Guide to CNC Machining Tolerance Control
The process of CNC machining relies heavily on accuracy. However, no CNC machine archives absolute accuracy. Variances occur due occur either because of material factor or the machining process used. Thus, at Creatingway we assign all CNC machining processes a specific tolerance for parts during the designing process. Machining tolerance is the amount of allowable variance in part dimension. It is also known as dimensional accuracy. It has its minimum and maximum dimensional limits. Any part dimension that falls within these limits meets tolerance.
Here Is Further Insight Into CNC Machining Tolerance
Types of Machining Tolerances
Machining tolerance is the sum number of specific dimensions permissible to vary from a given value. We express it in a few ways such as:
- The allowable amount below and above a specific dimension. E.g. 0.14999 ±0.0001
- Allowable variance by itself. E.g. ±0.0001
- Lower and upper limits. E.g. 0.1500, 0.1498
These defined areas of the allowable range of tolerance are known as tolerance band. A larger difference between the lower and upper limits causes a looser tolerance band. A smaller difference results in tighter tolerances.
Another way of expressing tolerances is by using decimal places in numbering. The more the number of decimal places, the tighter the value of tolerance. Here are examples of decimal tolerance expressions:
- One decimal place ±0.2”
- Two decimal places ±0.01”
- Three decimal places ±0.003”
- Four decimal places ±0.0005”
Importance of CNC Machining Tolerance
Most manufacturers decline to begin making parts until all features have well-defined tolerance. It is because this is a reference point for understanding how parts interact with other parts. Lack of information limits our understanding of what the final design will be. Thus, the chances of getting inaccuracies are higher.
For instance, consider a shaft to fit in a part design. The part has a hole of a specific diameter for the shaft to accurately fit in it. If the hole is smaller than the shaft size, then it will not fit the shaft. Here are the possibilities of either providing or not providing tolerance.
Tolerance provided. We start on the project immediately. It is because we are aware of the required dimension limitations. Thus, it improves lead time and minimizes cost.
Tolerance not provided. At Creatingway, we resolve to use standard tolerance. For example, a ± 0.005” for a part. means the part will have diameter bigger or smaller by 0.005”. in case it achieves tolerances lower than this limit it requires retooling. This only raises the turnaround time and increases cost.
Factors Affecting CNC Machining Tolerances
The Material
The behaviors of material under stress varies. Also, some materials are easier to work with than others. The particulars on material properties should be taken into account while finding out tolerance. These characteristics affecting the machining ability of the material. Here are a few material properties below:
- Heat stability: nonmetallic materials, for example, plastics, go through warping when exposed to heat. This limits the range of the machining process to use since we have to account for the heat. Considering this aspect will keep the tolerance in an acceptable range.
- Abrasiveness: highly abrasive materials are hard to tool. Examples of such material include phenolics and glass laminates. They cause wearing down of the cutting tool resulting in errors while machining.
- Rigidity and hardness: materials that are flexible and soft variably change dimensions. This makes it more difficult to machine them. Examples of such materials include: foam, polyisocyanurate, and polyurethane
The Machining Type
A CNC machining method significantly affects possible tolerances of the finished part. Some processes might be more precise than others.
- Rail Cutting: the process involves the manual operation of a rail saw. Due to its manual properties, it needs a larger tolerance band. This will achieve better accuracy.
- Shear Cutting: the process involves applying force great enough to cause material failure. We often apply a set of blades or a punch and die. Thus, it is not suitable for brittle or soft material. Since they can easily warp and break in response to the sheer force. It is thus not capable of holding tighter tolerances less than ±0.015”.
- CNC Screw Machining: the process uses a disc cam in feeding a workpiece. Since the part moves instead of the tool, it results in less vibration and deflections in the part. This allows for greater achievable precision. We recommend it for machining materials such as phenolic, foam, and plastic parts.
- Steel Rule Die Cutting: the process uses a custom die punch to make specific shapes. It is however not good for brittle or soft materials.
Plating and Finishes
These two properties add a small amount of material on the part surface. This alters the dimension of the final product and should be taken into account prior to design and production.
Cost
We recommend achieving precise tolerance. However, getting tighter tolerances than needed costs more. If working on a three decimal place tolerance, do not exceed a four decimal place one. Contact Creatingway now for your CNC machining services.
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