The fit and function of many plastic components rely on tight-tolerance machining and must be manufactured with precision. For example, if a male component must mate with a female component, tolerance can impact whether the two parts will fit together or, conversely, have a sloppy fit. The plastic part tolerances required depends on the part, the material, and the machining method.
Holding a tight tolerance on every dimension of a part will add considerable cost, even if it is possible. Dimensions of elements that slide, snap, interlock, press fit, rotate, align, mate, or have threads are ideal candidates for requiring tight tolerances.
Understanding Component Tolerance
The terms precision and accuracy are often used as synomyms in casual conversation. But in manufacturing, they have very specific meanings. Precision is the repeatability of a measurement from one part to the next. A batch of precision parts is nearly identical. However, that doesn’t mean they are acceptable.
Accuracy refers to how close a measured value is to the desired value. The more accurate, the closer to the desired dimension. Precision and accuracy together tell the machinist that they are consistently machining parts close to the desired values.
Machining a component to an exact value is virtually impossible because of material and machine variations. Tolerance or dimensional accuracy is how close a part needs to be to the dimension. This is the acceptable deviation from the nominal value with a positive and negative range, often expressed as +/-. Sometimes, there may only be a positive or a negative value.
Designating Tolerances
Proper dimensioning on your drawing will ensure you get the component you want most cost-effectively. Dimensional tolerances on your drawing let a manufacturer know which dimensions are critical. In some cases, general tolerances in the form of a table or note may be used for dimensions without specific tolerances stated.
When designing your part, consider tolerance stack up, which evaluates the cumulative effect of tolerances across an entire assembly or system of parts. It evaluates how variations in the dimensions of individual components, within their allowable tolerance limits, add up and affect the overall fit, function, or performance of the assembled product. Tolerance stack-up looks at worst-case scenarios, which assume all parts have minimum or maximum tolerance to see if it would affect assembly or function. For mating parts, it may be called minimum at maximum. A component or feature is at its smallest allowable size (minimum), while another related dimension or feature is at its largest allowable size (maximum). For example, if you have a hole and shaft assembly with dimensions of 10.00 mm with a tolerance of +/- 0.05 mm. You evaluate what happens when tolerances stack up to their extremes. In this case, the hole is at its smallest allowable size (e.g., 9.95 mm), and the shaft is at its largest allowable size (e.g., 10.05 mm) – the worst-case fit. In this case, the shaft wouldn’t fit into the hole.
Material Effect on Tolerance
The choice of material plays a significant role in determining the achievable tolerances during plastic machining. Unlike metals, plastics have unique properties that make them more susceptible to dimensional changes due to temperature variations, moisture absorption, and internal stresses. One of the key factors to consider is the coefficient of thermal expansion (CTE), which describes how much a material expands or contracts with temperature changes.
Plastics generally have a higher CTE than metals, meaning they are more prone to dimensional shifts as the machining environment or part heats up during cutting. For example, if a plastic part is machined at elevated temperatures (which can happen from friction) without controlling for these expansions, the plastic may shrink as it cools, causing the final dimensions to fall outside the required tolerances. Materials that have low melt points are more difficult to control dimensionally. Some materials are more susceptible to warping from redistribution of internal stresses as material is removed.
In addition to temperature, moisture absorption can affect certain plastics, such as nylon and other hygroscopic materials, altering their dimensions over time. This is why it is important to select a plastic with properties suitable for the intended operational environment.
Machinists can sometimes address these challenges by applying various techniques, such as annealing, using coolants, and rough machining. Fixtures can be used to prevent issues with warping.
Annealing is a process where the plastic part is heated and then slowly cooled to relieve internal stresses. This step is especially important when working with plastics prone to stress cracking or warping. By annealing the part before or after machining, machinists can stabilize the material, reducing dimensional changes due to internal stress release or thermal expansion.
Using coolants during machining is another effective way to control temperature increases. Coolants help dissipate heat generated from the machining process, reducing the risk of thermal expansion and ensuring more consistent dimensions.
A method to control dimensional changes is to perform rough machining first, leaving extra material on the part and then allowing the part to stabilize (cool down) before finishing to the final dimensions. This two-step process allows any stress or thermal-related movement to occur before the final precision cuts, helping maintain tight tolerances.
CNC Machines and Tolerances
Different types of machines offer varying capabilities when it comes to holding tight tolerances. For example, CNC machining centers often provide higher precision than manual machines because of their automated control systems and repeatability.
However, even among CNC machines, tolerances can vary. Considering the machinery’s limitations is important, especially when working with materials sensitive to thermal changes. For example, while certain high-end CNC machines can maintain plastic part tolerances within five-thousandths of an inch, others might only be able to achieve looser tolerances, particularly when processing materials with a high CTE.
Ensinger Delivers Accuracy and Precision
Ensinger Precision Components has the expertise to deliver parts with precision and accuracy. We can deliver CNC machined plastic parts with tolerances as tight as +/- 0.0005 inches, depending on the part material and processes used. Our development process, which all parts go through, will find the best combination of cutting tools, annealing, and coolant to maintain close tolerances.
Whether you require prototyping or high-volume production, our advanced equipment and machining expertise provide fast, reliable results. Contact us today to explore how our CNC machining capabilities can meet your specific requirements.