..at least in a single pass .
Totally agree with ^this^... Without getting into weaving, etc. But that's a convo for a different thread.
Amperage and penetration is truly determined by wire speed. IDK what machine you're using (miller 211) but there is no "automatic adjustment" strictly from a voltage setting.
Voltage and wire speed create a correct parameter which create a proper bead profile and penetration when adjusted correctly.
I respectfully disagree that amperage is determined by wire speed, but I agree that penetration can be affected by it.
There is absolutely and undoubtedly an automatic adjustment that takes place as long as the trigger is pulled. This is true for any MIG/FC machine. I learned this by paying attention to the digital readouts on a multi-process machine (CC/CV) in my welding classes. Ours was a Miller.
When welding with SMAW, we select a desired amperage (CC [constant current]) based on how much heat we need. Because arc length (resistance/ohms) constantly varies while making a weld, the laws of electricity state that voltage must be constantly adjusted to maintain a constant amp (current) demand. This was reflected in the readouts on the machine: amperage fluctuated very little, while voltage varied drastically by making changes to arc length in the demonstration. There is no "arc length" setting on the machine and it'd be awfully difficult to adjust it ourselves while we're welding if there were... so the machine takes care of that for us.
Using different sized rods is a good representation of how CC works. For example, using a 3/32" rod at 90 amps will make a hotter, smoother weld than using an 1/8" rod on the same amp setting. Why? Because 3/32" rods require less heat to melt, so more heat gets transferred to the puddle/base metal. It'd be more appropriate to set the machine at, let's say, 100 amps for an 1/8" rod in the same scenario.
Sound familiar?
The same is true with MIG/FC machines, but they use constant voltage (CV) instead of constant current. Similar to SMAW, there is no setting for arc length (resistance), so the machine compensates by constantly adjusting the amp output to give us the voltage we need.
We do have a setting for wire feed speed, but it's important to understand that, although it CAN affect penetration for any given wire size, it does NOT affect the amperage outputted by the machine. It is there so we can match the power setting to the wire size we're using (i.e., the "bead profile" you're referring to). This is how charts are made and why different size wires have different speed settings. It's also easy to confuse this as a fluctuation of amperage, but it's not... and here's why:
Bigger wire generally requires less feed speed than smaller wire for a given power setting.
If feed speeds were a representation of amperage output, then wouldn't we want to increase our amps for bigger wire? Why don't the machines tell us this?!
Because that's not how it works.
Anyone who has used a MIG/FC machine and had too low a wire speed setting has seen their wire ball up and fall off the end of the gun... maybe into the weld... maybe onto the floor... maybe into that tiny crack in the seam of your glove. In contrast, too high of a speed setting makes the wire shoot into the weld too quickly, causing it to pop and throw sparks everywhere (raise your hand if you've got burn scars).
If amperage fluctuated with wire speed, then neither of these situations would ever happen.
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