Process Capability: Drilling Performance

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.

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.