Originally Posted by Homestar
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Sweet. Something I can write about with my morning coffee.
Machine selection
I'm biased to a big red brand, but there are a lot of good machines out there. The ebay cheapies need to be treated as ebay cheapies, so keep that in mind for longevity and parts availability long(er) term.
That being said, even the big suppliers will hold parts for 10 years or so after the machine is discontinued, so there is a limit. Your budget and level of use will dictate.
Process selection
Not all processes (MIG/STICK/TIG) are equal. Stick is easy to get into, no gas needed, but lots of cleanup of slag etc. Can be very resilient to surface contamination. MIG is fast, can be run with a shielding gas with a solid core wire like an ER70S-6 spec, or 'gasless' with a flux-cored wire like an E71T spec (be careful, some gasless wires are single pass only...). Surface prep is important, as there is very little in the way of oxidants in the wire to carry away crap from your base metal to the surface of your weld pool.
TIG is the NASA spec welding process- everything need to be clean AF. You'd want to be able to eat your lunch off it. Fitup will ideally involve cutting, beveling, sanding and cleaning with acetone or similar to prep before even getting the welder turned on. So rusty work is not an ideal TIG candidate without a lot of prep.
Power source
Stick / TIG uses a constant current output, so you'll be able to tig (albeit scratch start) on even the cheapest little stick welders with a suitable torch.
This is opposed to MIG which is constant voltage output, and has extra magic inside to allow a 'multiprocess' power source capable of doing both CC and CV. Then you get into the world of the suitcase wire feeders which are fed CC from a engine drive unit, then switch it to allow MIG output.
Quality can vary... a welder is essentially a gearbox for power out of your wall... so it's level of efficiency, quality (smoothness) of output and lifespan come into play. 240V AC comes in, magic happens, and you get a DC output, Constant Current (CC) for Stick/tig, and Constant Voltage (CV) for MIG. Then there is the quality of the synergic maps that the machine uses to ramp up current/voltage, and choke short circuit current spike conditions in dip transfer...
TL;DR, buy what you can afford and justify getting value out of for your level of use.
Features
What's a nice to have and a must have? Keep in mind- is this machine making you your living, or is it something that you dig out of the shed for a play once a month?
Duty cycle- this reflects in part how efficient the machine is in converting your 240V into a welding output. Inverter machines chop and change the AC waveform to make a nice welding output, but this comes at the expense of power losses and heat... and the circuitry inside the machine can only handle so much heat before it ultimately melts into a puddle on the shed floor. 100% duty cycle would be nice, BUT unless you are literally a robot, you'll never reach the duty cycle. Aim for somewhere between 30 and 60% duty cycle at a current level which is honest for the kind of work you are doing- and clarify what the stated duty cycle is measured at. eg 60% @ 120A would be fine for a home gamer, but this might be a 180A machine, and be good for 30% duty cycle at 175A...
Accessories- Ensure you get what you need in the box to start welding. Commonly you can expect to get a MIG gun, earth clamp, a gas hose, some wire feed rolls (for MIG), a regulator.
Spec sheets- ensure you check / compare machines based off their spec sheets- if a supplier can't offer all the technical details of the machine, then consider them either a seller of crap goods, or someone who doesn't know their product. You wnat to know things like duty cycle at a given current output, input voltage (including a variance factor if you plan to run it off a smaller generator), output current (or voltage) range, operating temperature range, IP factor, weights and dimensions, and any applicable standards the machine meets...
Warranty / service- Find out what recourse you have with a faulty machine. They can and do break, like any other man-made thing. Also look at spares availability after the warranty period- many smaller companies will not hold an inventory of spares, so be wary.
DC TIG welding aluminium
I have not tried this, but there is myth and folklore spoken by crusty old boilies from the dark ages. Days where a TIG would cost 5x the yearly salary of the workshop foreman, machines of metal and Iron which required the building to be built around it on 10" thick reinforced concrete footings... Some say it is possible with DC for thick sections using Pure helium shielding gas. These days, with helium being a dwindling resource, the shielding gas would cost as much a couple of decent AC TIG machines... Interesting reading starts here if you are interested. If we (as humankind) were to put an accurate value on our helium reserves (ie for use in real useful technologies like cooling MRI machines etc), than the real cost of A helium filled party balloon would be about $300.
AC TIG for aluminium
Cheap ebay machines can be had from a couple of hundred, honest home / light fab machines start at approx $1200, a whizz-bang unit from the big manufacturers would set you back about $4K or more. You get what you pay for in these machines, it's equivalnet to watching a video in 240P vs 4K resolution. High end machines have AC pulse, and the welder can dictate where the AC waveforms sits relative to the 0V line (AC Balance), and then stipulate things like pulse time, background current, pulse ON time down to the poofteenth of a second, with a pulse rate of up to 200Hz (200 pulses per second...) I dream of something like this...
Machine country of origin
There was mention of 'cheap mass produced chinese stuff'. The unfortunate reality is that this is where most welding power sources are made. It's economies of scale and comsumer price points which dictate this. Most big brands have 80% of their range made in such places. Those who want higher quality and a given country of origin will have the option, but WELL out of the price range of a home gamer. Fronius, and EWM are two high end manufacturers who have in-country manufacturing, but these machine are an incredibly premium product. The rest are either partly or wholly made in china.
Yeah, can be quite involved, but have an idea of what you are comfortable to spend based on your usage of the machine (and what it'll realistically save you if you were to get the work done professionally by someone else) and be comfortable in having a throw-away unit if something does go wrong with a cheap unit. Mainly, get amongst it, have a crack and 'lay some sick beads' as the apprentices say.
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