In article ,
jim rozen wrote:
In article , Alden Hackmann says...
So at breakfast my wife asks me why the wheel always needs to be scraped, i.e.
why doesn't it come from the lathe with no
runout - and I have to say I don't really know. I then postulate that it's
because the wheel and shaft inevitably exhibit
some chatter when turned on the lathe. and I wonder what I can do about it.
First off, what surface(s) are you hand-scraping, and to what
are they fitted?
That would be the OD. I know this instrument. For my sins, I
was talked into producing a wheel for it from materials found by the
builder. The rim of the wheel is coated with rosin, and bows the
strings as the crank is turned by hand.
The wood he selected was not seasoned at all, and expanded and
contracted significantly with humidity changes -- along the grain
length. This resulted in the volume produced raising and falling with
the rotation of the wheel.
Another problem was the variation of the finish possible on the
end grain vs the side grain, resulting in an unpleasant rasping sound
beyond that introduced by the "trumpet" -- a vibrating bridge designed
to sound raspy. :-)
I suggested that he get some wood which was a slice out of the
center of the tree (starting with a naturally rather round piece, so the
expansion would be equal. He never did find something which was
reasonable, so I turned him one out of Plexiglas. That was perfect from
my point of view (and that of all others who knew him), as the polished
edge of the Plexiglas would not hold the rosin, and thus produced a very
quiet instrument. (He was never able to play it well, you see. :-) I'm
sure that you are making them for people who *can* play them, and I have
since heard some, and been amazed. My friend was not a good poster-boy
for the instrument.
Unfortunately, he fixed that by gluing some wood veneer around
the rim, with a diagonal joint where it closed, which worked
surprisingly well.
I would suggest that you consider simply mounting the phenolic
blanks in OD jaws and and profile both sides, then mount on your
shaft or mandrel and take a skim cut across the OD to clean it
up.
The shaft you are using is pretty thin and flexible, that may
be where some of your chatter is coming from.
I would consider a custom point for the live center with a
socket to hold the shaft and let the center get close to the wheel to
give it better support. (Or set up a steady rest close to the wheel to
control the vibration.)
I gather that the shaft can't be swallowed deeper into the
collet.
LE phenolic can be somewhat abrasive so if you are using HSS
tooling it will get beaten up pretty fast. Carbide if used
instead should be *un*coated as the coatings require a honed
edge and the phenolic really wants a sharp tool and fairly
large postive rake. My personal favorite for this is polycrystaline
diamond inserts from Valenite. I run the tpg221 sized ones
for composites and they last forever and give real sharp
edge.
That sounds like an excellent suggestion. And I'll bet that it
gets rid of some of the end-grain fuzz which you can get from turning
phenolic.
You will also find that much of the aroma from turning phenolic
goes away when cutting with a very very sharp tool. I think
most of it has to do with burning at the cutting edge.
A beefier mandrel as suggested would be a good way to go.
Or support closer to the wheel.
This is how we used to make sheaves, three operations - profile
first side, profile second side, and mount on mandrel and
turn OD.
I think that the sides are not really critical, assuming that
the hole is square to the surface. (Though it would probably look nicer
if they were faced, too.
Enjoy,
DoN.
--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. |
http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---