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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.
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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 manufacturing 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
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 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″)
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 manufacturing 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:
While these standard tolerances are useful tools for parts with non-toleranced parts, we still highly recommend that designers provide tolerances for their parts.
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:
For the best outcomes, it is suggested that designers define manufacturing tolerances as soon as possible.
There are many factors to take into consideration when determining tolerances. These include the following:
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.
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:
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.
In addition to the materials being used, the type of machining method used to produce parts will significantly affect the manufacturing tolerances. Below are a few common machining processes used and the tolerances they are capable of:
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.
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 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. Our machine shop focuses 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!
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