CNC Machining Tolerances: Overview, Benefits, and Tips

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CNC Machining Tolerances: Overview, Benefits, and Tips

CNC machining has significantly improved manufacturing results as part of the emergence of Industry 4.0. It utilizes various fabrication methods and advanced machines to create superior quality and high-precision components. These CNC machines help achieve the highest levels of accuracy and tight CNC machining tolerances.

So, a part must possess tight machining tolerances if it needs to fit perfectly with another. Therefore, you must communicate the preferred tolerances to the manufacturing partner. In addition, it would help to note that very tight tolerances incur more costs due to wear caused by CNC machine tools.

Although a new machine tool might easily attain tolerances of ±0.001 inches, the precision becomes less constant as it creates multiple copies of the same part. Suppose your order is above 1000 units using  ±0.001 tolerances. In that case, your manufacturing partner may need to adjust the machining speed or change the machining tool leading to high retooling costs and cycle time.

//  Material

Your choice of material for your project determines the specified machining tolerances. Material properties can influence the tolerance level attained with such material. Some of these features are:

Abrasiveness: Rough and coarse materials are typically harsh on machine tools and can make them wear out quickly. These CNC machining materials make it almost impossible to achieve certain tolerance because you have to change the cutting tools regularly, resulting in reduced accuracy.

Heat Stability: Heat stability is a common issue that affects non-metals. These non-metallic materials lose shape as heat builds up during CNC machining. More so, this factor limits the compatible processes with the particular material.

Hardness: Harder materials are easier to machine to tight tolerances than softer materials. This is because softer materials change dimensions as the machine tool touches the material. Hence, you must be more patient when machining more delicate fabrics.

//  Costs

Tolerances directly impact the quality of a machined part and its costs. Higher tolerances cost more since you must machine the parts more slowly and consistently with more expensive tools.

Also, parts with tighter operational tolerances need more extensive, thorough quality inspection since the acceptable margin of error is smaller. A high failure rate is another factor that increases the CNC machining cost for parts with tight machining tolerances. High-tolerance machining causes an increase in production costs and quality inspection costs.

//  Manufacturing Processes

Individual CNC machines have varying capacities. Some machines can achieve tolerances that others cannot. Likewise, a CNC machine’s tolerance decides the type of parts it can work on. However, you must perform secondary operations on machined parts before achieving the desired tolerance.

//  Inspection

When machining parts with tight machining tolerances, you will require a longer time to scrutinize the part. Also, you may have to utilize specialized tools and equipment to confirm the tolerance levels of the machined parts.

As a product designer, specifying the extent of acceptable variance for your machined part’s tolerance is essential. It significantly affects finding the right tolerance for your parts and the costs and turnaround time.

//  Examine the Machined Part’s Use

Although not all machined parts need to be designed with tighter tolerances, the specific use of the part will decide the degree of accuracy required when machining the parts. For instance, producing parts that do not fit with others doesn’t need higher milling accuracy. So, because machining parts with tighter tolerances is more expensive, you shouldn’t go for it if you do not need it.

Types of CNC Machining Tolerances

There are various types of tolerances in CNC machining. Here are the most common tolerances used in CNC machine shops:

// Standard Tolerances

Most product designers and engineers utilize standard machining tolerances for most commonly machined parts. Machined parts, such as pins, threads, pipes, etc., often use standard machining tolerances. Machinists often use normal tolerances when customers fail to specify the required dimensional limit. Many CNC milling services often provide standard tolerances of ±0.1mm.

You can easily locate these typical tolerances on a CNC machining tolerance chart. The standard machining tolerance range is often fixed by different international standards bodies. These bodies include the American National Standards Institute (ANSI), the International Organization for Standardization (ISO), and the American Society of Mechanical Engineers (ASME).

In addition, there are four separate tolerance categories based on their range for a workpiece’s dimension. These include fine (f), medium (m), coarse (c), and very coarse (v).

// Unilateral Tolerances

Unilateral tolerances have permitted variance in only one direction. It can occur either in the negative or positive direction only. An excellent example of a unilateral tolerance would be +0.00/-0.07mm. It means a finished workpiece can be below the size of 0.07mm but not more than the defined measurement.

You can utilize unilateral tolerance in designing a component that needs to fit with others. As a result, the components shouldn’t exceed the defined measurement because that can make them unsuitable for mating.

// Bilateral Tolerances

Bilateral tolerances permit the variation in the final measurement on either side of the dimension. That is, the deviation from the nominal value can either be positive or negative; the deviation can be bigger or smaller.

