CNC Machining Tolerances-What you should know about

CNC machines can achieve very high levels of accuracy. Some machines can achieve accuracies as high as +/-0.0025mm. However, running the milling of a part on a CNC machine’s highest level of accuracy is however an expensive and time-intensive undertaking. This is where machining tolerances come in. Since different parts require different degrees of accuracy, it is economical for a designer to specify the specific tolerance needed for that part.

What Are CNC machining tolerances?

In short, tolerances are measurements that signify the level of precision needed for a part that you want to manufacture. Specifically, machining tolerances indicate the degree of variation permitted in a part’s final dimensions or measured value.

Machinists measure machining tolerances by numerical values, typically preceded with a ± symbol. For example, you may assign a tolerance of ±0.001″ to a part measuring 2.550 inches in length. 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” to pass quality inspection.

Specified machining 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 a tighter tolerance—the more precision required. The larger the tolerance—also called looser—the less precision you need.

Things to Keep in Mind when Choosing Tolerances

One of the key things in determining the right CNC machining tolerances 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 a slightly larger manufacturing deviation will not affect them.  Apart from this, here are a few other things to keep in mind when choosing tolerances:

1. When do You Need Tight Tolerances?

Often, machining 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 the 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, you should communicate the required tolerances 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 machining 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.

2. High Costs for Tight Requirements

Tolerances directly influence the quality of a part, but also its cost. High tolerance machining is typically more expensive to carry out since you have to machine the parts more slowly and often with more expensive tools. Parts with tighter tolerances also require more extensive quality CNC inspection, since the window of error is much smaller—we’re talking fractions of a millimeter. Another factor that can influence cost with high tolerance machining is the failure rate, which tends to be higher. High tolerance machining can drive up production costs and quality inspection costs.

3. Tolerances Depend on the Material

You have to consider your choice of materials when specifying machining tolerances. Different materials have individual characteristics that can affect the level of tolerance that is achievable with them. Some of these characteristics include:

  • Hardness: Softer materials are more difficult to machine to tight tolerances. This is mostly due to their likelihood to change dimensions as the cutting tool touches them. You therefore have to exercise additional patience when machining these softer materials. 
  • Abrasiveness: Rough and coarse materials are usually tough on the cutting tools and can even cause them to wear out faster. These materials make it difficult to achieve specific tolerance because changes in the cutting tool result in less accuracy. Abrasive materials usually require the technician to change out the tool multiple times throughout the machining process. 
  • Heat stability: This is an issue that affects non-metals mostly. As heat builds up during the machining process, these materials start to lose their shape. This restricts what processes you can use on that particular material.

4. Choice of Manufacturing Methods

Each CNC machine differs in terms of capacity. Some machines can achieve tolerances that others can only dream of. The CNC machine tolerance determines what kinds of parts it can handle. In some cases, you will have to carry out additional operations on the part before it can achieve the fine tolerance you require.

5. Inspection

You will need quite a lot of time to inspect parts with very tight tolerances. You may also need to employ the use of specialized tools and equipment to verify the tolerance of parts.

How to Find the Right Tolerance

Sometimes, picking the tolerance is not the difficult part. It’s knowing if it’s the right one picked. 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 knowing the right tolerance, the turnaround time for the project as well as on its cost. A customer working with a CNC machining service could unwittingly spend double what it needs to be 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. Here are a few tips to help your decision-making process:

1. Consider the use of your part

Not all parts need to be designed with tight tolerance. The specific use of your part often determines the degree of accuracy required when machining it. Creating parts that do not combine with others for example often requires less milling accuracy. Considering how much more expensive it is to achieve tight tolerances, if you do not need it, do not go for it.

2. Look for a Reputable CNC Machining Company

One way for customers to find the right tolerance is to find a CNC machining service 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.

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.

CNC Machining Tolerance Types

1. Standard Tolerances

Machinists use standard machining tolerances for the most widely fabricated parts. There are standard machining tolerances for parts such as threads, pins, pipes, and so on. Some milling services offer typical tolerances of +/-0.1mm. Machinists usually apply these tolerances when the customer doesn’t specify tolerance levels. These typical tolerances can be found on a CNC machining tolerance chart. The range of standard machining tolerances is usually set by various international standards bodies such as the International Organization for Standardization (ISO), the American Society of Mechanical Engineers (ASME), and the American National Standards Institute (ANSI).

2. Bilateral Tolerances

When you use bilateral tolerance, the deviation from the given dimension can be either negative or positive. This means it can be a tiny bit bigger or a tiny bit smaller. An example of a bilateral tolerance is +/- 0.06mm. This indicates that the machined part can be 0.06mm shorter or longer than the specified measurement. Bilateral tolerances are used mostly for exterior dimensions.

3. Geometric Dimensioning and Tolerancing

Geometric dimensioning and tolerancing are much more thorough than the other systems of machining tolerances. It is a type of CNC machining tolerance that highlights the measurements and allowable deviations. It also outlines specific geometric characteristics for the machined part such as how flat it should be, its concentricity, and its true position. Geometric dimensioning and tolerancing are often used for parts that have extremely precise dimensions.

4. Unilateral Tolerances

Unilateral tolerances accommodate deviations in only one direction. The deviation is either positive only or negative only. An example of such tolerance is +0.00/-0.06mm. This means that the finished part can be at most smaller by measurement of 0.06 mm but must not exceed the specified measurement. You usually implement unilateral tolerance when designing a part that goes into another. The part must not be bigger than the specified measurement as that will mean it will be unable to go into its position.

5. Limit Tolerances

A limit tolerance is a type of CNC machining tolerance expressed as a range of values, where the part is fine as long as the measurement falls between that range. 13 – 13.5mm for example is a limit tolerance indicating that the part must have a measurement that falls between the upper and the lower limit. (13mm is the upper limit and 13.5mm is the lower limit)