CNC parts tolerances that every designer needs to know

Tolerance is the acceptable range of dimensions determined by the designer based on the shape, fit and function of the part. Understanding how CNC machining tolerances affect cost, manufacturing process selection, inspection options and materials can help you better determine your product design.

As a CNC custom parts manufacturer, we are obligated to help you save costs and provide you with professional advice and CNC machining prototypes. too tight tolerances may require the part to undergo a secondary grinding or EDM operation to complete, unnecessarily increasing cost and lead time. Tolerances that are “too loose” or inconsistent with the tolerances of mating parts can result in an impossible assembly, requiring rework, or at worst rendering the finished product unusable.

1. Tighter tolerances mean increased costs

It’s important to remember that tighter tolerances cost more due to increased scrap, extra fixtures, special measurement tools and/or longer cycle times, as the machine may need to be slowed down to maintain tighter tolerances. Depending on the tolerance callout and the geometry associated with it, the cost can be more than twice as much as maintaining standard tolerances.

Global geometric tolerances can also be applied to drawings of parts. Depending on the geometric tolerance and the type of tolerance applied, additional costs may be incurred due to increased inspection time.

The best way to apply tolerances is to only apply tight or geometric tolerances to critical areas when it is necessary to meet design criteria to minimize cost.

2. Tighter tolerances could mean changes to the manufacturing process

Specifying tighter tolerances than standard tolerances can actually change the optimal manufacturing process for a part. For example, a hole that can be machined on an end mill within one tolerance may need to be drilled or even ground on a lathe within a tighter tolerance, increasing installation costs and lead times.

3. Tighter tolerances can change inspection requirements

Remember that when adding tolerances to a part, you should consider how features will be checked. If a feature is difficult to machine, it is likely to be difficult to measure as well. Certain functions require specialized inspection equipment, which can increase part costs. Modo Rapid offer CMM(COORDINATE MEASURING MACHINES)report, and hardness testing machine.

4. Tolerance depends on material

The difficulty of manufacturing a part to a specific tolerance can be very material dependent. Generally, the softer the material, the harder it is to maintain the specified tolerances as the material will bend when cut. Plastics such as nylon, HDPE, and PEEK may not have the tight tolerances that steel or aluminum do without special tooling considerations.

5. Standardized tolerances for CNC machining

If the standard prototype and production machining tolerance is 0.13mm, which means that the position, width, length, thickness or diameter of any part feature will not deviate from the nominal value by more than this value. But if you need higher precision and need to judge based on the geometry and material of the part, please make sure to indicate this in the part design when uploading the file for quotation.

6. Surface Roughness for Machining Tolerances

In addition to length, width, hole size, there are also part tolerances such as surface roughness we need to take into consideration. In the standard product, the surface roughness of flat and vertical surfaces is equal to 63µin. And a curved surface equal to 125µin is better.

This is an adequate finish for most uses, but for decorative surfaces on metal parts we are usually able to improve the appearance with light sandblasting. If you need a smoother surface, please specify in your design and we will try our best to accommodate your request.

7. Geometric Dimensions and Tolerances

Concentricity: The rings on the bullseye are concentric, just like the wheels on a car are concentric with the axle. If the drilled or reamed hole must be exactly the same as the coaxial counterbore or circular boss, concentricity dimensioning is the best way to ensure this.

8. Flatness

Milled surfaces are generally very flat, but some warping may occur after the part is removed from the machine due to internal material stress or clamping forces during machining, especially thin-walled and plastic parts. The GD&T flatness tolerance controls this by defining two parallel planes in which the milled surface must lie.