Process Capability

Automotive manufacturing is unlike any other industry and occurs on a scale that is unmatched. Consider this interesting data:

• According to Statista.com, manufacturers produced 92.5 million motor vehicles in 2024.
• In 2024, Toyota built 10.3 million cars, VW built 9.2 million and Hyundai/Kia built 7.3 million.
• In the late 1920s and early 1930s Ford’s River Rouge plant could produce 4,000 cars per day — one car every 49 seconds. Ford’s annual capacity in 1929 was about 1 million cars.
• The engineering bill of material for a car can have 30,000 individual components or more.

Not only do auto makers deliver an immense volume of parts and assemblies, they adhere to stringent quality requirements while working with very short cycle times. Automotive manufacturers also work on smaller profit margins than many other industries, so there is a continuous drive to reduce costs.

Although automotive manufacturing presents a set of challenges not seen in other industries, there are some valuable concepts that machine shops can adopt from automotive manufacturers that will improve productivity. Process capability is the most important concept, and should be part of every shop’s lexicon.

Process capability

Process capability is a cornerstone of success in modern automotive manufacturing. A capable process reliably delivers components that adhere to quality requirements. The capability index (a number derived through statistical calculation) indicates the robustness of a manufacturing process. A high value indicates the process is very reliable and less susceptible to variation. In layman’s terms, a capable process requires less human intervention, thereby improving quality and productivity.

Admit that machining is more science and engineering than an art form.

Process capability is measured and used to drive improvement in other industries, but not with the same religious fervor as in the automotive world. In other industries it is common to find processes that are dependent on the person performing the task. This was common in my aerospace experiences; many of the special processes could only be performed by one or two people because they were the only ones who knew how to massage the machine to get good parts. By automotive standards, such processes are not capable.

Obviously, processes that can be executed by multiple people ensure business continuity, fewer errors and less volatility associated with today’s labor market. Although the benefits of having statistically capable processes are evident, there are few companies outside the automotive industry that have the resources to develop and deploy a process that could match the capability standards of the automotive industry.

Regardless, smaller shops can benefit by focusing on the primary tenet of process capability, which is all about reducing variation.

Drive improvements

So, what does a small shop with a few employees need to do to drive capability into their machining processes?

First, you must admit that machining is more science and engineering than an art form. This is not intended to diminish the skills of experienced craftsmen. Rather, it is intended to emphasize that a capable process minimizes the need for machinists to make manual adjustments. If a process requires continual input from a machinist to ensure a successful run, then there is an opportunity for improvement. It can also mean the wrong process has been used.

Once we agree that machining is more science than art, common machining processes should be documented and propagated throughout the shop. Consider a common process like drilling a 5/16" hole in 4140 steel. If 150 sfm and 0.005" per minute feed rate reliably produces holes in 4140, then the shop should adopt this as a standard. Tapping, turning, grinding and all other machining operations should be treated in the same manner. Yes, getting a shop full of machinists and programmers to follow the same procedures is difficult, but the benefits outweigh the effort.

Establish an inspection plan

Create a written inspection plan for your parts. Each dimension needs to be reviewed and a plan made for the inspection method and frequency before programs are made and tools purchased. When possible, this plan needs to be reviewed and acknowledged by the customer.

This will help alleviate a common problem shops encounter, especially when making a new part. A shop may measure with one type of tool while the customer measures with something different; this can lead to discrepancies, which usually result in rework or scrap for the shop. Plus, such issues may hurt the shop’s relationship with the customer.

In some cases, on the other hand, a shop that has implemented a capable process may find something within a customer’s job specs — like a close tolerance — that the customer acknowledges is not important, which makes the machining job easier.

In either scenario, having a well-documented inspection plan ensures everyone is measuring parts and interpreting drawings the same way, thereby removing the variation associated with inspection.

Reduce opportunity for errors

Removing variation might also include adjusting machining methods. Handling parts multiple times is the surest way to introduce variation. Moving parts around from machine to machine or fixture to fixture requires multiple setups and, possibly, multiple offsets. Adopting techniques and programming strategies that minimize parts handling will reduce the number of opportunities to make errors.

Consider a cylindrical part that is turned on both ends. It is common for the machinist to turn one end, then flip the part and machine the second end, which opens the door for numerous errors. A better way would be to employ some out-of-the-box thinking and figure out a way to turn the part in one setup to eliminate errors associated with handling the part in two setups. Producing parts with as few setups as possible is so effective, a famous machine builder coined the phrase “one and done” in the marketing campaign for its machines, which were designed to perform multiple operations without moving the part. Reducing the number of times a part is handled while relying on the capability of the machine tool is one of the best ways to reduce variation.

Monitoring production rates is a good way to find a process that is out of control. When production rates vary significantly it is almost certain the process has variation.

Unstable cycle times and production rates are indicators that machinists and operators are struggling to produce acceptable parts. These are also indicative of processes that require the machinist or operator to interact with the machine tool. Checking parts in the machine is a classic example. Thread gauges and micrometers are used to ensure that parts are good before being removed from the machine. These are common signs that a process is out of control (not capable) or that a machinist does not trust the process.

Despite what the lean consultants might tell you, small- or medium-size manufacturers do not have the resources to produce machining processes that can match those of auto makers. The concepts, however, are still valuable for shops of all sizes. Incremental improvements that reduce variation will improve productivity.

Parting thought

We all acknowledge the skills gap and look for ways to close it. Adopting the concepts that drive improvements to a shop’s process capability can help close the skills gap by reducing the amount of knowledge a new person needs in order to be successful.

Automotive manufacturers have measured process capability for decades because they were forced to solve cost, skills and quality issues years ago. As we move forward, smaller manufacturers will be forced to become more serious about process capability for the same reasons.