An excellent example of bilateral tolerance is +/-0.05mm. It shows that the machined workpiece can be shorter or longer by 0.05mm than the preferred measurement. Machinists often use the bilateral tolerance for exterior dimensions when machining parts.

// Geometric Tolerances

Geometric dimensioning and tolerancing are more complicated and advanced than typical machining tolerance systems. It typically involves highlighting measurements and permissible deviations.

Also, it indicates specific geometric characteristics of the machined components, such as their flatness levels, centricity, and true position. Some machined parts often require a specified diameter, which only geometric dimensioning and tolerancing can offer.

// Limit Tolerances

A limit tolerance is commonly indicated as a range of values where the CNC machined parts are considered tolerable, provided their measured value is within the allowable variance. For example, 19.996- 20.005 is a limit tolerance showing that the workpiece must possess a measurement within the upper and lower limits. In this case, 20.005 mm is the upper limit, while 19.996 is the lower limit.

// Geometric Dimensioning and Tolerance (GD&T)

GD&T is a versatile system of detailing and communicating the standard machining tolerances. It is more complex than the typical tolerance systems. It uses feature control frames to indicate specific forms and dimensional tolerances of parts.

Geometric dimensioning and tolerancing keep the final measurements of a part within the specified boundaries. Similarly, it indicates the part’s geometric properties, including its flatness, concentricity, and true position. More notably, GD&T symbols ensure the dimensional accuracy of part features in some parts with higher processing requirements.

Common CNC Machining Tolerances

CNC machining is a broad field that includes milling, turning, surface finishing, and more. CNC machining tolerances vary for each process due to the type of cutting tool used. The following are standard CNC machining tolerances for common processes:

• Router: ± 0.005″ or 0.13 mm

• Lathe: ± 0.005″ or 0.13 mm

• Router (Gasket Cutting Tools): ± 0.030″ or 0.762 mm

• Milling (3-axis): ± 0.005″ or 0.13 mm

• Milling (5-axis): ± 0.005″ or 0.13 mm

• Engraving: ± 0.005″ or 0.13 mm

• Rail Cutting Tolerances: ± 0.030″ or 0.762 mm

• Screw Machining: 0.005″ or 0.13 mm

• Steel Rule Die Cutting: ± 0.015″ or 0.381 mm

• Surface Finish: 125RA

If you compare these values with alternative remanufacturing technologies, you will find that the CNC machining processes involve tighter tolerances.

Importance of Tolerances in CNC Machining

Tolerances play a critical role in CNC machining, ensuring that the final product is functional, safe, and reliable. Tight tolerances are essential in various industries where parts must fit together precisely, and even small variations can lead to disastrous consequences. Therefore, mechanical designers must pay strict attention to tolerances in their designs and ensure manufacturing processes are monitored and controlled to meet their specifications. By maintaining tolerances, efficient production processes, cost savings, and safe, reliable products can be achieved.

// Meeting Quality Standards

CNC machines are built to produce precision parts that adhere to strict quality standards. To achieve the required precision, it is essential to maintain tolerances during the machining process. Even the slightest deviation from the required measurements can result in parts that do not meet quality specifications. This can lead to a number of issues such as parts failing prematurely or not fitting together properly.

// Ensuring Compatibility

In many industries, parts need to fit together precisely to function properly. If the tolerances are not tight enough, the parts may not fit together, leading to production errors, reduced functionality, and unsuccessful products. For example, in the aerospace industry, where tolerances can be as tight as a few microns, a single deviation can result in catastrophic damage.

// Promoting Efficiency

CNC machines are designed for accuracy and speed. By maintaining tolerances, it ensures that the machining process is as efficient as possible. This is because the machine does not have to stop and check parts repeatedly for accuracy, which can slow down the overall process. With the correct tolerances programmed into the machine, the process can continue uninterrupted, resulting in faster production times, increased productivity, and reduced costs.

// Cost-Effective Manufacturing

Maintaining tolerances in CNC machining can be cost-effective when done correctly. This is because the process requires fewer material and labor resources to produce quality products. When tolerances are not monitored and error rates increase, it can result in wasted resources through scrapped products, production delays, and rework. Additionally, it can result in additional costs associated with reworking and repairing parts.

// Minimizing Risk

Tight tolerances are crucial when manufacturing parts or systems that pose a safety risk. For example, medical devices, automotive, and aerospace industries require strict tolerance control to minimize the risk of malfunction. By maintaining tolerances within specifications, the risk of equipment failure, injury, and death can be minimized.

