Many products must be manufactured to exacting standards. Questions like: Is it straight enough? How flat is it? How uniform must the wall thickness be? are not abstract. The specified acceptable range of deviation from a given dimension is called the tolerance. For many applications where aluminum extrusion will be part of a component assembly, dimensional tolerances are critical. Designers should understand the standard dimensional tolerances for commercially produced extrusions. Tight tolerances reduce productivity, leading to higher production costs. Judicious use of high tolerances only where productivity of the extrusion is critical will help control costs and deadlines.
Here are six tips to help you use the most appropriate dimensions and tolerances for your aluminum extrusion parts.
Choose Only Critical Dimensions
Trying to achieve tolerances on non-critical dimensions is a major source of hidden costs. Often, designers over-emphasize tolerances that will not affect the form, fit, or function of the final product. These non-critical dimensions result in longer setups or repeat runs, leading to costly, delayed, or rushed deliveries. Designers can reduce these costs by only targeting the most critical product dimensions.
Understand What Tolerances Are Achievable
When designers have defined the most critical product dimensions, the next step is to understand what tolerances can be achieved based on the specific manufacturing process. Tolerances are affected by a variety of factors, including press size, billet temperature, extrusion speed, die shape and type, cooling time, and air temperature. To help designers, the Aluminum Association has established industry standard tolerances for extruded products. Designers can use these as a reference guide when designing a product. If the required tolerance is tighter than the standard, discuss the required tolerance with an aluminum extrusion expert.
Communicate Early with Your Aluminum Extrusion Partner
In order to understand tolerance expectations, it is important to involve aluminum extrusion experts in the initial stages of design. It is important that designers rely on extrusion experts to understand tolerance standards and how various factors such as aluminum temperature, cooling time, and extrusion speed affect each part of the design.
Establish Key Product Metric (CpK) Values
Establishing the CpK value to be used is a key factor in determining dimensional tolerance capability. Some CpK requirements will require a capability study to determine how well the extrusion process can meet the specified dimensions. Although this is an added cost, it will give the extruder an understanding of process capability and repeatability. The process capability index measures how well your process produces output that meets customer requirements. Determining the accuracy and precision of your process will allow you to estimate the number of failures that can be expected.
Understanding Geometric Tolerances
When tolerances are met, parts fit together well. They perform as expected and do not require unnecessary machining. Geometric dimensioning and tolerancing can be used to specify the shape of an extrusion on an engineering drawing. It has been likened to a technical language that has a uniform meaning for all; this can greatly improve communication throughout the design-to cnc manufacturing cycle. It makes it easier to capture design intent by giving designers and draftsmen better tools to communicate their needs. It adds a new dimension to drafting skills that define a part and its features. When engineers focus on fit and function, geometric tolerances can be designed to better control the fit and functional relationships of a part.
Design for Functionality and Manufacturability
Dimensioning a part’s functionality without considering manufacturability often increases cost and frustration. Keeping a simple dimensioning format will reduce costs by reducing redundant machining operations, re-clamping, and handling operations, while also reducing process variation.
These tips offer alternatives to default blocking tolerances. By working with your aluminum extrusion manufacturer early in the design process, you can plan and design for both functional and manufacturing goals.