Milling plastics is just easy enough to be difficult.
Milling plastics might at first thought seem to be a snap, but the variety of available plastic materials and their differing physical characteristics can make the task more complicated than it first appears.
Many plastics offer ease of fabrication, low weight, strength and the ability to hold close tolerances, which make them excellent for use in for many structural, wear and bearing applications. Although injection molding is arguably the most common way to form plastics, in some cases milling plastic parts is more cost effective or is simply necessary (see sidebar on page below).
Milling can be employed to make prototypes or short production runs of plastic parts without the expense of creating a mold. Implementing engineering changes for a milled part requires only changing a CNC program rather than reworking a mold. And milling can achieve tight tolerances and contours not possible via molding, making it possible to complete some parts in a single operation rather than a two-step mold-and-secondary-machining process.
While milling may be the best way to manufacture a particular part, machining plastic workpieces presents several challenges. Chief among them is controlling and dissipating the heat machining produces because plastics generally have much lower melting points than metals. For example, the acetal plastic Delrin has a melting point of about 350° F, while the melting point of aluminum is 2,000° F. Plastics also lose heat more slowly than metals, and, when heated, plastics expand up to 10 times more than metal alloys, according to Quadrant Engineering Plastic Products, Reading, Pa.
The variety of plastic material compositions presents a range of machining characteristics that basically defy generalization. For example, the Hytrel thermoplastic elastomer from Dupont Engineering Polymers, Wilmington, Del., is available in more than 50 grades. The selection includes “soft grades” with a flexural modulus below about 240 MPa as well as hard grades with higher flexural modulus values. There is no sharp transition point; machining conditions will vary gradually from type to type, according to Dupont.
Courtesy of Quadrant EPP
A 3 "-dia. cutter, tooled with six inserts and running at 2,500-rpm and a 10-ipm feed rate, mills a sheet of Quadrant Ertalyte TX PET-P.
Application support from plastic material suppliers is a starting point for developing milling processes. Quadrant, for example, has outlined basic plastic material characteristic information in its Quadrant Materials Triangle, available on its Web site. Intended to help designers pick the right plastic for their products, the matrix groups thermoplastic materials by material and performance criteria, including hardness, strength, heat resistance and dimensional stability.
Higher-performance materials have higher heat deflection temperatures and lower coefficient of thermal expansion rates and therefore are less affected by heat generation during machining, according to Quadrant. Machining parameters, however, reflect the advanced materials’ additional strength. Typically, with general engineering plastics, the company recommends the use of more aggressive feed rates than those applied with advanced materials. Cutting speeds can vary widely, as well. For example, Quadrant recommends starting-point milling speeds of 200 to 500 m/min. for some polyvinylidenefluoride (PVDF) and nylon (PA) plastics with melting points under 350° F, but suggests speeds of just 25 to 75 m/min. for polybenzimidazole (PBI) materials, which can perform reliably at temperatures well over 400° F. Quadrant added that PBI materials may also require the use of CVD diamond-coated or PCD tools for optimal tool life.
Fill It Up
As is the case with advanced metal alloys, increasing the performance capabilities of a plastic generally increases machining difficulty. One example is plastic filled with glass or carbon fibers, which increase strength, insulation abilities and dimensional stability. Lance Nelson, president of tooling and workholding manufacturer 2L Inc., Hudson, Mass., said the filled plastics are “wicked abrasive and chew up cutting tools,” and slower cutting speeds may be required to conserve tool life. “Glass is almost as hard as carbide,” he said. “If you run a very high surface speed, you will generate heat and the carbide tools will break down.”
Ed Padgett, co-owner of Padgett Machine Inc., Tulsa, Okla., which focuses on defense products, said his shop cuts a variety of plastics, but stays away from filled materials. In many cases, machining filled plastics requires special biohazard handling techniques. “From the envelope in which these plastics are being machined, that air has to be evacuated because it has glass particles in it,” Padgett said. He noted that shops may machine the filled materials on one or two dedicated machines in an enclosed room with its own circulatory system.
