They
really are very much related. If you think about it,
MIG welding is quite simply, the transformation of a
round wire into some other convenient shape (the weld
bead). The welding arc just acts as means to melt and
reform the wire into a weld bead. Having said this, then
it follows that that we can write a little formula to
help us calculate either travel speed or wire speed in
any given application. I call it my "SAP" formula.
SW *
Aw *
PW = Sb *
Ab * Pb
SW = Wire Speed (in/min)
Aw = Wire Area (Sq.In)
PW =
Wire Density (#/in)......the letter "P" was chosen over "D" to
avoid confusion over wire diameter.
Sb
= Bead Speed (in/min) ...which is equal to the
Torch Speed. I call it Bead Speed to simplify the formula.
Ab = Bead Area
(Sq.In)
Pb =
Bead Density (#/in)
Graphically the formula
looks a little like this.
The formula really says that wire consumption
(the left side of the equation)
must equal wire deposit (the right
side of the equation) if we assume no loss
of material in the form of spatter or slag. To simplify
it even further, if we assume solid wire as in a MIG
application, then the densities of both the wire and
deposit are the same and the equation now becomes:
SW *
Aw = Sb *
Ab
If Aw is
the cross section of the wire or profile and Ab is
the cross section of the bead or profile, then it turns
out that either the Wire Speed or the Travel Speed is
simply a function of the ratio of the area profiles.
SW = Sb *
(Ab/Aw)
or
Sb = SW *
(Aw/Ab)
Time for an example. We are MIG welding
with .045" diameter mild steel solid wire and our we
want to deposit a bead on plate with the following dimensions:
1/8" high x 1/2" wide. Our wire speed is 300 inches per
minute (about 250 amps). What does our travel speed have
to be, assuming no loss in spatter?
Sb = SW *
(Aw/Ab)
Sb = 300 *
{(.045*.045*.785)/(.125*.5)}
Sb = 300 *
(.0016/.0625)
Sb = 300 *
.026
Sb =7.7
inches/min
or
Torch Speed = 7.7 inches/min
_________________________________
Things get a little more complicated with
cored wire however. It's density is not the same as solid
wire. You can measure this yourself by taking a given
length of wire and weigh it to get a pounds/inch figure.
The density of solid steel wire and the steel deposit
is 0.2836 #/in. Now to do the same calculation as above
except with cored wire we have to go back to the original
formula which accounts for densities.
SW *
Aw *
PW = Sb *
Ab * Pb
So now the
calculation using a typical value of 0.2552 #/in
for a metal cored wire is as follows:
Sb =
SW *
(Aw/Ab)
* (PW/Pb)
Sb = 300 * {(.045*.045*.785)/(.125*.5)}
*( .2552/.2836)
Sb = 300 * (.0016/.0625)
* .900
Sb = 300 * .026
* .900
Sb = 7.02 inches/min
or
Torch Speed = 7.02 inches/min
Note how the torch travel
speed is now less than when we used a solid wire.
We have compensated for the slight loss in density or
weight of the cored wire.
I haven't included spatter into the equation
because this can vary according to welding parameters
and gas, but if you need to throw it into the equation,
it would look like this.
SW *
Aw *
PW * EW = Sb *
Ab * Pb
Where
EW is
the wire efficiency expressed as a percentage. For
example if the spatter is estimated at 2%, then the
wire efficiency would be 98% or EW would
be 0.98. That means in the above example the Torch
Speed would be 98% of 7.02 or 6.9 inches/min.
A word about cored
wire densities. It is best to get this number from
the manufacturer of the wire for the most accurate
number. Please don't assume that if you get a number
for 1/16" diameter wire and you calculate that it is
92% the weight of a solid wire, that you can use 92%
for the same product but in a different diameter. Chances
are that they are close but you may be surprised that
they are quite different. So it is best to check with
the manufacturer. Postle has a number of densities
already determined for many products. If we don't have
your density for your product, our lab will be glad
to measure it for you.
I have presented
a simple bead on plate for illustration, but the formula
is not confined to this configuration. The bead area Ab can
be a fillet weld, in which case the area would be
closer to a triangle. The more accurate you can estimate
the bead profile, the more accurate your calculations
will be.
This math stuff is heavy and I hope I haven't
wasted your valuable time. Hopefully it will come in
handy sometime. Enough said, on to something more light:
HUMOR. |
