Essential Knowledge on CNC Machining Tolerances

Leon Huang: Rapid Direct Author

Essential Knowledge on CNC Machining Tolerances

Learn more about tolerances when ordering parts from a CNC machining services.

If you are new to manufacturing, the word tolerance might not mean much beyond its original definition, synonymous with endurance and open-mindedness. But in the world of milling and lathes and CNC Machining service, tolerance takes on a whole other meaning.

cnc machining service tolerances

What are tolerances?

In short, tolerances are measurements that signify the level of precision needed for a part that is being manufactured. Specifically, tolerances indicate the degree of variation permitted in a part’s final dimensions or measured value.

Tolerances are measured by numerical values, typically preceded with a ± symbol. For example, a part measuring 2.550 inches in length might be accompanied by a tolerance of ±0.001”. This would indicate that the manufactured part would have a variable length measuring between 2.549” and 2.551”. If a part measuring 1.5 inches in height needs a tolerance of ±0.005”, the final part should fall into the range of 1.495” and 1.505” in order to pass quality inspection. You get the idea.

Specified tolerances tell manufacturers and CNC machining services what degree of precision to use when producing a part. The smaller the tolerance—in the manufacturing world this is called a tighter tolerance—the more precision is required. The larger the tolerance—also called looser—the less precision is needed.

Tolerances directly influence the quality of a part, but also its cost. Tighter tolerances are typically more expensive to produce, since parts must be machined more slowly and often with more expensive tools. Parts with tighter tolerances also require more extensive quality inspection, since the window of error is much smaller—we’re talking fractions of a millimeter. Another factor that can influence cost with tight tolerances is failure rate, which tends to be higher.Tighter tolerances can drive up production costs and quality inspection costs.

cnc machining drawing tolerances

How to find the right tolerance?

One of the key things in determining the right tolerance is figuring out at what point a part’s function and performance will be affected by a variation in dimension or size. Some types of parts, like critical components in engines, require a high degree of precision and any lapse in geometric form could have serious consequences. Other types of parts might not require the same degree of precision, and would not be affected by a slightly larger manufacturing deviation.

For product or part designers, it is imperative to determine how much leeway exists for a given part’s tolerances: it can have a big influence on the turnaround time for a project as well as on its cost. A customer working with a CNC machining services could unwittingly spend double what it needs to by asking for the “highest quality possible.” CNC machining is by nature a high precision process, so looser tolerances are usually still very close to the original specifications.

A suitable tolerance can be found in several ways. For one, customers should find a CNC machining services they trust. They can then discuss their goals and specifications with a manufacturing expert, and decide on the best tolerances for their project. In most cases, however, engineers or part designers will specify the tolerances before submitting a manufacturing request to a CNC machining services or rapid prototyping company. Having this information ready to go for your CNC machining services can save time and costs on the production side.

Tolerance measurement tooling
R&D Tolerance meaturement tooling

It is also important to bear in mind that if you do not ask about or specify any tolerances when submitting a part for production, most CNC machining services will automatically work with their standard tolerance, usually about ±0.005 inches (±0.127 mm). This is a very small deviation—imperceptible to the naked eye—but it could influence how your final part fits into an assembly. For instance, if a part has a hole that is too small, even by 0.005 inches, fitting its adjoining part inside the hole can be more challenging than necessary.

When are tight tolerances really needed?

This brings us to our next point. Often, tolerances can vary within a single component. While it may not be critical that the entire structure of a part adhere to the tightest tolerances, certain features of said part might require them. The highest precision is typically required at structural points where a part must fit or join with other components, such as holes.

If a part needs to fit with one or many components, it must have tight tolerances. In these cases, tolerances should be communicated to the CNC machining services, otherwise they will not have any indication of design intent. In other words, tolerances give your CNC machining services partner an indication of how your part needs to function.

It is also worth noting that the tightest tolerances can be more costly because of the wear caused by CNC machine tools. While a fresh tool might not have a problem achieving tolerances of ±0.001”, if the same tool has to produce multiple versions of the same part, this precision can become less consistent. If you order 1,500 units of a part at ±0.001” tolerance, your CNC machining services may have to replace the machining tool or adjust the machining speed, which can increase production time and cost.

RapidDirect, your professional CNC machining services partner.

At RapidDirect, we offer a range of tolerances to suit our customers’ needs. Our standard tolerances for CNC machining metals are compliant with DIN-2768-1-fine, while for plastics it is DIN-2768-1-medium, a full chart of these tolerances can be found here. A standard tolerance for a CNC machining services is typically ± 0.005”. The tightest tolerances possible are in the range of ±0.001”, roughly the width of a human hair.

R&D quality testing department
R&D Quality Testing Department

Our CNC machining services comprises many state-of-the-art systems, which enable us to meet our customers’ production demands and fulfill both low-volume and medium-volume production volumes. Among our many systems are five HERMLE 5-axis CNC machines, some of the top CNC machines on the market. Made by Germany-based HERMLE, the 5-axis CNC systems are capable of manufacturing parts with extreme agility, leveraging all five axes (X, Y, Z and A, B). This means the system can access the part from all directions and angles to achieve fast, accurate and highly detailed component production. RapidDirect’s extensive manufacturing facility in Shenzhen, China also hosts a number of 3-axis&4-axis&6-axis CNC machines, as well as auxiliary equipment like surface finishing solutions.

Hermle 5-axis cnc machine
Hermle 5-axis cnc machine

Our CNC machining services also covers quality inspection, including certified ISO 9001 QMS standards, material certifications and full-dimensional inspections with report. The full-dimensional inspection process, which relies on cutting edge metrology and measurement tools, is critical to ensure that prototypes or final parts meet the exact specifications and tolerances requested by the customer. In other words, we’ll check and double check that your specified tolerances are met, and if they’re not, we’ll work with you to deliver a part that meets your requirements.

If you have any questions about part tolerances for CNC machining or want to learn more about RapidDirect’s CNC machining services, please don’t hesitate to get in touch with a member of our manufacturing team.

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