Lol yes, maybe you're a bit feverish..
Mike Young wrote:
>----- Original Message -----
>From: "Alan King" <alan@...>
>
>
>>close enough for very small bits that need to have zero flexing. It'd
>>probably take a laser interferometer to do it reliably with a general
>>
>>
>
>:) Good one. No doubt you're right for something that small and into a
>larger chunk of something more substantial. For 24 mil holes in 1/16" FR4,
>though...
>
>
>
Within this range, the thinner bit is probably quite a bit less likely
to break as fast, bending in more of an arc due to having a bit more
flexibility from being thin. The 24 will try and stay more straight,
and with the same small displacement will tend to concentrate it more in
a single area.
>I'm having trouble forming complete thoughts this moment; running off a
>fever here in winter wonderland. Here's a dump of what flickered across the
>forebrain on reading your thoughts.
>
>Something very tiny will break before it can cut sideways, while something
>rather larger will cut rather than break. Somewhere in between is the
>continental divide between "woefully fragile" versus "bulletproof". The
>threshold size is smaller for: faster cutter speed; thinner material;
>smaller angular misalignment; slower feed rate; softer material; sharper
>tool edge. Small is good; it's the smallest hole your tool can drill instead
>of break. I'll put forth the notion that, various reasons, the size is
>rather sharp defined, and more or less fixed for a given tool. #73 is as
>small as I care to drill, and is on the bulletproof side of that line on my
>drill.
>
>
>
Try something. Start a hole with your 24. Move your drill so there
is a little bow in the bit. See just how long it takes you to do even a
small sideways cut. The flutes are to clear the hole, in the direction
of drilling. Carbide doesn't flex well, the normal bits are not
designed for side cutting. Even the routers that are designed to cut
sideways it is stressed to let them cut, not force them into the
material at any great rate.
>Flexural stress varies as the cube of the diameter for small deflections. A
>#73 is about twice the diameter of your #80. So, considering geometry alone,
>it sees about 8 times less stress for the same misalignment. It is also
>stiffer, requiring more side force to deflect a given distance, and so has
>stronger cutting action.
>
>
That's without the concentration point where the bit joins the shank.
For sure I've drilled with 80's bowing out with displacements that will
snap a 24 or 32 just about instantly. To a point, you can break thicker
wire faster than thinner wire when bending it back and forth by hand.
The thinner wire tends to bend in more of a radius and spread the load.
Use the same deflection and break some 30 ga wire and some 22 ga. I
think you'll find the 22 is far easier to break.. #80 bits are that bit
more flexible, they will radius bend and have very little actual force
at the tip. 24 will have most of the force concentrated at the tip and
board, and stress where the shaft meets the shank, and considerably less
bending in between. The extra strength only serves to concentrate the
stress into a tiny area. There are many such cases where things operate
backwards, a solid rod is weaker than a pipe of same diameter with a
certain wall thickness. If you stick a tight sized marble into the
pipe, it will bend at the marble first, it is a leverage point from one
side to the other that weakens the pipe. And think of the solid center
of a rod as being an infinite number of marbles, they all make the rod
actually a little weaker than if it were hollow with thick walls.. The
center doesn't really contribute much to the strength, what seems it
should have been stronger is actually weaker..
By doubling the diameter of an 80 to an .024, you'll make it
considerably less flexible and tend to concentrate stresses while not
increasing it's resistance to flex related damage by an equal margin.
You have to start getting into a large bit before it can start taking a
decent sideways cutting force instead of displacing, or pull your whole
axis into it's alignment instead of displacing the tip..
The real problem isn't the degree between the two anyway. It's that
you can run for long periods of time with .024 etc bits, thinking you're
in near perfect alignment, while you're actually stressing them the
whole time. The 80s will bow noticably when starting with the exact
same small misalignments, and you can see and correct it. When you go
back up to the .024 after that, they run that little bit smoother, and
start lasting a long time, usually until you screw up doing something
else and feed to fast or hit the bit. You see it immediately with the
80s and can correct, with the 24 you can run the bit and never notice
the same small level of displacement killing the bits because they won't
bow out so much and be as visible, you'll only see that they break now
and then. Don't think I've had one break from natural causes since
doing this, mostly from hitting it with other stuff, there has been one
or two from drilling, but was from leaving it at the same rate I use for
jogging the drill around, not at a normal drill rate. Another now and
then still beyond that, but with used bits and dropping the cases now
and then, I'm sure I have started fractures in more than a few before
they even get chucked up..
Note that I don't even consider all that important to actually do this
alignment. Bits are cheap, if you get close enough to do 500 holes it's
sort of a who cares number. But the same bits can likely still do 5000,
even if they're used. But it's important simply to understand that
there is another level of alignment to work on that is important to
really long life, before even considering that play etc or anything else
is what's actually breaking the bits. Everything else is simply a
secondary factor, even a tiny bit of misalignment is going to be more
important to bit life..
>3" per minute sounds significantly big, but to what do we compare it? A #67
>bit at 30k rpm is moving two flutes 500 fpm on the surface. 3 ipm is next to
>nothing in comparison, if comparison is reasonable.
>
>
>
These are applying forces in directions the bit can take. You could
drive thousands of miles with a 10,000ft drop off on your right, but
lets see how things work out when you turn right and then drive 10 feet
over it.. Independent variables don't relate at all.
Alan