Factors in CNC Machining Tolerances

CNC machining tolerances are crucial in ensuring the quality and functionality of the machined parts and components. Factors such as material properties, part geometry and size, cutting parameters and tools, machine capability, and application requirements should be considered when determining the appropriate tolerance levels. By understanding these factors and working closely with the customer, designers, engineers, and manufacturers can optimize the CNC machining process, reduce errors and rework, and deliver high-quality products that meet or exceed the customer’s expectations.

// Material Type and Properties

One of the most important factors that affect the CNC machining tolerances is the type of material that will be machined. Different materials have different physical properties, such as hardness, ductility, tensile strength, elasticity, and thermal conductivity, which can affect the machining process and the finished part’s accuracy. For example, aluminum is a lightweight, soft, and highly machinable material, while stainless steel is a hard, tough, and more challenging to machine. Therefore, you may need to adjust the tolerances to accommodate the material’s properties.

// Part Geometry and Size

The complexity and size of the part or component being machined can also impact the CNC machining tolerances. Parts with intricate features, tight corners, or small radii may require finer tolerances to achieve the desired accuracy, while bulkier, more robust parts may tolerate looser tolerances. Moreover, the part’s dimensions and orientation relative to the cutting tool can affect the machining tolerances, especially if the part needs to be refixtured or reoriented during the machining process.

// Cutting Parameters and Tools

The cutting parameters and tools used in CNC machining can significantly influence the tolerance levels. Factors such as cutting speed, tool diameter, tool wear, surface finish, and tool sharpness can affect the machining tolerances. For instance, slower cutting speeds and smaller tool diameters can improve tolerance accuracy, but may increase cycle time and cost. Additionally, different cutting tools, such as drills, end mills, taps, and reamers, have specific tolerance limits and can affect the part’s dimensional accuracy.

// Machine Capability and Calibration

The accuracy and repeatability of the CNC machine used for machining can have a considerable impact on the tolerance levels. CNC machines have various precision levels, ranging from standard to high-end models that provide micron-level accuracy. It is essential to use a machine that can maintain tight tolerances consistently over a large production run. However, the machine’s accuracy and performance can degrade over time due to wear and tear, so regular maintenance, calibration, and quality control checks are necessary to ensure the machine’s optimal performance.

// Application and Customer Requirements

The final factor to consider when determining CNC machining tolerances is the application’s intended use and the customer’s requirements. Some applications may require high precision and tight tolerances, while others may not need such accuracy levels. Moreover, the product’s functional requirements, operating conditions, and environment can influence the tolerance levels. Customer specifications and industry standards may also determine the acceptable tolerance ranges for the product, so it is essential to communicate these requirements clearly with the customer and align them with the production processes.

How Can You Find the Right Tolerance for Your Parts?

As a product designer, specifying the extent of acceptable variance for your machined part’s tolerance is essential. It significantly affects finding the right tolerance for your parts and the costs and turnaround time.

However, you should note that CNC machining is a typical high-precision process. Therefore, looser tolerances are almost equivalent to the original specifications. Here are tips to help you find the right tolerance for your parts:

// Examine the Machined Part’s Use

Although not all machined parts need to be designed with tighter tolerances, the specific use of the part will decide the degree of accuracy required when machining the parts. For instance, producing parts that do not fit with others doesn’t need higher milling accuracy. So, because machining parts with tighter tolerances is more expensive, you shouldn’t go for it if you do not need it.

// Partner with a Reliable CNC Machining Company

One of the most effective means of getting the desired tolerances for your parts is to partner with a reliable CNC machining service. You can discuss your project ideas and specifications with a manufacturing expert and determine the appropriate tolerances.

You must identify the tolerances before submitting a manufacturing request to a rapid prototyping company. Providing this information helps a long way in saving costs and time. More importantly, it is crucial to understand that most CNC manufacturers will automatically revert to their standard tolerances if you do not specify a particular tolerance margin.

Although this seems to be an insignificant deviation in tolerance, it can affect the fit and function of your final parts. For example, if you machine a part with a tiny hole even by ±0.127 mm, it won’t be easy to fit its adjoining part within it.

Conclusion

At IDEAL, we excel in providing top-quality custom machining services tailored to meet your project needs. Our advanced technology and skilled team ensure your specifications are met with exceptional accuracy and efficiency.

Contact IDEAL today to see how we can assist with your next project!