Tech Talk #57 – The CNC Fallacy

DavidTechArticlesBy David Reher, Reher-Morrison Racing Engines

“A CNC machine can’t distinguish between a good part and a bad part.”

Every decade has its buzzwords. In the ’30s, it was “streamlining;” in the ’50s, anything “atomic” was cool (except perhaps The Bomb). The ’80s were all about “turbo”, and ’90s were the Digital Decade. For many drag racers, the buzzword for the 21st century is “CNC,” an acronym for Computer Numerical Control.

Some racers have the mistaken belief that a CNC-machined part is superior because it is made by an infallible computer. When you’re spending money on race car parts, it’s important to understand both the promise and the pitfalls of CNC machining.

It seems I can’t pick up a racing newspaper or magazine without seeing “CNC” in advertisements and articles. Everything from motor mounts and throttle linkages to cylinder heads and engine blocks are touted as “CNC machined.” It’s become a stamp of approval. Some racers mistakenly assume that anything that’s CNC machined has to be perfect and therefore is a surefire performance enhancement. Unfortunately, that’s not true.

I’m certainly not an expert on CNC programming. About all I can do is hit the red “STOP” button. But I’ve been around these machines for decades, and I understand their advantages and limitations.

I’m an advocate of CNC machining. Like affordable flow benches and dynamometers, the advent of reasonably priced CNC machines has had a positive impact on racing by making advanced technology readily available. We installed our first CNC machining center at Reher-Morrison Racing Engines in 1988, and currently there are three CNC machines in residence at our shop. CNC machines are excellent “employees” – they don’t call in sick, they’ll work 24 hours a day without a break, and they don’t need medical insurance.

On the other hand, a CNC machine is incredibly stupid. It will do exactly what it’s programmed to do – which is not always what you intend it to do. It will cut right through a port wall or drill a bolt hole in the middle of a combustion chamber if there’s an error in the programming. That’s why we use a chunk of foam instead of an expensive new head casting when we’re testing a new program.

More importantly, a CNC machine can’t distinguish between a good part and a bad part. It doesn’t know whether a cylinder head port is efficient or a combustion chamber is shaped properly. It’s not capable of doing research and development, testing a part on a dyno or running it down a race track to determine how well it works. All it does is machine metal following a prescribed tool path.

Computer programmers have a name for this phenomenon: GIGO, which translates as “Garbage In, Garbage Out.” A CNC machine is just a big, fast tool, without intelligence or reasoning power. A human operator can recognize that it’s a bad thing to cut a cylinder head in half; a CNC machine can’t.

There are many types of CNC machines, from vertical mills and lathes to camshaft grinders and tubing benders. Not all CNC machines are created equal. There are giant CNC machines with 120-foot long gantries that can machine a spar for an aircraft wing from a solid piece of metal. There are CNC machines the size of a toaster oven that make precision subminiature parts. The point is that you wouldn’t machine a cylinder head on a desktop CNC, and you couldn’t economically machine header flanges on CNC mill that’s designed to make landing struts for F16s. Like any machine tool, a CNC machining center must be rigid and powerful enough to do its job, but not so massive and expensive that it can’t be operated profitably. As with any complex device, a CNC machine and its tooling must be serviced and maintained religiously.

There are several types of CNC machining centers. A three-axis machine is like an automated Bridgeport vertical mill: it machines side-to-side (X axis), forward and backward (Y axis) and up and down (Z axis). A four-axis machine adds a rotary feature, and a five-axis CNC has the ability to tilt the machining head. A four-axis CNC works well for machining relatively simple parts like exhaust port plates and carburetor spacers; a fifth axis is a necessity to machine complex shapes like cylinder head ports. We currently have one four-axis CNC machine and a pair of five-axis CNCs in our shop.

A CNC machine has the ability to make dozens, hundreds or thousands of identical parts. Unfortunately, if the original design is flawed, it repeats the same mistake over and over again with blinding speed. For example, I’ve seen CNC-machined connecting rods that have exposed bolt threads in the forks; these threads are an open invitation to stress risers and potential catastrophic failure. The CNC machine did exactly what it was programmed to do: it left exposed threads all the way through the bolt holes. Just because these rods were manufactured on a CNC machine didn’t make them right.

It’s a relatively simple process to digitize a cylinder head port and then duplicate the runner shape with a CNC machine. But if the original port shape isn’t good, the result is a series of identical and equally inferior clones. I’ve seen CNC-machined cylinder ports runners with sharp intersections and nasty tool marks. The ports may have been machined under computer control, but an experienced head porter with a hand grinder could have done a better job.

What’s important is not whether a part is CNC machined, but how well it performs. Whether it’s a washing machine, a ratchet or a competition cylinder head, the best buy is always the product with the highest value, not the cheapest price.