Contact pressure vs contact area

I'm learning to weld. I've realised my earth clamp is limited. The spring is weak and its contact area is small as it's tooth shaped.

Researching better clamps, I see clamps which have threaded bolts to clamp the jaws against the workpiece tightly. It is my understanding that unless you are clamping something malleable such as lead, increased clamping force wouldn't increase the contact area, unless such force is applied to leave indentations in the workpiece.

So I've been looking for clamps which have larger contact areas, and magnetic clamps seem to have large flat brass areas rather than pointed teeth which will be pressed against the workpiece with moderate spring pressure.

How does increasing clamp force increase conductivity, and is contact pressure really interchangeable with contact area size?

To be clear, I'm aware different clamp designs are more suited to different applications, but I'm specifically asking about the properties of electrical conductivity of a larger area being pressed lightly, vs a toothed (i.e, small) area being pressed very firmly.

Toothed clamp design: Magnetic design with brass flat contactor:

There is a proportionality between the pressure applied and the contact conductivity due the microscopic roughness of the contact surface that make the real contact surface much smaller. Applying a pressure flattens the surface increasing the contact area.

In some amount increasing the macroscopic area while keeping the same force doesn't increase the conductivity because the per square inch force will decrease proportionally.

See here a much detailed explanation.

• So the increased clamp force does increase conductivity because at a macroscopic level, it is increasing surface area by compressing together minute gaps. I was thinking too big... Also thanks for the reading material, I understand now how increased contact force of even small toothed clamps can increase conductivity due to increasing surface area at a macroscopic level and also forcing through surface oxides and films of dirt/debris. Thanks. – i-CONICA Oct 18 '18 at 0:34

Increasing clamp force does not increase conductivity (at least not in a linear way). What you are looking for is contact area size, which scales much better with conductivity than clamp force.

For constant pressure, clamp force and contact area size are interchangeable. But it is not pressure that you are looking for.

You can make an effective welding earth with a bare cable end and a g-clamp.

The real issue is the cleanliness of the workpiece...

Some objects have overhanging material so the toothed clamp works fine, once paint etc is removed.

Other objects have flat faces and need a magnetic clamp.

Other objects can be welded on a metal table and the earth cable is clamped to the table - especially handy for things that need rotation while welding ie pipes etc.