Marking sheetmetal

"Don W" wrote in message ...
: RST Engineering wrote:
: "Don W" wrote in message
: . ..
:
: RST Engineering wrote:
:
: Depends entirely on what tolerance you are marking/cutting/bending to. I
: want my students to be able to cut/bend to ten thousandths accurately and
: the only way to do that is to scribe.
:
: If you can bend aluminum to a 1/10,000" tolerance you are the MAN. You
: must have some really talented students.
:
:
: That's not the English of it. Ten thousandths is far different from one ten
: thousandth. Ten of the little thousandths rather than one of the tenth part
: of a thousandth. 0.010 as opposed to 0.0001. Two orders of magnitude
: difference. Get it?
:
: yeah, I got it the first time. Just having a
: little fun with you. Was it good for you too?
:
: That's why some people call it ten mils, or .010",
: or... sometimes ten one thousands of an inch, etc.
:
:
: Don W.
:

How about 1 hundredth of an inch?

".Blueskies." wrote in message
m...

How about 1 hundredth of an inch?


That was my thought when I read the whole silly thread. The problem is that
hundreths are not a "popular" dimension. It seems that in popular notation
the instant you segment an inch you are dealing in thou. So, asking someone
to remain within "ten thou" is instantly understood by almost anyone. By the
time you go the nest step and get into tenths people usually revert to
fractions and start dealing in /32s or /16s.

In any case stay away from pencils, I'm not that careful with a scribe, and
sharpies are common. I've got two sitting on my desk.

Orval Fairbairn wrote:
NOT ON ALUMINUM! The graphite from the #2 pencil will galvanically
corrode aluminum. Use a Sharpie and clean with either lacquer thinner or
denatured alcohol after you are done.

Why can't you just clean off the pencil marks?

Also, doesn't a galvanic reaction require an electrolyte between the
metals, so if you didn't erase or clean off the pencil mark but primed
and/or painted over the aluminum surface, no salt water or other
electrolyte could get in there to produce the galvanic reaction?

"Jim Logajan" wrote

Why can't you just clean off the pencil marks?

Also, doesn't a galvanic reaction require an electrolyte between the
metals, so if you didn't erase or clean off the pencil mark but primed
and/or painted over the aluminum surface, no salt water or other
electrolyte could get in there to produce the galvanic reaction?

The danger of using pencil for aluminum is not a "maybe" kind of problem.
The pencil works it's way into the molecules, and can not be cleaned off,
completely. It is an accepted fact, known to materials engineers as a
unacceptable practice. I don't know if it is really a galvanic reaction, or
something else, but people *way* smarter than you and me have proven the
problem. The metal will become brittle at the pencil line, and with enough
stress, *will* cause a crack to start.

Why risk it? Why argue? Use something else to mark your aluminum. Period.
--
Jim in NC

In article ,
"Morgans" wrote:

"Jim Logajan" wrote

Why can't you just clean off the pencil marks?

Also, doesn't a galvanic reaction require an electrolyte between the
metals, so if you didn't erase or clean off the pencil mark but primed
and/or painted over the aluminum surface, no salt water or other
electrolyte could get in there to produce the galvanic reaction?

The danger of using pencil for aluminum is not a "maybe" kind of problem.
The pencil works it's way into the molecules, and can not be cleaned off,
completely. It is an accepted fact, known to materials engineers as a
unacceptable practice. I don't know if it is really a galvanic reaction, or
something else, but people *way* smarter than you and me have proven the
problem. The metal will become brittle at the pencil line, and with enough
stress, *will* cause a crack to start.

Why risk it? Why argue? Use something else to mark your aluminum. Period.

And --- DO NOT USE A SCRIBE!!! Scribes, no matter how careful you are,
will damage the aluminum and create stress raisers. They can also damage
the Alclad coating and provide a path for corrosion.

It is not a matter of IF a piece will fail at a scribe line, but WHEN --
due to fatigue.

The admonition against pencils in aluminum shops also holds for engine
shops. The easiest way to fail a hot steel part is to mark it with a
lead pencil. The steel glows red hot and absorbs the carbon from the
mark, creating an instant stress concentration. An old friend related
how he saw a Lockheed Constellation exhaust manifold with a crack in the
shape of "OK", due to lead pencil.

Drew Dalgleish wrote:

Cumon Joe a shapie costs a dollar do we really need to make it last
longer ?

Sure, it's only a buck. But it only quits at night or on the
weekend when the stores that sell Sharpies are all closed.

Dan

Orval Fairbairn wrote:

The admonition against pencils in aluminum shops also holds for engine
shops. The easiest way to fail a hot steel part is to mark it with a
lead pencil. The steel glows red hot and absorbs the carbon from the
mark, creating an instant stress concentration. An old friend related
how he saw a Lockheed Constellation exhaust manifold with a crack in the
shape of "OK", due to lead pencil.

The steel absorbs the carbon and becomes brittle, whereupon it
cracks after being flexed while in use. The carbon doesn't cause a
stress concentration; that's caused by scratches or nicks that
interfere with the lines of stress in a metal part. A nick in a metal
propeller is an excellent example of a stress riser, as is a scratch in
an aluminum skin.
For a steel to be hardenable, it needs a carbon content of at
least 0.4% carbon. That's not much. Few steels have as much as 1%. 0.6%
is commonly found in hand tools and spring steel, and a steel having 1%
might be found in ball or roller bearings. A pencil mark could raise
the local carbon content a lot, probably to much more that 1%
,especially on a hot item like an exhaust manifold where the heat
allows rapid and deep absorption of the carbon.
Aircraft tech schools will teach you to leave the pencils at
home when working on aluminum.

Dan

Drew Dalgleish wrote:

Cumon Joe a shapie costs a dollar do we really need to make it last
longer ?

Sure, it's only a buck. But it only quits at night or on the
weekend when the stores that sell Sharpies are all closed.

Dan

It also quits just out revenge because you failed to cap it securely--for a
ridiculously short period of time. :-(

The obvious solution is to stock spares.

Peter

"Morgans" wrote:
"Jim Logajan" wrote

Why can't you just clean off the pencil marks?

Also, doesn't a galvanic reaction require an electrolyte between the
metals, so if you didn't erase or clean off the pencil mark but
primed and/or painted over the aluminum surface, no salt water or
other electrolyte could get in there to produce the galvanic
reaction?

The danger of using pencil for aluminum is not a "maybe" kind of
problem.

I'm not doubting galvanic reaction involved at all. Nor the inadvisability
of drawing a pencil line and leaving it there.

The pencil works it's way into the molecules, and can not be
cleaned off, completely.

This is the claim that bothers me. But assume it is true. I would expect
the rate of corrosion to be roughly proportional to the ratio of the
size of the exposed cathodic material (graphite) relative to the size
anodic material (aluminum). This ratio is usually expressed C/A, (where
C is the cathode surface area and A is the anode surface area). And I
did find a source on the net that verifies that [1]. So even if one
couldn't clean it all off, the corrosion rate would be greatly reduced.
And if the surface were sealed before any electrolyte (e.g. salt
water) could be introduced, no reaction would take place.

Why risk it?

Not asking anyone to risk anything.

Why argue?

Because rules of thumb are okay when deeper understanding takes too long
to impart to the practitioner, but understanding the deeper nature of
the problem allows so much more to be accomplished.

[1] http://www.ocean.udel.edu/mas/masnotes/corrosion.pdf

At the risk of beating this subject to death, use a sharpie, however a
Fine Point Sharpie is too thick. An Ultra-Fine Point Sharpie makes a
line approx 1 MM in width.

Don...