We received a request from a reader that wanted us to touch base on two very common sheet metal drawing mistakes. So we have created this blog post to strictly focus on these issues. When it comes to mechanical engineering and actual sheet metal fabrication, there are a couple grey areas that engineers should know about if they have never fabricated sheet metal. One has to deal with flat patterns and the other is sheet metal bend deductions.
Many times engineers will include the flat pattern on the dimensional drawings to illustrate what it looks like prior to forming and finishing. However, it is pointless to include flat patterns on prints if the part is formed and finished. Why? The dimensions of the flat pattern will not be taken into consideration when manufacturing. Reason being is that when manufacturing, prints need to illustrate the finished part dimensions. Since each fabricator or manufacturer has their own bend deductions, it is up to the manufacturer how to dimension the flat pattern to result in the finished part as per the drawing. Therefore, when designing a part, some engineering/design programs may ask for you to designate the K-Factor. Since the K-Factor is relative to the flat pattern, and the flat pattern is impractical to the fabricator, the K-Factor is not significant.
So to touch a bit more on the bend deduction or K-Factor (for the flat pattern), you might be asking, why do manufacturers have different bend deductions? Why can’t they all be the same and make things less complicated? The reason the flat pattern is insignificant to a manufacturer is because they have their own bend deductions based on the bend radius and angle, the material thickness and stiffness, the tooling used, and how much contact the tooling makes with the metal. Not all fabricators have the same tooling. Therefore, by providing prints that illustrate the complete finished part and its corresponding dimensions, the sheet metal fabricator is able to create their own bend deductions on the part that will meet the print.
It is important to add (from an estimator of manufacturing standpoint) that is it practical/necessary for the designer to always view the flat pattern of a formed part to ensure their design will unfold. We have seen numerous instances where the model created cannot be unfolded without conflict. On a side note, the flat pattern is definitely useful for estimating the fabrication of a part. If we have a flat pattern, we are able to use that as the part size and do not have to spend time calculating the estimated part size from the formed dimensions. It does serve a useful purpose for estimating, just not for manufacturing and/or inspection of the part.
When engineering a part that will be fabricated by an outside manufacturer, it can be helpful to speak to the manufacturer to find out about the machines they use and their capabilities. It’s also helpful to know what in-house tooling they have which can help save time and cost on your end because the parts won’t have to be customized with custom tooling. When providing the manufacturer with drawings to manufacture to, leave the ‘how-to fabricate’ up to them since that’s their expertise, and DO NOT include flat patterns!
For more information about manufacturing prints, check out these articles: