On Jan 2, 4:26*pm, "Jim Wilkins" wrote:
"Tom Gardner" Mars@Tacks wrote in message

...

I was always taught (while working toward my masters in Mechanical
Engineering) that the closer the tolerances the poorer the design.

I may have found the history of the 0.030" tolerance. A WW2 aircraft
sheet metal book I have shows sample prints for minor interior
sheet-metal brackets from Douglas Aircraft on which the default
tolerance for fractional dimensions is 1/32". The default for decimals
is 0.010" unless otherwise specified in the form of max over min, such
as 0.147 over 0.140 for clearance for the wood screws that secure cork
rings onto a thermos bottle holder.
jsw

Sheet metal parts and machined parts aren't in the same class.

I've been a machinist since 1993 (graduated adult education for
machining / toolmaking in 1994 with 1080 hours credited my
apprenticeship) I have worked in the machining trade consistently ever
since. I've never seen a + / - .030 wide open tolerance on a aerospace
machined part in the 20 years I've been in the machining trade.

Neither has the person who posted in this thread and who worked for
Boeing for many years.