Re: [CAD_CAM_EDM_DRO] Re: Rotary to linear translation - an interesting idea.

"D.F.S." wrote:

> > the tape must travel around
> > the radius faster in order to maintain it's length since the tape is
>
> > not stretchy (to that degree anyway).
> Nope. Not True.
>
> The length of the Tape/Wire/Cord/Shim stock is already of a length
> to allow for the extra bends it has in it.
>
> Please Excuse the ugly ascii art.
>
> C
> /0\
> 0/ \0
> J ------------------ --------------------------- K
> A B
>
> The presumption above is:
> The "Wire" must move from A to C to B in the same 'time' it would
> have taken it at a constant speed to travel from A to B.
>
> This in simply not true.

Yes, you are absolutely right. No way the tape moves from a to b in the

same time as the linear move. BUT, there is a slight change in the
effective
diameter of the ribbon passing around the wheel. This is just like the
pitch diameter of a belt, due to the inner surface of the ribbon being
in
compression. With a very thin ribbon, the effect would likely be
smaller than
good measuring instruments could measure (standard .0001" mikes,
for instance). With a braided steel cable, however, it will be
substantial.
If one were to make a DRO, and accurately measure the diameter of the
drum
to have, say, a circumference of 1.000", and then pull 1" of wire across
the
drum, you would definitely count MORE than one full revolution.
Assuming
a cable with a diameter of .020", it would be about the same as having
made
the drum .020" too big in diameter. (Cable diam .020, cable radius =
.010",
wrap it around a drum and the diameter through the center of the wire is

going to be .020" larger.)

Jon