The Right Tool
Top efficiency in any machining scenario requires tools that are engineered in response to the material’s specific characteristics. Jeff Davis, vice president of engineering at carbide tool maker Harvey Tool Co. LLC, Rowley, Mass., said specialty shops that process only plastics typically use machining processes and tools developed to maximize productivity in those materials. “Then, there are also plenty of shops that do plastics occasionally, and they are not looking to optimize—they are just looking to get through the job,” he said. That may involve applying a 2-flute, general-purpose HSS mill to cut plastic. “In many cases that works, but very often, the geometry, relief or the flute depth isn’t right.” That can produce poor surface finish, chipping, chatter, melting and even burning.
According to Davis, a fine surface finish is crucial in most plastic parts. As a result, cutters designed to mill plastic have geometries engineered that cut cleanly and minimize heat generation, including sharp edges, acute relief on the back side of the cutter and flute depths much larger than those common in metalworking.
Courtesy of Harvey Tool
The extreme flute depth of Harvey Tools’ single-spiral “O” flute tool provides maximum room for a large plastic chip to flow cleanly from the cut and carry away heat with it.
As an example, Davis cited Harvey Tools’ plastic-focused single-spiral “0” flute tool, which “almost looks like a corkscrew.” Depending on the individual tool, the flute depth extends to the center or past the tool’s core, eliminating the core diameter typical of a metalcutting tool. “The flute depth is gigantic, so there is plenty of room for the chip to exit,” Davis said.
The configuration is weaker than a tool with a large core diameter, but a tool for milling plastic generally isn’t subjected to the power and torque experienced by a metalcutting tool. When machining plastic, “The cutter can be more sleek and dynamic. It doesn’t have to be beefed up with corner radii, edge prep and special coatings.”
Harvey also polishes the tool’s large flute valley “to give the chip very little to stick to. We don’t want to have any texture or hangups that might keep the chip from being thrown,” Davis said. “If you can cut [the workpiece] solidly and cleanly [to create a big chip] you get the finish you are looking for and any incidental heat ends up in the chip.”
The differences in application requirements and of the machining characteristics of different plastic materials may prompt the use of different tool designs, according to Davis. For example, a tool with two straight flutes would be more appropriate than a single-flute, high-helix tool for milling abrasive-filled plastics. The dual flutes share abrasive wear and minimize fraying of fiber strands by cutting the workpiece without the upward or downward pull that a helix/spiral produces. Or, in an application where surface burrs are undesirable, a “downcut” tool with a right-hand cut/left-hand spiral design would force the chip, and the burr, down through the bottom of a through-hole so that they are not on the part surface.
The cutting strategies employed in plastic milling also differ from those employed in metalworking. On a typical metal workpiece, it is normal to rough a contour and leave a few thousandths of an inch excess material for a finishing pass. “You cannot do that in plastics, because in the finish cut you are taking very little material, and most of the heat either stays in the tool or the part because the chip can only absorb so much,” Davis said. “You want to take as big a chip as possible with the most feed and the least speed.”
Courtesy of 2L
Vacuum systems can help firmly locate plastic and oddly shaped workpieces.
Courtesy of Quadrant EPP
Vic Maturi, machinist (left), and Jim Hebel, manager of technical services and application development, for Quadrant Engineering Plastic Products discuss milling of a plastic workpiece at the company’s North American headquarters in Reading, Pa.
Quadrant’s Hebel concurred with this strategy, saying DOC is critical, and lighter is not better. “Too light of a pass will cause the cutter to essentially rub the surface. For finish cuts, we recommend a minimum of 1⁄32 " DOC,” he said.
Steven Lawver, prototyping machinist and CAD technician at experimental P/M lab Matsys Inc., Sterling, Va., uses a variety of machining technologies to process plastic materials for both professional and personal projects. Regarding a basic approach to milling plastic, he said, “Feed rates and speeds similar to those used for milling aluminum are a good starting point for most plastics. But if you have some scrap to work with, adjust the feed rates up from zero to find the point at which the bit will cause the material to climb, catch or chip on the bit,” and then back off. Some plastics are more forgiving than others. But the rule of thumb is that chip clearing and cooling are far more vital with plastics than would be otherwise.
Suppliers and end users agree that it is crucial to keep the plastic milling operation as cool as possible, but they don’t always agree on how to do it. 2L’s Nelson acknowledged that there is divergence of opinion on the use of coolant. “Some people say never use coolant, while some say you have to use coolant. As far as the cutting tool end of it, you should always use coolant. That is the bottom line answer,” he said.
