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The Mill: What Design Engineers Need to Know about This Important Orthopedic Tool

Editor's Note: This is the second of a year-long series of articles on design for manufacturability. We recommend that you read the series to refresh your own knowledge, and also share the articles with younger engineers.

Arguably, the most important tool for machining is the mill, also known as a milling machine or, if it includes computer numerical control (CNC), a machining center. These machine shop workhorses are used for a wide array of machining tasks from simple facing to complex surface profiling. Almost every orthopedic implant and instrument is at least partially fabricated on a mill, so understanding its use is valuable to all design engineers.

A mill can have multiple directions of travel. Typically, 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 the z-direction to cut the workpiece. A mill is different 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 armature that moves it in the x, y and possible z axes, depending upon design. Routers are less precise than mills and cannot hold the same tolerances because they lack rigidity due to their construction. However, they have the advantage of being able to operate over massive table sizes. Many plasma and laser cutters are configured as routers for this reason.

Mills come in two main varieties, vertical and horizontal. Vertical mills, such as the 3-axis mill described above, are more common and can be used manually. Open mills are necessary for manual operation. A machinist places a workpiece in a vise and clamps it to the table. Then, a corner is chosen as the origin and the dimensional controls are zeroed relative to this chosen location. All measurements are then made from the origin. When designing for manufacture, it is best to reference dimensions from a known origin that will not be obliterated during the machining process. If you have a hole pattern, the first hole is referenced to the origin and the remaining holes are dimensioned relative to the first hole.

A 3-axis mill is sufficient for most manufactured parts. It allows for three-dimensional contouring and is easy to program and visualize what you are doing, even when using CNC manufacturing to control the toolpath of the tool bit. However, some designs require more axes. Four-axis mills include rotation of the vise, usually as a special turret attachment. Parts such as acetabular reamers or helical blades should be machined on a 4-axis mill. Many enclosed 4-axis mills are horizontal mills, where the spindle is horizontal and the turret with clamps for multiple parts on multiple faces is vertical.

Then there are 5-axis mills that also allow rotation in an axis perpendicular to the fourth axis. When using more than a 3-axis mill, it is necessary to use CNC because of the difficulty in preventing collisions between the tool bit and spindle with the workpiece. Higher order mills also exist, but are rarely needed.

One common type of tool bit for a mill is called an end mill, so named because the cutting teeth are on the face of the tool as well as sometimes on the sides. The type of end mill to use depends on the desired machining, but the three most common types are face, flat and ball. Face end mills are large in diameter (1” to 3”) and are used to face a piece of material to make it flat and smooth. Flat end mills are used for machining features and pockets as well as for machining the outside profile of a part. Flat end mills come in a very large number of diameters, from less than 1/16th of an inch up to over an inch. It is best to start with a rough, bulk milling operation to remove excess material with a large diameter end mill and then do a tool change to a smaller diameter to a finishing operation. Ball end mills are ideal for surface contouring, such as the femoral component of total knee implants.

The mill—in its many varieties—is crucial to orthopedic manufacturing. A few final points to consider: orthopedic implants often begin as a forged casting. This eliminates a lot of the bulk for a faster and more efficient machining process. When working from a casting, it is important to remember to design the casting in such a way that it can be clamped securely into the mill. For all machining operations, it is also important to remember to clamp the workpiece in such a way that the end mill can machine the desired features. When necessary, it is 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. When using a through feature, that can also be the origin for the workpiece.

Dr. Deborah Munro
has worked in the orthopaedic medical device field for almost 20 years and holds numerous patents, mostly in the area of spinal fusion. She founded a Master’s in Biomedical Engineering at the University of Portland in Oregon and has since moved to the University of Canterbury in Christchurch, New Zealand, where she is developing a Minor in Biomedical Engineering within the Department of Mechanical Engineering. One of her areas of expertise is automated manufacturing, a course she created and taught for six years.