From: ndmead%gr.cs.utah.edu@cs.utah.edu (Nathan Mead)
Subject: Re: Oxy/Acet welding problems
Date: 26 Jan 93 14:59:28 MST
Organization: University of Utah

It's been a long time but I'll throw in my 2 cents.

In article <C1H4Bx.M96@dartvax.dartmouth.edu>, Charles.K.Scott@dartmouth.edu (Charles K. Scott) writes:
|> In article <2B656A90.13919@news.service.uci.edu>
|> dparsons@plasma.ps.uci.edu () writes:
|> 
|> > The 
|> > regulator pressures are not all that important as the flame adjustment is
|> > made at the knobs in the torch handle.
|> 

My recollection was that many welding tips were designed to work on 
approximatly equal pressure.  As Mr. Parsons states it's not all that critical
but proper flame adjustment is critical and is a lot easier if the regulator
pressures are just slightly above what you want at the tip.  I used to run
equal pressures starting about 5 psi and increase both if I needed additional
heat.  This is not true for cutting where the oxygen pressure is critical for
blowing away the molten steel.  For cutting I generally run between 25 psi and
50 psi depending upon the thickness of the steel.

|> Ok, so far so good.  Now a comment about technique.  I spoke with my
|> father in law who used to weld A LOT about welding thinking I could
|> interest him in what I'm doing and learn something.  Unfortunately, he
|> welded so long ago that the techniques he used have now been
|> discredited.  For instance, he said he used to chop up pieces of the
|> material he was gone to weld and used that as welding rod.  Everything
|> I've read lately tells me that you really should not be using the same
|> material for a rod that the pieces to be welded.  He also said he used
|> to heat the rod and let the molten material drip onto the weld area. 
|> This would produce a very poor weld from aircraft standards because the
|> base metal would not be heated enough to bond the dripping slag.  So my
|> attempt to include my father in law came to nought.   

One definition of welding is the joining of parts with the same filler 
material.  In my experience the two most important properties of a filler
material are that they be CLEAN and that they have a small enough cross
section to melt easily into the weld puddle without "stealing" all the heat.
My understanding is that the steel welding rods at supply houses are just
mild steel with a very thin copper plate to prevent rust.  I would have no
problem in welding with the same material as a filler IF I could get it in
a shape which was easy to use.  Generally this is not possible and, as welding
seriously messes with most of the fancy material properties that used to 
be in the material, mild steel filler rod is generally used.

|> Does anyone have any comment about the size of the rod (with regard to
|> the thickness of the material I'm welding,.080") and the technique I
|> mentioned about keeping the rod in the puddle as the puddle is moved
|> along.  Is this what everyone does?
|>

Welding is all about carefully controlling the temperature at the weld puddle.
It needs to be hot enough to get the desired penetration and not so hot that
it "blows out" or overheats the weld area.  In gas welding once a flame size 
is set temperature is controlled by the distance of the flame, the way the 
flame is moved about and by the addition of filler metal.  Generally the flame
control is hardest to master and requires lots of practice.  However controlling
the rate of addition of filler material should not be ignored.  Typically I
heat the material up to where a puddel starts to form, add enough material to 
get a good puddle and then work the puddle down the joint.  If I find that I am slightly overheating the base metal I'll add more filler and try to go faster and vise versa if I'm not getting the penetration.  In many tight joints little or
no filler material is required, particularly in thin materials.  Practice and
be flexable.

Also rember that welds are A#1 attractors for fatigue cracks.  They WILL NOT
be perfect and the WILL leave residual stresses in the structure.  I don't
care who does the welding.  Fatigue cracks will start in the welds or right 
next to the welded area.  Often due to residual compressive stresses cracks or unwelded areas cannot be detected by most common methods.  All you can do 
is accept it and think about how you are going to inspect each and every 
weld after the plane is finished and in use.  And a good design should have 
redundant load paths so a failure in one joint will not be fatal.  However the
failure must be detected or else....

Good luck.

Nathan Mead
Center for Engineering Design
University of Utah

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