The concepts of precision and repeatability may seem similar, but they have different meanings and implications, especially in a machine shop environment. Precision refers to the ability to hit a particular tolerance or dimension, it is a measure of the exactness of a process. Repeatability, on the other hand, is about the ability to consistently hit the same tolerance or dimension, it measures the variability of the process.
The Tolerance Floor Is Dropping
10 years ago, people were impressed if you could hold ±0.001″. Today, that’s just standard practice. “We are seeing increased part diversity in the production market and the ‘batch of one’ \[customized part production\] trend is exposing smaller machine shops,” says Bunty Bhullar, Managing Director of CGTech (North America), the software developer that creates VERICUT CNC machine simulation and optimization software. “These machines are often not pristine by any means, and they are being asked to make increasingly precise parts.”
What Machines Actually Need To Hold Tolerance Over Time
The long-term repeatability of a CNC machine is far more dependent on its mechanical architecture than its control system or software. It all starts with the frame. Heavy cast-iron construction absorbs vibration and maintains alignment over the years. Linear guideways and ball screws are equally important. High-precision ball screws transfer motion with minimal wear. Lesser ball screws will develop play over time as the components wear and the fit between the ball screw shaft and ball nut degrades.
Lesser linear guideways are similarly prone to wear, and some early CNC machines, with dovetail slides instead of linear guideways, ultimately wear out of tolerance through the tendency of the gibs that adjust the clearance on this type of slide to loosen over time. More premium machines utilize components that are held to higher tolerances, plus thermal compensation systems to offset changes in temperature, to hold their high level of positional accuracy for longer before maintenance is necessary.
Why Used Premium Machinery Beats New Budget Machinery
The decision to buy often contradicts common sense. If money is tight, many shop owners default to buying the cheapest new machine out there. We get it – it’s new, it’s under warranty, and the control is modern. But the fact is, a cheap new machine is built to a price point, not a level of performance. You get thinner castings, lower quality spindle bearings, and mechanical components that deviate out of tolerance faster than you’d like.
A well-maintained premium machine – let’s say a Mazak – is built on an entirely different foundation. The mass, rigidity, and precision are engineered into the DNA of the machine and designed to hold tight for years. Those qualities don’t go away when the original owner buys something new. An 8,000-hour Mazak that’s been properly serviced, lubricated, and test cut for accuracy can outperform a budget new machine for a decade or more.
For a shop that couldn’t write a check for six figures to get a new top-tier machine tool, a used mazak makes it affordable to access that kind of mechanical quality. It’s not about compromise. It’s about bidding on the work that calls for that kind of precision in the aerospace and medical-device supply chain. It’s about competing for the jobs you’d have to walk away from, otherwise.
The Mazatrol controller that comes with most of them is also a known quantity. The software is constantly updated and easy for new operators to learn. The built-in CAM functionalities and conversational programming cut setup times on complex jobs – an important factor if you’re bidding against shops who have more programming horsepower on staff.
The Financial Cost Of Getting This Wrong
The statistics provided are alarming. According to the American Society for Quality, expenses related to quality, including scrap, rework, and warranty claims resulting from out-of-tolerance components, could reach 15% to 20% of an entire manufacturing firm’s revenue from sales. That is not a negligible amount. It’s the determining factor in a shop’s profitability.
A Coordinate Measuring Machine has the ability to confirm that your parts are defective. Yet, it cannot repair them. And when you’re producing high-mix, low-volume work against the clock, rework stage creates an immediate impact on your profits. The machine that’s making the part is where you have to apply the pressure – everything that comes after that is about limiting the losses.
Matching Machine Capability To Contract Requirements
Before you choose to bid on a part with GD&T that includes tight positional tolerances or runout, the question is whether your machine can actually hold those numbers at production volume – not in a test cut, but over the full run.
If you’re not sure, that uncertainty carries a price. You either pad the quote a bit extra to account for the scrap you think you might make, or you win the job at a price that assumes everything will go right, which is a race to the bottom.
Shops that win and keep high-value contracts are the ones that match machine capability to part requirements before the contract is signed, not after. That means knowing what your equipment can actually deliver, and then making choices about where you put that machine time and talent when the answer is not good enough.
