Basics: 'DC only', what does it mean? What to look for in a beginner TIG?

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I've been looking for a TIG machine that I can use to do aluminum work with.  I have only worked with steel/iron using my MIG machine (Millermatic 175), and tried it with aluminum with not too good a result.  Cruising on Ebay just to see what's out there, I saw an ad for a Daytona machine or something like that and it said it was a DC only machine.  What does that mean?  And what does that keep you from doing?  I've got the Richard Finch book, so I'll check that out when I get home.  Also, what does scratch-start mean?  I assume that's how the arc gets started with a machine that has it?  And what is post flow and why is it important?  If there's a thread that has all this stuff in it, please point me to it!  Thanks,-Mike
Reply:AC is best for aluminum, though DC is still a Mil-Spec process.  It's just more difficult, is limited to the thicker weldments, and requires VERY clean, unoxidized aluminum, as there will be no cleaning action in the arc.These days, you pretty much buy an AC machine to do aluminum with.Scratch start means the tungsten is hot all the time, and you strike it like you do a stick electrode.  It CAN contaminate the tungsten a little, and the weld, but it is still an okay starting method, as opposed to "lift-arc" or "touch-start" which are electronically controlled, or HF start which is completely non-contact.Post flow is the flow of gas after the arc is out.  Keep the torch pointed at the weld.  It continues to provide a few seconds of shielding to the hot weld as well as the hot tungsten.
Reply:OK, so it's a good idea to go AC then...I was looking at the Maxstar 140 STR and the Syncrowave 180SD.  I know the Maxstar is way less money, and I figured it would be OK to start with, but the SD seems like it's just way beeter overall.I like the Miller stuff I have, so I'm looking there first though the Thermal Arc 185 seems to come up a lot in threads.
Reply:I owned a Maxstar 140 STR, and it was an excellent machine in my opinion and experience.I have the newest version now, the Maxstar 150 STH.  But I wouldn't weld aluminum with either.Without going to an AC/DC inverter, the Syncrowave is where it's at if within the power requirements.
Reply:If your planning on a fair amount of al welding the inverters  the TA you mentioned has a lot of functions in the AC mode that the SD doesn't.  The TA also has a full function pulser and 2t and 4t modes which allow you to control the amps with nothing but a push button on the torch.  It was a no brainer for me.DennisThermal Arc 185-TSWMillermatic Challenger 172VictorO/AAtlas Craftsman 12 by 24 LatheEsab PCM-875Wholesalem Tool Mill-Drill