Sheetmetal Design - Lessons from Experience

Flat-Pattern and DXF Files

When working with sheetmetal, the most important aspects are the flat pattern and DXF file. DXF file is underrated with design engineers. My early years working with sheetmetal parts, DXF file saved my life. DXF is a 1:1 scale file, and commonly used for laser cutters, plasma cutters, routers, etc. Very useful file format to be aware of.

A handy tip is to print the DXF file in scale 1:1, even if it prints on multiple A4 pages. Then use spray adhesive to stick these sheets of paper on the sheetmetal. Do your cutting, drilling, and bending according to the pasted drawing. When finished, clean the part with citrus cleaner. For a prototype, this is quicker than using a scriber and marking on sheetmetal, unless it has a precise/intricate feature.

Back to DXFs. Just exporting a DXF of a flat pattern from SolidWorks or Fusion 360 is not enough and will get you in trouble with the suppliers. First thing is to check the file and dimensions of the DXF. A free software that I highly recommend is called eDrawings Viewer from Dassault Systemes. Easy to use and quick software to inspect DXF files and part dimensions. 

Problems you will encounter with DXFs:

• Dimensions by default are imperial, if you work in Asia, Australia or Europe, this is a killer issue. Usually software doesn’t allow you to select units while exporting, but it is something that may be available in future releases. Scale factor to convert imperial to metric is 1/25.4 = 0.0393700787401575.
• Curved sections may over curve and in eDrawings these over curved features are visible when you hover your mouse over them. This usually happens when the radius is too large, so to fix this issue, you can try and reduce the radius. Another fix is to convert your model into splines when saving as DXF, if that option is available in your software.

Manufacturing/Fabrication

Sheetmetal fabrication sounds easy and it actually is when making a prototype, however as soon as you move into production stage, which is a typical requirement for all businesses, we run into limitations. These limitations are what you need to be aware of when dealing with fabricators.

Limitations include but are not limited to:

• Bending/Folding machine capacities - this will limit how many bends you can do and at what angles. A simple example is a U-Channel, if walls are larger than the base, most machines are not suited for this.
• Cutting technology used - plasma, laser, water jet, routing or guillotine - this will limit the thickness of your material. An example would be 10 or 12mm thick material, most commercial medium-sized laser cutters can cut only up to 8mm thick sheets.
• Welding type required - spot welding, TIG, MIG, etc. - not all fabricators have all types of welding equipment. A recent example is that we required a water-tight metal box but our usual fabricator didn't have spot welding machine and they could only tack weld or opt for a more expensive option which is unnecessary. So, I had to find a local fabricator who can achieve cutting and bending along with spot welding.

Typical Material Type and Thicknesses

Every country and region is different in terms of which raw material is readily available and which is not. This is very important because it will impact the cost per unit. An example is the Chinese industry, the sheetmetal thicknesses they have access to and use as common material, it is very difficult to find the same in Australia.

My advise on this issue is to pick your top three fabricators, whether local or overseas, ask them to list out all thicknesses available in Mild Steel (MS), Zincanneal Steel (ZA), and Galvanised Steel (GAL), as standard materials or locally available. These are your major material categories you will use.

Once you have this list in front of you, now you can design according to what's available and it will keep the cost low too. Additionally, it will save fabricators time to search for some odd material thickness we allocated. On the other hand, it will save us time and effort by not changing the model and drawings according to what the fabricator suggested on quoting stage.

Summary

1. Print DXF scale 1:1 and paste it on sheetmetal using spray adhesive to save you from scribing, do the operations, then clean with citrus cleaner when done.
2. eDrawings Viewer from Dassault Systemes is free of charge and versatile software to measure and check your DXF and DWG files.
3. Always check units of a DXF file after export, if in Imperial, convert it to Metric by scaling at 0.0393700787401575.
4. Always check the curved sections of the DXF, when you hover over the curve with your mouse, it should highlight only what you expect, anything abnormal, you need to fix the model by either reducing the radius/diametre or converting the flat pattern to splines.
5. Find out from your fabricator what machines they have and what they can achieve and can't achieve. Site visit will also be very helpful. Use this information when designing.
6. Find out from your fabricator what material thicknesses and steel types are locally and readily available. Use this information when designing.