We pride ourselves on offering precision machining services. At the same time, we know not everyone has the same understanding of what this means. Rather like the word, “accuracy”, “precision” is a surprisingly ambiguous term, so here we’ll explain how we go about undertaking CNC precision machining.

Components of Precision Machining

To understand what precision machining is, you need to know the various terms used to define measurement performance. You should also know what goes into achieving high levels of precision. We’ll address these points under two headings:

• Terminologies relating to measurement
• Achieving precision machining

Terminologies Relating to Measurement

We’re sometimes asked about the accuracy we can achieve in CNC machining. The problem with this question is that accuracy means distance from, or closeness to, the target dimension. We might get close once, but can we repeat the performance on every part we make? This is why repeatability is a more important term.

Think of repeatability as the amount of scatter in measurements of the same feature on a part. If we measure a diameter on 100 pieces, and the max to min variation is only 0.0002”, that’s a repeatable performance.

Customers in some industries are very interested in repeatability. They follow GR&R procedures, short for Gauge Repeatability and Reproducibility. Repeatability refers to the parts coming off one machine. Reproducibility means can we get the same result when the parts come off other machines?

Two other precision machining terms to know are resolution and calibration. Resolution is essentially the number of decimal places the measurement device reads to. Calipers typically read to 0.001” while a micrometer reads to 0.0005” or better, meaning the micrometer has higher resolution.

Resolution also ties into the dimensions and tolerances on the part prints. If a feature is toleranced to two decimal places it means no higher precision is required. Tolerance it to four decimal places and the measurement device must have at least the same precision. (In practice, we like the measurement device to have an order of magnitude higher resolution than the dimension being measured.)

Calibration is a very important part of precision. This relates to knowing the uncertainty in the measuring devices used. A measuring device may have high resolution, but it can’t be said to be accurate unless it has been calibrated and the error or uncertainty established.

The quality system certifications we hold, (ISO 9001, ISO/IATF 16949, ISO14001, and AS 9100) require that we operate effective calibration systems, and we are audited to verify we do what we say we will.

Achieving Precision Machining

Measurement capabilities are part of delivering quality precision machined parts, but arguably, the machinery used is more important.

This starts with the use of computer numerical control (CNC) machine tools. These have motorized axes that rotate and move tools and workpieces. Each axis has a high resolution encoder that reports position to the computer controlling the motors.

The movements needed to perform the required metal removal operations are programmed into the computer using CAM (computer aided manufacturing) software. This takes the part dimensions from the CAD geometry and generates the cutter paths, feeds and speeds needed.

Achievable precision depends on the type of machining operation. Milling and turning are less precise than grinding, honing and lapping. This is a function of the metal removal process. As a rule, processes with lower metal removal rates are more precise.

Precision is maximized by ensuring machines are in good condition, and where possible, by performing a combination of machining steps in a single setup.

Machine condition is a function of maintenance, age and design. While it’s important to carry out all required maintenance, machines are subjected to high loads and are worked hard. At Impro machine tools are replaced before wear and tear start to affect the precision achievable.

Combining operations in a single setup prevents the variation that occurs when parts are moved through a number of machines. This is one reason for using multi-spindle machines and milling and turning centers.

Understanding Precision Machine Shop capabilities

Answering questions about achievable accuracy requires long, complicated explanations. Here, we’ve explained a few of the most commonly used terms relating to accuracy and precision. We’ve also provided an overview of what’s involved in achieving high precision CNC machining. If you’d like to know more about what we can do, contact us for a discussion.