wrote in message
On Wed, 27 Jul 2011 18:46:58 -0400, "Stephen B."
wrote:

"Tim Wescott" wrote
On Wed, 27 Jul 2011 21:20:13 +0000, etpm wrote:

The standard taper 3/4 inch per foot is listed in Machinery's
Handbook
as 3, 34, 47. When I calculate it I get 3,34,34. I get this
answer
using
my TI calculator and my computer. Looking on the web I find the
3,34,47
answer. It's the angle I've always used. So how come I get
something
different? On my TI-30X calculator I divide .75 by 12 which
equals
.0625. This is the tangent. I then use the inverse tangent
function
to
get the answer: 3.576334375. Then I use the DDDMS function which
returns the answer: 3,34,34 80. So what's going on??????
Thanks,
Confused machinist Eric

A long-ago typo that never got corrected? Was the thing on the
web
a
table? Maybe they cribbed it out of Machinery's, complete with
typo?

I get your answer, by the way.


It is a question of significant digits.

0.75 in/ft calculated to 3 significant digits is 3.58 which comes
out
to exactly 3 deg 34 min 48 seconds.

the math you are calculating is the equivalent to 0.750000000 in/ft
a
far more precise taper.
Thanks Steve and Tim for your replies. It seems that the table on
the
web for prop shafts AND Machinery's Handbook are wrong. Kinda
destroys
my faith in Machinery's Handbook. It's the first mistake I've ever
found in it. Maybe I should write them a letter.
Eric

I would not go so far as to say they are Wrong. It is just less
precise. Calculating the taper from the angles I get using my excel
sheet:

3 deg 34 min 47 sec = 0.750712... In/ft an error of 0.000712...
3 deg 34 min 34 sec = 0.749953... In/ft an error of 0.000046...
3 deg 34 min 35 sec = 0.750011... In/ft an error of 0.000011...

As you can see the it disagrees with your calculations as to the
correct number of seconds is closest.

Remember the problem with IIRC the Pentium processors way back when?
No one was bothered except those that needed extremely high precision
theoretical math.