On Fri, 30 Sep 2011 08:14:50 -0700, Winston
wrote:
Larry Jaques wrote:
On Thu, 29 Sep 2011 22:33:35 -0700,
wrote:
(...) -- Indicates snippage, *not* three nostrils.
Me too, since the same blade cuts through steel
really nicely when the workpiece is clamped in
a bench vise and the saw is guided by hand.
Are you absolutely sure it's not caused by vibration of the workpiece?
If the piece is hopping, it could -induce- a waggle, oui?
I haven't seen any high speed video, but you are probably right.
Probably the first stroke of the blade goes well
then *any* contact of the blade to the workpiece on the return
stroke induces movement in the handheld workpiece, which causes
the largely undamped blade to 'violin', side to side.
Question: Why would it be so bad upside down when it isn't right-side
up? Could it be the addition of the table thickness? Pull the shoe
and mount the beast to the new, thin table. Let me know how it works.
Once this lateral oscillation of the blade starts, cutting just
about ceases as the workpiece gets trapped by the displacement
caused by the resonant standing wave in the blade. (It doesn't
really matter if it is fundamental or harmonic resonance.
A dynamically floppy blade is effectively wider than the kerf
at most points along it's length.)
Ayup. I've felt that with the recip saw many times. The 12" blades
are the worst.
http://www.youtube.com/watch?v=-n1d1...eature=related
clap, clap, clap OOH, fun, Daddy!
At that point, the workpiece spends much of it's time riding
the blade.
So the guides dampen the oscillation of the blade at it's
resonant frequency. The blade remains 'straight enough'
laterally and therefore does not trap the workpiece.
The blade probably still oscillates laterally when it cuts a
clamped workpiece and the saw is hand held. It does not
approach resonance because the base pressure of the saw
against the clamping device (vise, workbench)
prevents the workpiece relative movement that causes blade
lateral resonance.
Drill the tip of the blade, run a guitar wire through it, and support
it at the top. It should end your troubles. Use the beaded end on top
so you can drop a holey piece on it for further cutting.
A first - order test would be to clamp a workpiece in a vise
and attempt to cut it with much less base force than usual.
"base force"? Or run a ball bearing holddown right next to the blade,
springloaded at the top. But if it's a positional resonance, that
prolly wouldn't help. The original shoe is probably tuned to the zero
point of some wave already, probably at midspeed on a variable.
I suspect that the lowered mass coupling would cause the
blade to enter resonance at some FPM and pressure settings,
resulting in a bouncy ride for the user. BTDT
(Queue SFX: "trombone gobble")
That is my story and I'm sticking to it.
All the way to the San Josie Sanitarium, eh?
My take-away:
What if you made a 'virtual' top guide, immune to swarf?
Several displacement sensors and electromagnets dynamically
phase - cancel lateral acceleration of the blade,
straightening it in real time.
Quick, cheap, and easy. You'll have it done in half an hour, wot?
Hurry, the patent office closes at 4 on Fridays.
--
In the depth of winter, I finally learned
that within me there lay an invincible summer.
-- Albert Camus