With the development of CNC technology, CNC machining is being needed to be finer, while undesirable tolerance still exists in the machining process. so how do we avoid undesirable tolerance to the most extent? The following passage might solve the problem.

the tolerance for CNC machining

In CNC machining, the term tolerance is usually used in two different contexts: one is CNC machines and another one is designed for CNC machining.

In CNC machine, tolerance is the certificate of dimensional accuracy a machine can approach when machining a part. And the certification can be highly accurate for a CNC machine, because their machining parts can get at an accuracy at ± 0.0025mm, which is just as one-fourth of a human’s hair’s size. While the tolerances of different CNC machines go up and down, it usually is specified by manufacturers, for example. 0.02mm is a typical average tolerance, while providers would specify it through requirements from customers.

Tolerance is the acceptable range of variation of the dimension of a part in design and manufacturing, on the condition that all parts’ functions can not be affected. Different from tolerance in normal precious CNC machining, designers determine it through part’s function, fit and form, which are the essential keys for components that mated or interfered with. For example, compared with the handle of door, parts of an electric engine would need to have a higher tolerance. This is because the latter has many features that mate with other components. Tolerance is always represented by the marked number written beside the dimension to which it applies.

Linear dimension rangef(fine)m(medium)c(coarse)v(very coarse)
0.5 up to 3±0.05
±0.1
±0.2
over 3 up to 6±0.05±0.1±0.3±0.5
over 6 up to 30±0.1
±0.2

±0.5
±1.0
over 30 up to 120±0.1±0.3±0.8±1.5
over 120 up to 400±0.2±0.5
±1.2

±2.5
over 400 up to 1000±0.3
±0.8
±2.0±4.0
over 1000 up to 2000±0.5
±1.2
±3.0
±6.0
over 2000 up to 4000
±2.0±4.0±8.0

The acceptable standard tolerance in CNC Machining. 

General tolerances may be defined for linear or angular measurements, as well as for chamfers or other rounded parts. These tolerances specify standard tolerances for 4 different classes based on their range for a part dimension.

These classes are arranged into a chart that separates the different tolerance limits as very coarse (v), coarse (c), medium (m), and fine (f).

The range for each tolerance limit for a particular dimension bracket is based on International Standards (EN 20286, JIS B 0401, ISO 286, ISO 1829, ISO 2768, ANSI B4.1, ANSI B4.2).

What caused the tolerance during CNC Machining?

1)The materials

Common materials suitable for CNC processing are stainless steel, copper, steel, titanium and aluminum, etc. The hardness of the material is part of the reason for its tolerance, as well as pressure, temperature, shape, etc.

 the material for CNC machining

2) The operator

The inappropriate operation would cause tolerance

the process of CNC machining

3) Requirements from customer

As I mentioned above, some designs need specific tolerances to ensure matching with or interfering with other components. usually in custom CNC machining, required tolerance follows with order more often.

How should we avoid undesirable tolerance?

Modern machining processes are extremely precise. Yet, no machine can achieve precise accuracy. From the material of the part to the machining process used, various factors can cause differences. Thus, engineers assigned machining tolerances to the parts during the design process. To avoid undesirable tolerance engineers should work on many aspects.

the CNC machining process

The program should go one by one in order. Using the machine tool SF function, the operator fine-tunes the speed so that the machine tool can achieve the best cutting results. When clamping the tool, it is necessary to repeatedly check the program sheet and the program before loading and the operator needs to clamp as short as possible, and the cutting edge should not be too long. Cutting parameters, tolerances, margins, and speed feed settings should be reasonable. The tool requires the operator to inspect and replace it irregularly. Chip removal, air blowing, and oil flushing are not good: For down cutting of flat knife, R knife, and round nose knife, the speed and feed set should be reasonable. It is directly related to our machine tools, cutting tools, and cutting methods. So we have to understand the performance of the machine tool and make up the edges with burrs.

Too tight or too large tolerances will only increase machining time. In fact, unique tolerances are not required in many CNC parts, while there do exist used standard tolerances among different machines. If you are not sure about the tolerances, use the standard values as dimensions. Unless it’s necessary, try not to specify tolerances.

Our tolerance of CNC machining

ProcessTolerance
CNC Milling± 0.005″(metal)/±0.010″(plastic)
CNC Turning± 0.005″(metal)/±0.010″(plastic)
CNC Lathe± 0.005″(metal)/±0.010″(plastic)
Engraving± 0.005″(metal)/±0.010″(plastic)
Surface Finishing125RA