Many products must be manufactured to precise standards. Questions like: How straight is straight enough? How flat is flat enough? How uniform must a wall thickness be to be acceptable? are not abstract. The specified, acceptable range of deviation from a given dimension is known as a tolerance. For many applications, in which an aluminum extrusion will be part of an assembly of components, dimensional tolerances are critical. Designers should be aware of the standard dimensional tolerances to which extrusions are commercially produced. Tight tolerances can decrease productivity, which leads to higher production costs. Judicious use of high tolerances only where they are essential to the productivity of the profile will help keep costs and deadlines in check.
Here are six tips to help you use the most appropriate dimensioning and tolerance for your aluminum extrusion parts.
Choose Only Critical Dimensions
Trying to achieve tolerances on non-critical dimensions is a major source of hidden costs. Frequently, designers put too much emphasis on tolerances that do not affect the form, fit or function of the final product. These non-critical dimensions can result in longer setups or repeat runs, which can lead to costly, late or rush deliveries. Designers can decrease those costs by identifying only the most critical product dimensions.
Understand Which Tolerances are Achievable
When the designer has defined the most critical product dimensions, the next step is to understand which tolerances are achievable based on the specific manufacturing process. Tolerances are affected by multiple factors, including press size, billet temperature, extrusion speed, die shape and type, cooling time, and air temperature. To help designers, the Aluminum Association has developed industry standard tolerances for extruded products. Designers can use these as a reference guide when designing a product. If tighter tolerances than the standards are needed discuss the required tolerances with your aluminum extrusion expert.
Collaborate Early With Your Aluminum Extruder
To understand tolerance expectations, it’s important to involve the aluminum extrusion experts in the initial stages of design. It is important that the designers rely on the extrusion professionals to understand tolerance standards as well as how various factors, including aluminum temperature, cooling time and the speed of extrusion – impact each part of the design.
Establish Critical Product Measurement (CpK) Values
Establishing the CpK value to be used is a critical element in determining capability of dimensional tolerances. Some CpK requirements will necessitate a capability study to determine the extent to which the extrusion process can meet specified dimensions. Although this is an added cost it will allow the extruder to understand process capability and repeatability. Process capability indices measure the degree to which your process produces output that meets the customer’s specification. Determining the accuracy and precision of your process will allow you to estimate the number of failures that can be anticipated.
Understand Geometric Tolerancing
When tolerances are met, parts fit together well. They perform as intended, 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 is likened to a technical language, which has uniform meaning to all; this can vastly improve communication in the cycle from design to manufacture. It more readily captures the design intent by providing designers and drafters better tools with which to communicate their needs. It adds a new dimension to drawing skills in defining the part and its features. When an engineer is concerned about fit and function geometric tolerancing is structured to better control parts in a fit-and-function relationship.
Design for Both Functionality and Manufacturability
Dimensioning a part for functionality without considering manufacturability often creates added cost and frustration. Keeping the dimensioning format simple as will help keep costs down by reducing excess machining operations, re-clamping and handling operations, while also reducing process variation.
These tips offer an alternative to defaulting to block tolerances. By partnering with your aluminum extrusion manufacturer early in the design process you can plan and design for both functional and manufacturing goals.