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Different between resistance and spot welding?


#1

Can someone explain what is the different between resistance
soldering machine and spot welding machine? both seems to work
similar but they are not the same. one use a copper lead and carbon
rod and the other one use copper or brass tip on both (+ -).
apparently in spot welding the fusion occur in the surface of both
base metal where they are connected and in resistance units the heat
occur and the end of the carbon. can somebody tell me what is the
different between the two process?. and why in spot welding sometimes
there is not need of flux to avoid oxide in metals like stainless
steel and titanium. also I need to know where can I buy a small spot
welding units since it looks like they work better than resistance machines…


#2

Luis, The spot welders that are used for jewelry are what is called a
capacitive discharge welder. It stores up a charge of electricity in
a capacitor and then releases it all at once this discharge provides
enough energy to melt and even vaporize a small area of metal. They
are sold in two styles one is used to apply fusion findings which are
a specially formed finding with a little point or nib at the point of
contact with the work. The finding is driven down onto the work by a
spring or air cylinder and this completes the circuit and fuses the
finding to the work. It makes a permanent joint and is most useful in
production assembly because you often need to make jigs or fixtures
to hold the work for proper welding to occur. The second type of spot
welder is the “Tack” welder this is the one I think you are referring
to. It is used to temporarily tack two pieces together. In use each
piece to be joined is connected to the power supply via cables and
clips or fixtures and the current is discharged through them at the
point of contact between the pieces a spot weld occurs. This is a
very weak bond in mots cases and cannot be used as the only means of
attachment. It is often used to position parts for soldering or
brazing.

Resistance soldering machines also pass current through the work but

the current is at a much lower level for a longer time. This heats
the work and can heat it to the point where hard solders will melt.
It is really almost like torch soldering but the heat is from the
electric current.

In welding fusion findings the nib literally vaporizes and this
explosion blows away the surface oxides and dirt allowing for a good
solid weld. In tack welding one of the reasons for the weak joint is
the oxides and dirt that are present during the welding process but
if you put flux on it would not weld at all as the flux would conduct
the electricity around the point of contact.

If you are interested in attaching findings to your work then a
fusion finding tack welder may be a good choice if so look at the
Sparkie machine made by Triad which is available from many jewelry
supply companies. If you are looking to weld two parts together then
you may want a tack welder like the Tack II from ABI also from most
jewelry supply companies but I would find some one that will let you
try it out before you buy as they are only intended to temporarily
tack parts together before soldering.

Jim


#3
    Can someone explain what is the different between resistance
soldering machine and spot welding machine? both seems to work
similar but they are not the same. 

The fundamental difference between welding and soldering (or
brazing) is that in welding the two pieces of metal to be joined are
locally melted and the liquid zones (in spot welding called the
"weld nugget") fuses together, making the two one on cooling. The
crystal structure of the metals is continuous across the join.

In soldering or brazing the workpieces are not melted, nor do they
come near to melting. The workpieces are heated and some other,
lower melting point alloy is added which melts, flows and coats the
contacting surfaces of the work. On cooling this lower melting
solder or braze solidifies and sticks to the workpieces, gluing them
together. However there is a definite physical surface between the
work and the solder, and the two workpieces will not actually be
touching each other, they are held apart (and together) by a thin
layer of solder.

Welding tends to be stronger, soldering tends to be easier to do and
usually requires fewer safety precautions.

 can somebody tell me what is the different between the two
process?. 

Other than the fundamental difference between welding and soldering,
I cannot tell you the difference, because I’m not familiar with
resistance soldering (or possibly do not know it by that name).

 and why in spot welding sometimes there is not need of flux to
avoid oxide in metals like stainless steel and titanium. 

Because the initial flash of heat drives air and contaminants out
from between the workpieces and there is then a clamping force that
holds the work surfaces firmly together so that air cannot reach the
main molten part of the weld nugget, only a small ring around the
nugget edge.

 also I need to know where can I buy a small spot welding units
since it looks like they work better than resistance machines.. 

Which is “better” depends on what you want to do. There are
applications for which soldering is right and applications for which
welding is right. Also some metals just wont solder/braze easily and
some wont weld.

Iain Fielden
Sheffield UK