One of the key things in determining the right CNC machining tolerances is figuring out at what point a part’s function and performance will be affected by a variation in dimension or size. Some types of parts, like critical components in engines, require a high degree of precision and any lapse in geometric form could have serious consequences. Other types of parts might not require the same degree of precision, and a slightly larger manufacturing deviation will not affect them. Apart from this, here are a few other things to keep in mind when choosing tolerances:
1. When do You Need Tight Tolerances?
Often, machining tolerances can vary within a single component. While it may not be critical that the entire structure of a part adhere to the tightest tolerances, certain features of the said part might require them. The highest precision is typically required at structural points where a part must fit or join with other components, such as holes.
If a part needs to fit with one or many components, it must have tight tolerances. In these cases, you should communicate the required tolerances to the CNC machining services, otherwise, they will not have any indication of design intent. In other words, tolerances give your CNC machining services partner an indication of how your part needs to function.
It is also worth noting that the tightest machining tolerances can be more costly because of the wear caused by CNC machine tools. While a fresh tool might not have a problem achieving tolerances of ±0.001”, if the same tool has to produce multiple versions of the same part, this precision can become less consistent. If you order 1,500 units of a part at ±0.001” tolerance, your CNC machining services may have to replace the machining tool or adjust the machining speed, which can increase production time and cost.
2. High Costs for Tight Requirements
Tolerances directly influence the quality of a part, but also its cost. High tolerance machining is typically more expensive to carry out since you have to machine the parts more slowly and often with more expensive tools. Parts with tighter tolerances also require more extensive quality CNC inspection, since the window of error is much smaller—we’re talking fractions of a millimeter. Another factor that can influence cost with high tolerance machining is the failure rate, which tends to be higher. High tolerance machining can drive up production costs and quality inspection costs.
3. Tolerances Depend on the Material
You have to consider your choice of materials when specifying machining tolerances. Different materials have individual characteristics that can affect the level of tolerance that is achievable with them. Some of these characteristics include:
Hardness: Softer materials are more difficult to machine to tight tolerances. This is mostly due to their likelihood to change dimensions as the cutting tool touches them. You therefore have to exercise additional patience when machining these softer materials.
Abrasiveness: Rough and coarse materials are usually tough on the cutting tools and can even cause them to wear out faster. These materials make it difficult to achieve specific tolerance because changes in the cutting tool result in less accuracy. Abrasive materials usually require the technician to change out the tool multiple times throughout the machining process.
Heat stability: This is an issue that affects non-metals mostly. As heat builds up during the machining process, these materials start to lose their shape. This restricts what processes you can use on that particular material.
4. Choice of Manufacturing Methods
Each CNC machine differs in terms of capacity. Some machines can achieve tolerances that others can only dream of. The CNC machine tolerance determines what kinds of parts it can handle. In some cases, you will have to carry out additional operations on the part before it can achieve the fine tolerance you require.
You will need quite a lot of time to inspect parts with very tight tolerances. You may also need to employ the use of specialized tools and equipment to verify the tolerance of parts.