Marking sheetmetal

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.


I've always puzzled over why John Thorp called for using a pencil to
mark lines in his "building the T-18" articles from the mid 60s. Surely
they know about it then?

John

You are kidding, of course? You are marking a line 40 mils wide and
expecting a piece to come out to 20 mils or better accuracy?

Jim



"DonMorrisey" wrote in message
ups.com...

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...

Because he knows what in the hell he is talking about?

Jim



"J.Kahn" wrote in message
.. .




I've always puzzled over why John Thorp called for using a pencil to mark
lines in his "building the T-18" articles from the mid 60s. Surely they
know about it then?

John

("RST Engineering" wrote)
You are kidding, of course? You are marking a line 40 mils wide and
expecting a piece to come out to 20 mils or better accuracy?


If the line were 20 mils thick, where would you cut? The middle of the line?
The edge of the line?

But yes, a thin line is nicer.


Montblack
"Good enough. No one in the front row will ever notice it."
Theater set construction motto! g

"Montblack" wrote in message
...
("RST Engineering" wrote)
You are kidding, of course? You are marking a line 40 mils wide and
expecting a piece to come out to 20 mils or better accuracy?


If the line were 20 mils thick, where would you cut? The middle of the
line? The edge of the line?

But yes, a thin line is nicer.




When I started building I made up my on rule so I'd know which side to cut.
I always cut on the left edge of the mark. My reasoning is because I'm right
handed I will be holding the straight edge with my left hand and marking
with my right. With a very few exceptions this rule has served me well.

If I think there will be a question I put a little on the right(as opposed
to left) side.

RST Engineering wrote:
You are kidding, of course? You are marking a line 40 mils wide and
expecting a piece to come out to 20 mils or better accuracy?

Jim



"DonMorrisey" wrote in message
ups.com...
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...




And if you could put the bend within 10 mils of where you intended every
time, would anyone care?

If the designer calls for that amount of accuracy in anything other than
bearings, it's time to find a new designer. I was told by a old-time
sheet metal worker that unless the plans state otherwise, standard
accuracy requirements are 1/32". That's plus or minus .03125 in either
direction for a .0625" window.

On parts that I do want to cut straight because I want them to be
pretty, I mark with a sharpie against a metal ruler, then aim for the
side of the line. The metal ruler limits where the sponge on the tip of
the marker can go.

I take needle nose pliers and pull the point out about an 1/8 in or so then
use my razor knife to repoint the wick. Works for awhile and the wick is
over an inch long so it still. hmmmm. well it still wicks!

"Ebby"
Hatz Classic s/n37
Camden, NY

"Morgans" wrote in message
...

"Drew Dalgleish" wrote

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

They don't last very long when you forget and leave the cap off, do they?
g

There is a version of a marker that clicks like a retractable ink pen,
though, and they work pretty good. The problem is that the point is not
nearly as sharp.

Anyone ever try to sharpen a sharpie? It seems like there should be a
way,
somehow.
--
Jim in NC

"Ernest Christley" wrote in message
m...
RST Engineering wrote:
You are kidding, of course? You are marking a line 40 mils wide and
expecting a piece to come out to 20 mils or better accuracy?

Jim


And if you could put the bend within 10 mils of where you intended every
time, would anyone care?

I care very much. If you got a chassis from me and it had 20 mil gaps in
the corners, you'd think I was the sloppiest designer in the world. All my
freshman students bend within 10 mils on everything to pass the course.
Most of them can hold 5 if they try.




If the designer calls for that amount of accuracy in anything other than
bearings, it's time to find a new designer.

I *BEG* your pardon? In my part of the world, 0.0 is within 50 mils, 0.00
within 20 mils and 0.000 within 5 mils.

Jim

RST Engineering wrote:
"Ernest Christley" wrote in message
And if you could put the bend within 10 mils of where you intended every
time, would anyone care?

I care very much. If you got a chassis from me and it had 20 mil gaps in
the corners, you'd think I was the sloppiest designer in the world. All my
freshman students bend within 10 mils on everything to pass the course.
Most of them can hold 5 if they try.



Oh, no. Now you done gone and did it.

First, I need to make sure we're talking apples to apples. A mil is
1/1000th inch. Consulted http://www.onlineconversion.com/length_all.htm
just to make sure.

1 mil = 0.001 inch [international, U.S.]

I have here in my hot little hands (queue paper rattling for the ditto
heads) one gen-you-wine "RST-443 Panel Mount Intercom", the serial
number being 1014, which went together perfectly and works wonderfully.
I LIKE it, and you can't have it back. Starting at the front right
corner and proceeding counter-clockwise, the gaps in the bends on the
lower half of the chasis measure .020, .030, .032, .035. The bends in
the upper half required a different technique. The edge of an envelope
fit loosely in the gap of the rear corner bends, the envelope measuring
..010.

We must be talking different measurements; either that, or I have
extremely low expectations and you're headed out to chew some hides in
your quality control department. 'Cause, I think this box is tighter
than a cheerleaders butt.


If the designer calls for that amount of accuracy in anything other than
bearings, it's time to find a new designer.

I *BEG* your pardon? In my part of the world, 0.0 is within 50 mils, 0.00
within 20 mils and 0.000 within 5 mils.


I have only a couple measurements on my set of plans that call for a
specific tolerance. One is for the pins locking the gear leg halves
together. The other is for the wing lock pin. The rest are just
measurements with the smallest being with 1/16". Everything broken down
into feet, inches and fractional inches. But no "plus or minus"
anywhere. So I asked a recently retired machine shopman and sheetmetal
worker, because, heh, if you can't trust somebody that's been doing it
40yrs, who can you trust? He told me 1/32" if not specified. Does the
difference come down to how the numbers are specified? Fractional
inches resolve to 1/32, and decimal inches have the tolerances you state?

"Ernest Christley" wrote in message
m...
He told me 1/32" if not specified. Does the
difference come down to how the numbers are specified? Fractional inches
resolve to 1/32, and decimal inches have the tolerances you state?

I've watched this thread with some interest, I like being accurate but I'm
realistic. I have no doubt that Jim Knows what he's talking about. On the
other hand I'm fairly sure I can live with the accuracy that a Sharpie
affords almost any time. The most accurate project I've had recently was
replacing an engraved bezel. it had to inlay with no gaps and had four holes
for push studs. I took no measurements at all. I simply filed to shape, no
gaps, no slop.