A Miller 175 MIG I am looking at buying has a 30% duty cycle. It also has a light on the face that is made to look like a thermometer. Exactly how does one keep track of just how much welding they are doing, and how do they know when they hit the limits of the duty cycle? Does it just shut down?
Duty cycle is a bit of a game. The Millermatic 175 is not really a 175 amp welder. It is really a 120 amp welder, but marketing people like to increase the model number every few years so they started doing schizoid math to try to justify it.
Originally welders were designated by the amperage output that they could sustain indefinitely. This would be the 100% duty cycle level of output. Later they switched to 60% duty cycle so all the Hobart Betamig 200's miraculously became Betamig 250's. On the smaller machines they often go for 20% for 110 volt MIGs and 30% for the smallest 220 volt MIGs. It is al just goofy math.
In reality a Millermatic 175 can weld 1/8" steel pretty much indefinitely, 1/4" steel intermittently and 3/8" steel occasionally.
I especially like the justification for the Millermatic 145 that requires a 45 amp 110 volt circuit to achieve 145 amps output. Gotta love those marketing guys.
I don't see that going by the maximum current is that unreasonable providing they make the duty cycle information readily available, after all how many people in the target market, light use and hobbyists, for most of these machine are going to exceed the duty cycle. It is something that users should be aware of and I wonder how many people have an idea what amount of the time they're actually welding. I think the maximum current can give a good idea what the machine is capable of welding and the duty cycle may be acceptable for most use at maximum at least if it's sensible. About 25 years ago I bought an Eland, a well thought of UK brand, 160A MIG and it is 40% @ 160A, 60% @ 140A, and 100% @ 95A, I've never had a problem with that for what I do and these days it is used on the upper 2 out of 6 settings almost exclusively giving me
4 minutes out of 10 @ 160A.
Having been asked to look at a couple of MIGs recently for someone I now realise that the Eland is quite a good spec compared to some these days. I was asked to look at a SIP TopMIG 150, don't know if you have those in the US, but basically it looked like a small (cheap) MIG pretending to be a big one and the duty cycle at maximum current I couldn't find it quoted but I suspect you could count it on the fingers of one hand, that might be low enough to be an issue. The Eland was classified as light industrial when bought and I think that is an apt description, in the intervening years the terms "professional" and "heavy duty" have come to be applied to things which are neither.
I sort of kept track on a welding project a few years ago. I was burning a
1/8" rod every ten to twenty minutes, with the rest of the time spent chipping, grinding, and setting up the next joint. The electric meter ran up less than one KWH per hour.
My cheap import angle grinders with a wire brush and cutoff wheel also overheat quickly and need time to cool. They were good enough for the low duty cycle of digging out weld flaws but not for general prep and cleanup.