CNC machining with tight tolerances can be challenging and costly. Tight tolerances result in increased scrap, the use of special measurement tools, and longer lead times. In addition, they can require additional fixturing. This can result in a cost that can be up to two times that of a standard tolerance. However, tight tolerance machining is imperative when part design requires the highest level of accuracy for proper performance of the final parts.

What is Tight Tolerance Machining

When machining a part, it is important to ensure that the tolerance is within acceptable limits, or else the part will be scrapped. Tolerances vary from part to part, and the tighter the tolerance, the more expensive it will be. In some cases, tight tolerances are required for certain structural features of the part.

Tolerances are often referred to as GD&T, or geometric dimensioning and tolerancing. The limit tolerance refers to the range of dimensions in the part that can be produced. Parts that fall outside of these limits are considered unusable, so tight tolerances are required.

Tight tolerances also mean longer cycle times and more specialized inspection tools. These features require greater costs as well as additional set-up and fixturing. In addition, the increased cycle time may result in increased scrap. To keep the part within tight tolerances, CNC machines have to lower their speeds. Increasing the cycle time can cost as much as two or three times as much as a standard tolerance. Since tight tolerances can be difficult to maintain, they should be used only when strictly necessary to ensure that the part meets its design criteria.

Common Types of Machining Tolerances

CNC machining tolerances define the limits of acceptable parts and scrap parts. These tolerances are often set to reduce costs and ensure desired quality. Tighter tolerances may require more time and extra processes, while parts with looser tolerances can be finished using a basic machining process. For these reasons, CNC machining tolerances should be specified during the design process and before the start of the manufacturing process.

CNC machining tolerances are measured using numerical values. Typically, a tolerance band of +/ 0.001″ is acceptable in most applications. However, it’s important to note that the tolerances you receive will vary by material and application. For example, a five-hundredths tolerance, which is equivalent to a single sheet of paper, may be too tight for some products.

Tolerances can be divided into two types: general and form. The first is the most general and specifies general characteristics, while the second specifies more specific characteristics, such as flatness and true position. It is important to understand and interpret these tolerances, as improper application of them can cause expensive problems. A good CNC machine shop will know the proper machining tolerances and how to apply them correctly for tight tolerance CNC machining services.

Tips for Maintaining Tight Tolerance Machining

When it comes to CNC machining, it’s important to maintain tight tolerances. To do this, manufacturers must use high-precision equipment. This means more expensive tooling, but it also means more frequent maintenance. While it’s tough to hold to such tight tolerances, it is possible to keep them within a reasonable range.

Here are some important factors to achieve very tight tolerances.

  • In addition to using high-precision equipment, you should also avoid using dull cutting tools for low-melting-point materials.
  • To get tight tolerances, target a tolerance band that is half the size of the part’s overall dimension. This way, the related dimension of all machined parts will be on the high side for external surfaces and on the low side for internal surfaces.
  • It is also important to understand that a very small tolerance band will likely mean that you must adjust the sizing of several workpieces to achieve the desired precision.
  • Checking runout is another way to maintain tight tolerances. Runout refers to when the machining shaft deviates from its true circular rotation. Ensuring that the central axis of the cutting tool is aligned will provide the desired result for the machined part.

Tolerances are essential for repeatability and specificity. Even a thousandth of an inch (the thickness of a human hair) can make or break a final product. A CNC machinist understands the importance of how even the smallest degree of variation can impact the quality of a machined part.

Tolerance Expectations and Material Machinability

When CNC machining with tight tolerances, you have to take into account both material machinability and tolerance expectations. Tolerances that are too tight can increase production and development costs. To avoid excessive costs, it is best to keep tolerances as generous as possible during the design phase. Tolerance expectations are the limits of a component’s dimensional accuracy. Tolerances can be negative or positive. A positive tolerance means that the part is within a certain tolerance, while a negative tolerance means the part cannot be within a certain measurement. For instance, a +/-0.06mm tolerance means that the part can be at most 0.06mm larger or smaller than the specified measurement. Generally, a part with a 0.063mm tolerance will not fit into the same position as an identical part with a higher tolerance. Material selection is a key factor in producing tight tolerance parts.

Finding a Machining Service for Tight Tolerance Parts

Knowing how to properly communicate these expectations to a CNC machining service is essential for making sure your project meets your goals. Tolerance expectations should be outlined with the manufacturer before the project begins. In addition, you should consult with the CNC machining service provider to ensure that you get the exact CNC machining tolerances you need. Avanti Engineering has machined to a variety of different tolerances for small parts and high volume metal parts for our customers. Our team listens to the needs of customers and we deliver their parts with a high degree of precision. With years of experience, Avanti Engineering offers competitive pricing and unmatched customer satisfaction. Give our team of experts a call today to discuss your CNC machining project.