But those promoting dry machining of plastics using air-jet cooling point out that some plastics are hydroscopic: they absorb water. “Nylon is very hydroscopic, and some people don’t want to use coolants with nylon because it causes it to swell and change dimensions,” Nelson said.
While it is true that some materials absorb moisture, Quadrant has found that components are not on a machine long enough for this to be a factor, according to Jack Sharp, toolroom/machining manager. “The only time this may be an issue is if a part were to sit in a puddle of coolant overnight or during an off shift,” he said.
The company always recommends the use of a water soluble coolant to keep the part cool and reduce the effects of heat generation, noting that coolant also helps clear chips. Water-soluble products are specified because some plastics with lower chemical resistance can be discolored or cracked by the rust inhibitors in oil-based coolants.
While plastic milling generally is best performed with coolant, Nelson agreed there are specific applications, such as medical parts, that must be run dry to avoid the chance of contamination of a component that may later be implanted in a patient.
Plastic’s relative softness and flexibility, combined with the need to maintain a rigid machining system to assure fine surface finish and dimensional accuracy, dictate special workholding considerations.
One solution is vacuum workholding. Nelson of 2L said the vacuum workholding equipment his company offers is an outgrowth of the company’s original business of providing solid-carbide and spring-loaded engraving tools. When end users sought ways to hold unevenly shaped items for engraving, 2L developed vacuum workholding systems as a way to immobilize a workpiece without clamps.
The systems are based on plates with horizontally gundrilled passages connected to the plate surface via holes blocked by removable screws. A vacuum pump is attached to the passages, and taking out a screw creates a vacuum at the plate surface.
The strength of the suction on a part is directly proportional to its surface area, so “if you have small parts, you are not going to be able to use a 3 "-dia. facemill and take a ¼ " DOC; the part will just slide,” Nelson said. As a result, the plate surfaces feature rows of threaded and tapped holes that enable users to bolt work stops or adjustable edge clamps to grip the side of a part.
Vacuum systems provide strong grip on nonporous plastics, but because their effectiveness is based on surface area of the part, machining small parts requires other methods, such as using vacuum to hold a single large sheet of plastic, then applying gang or nesting machining techniques to cut a large number of parts in the sheet. “So the parts don’t go flinging around inside the machining center when they are making them, a shop will often cut three sides of the part and on the fourth side leave a very small web of material,” Nelson said. “When machining is completed, the sheet is taken off the chuck and the parts snapped off of it like you do on a kid’s model set.”
For simple or low-volume part applications, some shops use two-sided tape to hold plastic parts. Nelson said the special engraving tape is sticky on one side and not as much on the other side. To ease part removal after machining, the tape sticks strongly to the table but not as strongly to the part.
Machines typically used in metalcutting can be used to mill plastics, but shops that make plastic parts need to select machines that are most appropriate for the application. Bill Howard, vertical machining center product manager for Makino Inc., Mason, Ohio, said the company’s machine tools are employed in many plastic-milling applications. One major application is milling acetal resins, such as Delrin and Celcon, for use in wear components between the metal elements of artificial knees. The cutting forces when milling those plastics are minimal compared to metalcutting, and the knee components usually have complex, 3-D shapes. Therefore, the machines used for the knee components generally are high-speed units with 4- or 5-axis capabilities.
Virtually all medical parts are cut dry so a coolant or a lubricant doesn’t migrate into the material. “We recommend through-spindle air and air nozzles that move the chips from the cutting area down into the chip handling systems of the machine,” Howard said.
Howard pointed out that machining graphite EDM electrodes is somewhat similar to plastics in that it typically is performed dry, and removal of finely cut material can be an issue. As a result, “graphite” versions of its V-22, V-33 and other VMCs, featuring systems that pull cut material out of the work area, can be used in milling plastics. “We actually are pulling a slightly negative pressure on the work zone of the machines,” he said.
