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Machining Tolerances

Modern machining processes are exceptionally accurate. However, no machines are capable of exact precision. Variances can be caused by a variety of factors from the part material to the machining process used. For this reason, engineers assign machining tolerances to parts during the design process. 

Machining tolerance, also known as dimensional accuracy, is the amount of acceptable variance in the dimension of a part. This is expressed as a maximum and minimum dimensional limit for the part. Parts are considered to be within the tolerance if their dimensions fall between these limits. If the part's dimensions fall outside of these limits, however, these parts are outside the acceptable tolerance and considered unusable.

For designers, determining the appropriate tolerances for a part is an essential task in preparing a design for an order. However, it can be difficult to determine appropriate tolerances for a part, especially parts that are made of non-metallic substances. To develop appropriate machining tolerances for your designs, understanding standard machining tolerances and the tolerances that certain materials and machining processes are capable of will be essential. For this reason, we've created some machining tolerance guidelines to help you determine machining tolerances for your nonmetallic parts

Overview of Machining Tolerances

Machining tolerance is defined as the total amount a specific dimension is permitted to vary from the given value. This can be expressed in a few different ways:

  • The upper and lower limits (e.g. 0.2500, 0.2498)
  • The allowable amount above and below a defined dimension (e.g. 0.2499 ±0.0001)
  • The allowable variance by itself (e.g. ±0.0001)

The range of allowable dimensions within this defined area is known as the tolerance band. The larger the difference between the upper and lower limits, the larger or "looser" the tolerance band. The smaller the difference between the upper and lower limits, the smaller or "tighter" the tolerance band. 

Tolerances may also be expressed with any number of decimal places. The more decimal places are included, the stricter the tolerance is. These various types of tolerances are expressed as follows:

  • One decimal place, expressed as (.x), (e.g. ±0.2")
  • Two decimal places, expressed as (.0x), (e.g. ±0.01")
  • Three decimal places, expressed as (.00x), (e.g. ±0.005")
  • Four decimal places, expressed as (.000x), (e.g. ±0.0005")

What Are the Standard Machining Tolerances?

There are no true standard machining tolerances, primarily because different applications necessitate different tolerances. Some manufacturers and industry organizations, however, have established standard tolerances they use or recommend for certain parts and materials. This is especially the case for the military and aerospace manufacturing industries. 

Often, customers provide machine shops with tolerances for their projects. Some machine shops require customers to provide tolerances, while others work from a list of common tolerances if none are provided. For example, at AMI, our standard machining tolerance for our non-metallic parts is ±0.01". While this may seem to be a relatively loose tolerance, this tolerance was specifically chosen as it balanced out the benefits and drawbacks. Some benefits of the two-decimal-place standard tolerance include:

  • Reduced costs: Unnecessarily tight tolerances can increase the cost of production. This is because tight tolerances require more time and labor to meet requirements. Tight tolerances also increase the chances that a part will fall outside of the acceptable tolerance band, increasing the potential for scrapped parts that can further add to the costs. A ±0.01" tolerance finds the balance between precision and cost-effectiveness.
  • Faster run times: The ±0.01" tolerance allows machinists to achieve faster run times as they do not need to spend as much time retooling the part to meet tighter tolerances. This not only improves turnaround times but also decreases tooling costs overall.
  • Best material standard: AMI handles a wide variety of non-metallic parts on a daily basis, made with a range of materials. A ±0.01" tolerance is the best range for all materials we handle.

While these standard tolerances are useful tools for parts with non-toleranced parts, we still highly recommend that designers provide tolerances for their parts.

Why Are Machining Tolerances Important?

Despite the availability of standard tolerances for non-toleranced dimensions, many manufacturers do not use them. Many manufacturers will refuse to start making parts until the engineer has defined all features with a tolerance. The reason for this is that the manufacturer has no frame of reference for understanding how the part will interact with other pieces. As a result of this lack of information, the manufacturer does not know how important a specific dimension will be to the final design.

To illuminate the issue mentioned above, consider a part designed to fit on a shaft. The part design includes a hole diameter that is designed to fit the shaft perfectly. If the hole is any smaller, the part will not fit the shaft. One of three things can happen, depending on whether or not a tolerance is provided:

  1. A tolerance is provided: When a tolerance is provided, the manufacturer can start work on the part as soon as they receive the design and are made aware of the dimensional limitations required. This minimizes costs and improves turnaround time.
  2. A tolerance is not provided, and the manufacturer refuses to work on the part: Some manufacturers will not work on a part without the tolerance being provided to ensure the satisfaction of the customer and reduce retooling costs. While this can increase turnaround time, it saves the engineer from potential retooling costs.
  3. A tolerance is not provided , and the manufacturer moves forward with production: In this case, the manufacturer is not aware of the lower limits of the design and may apply a standard tolerance of, for example, ±0.005" to the part. This means that the diameter may be up to 0.005" smaller or 0.005" bigger than the specified diameter. If the machinist produces the design with a diameter that's 0.005" smaller than required, the part will not fit the shaft and will need to be retooled. While the manufacturer is perfectly capable of achieving this, the retooling process increases costs and turnaround time, which can significantly impact the designer's business.

For the best outcomes, it is suggested that designers define machining tolerances as soon as possible.

