Cheat Sheet for Designing Sheet Metal Pieces

If you have to design a sheet metal piece, you need to be careful. Sheet metal uses completely different fabrication techniques, equipment, and expertise as compared to a standard solid piece. For better results, we put together this cheat sheet for designing sheet metal pieces. 

Sheet Metal Isn’t Like a Solid Part

The biggest takeaway is that sheet metal doesn’t act the same way a solid block of metal does. Even if both pieces use the same alloy of aluminum, the machining process is completely different.

This is strictly due to the thickness of the material. 

A CNC mill can quickly cut a complex geometry into a piece that’s 1/4” thick. If the same process was attempted on a piece less than 1/10” thick, the piece will bend, chatter, and deform. Even using a vice grip to hold the part down can ruin the piece.

When you’re designing a sheet metal piece, you can’t use the same methods that you would use on a solid part.

Understanding the Flat Layout

One of the big differences between the two design styles is the use of a “flat layout”. If you’re making a four-walled box out of sheet metal, the flat layout is what the raw material looks like before all the bends are made.

In other words, you can unfold your final part to get the flat layout. This can be automatically done on your favorite 3D CAD software (assuming you do the design correctly). 

In a perfect world, a machinist will create a replica of your flat layout then do the final bends to get the finished piece. 

If the piece is getting CNC cut via laser, plasma, or water, then this flat model can be fed into the machine to automatically create the cuts.

If you give a machinist a dimensioned folded part, then they’ll have to do a lot of work to generate the flat layout which will be used for fabrication.

What Tolerance Should You Use?

Another critical difference is the type of tolerances you can allow. With a solid piece, you can design for 0.125” tolerances and the piece might still be perfect in the end. With sheet metal, you need to get more strict.

A ±0.005” tolerance is basically the industry standard. It’s tight enough to get the right results but loose enough to account for some of the equipment used in fabrication.

For your bends, a ±0.010” tolerance is fair. Precision brakes can typically get within 4 mils (0.004”), but it’s good to give some extra wiggle room when the design allows for it. Tighter tolerances will cost you a lot more money for each part. 

Use Built-In Sheet Metal Processes on CAD

As we mentioned, 3D CAD has a lot of useful tools for you to use. With a trained draftsman/ designer, your modeled part can automatically include relief cuts, the correct bend radius, flanges that are feasible, and realistic radii.

It’s worth noting that it’s easy to make a mistake, even with CAD helping you. It’s only as good as the parameters that are initially inputted. Without the right amount of expertise, you could submit a design that can’t be machined by a sheet metal fabricator.

Our advice? Use CAD as a starting point and interface with a fab shop before submitting it for fabrication. They’ll be able to fill the gaps in knowledge and help you design a part that will be easily fabricated.

Look at the Corners

The corners are critical. A “corner” is where two bent edges come together. The corner of a solid part might not seem really important, but you need to carefully look at your sheet metal.

At this interface, the two bent flanges will either underlap, overlap, or butt. 

This matters because the metal that’s being bent has a certain level of memory. If you bent a piece of stainless steel sheet 90-degrees, the resulting piece will not be exactly 90-degrees. It actually has to be overbent to achieve the exact angle.

In simpler terms, it’s hard to fabricate perfectly perpendicular corners. With overbending and these interface styles, your design will be limited when it comes to geometric perfection.

The Flange Depth Matters

A flange is any part that’s bent and has a flat surface after the bend. If you look at an L-bracket, the top (or bottom) face can be considered the flange.

Again, this might not seem like an area that you need to focus on. However, the flange depth or length has some constraints.

If the shop is using a V die to make the flange, there is a minimum and maximum length that any flange can be. In general, the flange needs to be at least 3 (though we suggest 4 times) longer than the thickness of the material.

For sheet metal that’s 0.120” thick, that means a minimum flange length of 0.480” long.  If it’s shorter than that, there’s no way a standard bending machine can make the flange.

Conclusion

Designing a sheet metal piece is very different than a solid piece. Using these design considerations will help you get the best results for your upcoming sheet metal piece. For even better results, you need to use the right fab shop that has experienced sheet metal machinists. You can choose Rapid Axis for your sheet metal needs. Reach out to us today for a free quote.

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