Hey, I’m Barry Zeng. I’ve been in manufacturing for 12 years at Shanghai Yunyan Prototype & Mould Manufacture Factory, and if there’s one question I’ve never stopped thinking about, it’s this: what is precision actually worth? I’ve seen clients pay triple for tolerances they didn’t need. Conversely, I’ve also seen clients save a few hundred dollars on a part that failed in the field and cost them tens of thousands. The difference between those two outcomes wasn’t the machine, the material, or the drawing. Instead, it was understanding where precision matters and where it doesn’t.

CNC machined parts are the most versatile precision components in modern manufacturing, but “precision” isn’t a single thing. Rather, it’s a spectrum. Consequently, knowing where your part sits on that spectrum is one of the most valuable skills you can develop. In this article, I want to walk you through what precision actually means in real terms, where it pays off, and where it’s a waste of money. No numbered lists. Just real stories from my shop floor. Grab a coffee, and let’s get into it.

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Exploring CNC Machined Parts: The Value of Precision

Let me start with a story that I still think about. A few years ago, a client sent me two drawings for essentially the same part — a mounting plate. One version had ±0.01 mm tolerances on every dimension. The other had ±0.1 mm tolerances, except for two features that needed to mate with dowel pins. Those two features were specified at ±0.01 mm.

I quoted both. The first version came in at $48 per part, whereas the second version came in at $31 per part. Same material, same quantity, same machine. The only difference was where the precision was applied.

That’s the value of precision in a nutshell. Precision is expensive, but it’s also targeted. When you apply it where it matters and relax it where it doesn’t, you get a part that works and a price that makes sense. However, when you apply it everywhere, you pay for precision you’ll never use. CNC machined parts give you the ability to make that choice — feature by feature.

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What Precision Actually Means in Real Terms

When people say “precision,” they usually mean “tight tolerances.” However, precision in machining is really three separate things, and they don’t always come together.

The first element is dimensional accuracy — specifically, how close a feature is to its nominal size. For example, if a bore is supposed to be 20.00 mm and it comes out at 20.01 mm, the dimensional error is 0.01 mm.

The second element is geometric tolerance — namely, how close a feature is to its ideal shape and position. For instance, a bore can be the right diameter but not perfectly round. Similarly, a surface can be flat on average but wavy. A hole can also be the right size but in the wrong location. Therefore, geometric tolerances cover roundness, flatness, parallelism, perpendicularity, concentricity, and position.

The third element is surface finish — essentially, how smooth or rough the surface is. A part can be dimensionally perfect, but it might still have a surface finish that causes leaks, friction, or fatigue failure.

When you specify a tight tolerance on a CNC machined part, you need to be clear about which of these three you actually need. For example, a sealing surface needs a smooth surface finish and roundness. A bearing journal needs dimensional accuracy and concentricity. Meanwhile, a mounting hole primarily needs accurate position. If you specify all three on every feature, you’re paying for precision that only matters in a few places.

Where Precision Pays Off: Real Examples

Let me show you where precision actually earns its keep, using real parts I’ve made in my shop.

Sealing surfaces. I made a hydraulic manifold once with a sealing face that had to hold 300 bar of pressure. The surface finish needed to be Ra 0.4 µm or better, and the flatness needed to be within 0.005 mm. If either of those had been off, the seal would have leaked. As a result, that’s precision that pays for itself — because a leak in a 300-bar system isn’t just an inconvenience; it’s a safety hazard.

Bearing journals. On another project, I turned a motor shaft with two bearing journals that needed ±0.005 mm diameter tolerance and 0.01 mm concentricity between them. If the concentricity had been off, the shaft would have vibrated, the bearings would have worn out prematurely, and the motor would have failed. Therefore, that precision was non-negotiable.

Mating features. For instance, I machined a set of aluminum housings once that had to fit together with a dowel-pin alignment system. The dowel holes needed ±0.01 mm position tolerance. If they’d been off, the housings wouldn’t have aligned, and the assembly would have failed. Meanwhile, everything else on the housing could be ±0.1 mm, and it worked fine.

In each of these cases, precision was applied to a specific feature that had a specific function. It wasn’t applied everywhere. That’s the discipline.

Where Precision Is a Waste of Money

Now let me show you the other side. These are the places where tight tolerances are usually wasted.

Clearance holes. A hole that a bolt passes through doesn’t need to be precise. For example, if the bolt is M6 and the hole is 6.5 mm, the bolt has 0.5 mm of clearance. Making that hole 6.50 mm instead of 6.55 mm costs money and accomplishes nothing. I’ve seen drawings with ±0.01 mm on clearance holes. That’s precision that does nothing.

Non-critical cosmetic surfaces. If a surface is visible but doesn’t seal, slide, or mate, it needs a good surface finish but not a tight tolerance. For instance, a brushed aluminum panel looks great at Ra 0.8 µm. Specifying Ra 0.2 µm on that panel adds polishing time and cost without any visible improvement.

Overall dimensions. The overall length of a bracket often doesn’t matter much. As long as the mounting holes are in the right place, the overall length can be ±0.5 mm and everything still works. Therefore, tightening that tolerance is a cost with no benefit.

Non-mating surfaces. If two surfaces don’t touch, don’t seal, and don’t align anything, their dimensional relationship doesn’t matter. In fact, I’ve seen drawings with tight tolerances on features that are purely structural — a flange thickness, a web dimension, a corner radius. Those tolerances are usually unnecessary.

The pattern is simple: precision matters where two things interact. However, it doesn’t matter where nothing interacts. Applying precision everywhere is like tightening every bolt to the same torque — some need it, but most don’t.

What Precision Actually Costs

I want to give you a sense of what you’re actually paying for when you tighten a tolerance on a CNC machined part.

Going from ±0.1 mm to ±0.05 mm usually doesn’t cost much. The machine can hold that easily, and the inspection is straightforward. Consequently, expect maybe a 5–10% increase in price.

Going from ±0.05 mm to ±0.01 mm is a bigger jump. Because the machining becomes slower and the inspection becomes more involved, you might need a temperature-controlled environment for measurement. Therefore, expect a 20–40% increase.

Going from ±0.01 mm to ±0.005 mm is a significant undertaking. The machine needs to be in pristine condition, and the tooling needs to be fresh. Furthermore, the inspection likely needs a CMM, and you might need multiple passes to hit the number. As a result, expect a 50–100% increase, or more.

Finally, if you need something like ±0.002 mm, we are talking about specialized equipment, climate control, and an extraordinary level of care. I’ve done work at that level, and it takes everything I’ve learned in 12 years. It’s certainly not a standard job.

The point is this: every step tighter costs real money. Therefore, applying precision where it isn’t needed is one of the most common and expensive mistakes in manufacturing.

👇 Not Sure Where Precision Matters on Your Part? Let’s Talk.

Send me your CAD file or drawing. I’ll review your tolerances, identify where precision is costing you money, and provide a free DFM report and quote within 24 hours. No robots, no voicemail. Just me and my honest opinions.

P.S. Mention “precision guide” when you email, and I’ll send you my personal tolerance checklist for CNC machined parts. It’s saved my clients thousands. And it’s free. Because I’m nice like that.

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Barry Zeng
Senior Manufacturing Engineer, Shanghai Yunyan Prototype & Mould Manufacture Factory
(12 years of CNC machining experience. I’ve made precision parts for everything from hydraulic pumps to medical devices. I know where precision matters and where it doesn’t.)