Why the tighter dimensional tolerance means higher CNC machining Cost?

As we all know, there are many factors will affect the machined cost, time, and ease of manufacturing in CNC machining process, such as fillets, undercuts, threads, etc. However, we’d love to share why and how the tighter tolerance can increased the CNC machining cost today. 

Well, the tolerance is the acceptable range for a dimension for a CNC machined parts, which is determined by the designer based on the form, fit, and function of a part. It is important to keep in mind that a tighter tolerance can result in additional cost due to increased scrap, additional fixturing, and/or special measurement tools.

Longer cycle times can also add to the cost if the CNC machine needs to slow down to hold tighter tolerances. Depending on the tolerance call out and the geometry associated with it, costs can be more than double of what it would be to hold the standard tolerance. Tighter tolerances should only be used when it is necessary to meet the design criteria for the part.

Furthermore, overall geometric tolerances can be applied to the drawing for the machined parts. Based on the geometric tolerance and type of tolerance applied, cost may rise due to increased inspection times.

The best way to apply tolerances is to only apply tight and/or geometric tolerances to critical areas, which will help minimize costs of CNC machining.

If a drawing or specification sheet has not been provided by the customer, a company may provide general specifications to follow to manufacture a model. 

These specifications of the CNC machined parts may change from one company to another. In addition, some companies do not have default tolerances and will require the customer to provide the CNC machining specifications.

Listed below are what NINGBO OEM will follow when a customer has not provided any tolerance spec.

• Tolerance for all dimensions will be ±.005 in(0.127mm). for all metal parts and ±.010 in(0.254mm). for all plastic parts.

• The finish will be an as machined finish to a maximum 125 micro inches RMS.

• If tapped holes are not added on the quote and a provided drawing, they will not be added to the part and will be machined to the diameter specified in the model.

• No surface treatment (bead blast, anodize, powder coat, etc.) will be applied unless specifically requested by the customer.

• For metal machined parts, walls should be a minimum 0.030 in. (~0.75 mm) thick.

• For plastic machined parts, walls should be a minimum of 0.060 in. (~1.5 mm) thick

Finally, unless specifically called out by the designer, the standard tolerance used by Ningbo OEM is ±.005 in. for metal parts and ±.010 in. for plastic parts. If tighter tolerances (less than the standard, e.g. ±.002 in.) are required, the information must be communicated as to which dimensions require a more narrow range. As a point of reference, a piece of paper is about 0.003 in. thick.

To learn more about CNC machining recommendations to optimize your designs, reduce machining time, and lower your costs, please inquiry our engineer team instantly.