Would anyone like to add a comment concerning the shape of the tip of the electrode Steel vs. Aluminum and how to form that shape ?
I use an import inverter setup much like the one that you have, and have found that 2% Thoriated tungsten ground the same as for steel works very well. There is something a little different between the inverters and the old copper wound welders that makes this work. I find that with pure tungsten on the inverter the tip tends to melt and erode way too fast when welding aluminum, and will contaminate the weld.
I've only been TIG welding a short time myself, and can share a few things that seemed to work for me as far as getting started.
Make sure that your using an appropriate size electrode. The diameter of the electrode should be close to the thickness of the material that your welding. Using a large electrode on thin material is a recipe for disaster in my experience.
On getting your puddle started on both pieces, I find it best to have no space between the parts I'm joining. When starting your puddle, especially on thin material (less than .100") pick a spot a little ways in from the end of the joint to begin the weld so that the corners don't melt away and fall off. (Once the weld is started you can go back and finish it to the edge.) Put as much heat into it as you can as quickly as you can while still leaving yourself a little pedal to work with to get the puddle started. Watch the area closely, you will see the "texture" start to change in the area you are heating before it actually melts, if one side melts and the other isn't really close move your torch slightly to concentrate more heat on the side that isn't melting. Once one side is molten and the other is really close, simultaneously add a dab of filler rod and mash the pedal for a second to get extra heat to melt the filler to both sides and create the weld pool. This is one of the most critical parts to me, because in order to insure good penetration you must have full contact with the pool with both pieces that you are joining, but you have to develop the feel for it to not burn through. (Does that make any sense
) When I was starting I burned through a lot, and practiced till I was able to pretty much keep it right on the edge of burning through for full penetration. If you don't go over the edge a few times, you'll never really know exactly where it is.
Once you have a pool started, it's a relatively easy matter to run the bead. I like to pulse the pedal, add a dab of filler, let off the pedal, then move a little and repeat. Just watch the pool carefully and add filler when the edges start to melt back. As your weld progresses the aluminum absorbs heat and it takes less amperage to continue the weld, keep an eye on this also and slowly back off on the pedal to maintain an even bead width. If you keep welding at the same amperage it took to start the bead, you will eventually blow a hole in it.
Ending the bead is another tough spot. I find it best to taper off the amps and slow my weld speed when I'm getting to the end of the weld. When I put my last spot of filler in I'll give a little extra pulse to the pedal and then let off completely and pull the torch away slowly in the direction of travel, rotating the head to keep the post flow gas on the weld as the machine ramps down. I find that this move helps to minimize pitting at the end of the bead.
Everything that was said above about cleanliness is spot on, don't get tempted to try to bypass any of it, it will not be good. In fact, if you've cleaned your metal, and have to leave it for some reason for more than an hour or so, clean it again before trying to weld, wire brush and all. Bare clean aluminum starts oxidizing instantly. I know you machine parts, just think how dull that beautiful shiny freshly machined surface looks 1/2 hour after you're done.
IMPORTANT SAFETY WARNING>
Use ONLY acetone or alcohol to clean the metal. DO NOT use brake cleaners or other shop chemicals ever, as they may have nasty reactions with heat and atmosphere and try to kill you. Make sure that whatever you use to clean is completely evaporated and the container is well away from the welding area before striking an arc so that you don't end up on fire.
The main thing is to practice, practice, practice. When you think it looks good, cut it and look inside the weld. Bend the parts you've welded and then cut them to see how they hold together, and make sure you're comfortable with it.
I always viewed welding as an evil necessity until I started TIG welding. Now it's my Zen place and something that I absolutely love to do. The total control and concentration needed to do good work drives everything else from my mind like nothing else I've ever found. It's better than recreational medication, LOL.
Another thing that I highly recommend is to get a water cooled torch if you're planning on doing a lot of aluminum. It takes a lot of amps and the torch can get very hot in the hand quickly. It really sucks having to stop in the middle of a good bead because your torch is about to melt in your hand. I bought a water cooled torch for about $150 and built my own cooler from a 5 gallon plastic bucket, a small pond pump, and some fittings from the welding supply shop for less than $100. It makes all the difference in the world and is the best thing since sliced bread in shirt pockets.
It looks like your off to a good start, and I hope some of what I've put up here helps. Like I said at the start, this is what works for me, your mileage may vary and it seems that everyone develops their own style and technique over time. Just remember to relax and enjoy it and have fun.