Is Zero-Tolerance Machining Possible?
Zero-Tolerance Machining is a marketing term that describes extremely tight tolerances, such as +/- 0.0005mm. Machining parts to a tolerance of absolute zero is impossible: even a molecular difference would be rejected if an order specified zero tolerance.
However, a tight tolerance to a fraction of a millimetre is achievable with CNC machining services. Technically we should refer to these as Near Zero Tolerance. These strict tolerances could be beneficial for components that are very small or need to fit in specific machinery such as medical implants.
What are CNC Machine Tolerances?
In Machining, tolerance specifies the accuracy a part requires. They show the amount of variation that is allowed for specific dimensions of a machined part.
Tolerances are stipulated in quality control documents to ensure parts that vary too much are not accepted.
Why does CNC Machine Tolerance matter?
If parts are too wide or too narrow, or grooves are too deep or too shallow, for example, then finished parts won’t fit together as intended.
However, no machine is perfect, even our CNC sliding head lathes. There is no true zero-tolerance machining because there is always some variation.
While CNC turning services are extremely accurate, no two parts will be truly identical, even on the same machine with the same program. We account for this by setting a standard tolerance limit.
Standard tolerances allow us to create parts as close to identical as possible. For extremely tight tolerances, such as in small turned parts, it’s crucial to use well-maintained machines and quality materials.
A CNC lathe tolerance is usually set at around +/-0.127mm, slightly thicker than a hair. For most parts, such a tiny variation in dimensions will have no impact.
+/- what?
To explain how accurate machines are, tolerances are usually communicated as follows:
+/- 2mm
This phrasing means: the manufacturer will cut to the required line, up to plus or minus 2mm. 2.1mm in either direction will not be accepted, but 1.8mm will be.
How are “Zero Tolerance” cubes made?
Products marketed as Zero Tolerance, such as the zero-tolerance desk toys seen on social media in recent years are, of course, not entirely perfect cuts. They are what is know as ‘slip fit’: you can pull the parts apart with your hands. This means there is a gap between the parts of 0.0005mm. The join disappears when the parts are put together because the side has been machined as one, and the edges are left sharp, not bevelled. This is not particularly useful for machine components, but it does look impressive.
Does every part need a near-zero tolerance?
It’s not sustainable to attempt near-zero tolerances for every part. For large turned parts, such as axles, bolts, shafts or gears, even a variation of 10mm (+/-5mm) may not make a difference to their functioning in the whole system.
Larger parts can be made with larger tools, which remove more material in less time. Trying to achieve a near-zero machine tolerance for a 4m shaft will require a small tool, and take too long to be worthwhile.
However, when you’re machining a 6mm bolt to fit into a 7mm hole, your tolerance needs to be below +/-0.5mm. Precision turning with advanced CNC machines can achieve these tight tolerances.
Engineers favour Wire EDM machines for precision measured in microns and nanometers, but this is unnecessary and expensive for most parts.
Precise machining is beneficial for:
- Aerospace engine & navigation components
- Medical implants & surgical instruments
- Semiconductor components for microchips
- Automotive parts in high-performance engines
Balance Precision with Speed and Affordability
The achievement of 1 micron tolerances is impressive, and many engineering universities have been working to achieve even more precise cuts in metal.
For most jobs, there is still huge value in +/-1mm as a precise enough tolerance. This is easily achievable with standard tooling, making it affordable and quicker than attempting unnecessary uniformity.
Turned parts manufacturers set tolerances for each job, depending on:
- Machine capabilities
- Customer requirements
- Material properties
We offer precision machining for turned parts to tight tolerances. With over 30 CNC lathes on site, our lead times are unmatched, and we can often offer large-batch machining within days of receiving the order.