Hard to Mill Well
While plastics generally cut easily, there are other factors that raise the machining difficulty factor. A tool may fly through a plastic workpiece but leave a melted mess behind. Or a glass-filled plastic material may produce tool wear at unexpected levels. The key is to be aware of the varying machining characteristics of different workpiece materials, proceed carefully and don’t hesitate to consult experts, according to Harvey Tools’ Davis. “It’s more complex than one thinks, but it’s not difficult to understand.” CTE
bout the Author: Bill Kennedy, based in Latrobe, Pa., is contributing editor for Cutting Tool Engineering. He has an extensive background as a technical writer. Contact him at (724) 537-6182 or by e-mail at email@example.com.
Courtesy of Padgett Machine
Rather than mold a part, Padgett Machine milled this 5⁄8 "-long × ½ "-wide × ¼ "-thick high-grade Delrin-polymer aerospace electronics insulator because its ±0.002 " tolerance and squareness requirements didn’t allow for the taper that would be required to remove the part from a mold.
Mill or mold?
Ed Padgett, co-owner with his brother, Randy, of Padgett Machine Inc., mills parts from plastics because “sometimes plastic injection is not the answer. Sometimes a part has to be machined.” He cited a 5⁄8 "-long × ½ "-wide × ¼ "-thick high-grade Delrin polymer insulator for aircraft electronics equipment. The part was not a candidate for injection molding because a molded part “has to have a taper on so it can be pulled out of the die; this part had a ±0.002 " tolerance that didn’t allow for taper on the edges.”
And even if the part’s features were compatible with the injection molding process, it wouldn’t have been cost-effective to mold. “The application, just 6,000 parts over 3 years, didn’t warrant building an injection die. Normally, if you’re going to build an injection die, you’d be running 40,000 to 50,000 parts.”
Padgett milled prototypes on a Haas VF-2 vertical machining center. Then, after receiving approval for production, switched to a Daewoo Puma 2000 SY 4-axis lathe, which permitted milling and drilling features that are not parallel or perpendicular to the spindle center. Padgett said Delrin is abrasive, so the shop applied coolant intermittently when machining the parts to extend the life of the carbide tools.
Milling tips and hints
Quadrant Engineering Plastic Products patented the first process for extruding nylon stock shapes for machining in 1946 and has become a global manufacturer of cast, extruded, compression-formed and molded engineering plastic shapes intended to be machined. Its tips for machining plastics includes the following:
Part tolerance. Depending on the part and plastic involved, machining can typically achieve tolerances of 0.1 to 0.2 percent of the nominal part size. For tighter tolerances, consider rough machining to within 0.030 " of final size, then allow the part to stress relieve for 24 to 48 hours before final machining.
Cutter size. Use the largest cutter possible, because more passes over a surface result in machined-in stress. Note that the manufacture of thermoplastic materials produces a degree of internal stress; be sure stock is stress-relieved by the plastic shape provider.
Part flatness. When flatness is critical, start with a flat plate; don’t use a vacuum table to suck a bowed plate flat prior to machining. The plastic has a memory and will spring back to the bowed condition when released from the vacuum. Instead, place the material belly up, hold it on the edges, and machine the top surface flat. That surface now is truly flat and can be flipped over and fixtured or held via vacuum for further operations.
Climb milling. Maintaining sharp cutting edges, providing plenty of chip clearance and employing climb milling can help control heat.
Hard plastics. When milling hard plastics, avoid generating sharp internal corners. The radius of curvature should be at least 1mm. Chamfered edges are also helpful in preventing chipping of part edges during milling by providing a smoother transition between the cutting tool and the plastic workpiece.
Dupont Engineering Polymers
Harvey Tool Co. LLC
Padgett Machine Inc.
Quadrant Engineering Plastic Products
Related Glossary Terms
Way of displaying real-world objects in a natural way by showing depth, height and width. This system uses the X, Y and Z axes.
Substance used for grinding, honing, lapping, superfinishing and polishing. Examples include garnet, emery, corundum, silicon carbide, cubic boron nitride and diamond in various grit sizes.
Substances having metallic properties and being composed of two or more chemical elements of which at least one is a metal.
Rotary tool that removes hard or soft materials similar to a rotary file. A bur’s teeth, or flutes, have a negative rake.
Stringy portions of material formed on workpiece edges during machining. Often sharp. Can be removed with hand files, abrasive wheels or belts, wire wheels, abrasive-fiber brushes, waterjet equipment or other methods.
