On 1/3/24 18:54, andy pugh wrote:
On Wed, 3 Jan 2024 at 22:01, andy pugh <bodge...@gmail.com> wrote:
This means that the large and small arcs are tangent to each other,
which requires that their centres lie on a perpendicular line, and
that in turn means that the arc centres of small and large arcs must
be coincident.
Close, but no cigar, it seems.
There is an equation in an online document
(https://archiwum-bazakonkurencyjnosci.funduszeeuropejskie.gov.pl/file/download/587886)
that isn't actually correct:
x'= Dm/2×cosØ −2×e×cos(2Ø)+e×cos(4Ø)
y'= Dm/2×sinØ -2×e×sin(2Ø)+e×sin(4Ø)
However, after getting hold of a pirate copy of the standard, it turns
out that it was only slightly wrong. This is what it should be. ( -+++
not -+-+)
x'= Dm/2×cosØ −2×e×cos(2Ø)+e×cos(4Ø)
y'= Dm/2×sinØ +2×e×sin(2Ø)+e×sin(4Ø)
Plotted out this is very close to the shape that I had but differs by
about 30 microns at the worst spots.
The good news is that this is very easy to convert to either XY
offsets for boring, or a radius for computed offset on a lathe as it
is already in terms of spindle angle Ø.
Humm, that may call for a higher resolution spindle encoder than I have
on my Sheldon, I'm watching the 60 tooth gear on the spindle si my
ultimate accuracy given perfct gears is 1.5 degrees, and that is noisy
as hell because of the wear from 75 years of back gear over engagement
which has rounded the taller teeth quite noticeably. I would not swear
that any one edge is any closer than two degrees because of this wear.
The vfd makes a good filter.
I'd bet a $20 you could see the teeth as a pattern in an otherwise good
grind.
Letting my mind out to play w/o a chaperone, a 1000 line Omron encoder
for $20.00 shipped on fleabay, driven by printed 1/1 timing sprockets
with the driving gear pressed onto the smaller gear that is now unused
on the back of the spindle would give a .36 degree accuracy. I just
looked to see whats in the way, nothing as thats all been removed.
Plenty of room for a 6mm belt sprocket, as actually there's 2 gears, one
on each side of the rear bearing. And the outside one is totally unused
now. This might be a good excuse to put a higher resolution encoder on
my Sheldon. And teach it yet another new trick. The wheels are
spinning, but need lubed, at 89 they are getting rusty. :o)>
Thanks Andy.
--
atp
"A motorcycle is a bicycle with a pandemonium attachment and is
designed for the especial use of mechanical geniuses, daredevils and
lunatics."
— George Fitch, Atlanta Constitution Newspaper, 1912
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Cheers, Gene Heskett.
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