Young orthopedic engineers face a knowledge gap when designing a device that is to be manufactured with traditional machine tools.
Industry and academic leaders have asked BONEZONE to address these design for manufacturing issues, so we asked Deborah Munro, D.Eng., to author a series of articles that could provide basic knowledge of major machining tool operation, as well as strengths and limitations of each technology to help improve engineers’ designs.
Dr. Munro, who has worked at orthopedic device companies and taught engineering at the university level, says that overarching issues stem from a lack of hands-on education required for today’s engineering degrees. Universities arm engineers with the ability to create complex designs, but don’t teach how to machine them. Therefore, engineers often create designs that overcomplicate or even prohibit the fabrication of a part, Munro says.
Here, we’ve taken Dr. Munro’s articles on the drill press, mill, lathe, EDM and cutting tools and condensed them into a single guide for you to reference.
The drill press is used to create a hole in metal, wood, plastic or other material. In a typical setup, a tool bit is mounted and tightened into a chuck. The tool bit rotates at a high speed and is driven downward into the surface of the part.
For a drill press, the face of the tool bit and its sides must be able to cut material. The cutting surfaces are called “flutes,” and they often spiral down the surface of a tool bit. The more flutes, the smoother the finish of the resulting hole. There are five main types of tool bits, including a centering bit, drill bit, reamer, countersinks and counterbores, and a tap. Each bit has a specific purpose and could create threads, divots, recesses or smooth holes.
Drilling is a multi-step process. It’s important to remember that holes are first drilled and reamed to their shank diameter, then tapped to create the thread pitch. Speed is also an important factor to consider. The larger the diameter of the drill bit, the faster the outside flutes will travel; the smaller the diameter, the slower the speed. Therefore, the RPM of the drill press chuck needs to be adjusted to be optimal for the diameter.
When a drill pierces a material, it produces chips and generates heat. Manufacturers usually prefer to “peck” at a cut to clear chips and reduce this heat. For hard metal parts, a liquid coolant is used; soft metal and wood can use air as a coolant, if needed.
The Engineering Takeaway:
When you’re drawing a part to be drilled, design holes as either a rectangular, linear or circular array, and always dimension from an outside corner of the part, not the centerline or from a location that will be machined away. After that, the remaining holes should be dimensioned from the first hole, not the edge of the part.
The mill, also known as a milling machine or machining center (if it includes a CNC machine), is used for a variety of machining tasks, from simple facing to complex surface profiling. Almost every orthopedic implant and instrument is at least partially fabricated on a mill.
Mills can have multiple directions of travel. A 3-axis mill has a horizontal table that moves in the x-y plane and holds your workpiece, and a vertical (z-axis) high-speed rotating tool mount called the spindle. The spindle holds the tool bit and can plunge in a vertical direction to cut the workpiece. A mill differs from a router in that the horizontal table is stationary in the x-y plane and the high-speed rotating tool (router) is affixed to an armature that moves it in the x, y and possible z axes. Mills are more precise and rigid than routers, enabling them to machine to tighter tolerances.
There are vertical and horizontal mills. A three-axis mill is sufficient for most manufactured parts, allowing for three-dimensional contouring, but some designs need more axes. Four-axis mills include vise rotation, typically as a special turret attachment. Parts like acetabular reamers or helical blades are manufactured on a four-axis mill. Five-axis mills allow rotation in an axis perpendicular to the fourth axis. Anything more than a three-axis mill requires the use of a CNC because of the difficulty in preventing collisions between the tool bit and spindle with the workpiece. There are some higher order mills, but they aren’t as common.
Tool bits called end mills are used to shape the material. Face-end mills are used to make material flat and smooth; flat end mills machine features and pockets as well as the outside profile of a part; and ball end mills are ideal for surface contouring, such as on the femoral component of a total knee implant. Often, a large-diameter end mill is used to rough away bulk material before performing a tool change to complete a finishing operation.
Engineers must consider the following when designing for a mill. Orthopedic implants often begin as a forged casting to eliminate bulk material removal in a more efficient process. When working from a casting, it’s important to design the casting in such a way that it can be clamped securely into the mill. In all milling operations, one must clamp the workpiece so that the end mill can machine the desired features. It’s useful to make a “through feature,” such as a hole pattern, that can be used to orient the part when it is flipped over to machine the back side (and can also be used as the origin for the workpiece).
