Design Considerations and Standard Steel Thicknesses

A long time ago I used to be an engineer. My strength was designing railway systems so that trains didn’t crash into each other at high speeds. One of the big challenges in doing this was working with all the constraints – geography, line speed, safety restrictions, layout, etc. – to produce an elegant design that delivered the requirements with the least amount of resources. I’d always find it really frustrating when we had to change the design for trivial reasons and this thought has stayed with me in the steel business.

One frustration that I was talking about with a design engineer the other day was how the use of metric and imperial measurements could distort design and production.

Metric and Imperial

The design engineer was designing a pressure vessel that according to the ASME code needed to have an 18 mm shell. Because many of their suppliers used imperial measurement she then converted that into inches 3/4″ or 19.05 mm and the procurement team sent the the RFQ out. We received the RFQ and because we work in a metric system went UP to the nearest thickness to quote 20 mm.

Over Budget

And then for a mysterious reason we were over budget by 11%. The additional weight of the shell as it was increased from 18 mm to 20 mm with no design justification at all. Not so mysterious really – both of us had built into our requirements assumptions about what the other required or could do – and they were wrong.

Safety Padding

Now I don’t know how your design department works but from experience I can see how we all pad the thickness a little bit (whatever the measurement system) for safety’s sake. When you look at the overall impact though the combined result can be expensive over engineered kit that provides an uneeded and unpaid for level of safety.

Ask For What You Need!

So how to avoid the problem? The easiest way is to ask your stockholder for the dimensions that you require – based on the design. Then you leave it to him to find a plate that meets your requirements. It also makes it very clear the premium that you pay for availability rather than waiting for new rolling.

Or Know Your Stockholder

The alternative is to have a very good understanding of the thicknesses that your stockholder stocks so that you don’t make false assumptions, but that means more work.

This won’t save you money every time, but every so often do a reality check and ask: “is my stockholder offering what I really need, or what he thinks I want?

Standard Thicknesses

And as I’ve been talking about thicknesses here’s a quick list of sizes that it’s reasonable to design to if you’re buying from stock:

5163890180
61740100190
81845110200
92050120210
102255125230
112560130250
122865140
133070150
143275160
153580170

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