Producing parts with a silky smooth finish

Work produced by toolmakers and machinists reflects years of experience, training and difficult lessons. These people are a proud lot who are dedicated to the craft and work hard to demonstrate their skill. All have their own specialty and gravitate to a specific process, such as turning, milling or grinding, but achieving superior surface finishes is a common goal.

Producing a part with a silky smooth, bright appearance is regarded as a mark of expertise. This goal is accomplished in many ways, from adjusting cutting speeds and depths of cut to trying different geometries for cutting edges and strategically applying an emery cloth when nothing else works.

These actions are reinforced by quality control, customers and others who make appearance-based quality assessments. Evidence can be found in some of the old machinist sayings like “If you can’t make it right, then make it bright because they check the ugly ones first.” Appearance is important, but improper application of surface finish requirements can lead to lost productivity, increased costs and poor quality.

Design engineers must consider and balance the trade-offs among mechanical properties, how a part interacts in a system, appearance requirements and the cost of manufacture.

It is easy for machinists to become frustrated with stringent surface finish requirements, but surface finish is a critical component of part life. In general, a finer surface finish yields a stronger part than one with a rough surface finish. Research has demonstrated that failure modes like fatigue are exacerbated by rough surface finishes, so it is important for design engineers to specify a finish that will deliver the desired strength. Likewise, it is important for machinists to deliver the required finish to ensure that a part lasts as designed. This is one reason that aerospace manufacturers are sticklers about surface finish needs.

Producing a part with a silky smooth, bright appearance is regarded as a mark of expertise. Image: Cutting Tool Engineering

Processes like shot peening, a procedure that distributes small steel shot, or BBs, at high velocity, or roller burnishing, which passes a hard roller over a machined surface, can reduce the risk of fatigue cracking by imparting compressive stress into the surface of a part. Both practices are used extensively in the manufacture of turbomachinery components and provide a glassy, smooth finish.

Improper surface finishes also can cause other failures. Sealing surfaces for hydraulic actuators are a good example. Earlier in my career, I worked for an automotive company that manufactured power steering gears, which basically are specialized hydraulic cylinders. Keeping the 103 bar (1,500 psi) oil from leaking everywhere required having a 0.5 R_a — very slick, almost like a mirror — surface finish on the shaft so the seals would function properly. Any surface defect, including a poor surface finish, is a leak path and an opportunity for warranty claims. On more than one occasion, we had to quarantine and rework parts because a poor surface finish caused leaks upon assembly.