What are CNC Machining Tolerances? Accuracy vs Repeatability: What Buyers Should Know

Have you ever looked at two CNC machine spec sheets side by side and wondered whether to trust the
accuracy number or the repeatability number — and whether they even mean the same thing? They do
not, and confusing the two is one of the most common mistakes buyers make when evaluating machines.

Both specifications appear on almost every datasheet, yet they describe fundamentally different
aspects of performance. Understanding the distinction will help you cut through the marketing
language and choose a machine that actually delivers consistent results on the shop floor.

What is CNC Machine Accuracy?

Accuracy describes how closely the machine reaches the commanded position. If you instruct the
machine to move to X = 100mm, accuracy tells you how near the tool actually lands to that exact
coordinate.

Accuracy is typically expressed as a ± value — for example, ±0.05mm or ±50 microns. A machine with
higher accuracy places the tool closer to the intended target position.

What is Repeatability?

Repeatability — sometimes called precision — describes the machine’s ability to return to the same
position consistently across multiple attempts, regardless of where that position falls relative
to the true target.

If you command the machine to go to X = 100mm ten times, repeatability measures how tightly those
ten landing points cluster together. Even a machine that is not perfectly accurate can have
excellent repeatability if it lands in nearly the same spot every single time.

Key Difference Between Accuracy and Repeatability

• Accuracy = How close you get to the target position (systematic error)
• Repeatability = How consistently you return to the same position (random error)

Think of it like target shooting: accuracy is hitting the bullseye, while repeatability is grouping
your shots tightly together. A machine can group its positions perfectly yet always land 0.04mm
off-centre — great repeatability, poor accuracy. Another machine might average out close to the
target but scatter its positions unpredictably — decent accuracy, poor repeatability.

Why Repeatability Often Matters More

For most practical machining work — batch runs, multi-pass operations, fixture-based production —
repeatability has a more direct impact on finished part quality than absolute accuracy. Here is
why:

• Consistent inaccuracy can be corrected. A machine that always lands 0.03mm off can be dialled
  in with calibration or offset values. Random drift cannot be compensated reliably.
• Poor repeatability produces variation in finished parts, which is costly and difficult to
  manage at the inspection stage.
• Most machining workflows depend on the machine returning to the same position reliably — tool
  changes, multi-pass cuts, and fixture alignment all require this.

What Buyers Should Focus On

• Insist that the spec sheet lists both positioning accuracy and repeatability as separate,
  clearly defined figures — not a single vague “precision” claim.
• Pay particular attention to bi-directional repeatability, as it reflects real-world
  performance better than uni-directional figures, which can hide backlash and mechanical play.
• Evaluate overall build quality and structural rigidity, as these are the underlying factors
  that determine long-term repeatability under load and temperature changes.
• Ask for actual test data from a physical machine rather than design-intent figures from a
  datasheet.
• Remember that real-world performance also depends on proper setup, tooling condition, and
  ongoing maintenance — no specification number accounts for those variables.
• Akriti Precision Systems CNC machines are all certified for

Final Thoughts

While both accuracy and repeatability are important CNC machine specifications,
repeatability generally has a greater impact on consistent part quality and reliable production.
When buying a CNC machine, prioritise machines that demonstrate strong repeatability and solid
overall build quality over those that simply quote impressive accuracy numbers.

Understanding this distinction means you can evaluate machines on what actually drives results on
the shop floor — not just what looks good on a datasheet.

Akriti
Precision Systems
manufactures desktop and benchtop CNC machines in India with a focus on real-world
performance and reliability. Our machines are designed to deliver consistent repeatability and
dependable machining quality, helping workshops and MSMEs produce reliable results run after run.
When evaluating CNC machines, we encourage buyers to look closely at both accuracy and
repeatability alongside overall build quality and after-sales support.