still “taking a piece of metal and hammering on it,” the process can now produce complex, precision parts economically in large quantities.

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.