A lathe’s primary role is to create cylindrical objects with a central axis of symmetry by rotating about an axis. They’re used to make shafts, screws, pulleys, pins and wheels, among other items. With a CNC lathe, it’s possible to machine nonsymmetrical parts such as cams on a crankshaft.
Working with high-powered lathes can create a stressful and dangerous environment for machinists, because any loose item (hair, clothing, etc.) can wrap around the spindle in a fraction of a second, with consequences that can be fatal. At high speed, an operator also cannot see if a component is out of round or has knobby protrusions or sharp metal shavings clinging to it. Before bringing the tool bit into contact with the component, the operator must turn off the lathe and rotate the part through 360 degrees of movement to check, or else risk severe injury or breakage of the tool bit, part or lathe.
When designing a component for lathe machining, the reference origin is at the end of the part, never anywhere in between. The headstock is usually used as the reference, and all intermediate dimensions are referenced to the head. This avoids tolerance stack-up; if one measurement is off, none of the other dimensions are affected and there is always a reference plane for the machinist to check the part’s dimensional accuracy during machining.
All dimensions must be provided across the diameter, never the radius or from the outside to the depth of an indent. Machinists measure with a caliper that they place across the diameter of the part or with a depth gauge that can be slid along the profile. All bolt circles, such as a pattern of drilled holes, should be dimensioned as the diameter through the center of the holes, the diameter of the hole and the angular dimension between two holes.
Lathe operators have a lot on their minds to make sure they’re machining safely, and without undue wear or damage to the machine. Lathes operate differently than other machine tools, so engineers need to incorporate design considerations for a lathe-machined component. When designing and drawing a lathe component, always provide a cross-section through the axis view with the central axis clearly shown. If using GD&T, the axis should be a datum reference along with the component’s head normal to the axis.
EDM and Other Alternative Metal Cutting
Aside from traditional machining methods, there are other ways to cut metal parts, including electric discharge machining (EDM), laser cutters and plasma cutters. EDM is used to create high-precision internal geometries, sharp inside corners and very long holes, while laser or plasma cutters are used for rapid machining of sheet material.
EDM is a controlled metal removal process that uses electric spark erosion to produce a finished part. There are two kinds of EDM: block and wire. Block EDM uses a block of electrically charged, machined copper or graphite to erode an exact, reversed copy of itself in a metal part. It’s often used to make molds and is good for creating sharp inside corners and tapered walls with a mirrored finish.
Wire EDM is more common, because it can make three-dimensional objects and tiny, long holes. It also uses electrode erosion, but the cathode is a wire, often submerged in a cooling bath. Both block and wire EDM are very slow, and a complex machining operation can take more than a day to complete. Wire erodes during use, necessitating a fresh wire fed from a spool during operation.
Laser cutting works by directing the output of a high-powered optical laser. It works on anything from wood to acrylic to metal. The laser can melt, burn or vaporize the material, using a jet of gas to clear debris and leaving an edge with a high-quality surface finish. There are three types of laser cutters: gas, crystal and fiber. Gas laser cutting is good for thin, nonmetal sheets in most cases. Crystal laser cutting can be used with both metals and nonmetals, and has a huge range of applications, but it is expensive and has a shorter lifespan than other machines. Fiber laser cutters use continuous wavelengths of light or pulsed wavelengths to cut material. These have a much longer service life and require little maintenance.
Plasma cutters work by sending an electric arc through a gas in a constricted opening nozzle. The gas squeezes through the nozzle at a high speed, cutting through the metal by melting it. This is a low-precision cutting technique, but it is very fast and can cut through up to six inches of steel. Plasma machining is good for parts that will be finish machined in a second operation or parts in which tolerances aren’t critical.
EDM, laser and plasma cutters are important tools in any orthopedic manufacturing operation, and different methods are used for different part requirements. When designing a part that will be made using one of these alternative cutting methods, know the capabilities of each type of machine.
We hope that this guide was helpful. What types of machine tools would you like to learn more about? We welcome your ideas for future installments of this series.
Heather Tunstall is an ORTHOWORLD Contributing Editor.