Car trouble: CMM Inspection

Machining operations are fundamentally the same across all industries. However, machine shops and the way they operate can be vastly different. For example, aerospace shops generally are considered clean, sterile and deliberate whereas small job shops often have a Wild West feel in which anything goes. Shops obviously differ because of the work they do. I have had the opportunity to work at different shops and have found automotive machining to be the most challenging.

In every industry, machining comes with challenges that alone are not unique but, when combined, create unique situations that can be very difficult. It is the combination of common machining challenges that makes machining in the automotive industry the most difficult.

Part cost is important to all manufacturers, and cycle time influences part cost more than any other variable. In the power generation industry, we machined large forgings and chased hours for cost savings. At the aerospace shop, we machined castings and forgings and chased minutes. At the automotive shop, we machined bar stock, castings, forgings and pretty much anything else that you can imagine, and we chased seconds. In fact, a significant saving of time in the automotive industry was frequently one or two seconds, and saving those was never an easy achievement.

I have had the opportunity to work at different shops and have found automotive machining to be the most challenging.

Seconds are hard to come by because machining operations in automotive manufacturing are highly developed. Much of the machining process has been honed over many years, so the fat already has been trimmed. Takt times — the time in which a product must be completed to satisfy demand — are very low, and it is typical to see takt times of 30 to 40 seconds at shops making automotive components. Machining parts with short takt times usually requires breaking machining operations into multiple steps across multiple machines. It also means using tools like multistep drills that generate several pieces of geometry at once. These activities naturally drive out the normal forms of waste, such as tool changes and noncutting moves, from the process. Therefore, little waste is left.

Aerospace parts normally are associated with close-tolerance machining, which is understandable. Automotive machining does not share that stereotype, but automotive machining tolerances are often as tight as those in aerospace machining. Not only do automotive parts have very close tolerances — the parts have the added pressure of statistical process control requirements. SPC specifies control limits, which essentially close up the tolerance band, necessitating highly developed processes that are both accurate and repeatable.

Entire books have been devoted to SPC tools, so a full definition here would be impossible. SPC sets upper and lower control limits to ensure that the process delivers dimensions that are as close to nominal as possible. There are statistical tools like Cpk and Ppk that quality professionals use to measure accuracy and repeatability of the machining process, with an ideal value being 1.66 or better. Designing a machining process that achieves the desired 1.66 Cpk essentially eliminates most of the tolerance band. So if a 10 mm hole has a tolerance of ±0.03 mm, achieving 1.66 Cpk would require a process that consistently delivers a hole that deviates from the 10 mm target by only 0.01 mm (estimated). It is possible to drill holes that consistently maintain the ±0.03 tolerance, but achieving 1.66 capability probably would need an additional reamer or a custom drill. So SPC adds another layer of complexity to machining process development.