Welding prep, cutting notches and stress risers

I'm reading about welding. Here's an article about how to cut a
'fishmouth' joint (and I visualize clamping the tube in a vice, in the
meanwhile making small marks in the metal), then the pre-cleaning
(using an abrasive, another thing my A&P told me not to use when
cleaning tubing) then the actually welding.

On the other hand, sometime ago someone mentioned the harm that might
occur if one were to mark on the metal with a pencil.

In one case you're handling the material and putting in a nominal
number of nicks and scratches; on the other hand I'm told not to mark
with a pencil?

What am I missing? - MIke

On Fri, 19 Aug 2005 02:18:06 -0400, Michael Horowitz
wrote:

I'm reading about welding. Here's an article about how to cut a
'fishmouth' joint (and I visualize clamping the tube in a vice, in the
meanwhile making small marks in the metal), then the pre-cleaning
(using an abrasive, another thing my A&P told me not to use when
cleaning tubing) then the actually welding.

On the other hand, sometime ago someone mentioned the harm that might
occur if one were to mark on the metal with a pencil.

In one case you're handling the material and putting in a nominal
number of nicks and scratches; on the other hand I'm told not to mark
with a pencil?

What am I missing? - MIke

fishmouth joints help to get the weld penetration even. with properly
prepared setups you can adjust your welding speed to get structurally
reliable joints.

the chemistry of the pencil will be what your man is concerned about.
some materials introduced as contaminants in welds can substantially
reduce the weld strength.

you'd never see this in aircraft but braze through a lump of solder
and you will have a stuffed brazing.

Stealth Pilot

I'm not sure I made myself clear. My A&P is concerned about 'stress
risers', but the articles I read about making joints involve a process
which scratches the surface of the tubing and appears to actually
produce those 'stress risers' - Mike

mhorowit wrote:
I'm not sure I made myself clear. My A&P is concerned about 'stress
risers', but the articles I read about making joints involve a process
which scratches the surface of the tubing and appears to actually
produce those 'stress risers' - Mike


The 'stress risers' will be melted away if the weld is done properly.

If you want to make perfect joints, get a copy of Tubemiter. You input
the dimensions of your tube and the angle of intersection. It prints a
template that you wrap around the tube, then tape in place. Cut, grind
file, up to the marks and be amazed at the 32nd or less of gap. If you
have to fishmouth both ends, print both templates and line them up by
laying the tube on a flat suface next to a board and then lining up the
templates.

You can find a copy he

http://ernest.isa-geek.org/Delta/Library/tubemiter.exe

--
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."

The warning about using a lead pencil for marking is usually with regard to
aluminum. Lead graphite will make cracks in aluminum.

Soapstone is works for welding as it with stands the heat.


--
Cy Galley - Chair,
AirVenture Emergency Aircraft Repair
A Service Project of Chapter 75
EAA Safety Programs Editor - TC
EAA Sport Pilot
"Michael Horowitz" wrote in message
...
I'm reading about welding. Here's an article about how to cut a
'fishmouth' joint (and I visualize clamping the tube in a vice, in the
meanwhile making small marks in the metal), then the pre-cleaning
(using an abrasive, another thing my A&P told me not to use when
cleaning tubing) then the actually welding.

On the other hand, sometime ago someone mentioned the harm that might
occur if one were to mark on the metal with a pencil.

In one case you're handling the material and putting in a nominal
number of nicks and scratches; on the other hand I'm told not to mark
with a pencil?

What am I missing? - MIke

On 19 Aug 2005 04:58:37 -0700, "mhorowit" wrote:

I'm not sure I made myself clear. My A&P is concerned about 'stress
risers', but the articles I read about making joints involve a process
which scratches the surface of the tubing and appears to actually
produce those 'stress risers' - Mike

Has your A&P never welded a tube or marked one? You don't have to
scratch it with anything. You can use any number of writing
instruments to make a mark including a magic marker. Use your
imagination.

In the event, even if you do scratch the surface to mark the length,
you end up grinding right up to that mark, and then welding a large
bead right over it. It becomes one with the weld, so to speak.

There is no need to scratch a mark in the middle of the tube.

You will find that the act of welding tubing together is intuitive and
relatively simple, if you just start doing it. You can be very very
precise and use things like a lathe to mill the angle, or something
like the "Ole Joint jigger" that does basically the same thing,
although with a lot of noise and smoke. You can also just round off a
grinding stone and hold the tube against it until you approximate the
curve you need. You aren't gluing this thing remember, it doesn't
have to be accurate to within a fraction of an inch all around. The
filler wire can cover a lot.

Corky Scott

In article FRkNe.264882$_o.217775@attbi_s71,
"Cy Galley" wrote:

The warning about using a lead pencil for marking is usually with regard to
aluminum. Lead graphite will make cracks in aluminum.

Soapstone is works for welding as it with stands the heat.


Lead pencils are not lead, but graphite (carbon). If you mark, ay, an
exhaust pipe with a lead pencil, you are introducing a localized
high-carbon area, which the metal will absorb when it gets red-hot. This
will cause localized stress concentrations, leading to cracks. An old
friend related that he was looking at the exhaust pipe on a Lockheed
Constellation and saw a crack shape "OK" on the pipe.

Carbon absorption is the reason that good engine shops banish lead
pencils from their shop.

The problem with aluminum is that the carbon sets up a battery action
with the aluminum, and subsequent corrosion.


Cy Galley - Chair,
AirVenture Emergency Aircraft Repair
A Service Project of Chapter 75
EAA Safety Programs Editor - TC
EAA Sport Pilot
"Michael Horowitz" wrote in message
...
I'm reading about welding. Here's an article about how to cut a
'fishmouth' joint (and I visualize clamping the tube in a vice, in the
meanwhile making small marks in the metal), then the pre-cleaning
(using an abrasive, another thing my A&P told me not to use when
cleaning tubing) then the actually welding.

On the other hand, sometime ago someone mentioned the harm that might
occur if one were to mark on the metal with a pencil.

In one case you're handling the material and putting in a nominal
number of nicks and scratches; on the other hand I'm told not to mark
with a pencil?

What am I missing? - MIke

What about all the scratches made by clamping it in the vice and using
an abrasive to clean before welding? arn't those sources of 'stress
risers'? - Mike

mhorowit wrote:
What about all the scratches made by clamping it in the vice and using
an abrasive to clean before welding? arn't those sources of 'stress
risers'? - Mike


You don't need to clean back any further than you will be welding.
'Scratches' should be melted away. Use a slightly worn out abrasive pad
(sand paper on foam), or 120 or greater grit paper. It will give you a
mirror finish. A 'scratch' has to be a significant depth into the
parent material to be of consequence.

'Significant' in this case is a weasel word without definite meaning.
The precise depth is different for various materials. Consult 43.13B
for specific details.

--
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."

I believe the original poster was also talking about the vice making
marks in the tube as it is clamped in. I would think that you could use
some bronze "shoes" to cover the vice jaws. Blocks of wood maybe. Im
also thinking that you shouldnt need to clamp it so hard that it would
cause scratches. Just my 2 cents.
Jesse M
A&P