Factors That Impact Machining Tolerances

There are many factors to take into consideration when determining tolerances. These include the following: 

  • Material: Materials behave differently under stress, and some are easier to work with than others. These material properties must be taken into account when determining tolerances. The specifics of how materials affect machining tolerances are discussed further down this page.
  • Machining type: The method of machining used will significantly impact the possible tolerances for the finished part, as some processes are more precise than others. The specifics of how machining processes affect tolerances are discussed in more detail further down this page.
  • Plating and finishes: Any plating or finishing processes should be taken into account when determining part dimensions and tolerances. While plating and finishing add small quantities of material to the surface of a part, these small amounts still alter the dimensions of the final product and should be taken into account before production.
  • Cost: Tolerances should be precise but never tighter than necessary, as tighter tolerances are more expensive to achieve. If your part will work with a three-decimal-place tolerance, do not make it a four-decimal-place tolerance.

It's also important to remember to double-check tolerances. Old part specifications that you wish to reuse may be using tolerances that are unnecessarily tight or may have tolerances that have been transcribed incorrectly. Even new part specifications may contain errors. Taking an extra few minutes to double-check existing tolerances on old and new projects can help avoid retooling costs in the future.

When these factors are considered, and tolerances are used correctly, engineers can rest assured that their parts will fit properly when the manufacturing process is complete.

Tolerances by Material

The qualities of a part's materials must be taken into consideration when defining part tolerances. The designer must define the characteristics of the material being used and take into consideration how each of these characteristics may affect the ability of the material to be machined and the acceptable tolerance. Just a few of these characteristics are defined below: 

  • Abrasiveness: Certain materials that are very abrasive can be hard on the tooling process. Phenolics like G10/FR4, G11, GP03, and any glass laminates fall into this category. Because of the abrasiveness of the material, these materials can affect the tolerances of the design as they wear down the cutting machinery.
  • Hardness and rigidity: Soft, flexible materials are generally more difficult to machine to specified tolerances because of their ability to change dimensions. Polyisocyanurate, polyurethane, and XPS foam all fall into this category. As a result, extra measures may be needed to cut the material to fit tolerances.
  • Heat stability: Some non-metallic materials, especially plastics, tend to warp in the presence of heat. This limits the types of machining processes that are acceptable and affects the tolerances of the part.

It is also important to consider what type of material you will be using when choosing the machining process for manufacturing, as some materials are incompatible with certain machining operations.

Tolerances by Machining Type

In addition to the materials being used, the type of machining method used to produce parts will significantly affect the machining tolerances. Below are a few common machining processes used and the tolerances they are capable of:

  • CNC screw machining: This method of machining uses a disc cam to feed a workpiece through a guide bushing, moving the part around the tool instead of vice versa. Because the part is moving instead of the tool, there is less vibration and deflection, allowing for extreme precision. This method is recommended for foam, phenolic and plastic parts with extremely high dimensional tolerances and can achieve tolerances of ±0.005”.
  • Shear cutting: Shear cutting is a process by which a part's dimensions are altered by applying a force to the material great enough to cause the material to fail. Shear cutting is often applied using a punch and die or a set of blades. For this reason, it is typically not recommended for particularly soft or brittle materials, as these materials can more easily break or warp in response to the applied force. This method is able to hold good tolerances but is not able to hold tolerances tighter than ±0.015" as the part is not held by a fixture or mechanical device but by a human.
  • Steel rule die cutting: This machining method uses a custom steel tool called a die to punch out specific shapes on a material. This method is preferred for producing custom die-cut components on a budget, as it is an accurate, simple and cost-effective method of production. While this machining method is typically not a good choice for very brittle or soft materials, it works well for materials like foam, rubber and plastic. To account for all the different consistencies of all the types of materials that AMI die cuts from, we typically give steel rule die cutting a machining tolerance of ±0.015" to provide the most leeway.
  • Rail cutting: In rail cutting, a rail saw is manually operated to alter the part. Because of its manual nature, rail cutting requires a larger leeway in the form of a larger machine tolerance. For this reason, AMI typically uses a machining tolerance of ±0.031" for this machining method.

If you are unsure as to the best machining method for your product and require tolerance, don't make an educated guess. Always reach out to a machining company like American Micro Industries to discuss your options and figure out the best solution.

Choose American Micro Industries

Dimensional tolerance is something no company should take for granted. A good set of machining tolerances can be the difference between success and failure for a project, improving processes and reducing costs significantly. If you're looking for a company that understands machining tolerances and can help you achieve your project goals, American Micro Industries can help.

Since 1995, the Custom Components division of American Micro Industries has grown to become the leading manufacturer of exceptional custom non-metallic parts. We've partnered with a wide range of OEMs and contract manufacturer customers in the aerospace, defense, electronic, marine, technology and medical industries, meeting some of the most demanding machining requirements. Based in Chambersburg, PA, our team has experience working with a range of materials, including foams, electrical insulators, phenolics, plastics and rubbers.

At American Micro industries, our team has one goal — to provide complete customer satisfaction. We focus on delivering exceptional quality with fast turnaround at an affordable price point. We base our operations on the strictest quality standards and inspect all parts individually before shipment to ensure that our parts exceed expectations. We also offer most of our in-stock items for same-day or next-day shipping. Everything we do, from our stringent inspections to our customer care team operations, is designed with the customer in mind. 

Learn more about American Micro industries, our products and our services or get a custom project quote by contacting us today!