Through my website, someone from a manufacturing company asked how to
calculate thinning in a sheet metal part made from 2mm thick high
strength steel. My response:
First, I assume the nominal or expected thickness is 2mm. But this is
just the aim thickness. The steel mill is allowed to ship product that
has some deviation from this gauge - maybe 1.85mm to 2.15mm as an
example. So you must get an accurate measurement of the actual starting
thickness. You should try to get it from the blank you will use to form
the part in question, or at least from an adjacent blank. The reason for
this is that there is likely a thickness variation down the length of
the coil. This normal and inherent variation occurs even when the
product is completely within specification. Similarly, you do not want
to make your thickness measurement at the coil edge. Due to a rolling
phenomenon called "crown," the edges of the coil are usually thinner
than the rest of the width. Again, this is allowed within most
specifications in that all minimum gauge measurements are to be made no
closer than 25mm from the coil edge (as an example). Across the coil
width, the thickest part is at the center width position of the master
coil. For this reason, I recommend taking your thickness measurement at
the 1/4 width or 3/4 width position.
The next step is to form the part. In the areas of interest on this
formed part, it is necessary to get an accurate thickness measurement.
Some companies will cut the part and use a micrometer to measure the
as-formed thickness, while others will use a calibrated ultrasonic
thickness gauge. The second approach does not destroy the sample, and
the part can be put back in for normal processing after it is measured.
If your area of interest has feature lines or small radii, it is
necessary to use the proper tools and techniques to get an accurate
thickness reading.
At this point, you have two measurements: the starting thickness of your
sheet metal (call that S) and the formed thickness of your part at the
location of interest (call that F). The percent thinning is calculated as
T% = 100 * (S-F)/S
If you have accurate measurements, you can have confidence in your
results. Best of luck!
Danny Schaeffler
Engineering Quality Solutions, Inc. 4M Partners, LLC
http://www.EQSgroup.com http://www.Learning4M.com
