Promoted content: The Intolerance of Tolerance in Manufacturing Design and CNC Machining

4 min read

Faulty products.


When does right become wrong? How do complicated projects get off the design board and break ground? The answer is having faith in tolerance because its importance cannot be underestimated.

Understanding how to achieve precision tolerance during the manufacturing process is crucial in succeeding in the supply chain. It certainly helps to have the right CNC Services in the UK on your side. There are no ifs and buts in tolerance, it either conforms, or it doesn’t.

Why Is Tolerance Important?

Without tolerance, parts wouldn’t fit together, dimensions would vary from one manufacturer to another, and with so much inconsistency, engineered products and even entire buildings wouldn’t function as designed. Any engineer or designer passing on the responsibility for tolerance parameters does so at significant risk to the success or suitability of the end product (and their reputation). Everyone knows that tolerance is ultimately one of the most critical factors in the journey from the drawing board to the finished product. 

But to what degree is tolerance essential? Because there are many types used in varying environments and different materials. For instance, building a gateway over a road at Salford University that incorporates a fully clad wall isn’t the same as building a bridge, as Simon Atkinson, BAM’s (steelwork) project manager, explains: “Bridges are designed to be flexible, [but] when you put glazing on a bridge it becomes problematic. There is a much lower level of permissible deflection.”

Factors Affecting Tolerance Measurement

Tolerance needs to be understood in terms of thickness, straightness, twist, and a part’s mass. Then there is the tolerance that recognises the effects of temperature and humidity on a product. For most designers, there are three key considerations to address:

Additional coatings – Machined parts can also have powder coating, anodising, polishing, electroplating, galvanising and heat-treating processes, all of which add microlayers,  increasing exterior measurements.

Thermal expansion is when material changes its volume in response to temperature alterations, typical in soft metals and composite material.

Welding process – High temperatures are involved in welding, which can affect the material. What’s more, welding isn’t as precise as CNC machine services, so tolerances cannot be measured to the same precise margins.

The capability to produce parts to tight tolerances and even to extreme precision tolerances on CNC Machines like those available with Geomiq can become unilaterally critical. Mainly when designers and engineers impart both necessary and unnecessary narrow tolerances or when other factors affecting them are in play.

CNC Services Achieving Precision Tolerance

In practice, several minor considerations can affect manufacturing tolerances using cutting, milling and drilling tools on a CNC Machine. The primary concern for a manufacturer is overcoming tool deflection, by which we mean the sharpness of the machine tooling and how it can affect the material’s edge geometry. Which is the very point at which the part is going to be measured for tolerance adherence. Ensuring they are accurate through testing and dummy runs is generally good practice.

It helps if the tolerance the manufacturer applies to their product has little margin for error. At Geomiq, the standard tolerance is +/-0.127mm, but precision tolerance comes in at +/-0.051mm for all CNC services, and it is possible to achieve accurate precision at +/-0.005mm.

Perhaps the most essential characteristic of a CNC machine is its capability to return the exact measurement repeatedly. Ensuring a CNC Milling tool is highly accurate should be its primary focus. For milling a piece of metal that needs to be 250mm long with radiused grooves at 25.7mm intervals, the CNC machine’s tooling needs to cut the piece exactly. If it is accurate, it will be closer to 25.7mm every time, without any errors.

For parts that don’t need to be milled, CNC Turning can be used to produce tubes, pipes, or anything cylindrical in shape. The accuracy of a turning often means parts have to marry up to or match with or slide into other components. The lathe it is made on, and the tooling used need to be precise, often achieving precision tolerance as a minimum for it to suit the purpose.

When Tolerance Goes Wrong

Ambiguity, though, is never good. Leaving out tolerances means no one else further down the supply chain will understand its importance (or non-importance). Get it wrong, and it can cause a chain reaction to the end-user culminating in what can only be described as one of the four worst scenarios a supplier can experience:

  • Faulty products
  • Remanufacturing costs
  • Delays to customer
  • Reputation damage
  • Redemption costs applied

Tolerances allow measurements to be accurate when it comes to Quality Control. When multiple materials come together to produce component parts, tolerance, measurement and specification all play their role in determining conformance.

Tolerances allow measurements to be accurate when it comes to Quality Control. When multiple materials come together to produce component parts, tolerance, measurement and specification all play their role in determining conformance.

  1. The true value of the part is within specification limits, and the measured result is also within conformance limits. This is a true negative as the test for non-conformance would result in a negative (Conforming).
  2. The true value of a part is outside specification limits, but the measurement is within conformance limits. This result is a false negative (Conforming/Non-conforming).
  3. The true value of a part is outside specification limits, and the measurement result is also outside conformance limits. This is a true positive (Non-conforming)
  4. The true value of the part is within specification limits, but the measurement is outside conformance limits. This is a false positive (Non-conforming).

When manufacturing tolerances are missed, it makes conformance of product almost impossible to achieve.

Tolerance as an Innovative Design Feature

For the built environment, the prefabricated or modular building has become an increasingly popular way of minimising construction costs for new builds in both residential and urban planning. Tolerance is the one consistent feature of modular construction when multiple different parts from different sources all arrive together on site to become a whole.

One award-winning design was the 43-storey Atira Student Living tower, built-in central Melbourne for RMIT University. Because of its unique design, it is one of the tallest modular buildings in the world. Its Chief Engineer Shan Khan was clear about the foundation for its success: “Tolerance was a key consideration of the building, and it was the most innovative feature”.

Accuracy, precision and tolerance used in machine processes like CNC turning, drilling, milling and cutting allow projects to be manufactured as planned and designed. Get it right, and projects flow smoothly and without costly delays. Get it wrong, and it can have an unfortunate consequence on reputation and the bottom line. Tolerance is a guiding principle that makes so many project stakeholders reach a goal and makes complicated building projects possible.