Homebrew PCBs

"Stefan Trethan" <stefan_trethan@...> writes:
> (is it inherent to the material or a code)?

Sometimes.  Check the code on the wheel, it will have some letters
(material), numbers (grit), and letters (hardness).

http://www.woodcentral.com/cgi-bin/readarticle.pl?dir=handtools&file=articles_557.shtml

http://www.peterchild.co.uk/info1/wheelinfo.htm