Tolerance Guidelines for CNC Machining
In the manufacturing and fabrication process, variation is almost always involved between individual parts produced and between projects. Regardless of these variations, it is needful to have a method of making sure all components function as designed. This is where the idea of tolerances comes in to the mix. Tolerances are the margins of acceptable error for an individual part in the manufacturing process. At JBC Machine, we offer CNC machining services that are able to fabricate parts based on a variety of tolerances – including tight and standard – based on the customer’s requirements.
Optimizing Tolerances for CNC Machined Parts
Standard tolerances used in machining operations are typically as follows:
- For metal parts: +/- .005
- For plastic parts: +/- .010
However, some parts require an even smaller deviation for certain dimensions to ensure a correct fit.
Some applications may require tighter tolerances. In these cases, it is beneficial to restrict the use of tighter tolerances to only required surfaces or areas of the part in order to avoid unnecessary additional expenses. Using the standard tolerance is often the way to go if extremely tight tolerances are not needed for a part – this will also minimize the time required for machining and reduce costs.
The designer should establish the tolerances for a part before handing it over to the manufacturer or fabricator for CNC machining operations. This will help prevent the need for retooling and ensure customer satisfaction.
When the customer does not designate a degree of tolerance for a part, the machinist will often apply the standard tolerance, which can often be found on a machining tolerance chart. Standard tolerances are commonly set by various standards bodies, such as ANSI, ASME, and ISO.
Other types of tolerances used by manufacturers and fabricators include bilateral, unilateral, limit, and geometric dimensioning and tolerancing.
With a bilateral tolerance, the deviation from the dimension in question may be positive or negative – in other words, slightly smaller or slightly bigger. An example would be +/- 0.02.
With a unilateral tolerance, the allowed deviation is only in one direction – positive or negative.
A limit tolerance is machining tolerance designated by a range of measurement values in which the part must be manufactured – for instance 15 – 15.5 mm. The part must be manufactured to fall somewhere between that lower and upper limit.
Geometric Dimensioning and Tolerancing
The type of machining tolerance defines specific geometric features of the machined component such as its concentricity, flatness, and true position.