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Pencil to mark parts?

M

mdredmond

Well Known Member
I've heard (not sure where) never to use a pencil to mark aluminum parts.

I have a sweet set of steel T-Rules with holes punched every 1/64" inch. You can draw straight lines parallel to an edge by putting a mechanical pencil point in a hole and 'dragging' the ruler down the edge of the part. My super-fine sharpies are too big to fit in the holes in the ruler. They are made for 0.5mm pencils.

I'd love to be able to use this to mark holes on the parts I have to fabricate/drill, but am concerned about what I've heard re graphite and aluminum. Anyone know whether I can can use a pencil if I clean the graphite off when done?

Thanks!
 
You heard right! NEVER mark on aluminum with a lead pencil. We spent many many hours doing corrosion control on old C-124s. One of the worst things we found is where someone had marked aluminum with a pencil. Marking aluminum with a pencil is just like using a scribe. You can never get all the graphite off.
 
fab a new set

Looks like you could fab up a new set using some aluminum angle and maybe some .030 or .040 aluminum sheet, with holes big enough for the fine-point sharpie. Who knows, you could even start a business manufacturing them.
 
A Major No-No

mdredmond said:
I've heard (not sure where) never to use a pencil to mark aluminum parts........Thanks!
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To be caught using a lead pencil to mark aluminum in the production environment would expose the miscreant to a verbal warning or worse....disciplinary action. :eek: This point was well covered in basic sheet metal class at McDonnell-Douglas, the consequences (or more precisely the reaction of lead on aluminum structure) has been well documented over the years. We were issued specially designed and approved markers, very similiar in appearance to a common lead pencil but the lead was replaced with a red or blue marking agent. I cannot say where you might find them. When I built my 6A, I mostly used a fine point Sharpie.
 
It's not "lead", but rather carbon - in this case graphite - and it will indeed cause problems by getting into the microcrevices inherent in the aluminum surface. The combination of carbon and aluminum in contact with each other will set up galvanic corrosion that will only become evident after some time, and you will NOT be able to clean the carbon off the aluminum surface sufficiently to stop this without removing a couple thousandth's of aluminum.

Sorry, I make my living dealing with corrosion - the sheet metal guys are right on this one.
 
Bob Collins said:
I was kinda surprised when I saw Joe Schumacher lay out his panel with pencil in one of the "From the Ground Up" episodes.
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I would assume it wouldnt be all that bad considering it is not a structural component, will be in a dry environment, and probaly will be sanded or bead blasted before painting or powdercoating.
 
"you will NOT be able to clean the carbon off the aluminum surface sufficiently to stop this without removing a couple thousandth's of aluminum"


Well, in my enthusiasm to get started I drew a line with pencil across the HS-810 spar angle - then read section 5C. Doh. :eek: :eek: :eek:

I've scrubbed away with scotchbrite to get rid of the mark, but should I just discard the part and get another one? I guess I can use the original to practise bending to 6deg.

Thanks

Chris
 
If you have polished the aluminum to a nice bright finish, you've effectively scrubbed away the top several microns of surface. The problem with the carbon graphite is that it will get into the microcrevices and fissures that are inherent with worked aluminum. These crevices and microfissures are typically only a micron or two deep, several microns at best. Once you polish it to a mirror finish, you've removed this surface (and the graphite with it), leaving a fresh metallic aluminum surface that will quickly oxidize from exposure to the air. Aluminum oxide will effectively seal the metallic surface under it by tightly clinging and preventing further oxygen exposure.
 
We had this discussion in the UK about 9 months ago. I decided to set up a test piece. I took an offcut of aluminium and drew 5 lines on it.

HB, B, 2B, Crayon and Sharpie.

I then hung the piece in a tree !

This month, I scrubbed it clean and examined it under a glass - the pencil lines all showed feint traces of corrosion at their edges, the crayon and Sharpie did not.
 
rib

i have a tank rib with various different types of primer and some some bare spots. been out side for three years now in south carolina.80 miles from the coast . will have to check it closely and pencil it in to watch that.
 
"So..... what do you do in South Carolina for fun?"

"Oh, we just sit on the back porch and watch sheet metal hanging in the trees corode..." :D :p ;)
 
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