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Posted on June 15, 2020

Enrich your knowledge about tolerances for CNC machining, through learning about implications when tightening the range and the ways your existing tolerance range relies on various factors.

1. Tolerances are the allowable range for a dimension, usually expressed as “+/-’’. The designer determines this based on the fit, form, and function of the part.

If a CNC machining tool has a tolerance of +/- 0.01 mm this means that the machining tool can have a deviation of 0.01 mm in every cut. It can be 0.01 more than the standard value or 0.01 mm less than the standard value.

2. The tighter the tolerance the more money it’s going to cost since there are greater set-up needs, lengthier cycle times, and additional fixturing or specialized tools.

When CNC machines reduce speed to hold tighter tolerances leads to longer cycle times resulting in a jump in costs. Extending the tolerance out just one additional decimal point can surge costs by a factor of two or three. Trying to set-up tighter tolerances involves a lot of intensive labor and inspections that can be very time-consuming. Therefore, the only time tighter tolerances should be used is when there is a requirement in the design criteria for the part.

3. If you ask for tighter than the standard tolerance you might be altering the optimal manufacturing process for your segment.

There are times when a tight fit tolerance is mandatory. However, when carefully considering end-use you can determine between critical and non-critical tolerances, avoid unnecessary tight tolerances, and reduce your cost of quality.

For instance, a hole that is able to be machined on a vertical mill at a single tolerance could require it to be bored on the lathe for a tighter tolerance. This increases set-up cost along with lead time. If you decide to go tighter with your tolerance you may be required to use grinding or lapping.

4. Be aware that the feature will go through an inspection process when additional tolerances are placed on your parts.

The harder the feature is to machine, the harder it is to measure. Some features need unique inspection equipment which can raise the cost of parts. The smaller the part is the harder it is to manufacture. For example, features smaller than 0.005” are hard to see without the use of equipment. Accurate inspection is a huge challenge for manufacturing small parts or large parts that contain small features.

5. The type of material selected can determine the difficulty of manufacturing a part to a specified tolerance.

Typically, if the material is soft it will be hard to hold to a specified tolerance. Since the material is so soft it can easily bend and move while being cut. Also, plastic material like Nylon might not hold a tight tolerance like an aluminum material can without altering the tools being used.