Condition of vibration involving the machine, workpiece and cutting tool. Once this condition arises, it is often self-sustaining until the problem is corrected. Chatter can be identified when lines or grooves appear at regular intervals in the workpiece. These lines or grooves are caused by the teeth of the cutter as they vibrate in and out of the workpiece and their spacing depends on the frequency of vibration.
- chemical vapor deposition ( CVD)
chemical vapor deposition ( CVD)
High-temperature (1,000° C or higher), atmosphere-controlled process in which a chemical reaction is induced for the purpose of depositing a coating 2µm to 12µm thick on a tool’s surface. See coated tools; PVD, physical vapor deposition.
- chip clearance
In milling, the groove or space provided in the cutter body that allows chips to be formed by the inserts.
Workholding device that affixes to a mill, lathe or drill-press spindle. It holds a tool or workpiece by one end, allowing it to be rotated. May also be fitted to the machine table to hold a workpiece. Two or more adjustable jaws actually hold the tool or part. May be actuated manually, pneumatically, hydraulically or electrically. See collet.
Space provided behind a tool’s land or relief to prevent rubbing and subsequent premature deterioration of the tool. See land; relief.
- climb milling ( down milling)
climb milling ( down milling)
Rotation of a milling tool in the same direction as the feed at the point of contact. Chips are cut to maximum thickness at the initial engagement of the cutter’s teeth with the workpiece and decrease in thickness at the end of engagement. See conventional milling.
- computer numerical control ( CNC)
computer numerical control ( CNC)
Microprocessor-based controller dedicated to a machine tool that permits the creation or modification of parts. Programmed numerical control activates the machine’s servos and spindle drives and controls the various machining operations. See DNC, direct numerical control; NC, numerical control.
- computer-aided design ( CAD)
computer-aided design ( CAD)
Product-design functions performed with the help of computers and special software.
Fluid that reduces temperature buildup at the tool/workpiece interface during machining. Normally takes the form of a liquid such as soluble or chemical mixtures (semisynthetic, synthetic) but can be pressurized air or other gas. Because of water’s ability to absorb great quantities of heat, it is widely used as a coolant and vehicle for various cutting compounds, with the water-to-compound ratio varying with the machining task. See cutting fluid; semisynthetic cutting fluid; soluble-oil cutting fluid; synthetic cutting fluid.
- electrical-discharge machining ( EDM)
electrical-discharge machining ( EDM)
Process that vaporizes conductive materials by controlled application of pulsed electrical current that flows between a workpiece and electrode (tool) in a dielectric fluid. Permits machining shapes to tight accuracies without the internal stresses conventional machining often generates. Useful in diemaking.
Milling cutter for cutting flat surfaces.
Rate of change of position of the tool as a whole, relative to the workpiece while cutting.
- finish cut
Final cut made on a workpiece to generate final dimensions or specified finish. Often made using reduced feeds and higher speeds. Generally, the better the surface finish required, the longer the finish cut takes. Also, the final cut taken on an electrical-discharge-machined part.
- flat ( screw flat)
flat ( screw flat)
Flat surface machined into the shank of a cutting tool for enhanced holding of the tool.
Grooves and spaces in the body of a tool that permit chip removal from, and cutting-fluid application to, the point of cut.
- gang cutting ( milling)
gang cutting ( milling)
Machining with several cutters mounted on a single arbor, generally for simultaneous cutting.
Hardness is a measure of the resistance of a material to surface indentation or abrasion. There is no absolute scale for hardness. In order to express hardness quantitatively, each type of test has its own scale, which defines hardness. Indentation hardness obtained through static methods is measured by Brinell, Rockwell, Vickers and Knoop tests. Hardness without indentation is measured by a dynamic method, known as the Scleroscope test.
- high-speed steels ( HSS)
high-speed steels ( HSS)
Available in two major types: tungsten high-speed steels (designated by letter T having tungsten as the principal alloying element) and molybdenum high-speed steels (designated by letter M having molybdenum as the principal alloying element). The type T high-speed steels containing cobalt have higher wear resistance and greater red (hot) hardness, withstanding cutting temperature up to 1,100º F (590º C). The type T steels are used to fabricate metalcutting tools (milling cutters, drills, reamers and taps), woodworking tools, various types of punches and dies, ball and roller bearings. The type M steels are used for cutting tools and various types of dies.
