David,

As others have said here, spot welding titanium to stainless steel isn't practical, but spot welding titanium to itself IS possible without using TIG equipment & shielding gas (usually argon). There are several inexpensive spot welders on the market that do the job fine, as well as Ebay is a great source. Shipping is typically the costly part as they can be pretty heavy. I haven't seen Wren's plans, but I'm sure they would work also. Please be careful as you're working with a signifigant amount of heat & current.... be safe!

Biggest enemy of spotwelding is dirt & oils on the surface. Also, try to get as good of a fit as possible. In my work (I design & program robotic welding systems, specializing in thin metal TIG welding) most of the time is spent in perfecting the parts layout & fit. If those are good then the weld itself is simply a matter of getting the proper amperage & speed.

Be aware that if you do choose to spotweld titanium you'll most likely get some corrosion over time as it really needs to be shielded with an inert gas. The oxygen surrounding the molten metal will contaminate the weld.

Take a look at www.design2fab.com for a demo of some sheet metal layout software. I'm sure there's many others on the market geared towards HVAC systems.

Bob