Resources / Solutions / Effects of Dimensioning and Tolerance

In today’s fast paced world, it is sometimes hard to find the time to define the necessary tolerances that a design can live with. We tend to default to title block tolerances because we know they will work. However, this can add unnecessary cost to the part. I would like to give you some tips and solutions that will improve your designs for manufacturability and keep costs competitive.

One hidden cost driver can be trying to achieve tolerances on dimensions that are not critical. Many times we concentrate on getting dimensions in tolerance that do not affect form, fit or function. Usually this results in reruns, longer setups, or requests for print deviations causing late or rush deliveries. These are costs that can be decreased by identifying the critical dimensions thus reducing setup time, inspection time and expediting costs.

Now that we have identified the critical dimensions, we also need to understand tolerances achievable dependent on the manufacturing process. There are multiple extrusion parameters that affect tolerances such as billet temperature, extrusion speed, die shape and type, cooling time, amount of post-stretch, air temperature, and multiple die copies. My belief is that it is the extruder’s responsibility to educate the customer on the impact of these parameters on each part of the design. The Aluminum Association has developed industry standard tolerances for extruded product and the information is available on our Web site at http://www.alexandriaextrusion.com/aec/engineering/technicalguides.cfm. These tolerances may seem large but Alexandria Extrusion Company can hold tighter tolerances than what is stated. These standards can be used as a reference in designing and show how the tolerances are affected by the differences in feature or size of dimension.

Involving your extruder in the initial stages of your design is key to understanding what you can expect for tolerances. The following quick guide can be used as a rule of thumb in the early design stages:

• For linear dimensions use a tolerance of .008 per inch of dimension with the exception of wall thickness.

• For wall thickness use 10% of the wall dimension.

• For angular tolerances on surfaces longer than .50 inch use +/- 1 degree to +/- 2 degrees depending on the dimension location. For surfaces under .50 inch long the angular tolerance increases dependent on length. The shorter the length the larger the tolerance needed.

• For straightness use .0125 per foot of length.

• For twist use ½ degree per foot of length.

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 on those dimensions. Although this is an added cost it will allow us to understand process capability and repeatability. A 1.33 cpk requirement in effect reduces the tolerance band to 75%. Likewise, requiring a 1.67 cpk reduces the tolerance band to 60%. It is very important to verify your extruder’s ability to control their processes to attain your stated CPK’s. (Exhibit #1) This will eliminate many issues as the product goes into production.

Exhibit #1

Another critical factor that I would like to talk about is geometric tolerancing and its application to the extrusion process. Generally when an extruder talks about flatness of an extrusion he is talking about the cross-sectional flatness of the profile and when he talks about straightness he means the bow in the length of the part. This is just an awareness of the difference so if you are asking for flatness of the whole surface as defined with geometrics make sure the extruder understands. Some geometric tolerances such as profile of a surface can cause increased inspection time and add significant cost to the part. I normally recommend using profile of a line on the cross-sectional profile in conjunction with a twist and straightness call out to achieve your profile of a surface. This allows the extruder to verify the profile prior to machining to ensure its functionality. (Exhibit #2)

Exhibit #2

Dimensioning formats can also add cost and frustration when a part is dimensioned for functionality and doesn’t take into account manufacturability. Examples of this would be the use of multiple datums on the same plane or datums established from numerous manufacturing steps. Another example of this would be having a part that has an acceptable saw cut tolerance; however, the machined features are dimensioned from both ends of the part. Dependent on the manufacturing method this could cause an added milling to length, multi clamping or touch probing operation that adds cost. Keep the dimensioning format as simple as possible by using the traditional primary, secondary and tertiary datums whenever possible. This will help keep cost down by decreasing excess machining operations, reclamping and handling operations and thus reducing process variation. Working with the extruder in the early stages of the design can benefit in the long run. Another tip when designing a symmetrical shape is to add an identification mark to allow for proper orientation, thus reducing tolerance variance that is inherent in the extrusion process. (Exhibit #2)

If this type of engineering assistance would be beneficial to you, please forward your prints in the following formats: (.fldprt), (.iges), (.step), (.dwg) and (.dfx) to Rodney Floding, Estimating Lead/Designer, and his design team at rfloding@alex-extrusion.com. Many of our customers have found our complimentary extrusion seminar to be extremely valuable in helping design their products, with outcomes of 50% lead time reduction and a significant reduction in piece part cost. Please browse through the engineering section of our Web site for more useful information at http://www.alexandriaextrusion.com/aec/engineering/.