Turning machine capable of sawing, milling, grinding, gear-cutting, drilling, reaming, boring, threading, facing, chamfering, grooving, knurling, spinning, parting, necking, taper-cutting, and cam- and eccentric-cutting, as well as step- and straight-turning. Comes in a variety of forms, ranging from manual to semiautomatic to fully automatic, with major types being engine lathes, turning and contouring lathes, turret lathes and numerical-control lathes. The engine lathe consists of a headstock and spindle, tailstock, bed, carriage (complete with apron) and cross slides. Features include gear- (speed) and feed-selector levers, toolpost, compound rest, lead screw and reversing lead screw, threading dial and rapid-traverse lever. Special lathe types include through-the-spindle, camshaft and crankshaft, brake drum and rotor, spinning and gun-barrel machines. Toolroom and bench lathes are used for precision work; the former for tool-and-die work and similar tasks, the latter for small workpieces (instruments, watches), normally without a power feed. Models are typically designated according to their “swing,” or the largest-diameter workpiece that can be rotated; bed length, or the distance between centers; and horsepower generated. See turning machine.
- machining center
CNC machine tool capable of drilling, reaming, tapping, milling and boring. Normally comes with an automatic toolchanger. See automatic toolchanger.
- metalcutting ( material cutting)
metalcutting ( material cutting)
Any machining process used to part metal or other material or give a workpiece a new configuration. Conventionally applies to machining operations in which a cutting tool mechanically removes material in the form of chips; applies to any process in which metal or material is removed to create new shapes. See metalforming.
Any manufacturing process in which metal is processed or machined such that the workpiece is given a new shape. Broadly defined, the term includes processes such as design and layout, heat-treating, material handling and inspection.
Machining operation in which metal or other material is removed by applying power to a rotating cutter. In vertical milling, the cutting tool is mounted vertically on the spindle. In horizontal milling, the cutting tool is mounted horizontally, either directly on the spindle or on an arbor. Horizontal milling is further broken down into conventional milling, where the cutter rotates opposite the direction of feed, or “up” into the workpiece; and climb milling, where the cutter rotates in the direction of feed, or “down” into the workpiece. Milling operations include plane or surface milling, endmilling, facemilling, angle milling, form milling and profiling.
- milling machine ( mill)
milling machine ( mill)
Runs endmills and arbor-mounted milling cutters. Features include a head with a spindle that drives the cutters; a column, knee and table that provide motion in the three Cartesian axes; and a base that supports the components and houses the cutting-fluid pump and reservoir. The work is mounted on the table and fed into the rotating cutter or endmill to accomplish the milling steps; vertical milling machines also feed endmills into the work by means of a spindle-mounted quill. Models range from small manual machines to big bed-type and duplex mills. All take one of three basic forms: vertical, horizontal or convertible horizontal/vertical. Vertical machines may be knee-type (the table is mounted on a knee that can be elevated) or bed-type (the table is securely supported and only moves horizontally). In general, horizontal machines are bigger and more powerful, while vertical machines are lighter but more versatile and easier to set up and operate.
Strip or block of precision-ground stock used to elevate a workpiece, while keeping it parallel to the worktable, to prevent cutter/table contact.
- polycrystalline diamond ( PCD)
polycrystalline diamond ( PCD)
Cutting tool material consisting of natural or synthetic diamond crystals bonded together under high pressure at elevated temperatures. PCD is available as a tip brazed to a carbide insert carrier. Used for machining nonferrous alloys and nonmetallic materials at high cutting speeds.
Space provided behind the cutting edges to prevent rubbing. Sometimes called primary relief. Secondary relief provides additional space behind primary relief. Relief on end teeth is axial relief; relief on side teeth is peripheral relief.
Hole or cavity cut in a solid shape that connects with other holes or extends all the way through the workpiece.
Minimum and maximum amount a workpiece dimension is allowed to vary from a set standard and still be acceptable.
On a rotating tool, the portion of the tool body that joins the lands. Web is thicker at the shank end, relative to the point end, providing maximum torsional strength.