Choosing the Right Tungsten Size
The first thing to note when selecting tungsten size is that there are calculators available online that will tell you which size to use based on the thickness and material of work, such as this offering from Miller. However, using a calculator for every different weld is extreme overkill for most people: Tungsten grind angle can be adjusted to allow for crisper starts or more heat resistance.
'Blunt' Tungsten Angle
'Sharp' Tungsten Angle
'Super Sharp' Tungsten Angle
It’s easy to see in the images of 1.6mm and 2.4mm tungstens above, that tungsten grind angle affects the overall shape of the tip more than the diameter. This means that for the average welder who does a bit of everything, keeping a 2.4mm tungsten in the collet and sharpening it differently can be just as effective as changing out for a different tungsten size.
Why choose a different tungsten size/grind angle?
The simple answer is that a tip that is too fine will melt, and a tip that is too blunt will not start well or hold a stable arc. Starting and arc stability are easier at higher amperage, so it makes sense to use larger/blunter tungstens on thicker material and the opposite on thinner material. Aluminium and welding using AC will require a much bigger tungsten for the same amperage on DC (depending on AC balance, more positive balance heats the tungsten more).
If your current tungsten size and grind angle are starting well and stable, you will not see a significant benefit by changing to a ‘more optimal’ size shown by a welding calculator.