Posted May 13, 2021

How Accurate Is CNC Milling?

Robotic surgery equipment requires accurate CNC milling to hit tight tolerances.

The quality of CNC-machined parts depends on many factors, including part design, tool selection, toolpath programming, machinist skill set, workholding strategy, and more. Parts in critical-path functions, such as robotic arms used for laser surgery or landing gear components in high-performance aircraft, must perform reliably. Reliability requires accuracy, precision, and the ability to hold tight tolerances on the part of the CNC machine shop manufacturing the part.

When determining which machine shop to use for critical components, it is important to ask two questions: how accurate is CNC milling, and what’s the machine shop’s commitment to delivering that accuracy? A closer examination of what constitutes accuracy in CNC milling will help design engineers make the best choice in machine shops when getting their parts built. 

How CNC Machine Accuracy Impacts Different Industries

Although precision in machined parts is essential in every industry, there are some industries where the need for accuracy is more critical than others:

  • Medical: The equipment used for medical procedures, such as robotic arms for laser surgery, relies on precisely machined micro and macro-sized parts.
  • Aerospace: Aerospace components must adhere to the tightest tolerances for smooth operation—regardless of the environment—to ensure the aircraft’s safe operation.
  • Transportation: Buses, trucks, trains, and other vehicles require a wide assortment of shafts, couplings, pistons, and other parts working seamlessly together.
  • Energy: Whether it’s gas and oil, wind power, or other sources, the industrial equipment used to generate and manage the energy requires precisely machined parts to hold up under tremendous stress.

These constitute only a few examples of the industries requiring extreme precision in the fabrication of their machined parts. However, many others may not need as stringent of tolerances, but manufacturers must build parts the same way every time. Repeatability in precision parts is crucial for everything from consumer-packaged goods to essential replacement parts.

It is important to note that CNC milling accuracy is also critically important to the machine shop itself. Maintaining accuracy reduces manufacturing time and expenses for the shop and eliminates the need to redo incorrectly fabricated projects. The following section outlines the different components of accuracy in CNC milling, specifically.

How Accurate is CNC Milling: Three Measurements

Three different measurements determine a milled part’s overall accuracy: positional accuracy, precision or repeatability, and tolerance.

Positional Accuracy of the Mill

Positional accuracy equals the difference between the specified distance between points on a part and the actual measured distance of those points once milled. For a CNC mill, accuracy depends on how well it can follow its programmed paths. Machine shops decide a mill’s accuracy by taking multiple measurements and calculating the deviations’ statistical average value. 

Repeatable Precision 

Machine shops define the precision of a mill by how well it can precisely repeat the same commands, or Gcode, in order to deliver the same results part after part. Similar to accuracy, shops can calculate a CNC mill's precision with multiple measurements when commanding the machine to cut a path. The measurement of precision lies in the repeatability of cutting the same precise path, ultimately producing exact duplicates of parts over and over.

Tolerance of the Finished Part

Machinists and engineers use the term tolerance when referring to the amount of allowable deviation from the set value of measurement for a CNC mill. CNC programmers and machinists program the mill to manufacture a part within a client’s desired tolerance range. The mill should stay within those set tolerances from the specified measurement while cutting away material and producing the part.

The goal for a CNC mill is for it to stay within its specified tolerances by repeatedly maintaining high positional accuracy. For example, imagine shooting arrows at a target. How close the arrows get to the bullseye measures accuracy. How close the arrows find themselves grouped equals repeatable precision. Repeatability without accuracy when machining a batch of parts will not yield any usable results as the parts will all be equally out of tolerance. Alternatively, accuracy without repeatability will result in only one or two usable parts. Both accuracy and repeatability combined will produce an entire batch of sound products. A machine shop leverages this precision to ensure your parts’ quality.

How to Determine the Accuracy of Your CNC Machine Shop

To maintain the accuracy and precision necessary for milling parts, a good machine shop will prioritize the following in its processes and procedures:

  • Working environment: Accuracy depends on a working environment that provides stable conditions such as temperature and humidity.
  • Equipment condition: CNC milling precision requires consistent preventative maintenance to keep all the equipment in perfect running order.
  • Machine calibration: Machine shops must recalibrate all precision milling equipment at regular intervals to hold their factory settings.
  • Tool monitoring: Shops must also monitor cutting tools—which will wear down with time—for maintenance or replacement.
  • Workholding: To minimize vibration and guarantee precision, shops must use proper workholding setups to secure workpieces in place for machining.
  • Machinist Experience: Machinists with more experience are more likely to understand the subtle nuances that lead to the manufacture of accurate parts.
  • Programming: Skilled manufacturing engineers understand how to use different tools, with different approaches and speeds, to efficiently manufacture accurate cnc milled parts.

A high-quality machine shop should have a documented process for first article inspections whenever setting a part up on a mill. It will also have regular inspection points throughout manufacturing for quality control. As a customer, you can ask for these inspection reports to follow up on a project’s progress, and you should review the machine shop’s quality policies.

Plethora can help you answer the question: how accurate is CNC milling? We have our quality processes published, and you can review them by clicking here. Our standard tolerances for metals is ± 0.003″ (± 0.08mm), and for plastics is ± 0.005″ (± 0.13mm). Not only do we maintain and calibrate our equipment to ensure its precision, but we go the extra step to verify the CAD data from the drawings and specifications that you provide. As an ISO 9001 certified manufacturer, our goal is to provide the highest quality machining in all our work with accurate and precise CNC milling. Our online DFM and quoting systems are ready to receive your data at any time so we can immediately begin working with you on your next project. To get started, upload your design files to Quote My Part or call us at 415-726-2256.
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Topics: materials, processes, Manufacturing, finishing, CNC machining, Quality, Prototyping, threading, Custom Milling