Undercut is the loss of base metal caused by welding.  

Typically it means a reduction in thickness of the base metal.  So your 0.06" is down to, say, 0.05" at the place where the weld was made.  The seriousness of undercut as a weld fault depends upon the manner in which stress is applied.  In some cases undercut is no big deal.  In other cases it may be lethal.

In my line of work, if I can see it without having to squint, it's too much.

Some joints must be machined to form a groove, then the groove is filled with molten weld metal.  Undercut can sometimes refer to the melting away of the side wall of the groove.  This usually occurs only in thick joints, so I doubt that it's relevant to your situation.

I'd recommend you hire a qualified welding inspector as well as the metallurgist.

See ANSI/AWS B1.11

Undercut is a gouge/or groove adjacent to the toe of a completed weld. This gouge may or may not be detrimental.

Many codes and specifications limit the depth of the undercut to a certain depth or percentage of the thickness. This gouge or notch can be a location for failure due to stress concentration at the location of the groove. I'm not familar with any of the sheet metal welding codes but they may have some acceptance criteria that may help.

It would be my guess that the manufacturer/designer of the equipment would have specifications for what would be acceptable and what would not.

Some codes/specifications allow for the are where the undercut exists to be ground/polished to remove the undercut provided the minimum design thickness is not reduced.

Good day

Gerald Austin
Iuka, Mississippi
http://www.weldinginspectionsvcs.com

For sheet steel welding code try AWS D1.3...  The AWS D1.1 has an excellent visual representation of Undercut and other weld defects in fig. 5.4 page 170 of the 2000 edition.

I agree with Pipewelder and offer the following:

AWS A3.0 Standard Welding Terms

Undercut - A groove melted into the base metal adjacent to the weld toe or weld root and left unfilled by weld metal.

Acceptance Criteria (sheet metal)

ANSI/AWS D1.3-98 see 6.1.1.3 Undercut

"The cumulative length of undercut shall be no longer than L/8, where L is the specified lenght of the weld.... Depth of undercut is not a subject of inspection and need not be measured"

Hope this helps.

Thanks everyone for the excellent feedback and comments. Your guidance on the appropriate standards to refer to helped nail down the meaning of undercut.

Good definitions of undercut. However I must question the wisdom or need  of using 10X magnification to evaluate undercut.  If this condition is truly detrimental to the equipment in question then an evaluation of the Welding procedure and or process may need to be performed . Let us  know how things turn out or if you have any more questions. One last thing ....how deep is this undercut?

Steve