[Oscar]

Supposedly the aluminum wire can get pre-heated to the point where it could cause feeding issues if the actual tip diameter is too close to the actual diameter of the wire (due to expansion of the wire at the tip). Also, for example, say that the tip (which is supposed to be over-sized by about +0.002"-0.003" for the nominal wire diameter, is actually only at +0.001", and the wire just happens to be 0.001" over-sized from the factory, then it might encounter some drag/binding and bam you have a birdsnest. I do this as well as a precaution, but I supposed that it also depends on how hot the weld is. IOW if you're running at the top end of the amperage range for the wire and you have a high voltage, and your CTWD is on the lower-end of the recommended range, then perhaps. But if you're not having problems, then you're not having problems. Hard to argue with that.

 

[Oscar]

It's a good thing if the rated size is a "steel" tip.

i.e. you have three options for a 1mm wire: a 1mm steel tip, a 1mm "A" aluminium tip, or a 1.2mm steel tip.

If all you had is regular steel tips, then 1.2mm is the right size for 1mm aluminium... if that makes sense?

But the "best" option is a proper aluminium tip, it's the right size diameter to give good electrical characteristics without compromising on expansion. You'll get longer tip life with fewer burnbacks. I can't say I've noticed a huge difference in weld characteristics between a 1.2A and a 1.4 tip i used a while back on the same 1.2mm alu setup - you're in spray transfer so conductivity is less of an issue. But theoretically you'll get better contact = wetter bead, even if only slightly.

Not sure on Miller stuff, but Binzel always has an "A" option for all of their gun tip sizes.

Good clarification there. Some tips could be "aluminum tips" meaning that they are already over-sized beyond what the stamp mark says, so perhaps the 0.035" tip could actually be bored out for wire clearance + expansion tolerance clearance.

My MIG tips are over-sized explicitly, so for 3/64" wire, they are labeled as 0.052", so about +0.005" oversized. For 0.030" wire, one would simply use the 0.035" tip, and for 0.035" wire, one would use the 0.040" tip. Also you can look into CuCrZr (copper-chrome-zirconium) tips, as they handle the heat of pulse/spray arc better than standard 100% copper tips. And of course proper CTWD and arc length play a role as well.