[QUOTE=Donald Miller]
If I were looking to build a lightweight metal camera I would not choose stainless because it is not dimensionally stable...at least in the thicknesses that would allow it to be light weight.
You need to explain this further. Stainless is MORE dimenionally stable than aluminum - look up the coeffecient of expansion for the metals.

Stainless is a heavy metal. I know because I have worked with it for over thirty years.
It's nearly the same weight as carbon steel, so "heavy" is relative, especially when used in thin sections.

I would opt for titanium. It is lighter and stronger than aluminum and stainless. It would require machining too. One of the small "all in one machines" would probably do what is needed to build a camera.
You need to check on the mass of metals before you make statements like this. Titanium is 60% the weight of steel for the same cubic volume. Aluminum is 40% the weight of steel for the same volume - making aluminum 20% lighter than titanium.

The reason a final structure in titanium can be lighter than aluminum is the strength of titanium is about 6x aluminum. Meaning that depending upon the structure, a titanium piece could be up to 6x lighter than the same piece made from aluminum because of titanium's greater strength which results in thinner sections and reduced volume.

Also, titanium is a PITA to work. It chews tools up in a hurry, doesn't hold first pass dimensions readily so it requires a lot of finishing passes. I have a 6-inch square, 10-inch tall block of titanium in my workshop. You can attempt to polish it with carborundum, Al/Ox, ceramic, etc. - it either gums up the polishing wheels or just destroys them in a spectacular shower of sparks.

Titanium is extrememly expensive to purchase and expensive to work to a final finished piece. Magnesium alloy is a far, far better choice for any type of camera body. Way lighter weight then either titanium or aluminum and far easier to work than titanium.

In fact this would also be what I would use to build film holders since the ability to hold tolerances is greatly enhanced over conventional wood working equipment. If the machine can machine metal, it sure can machine wood.
Then you have no knowledge of computer numerical controlled wood working equipment - it's the same accuracy as CNC metal working equipment. Wood can be worked on metal working machinery with slightly different tooling.

Welding aluminum is usually done with TIG or MIG reverse polarity DC with high frequency generator.
Not quite. Aluminum is welded with "AC" (not real AC current) but with DC current and the electrode alternating between electrode positive and electrode negative. The DCEP portion of the wave providing cleaning of the oxides on the aluminum surface, and the DCEN portion providing the welding.

The high frequency does two things. First, it starts the arc without touching the electrode to the metal surface so you don't contaminate the electrode; secondly, a frequency imposed on the arc focuses the arc (makes it narrower) which can be useful for greater penetration. In most cases, the frequency is set between 120 Hz and 150 Hz. The Miller machines go up to 250 Hz which is almost useless - but it looks good. Kind of like a guitar amplifier volume control that goes to "11."

The weld is flooded with argon for lighter material and flooded with Helium for heavier material.
No. Argon can be used for welding any thickness of metal as can helium. Helium holds more heat that argon, so it can provide greater penetration, but with a narrower weld bead. It may be useful on thicker metal if the welder's maximum amperage cannot be matched to the thickness of metal. Believe me, a 600 amp TIG can weld almost any metal in a single pass with argon.

Welding stainless (especially light guage--22-18 ga) is best done with TIG equipment ...DC straight polarity and high frequency generator. The weld is flooded with Argon to keep the weld from having inclusion of oxygen which will tend to cause the weld to become brittle and subject to failure
Actually - it's far easier to weld thin metal with a MIG. That's why they're so popular for sheet metal repair. MIG welding thin stainless is a breeze. The problem with TIG on thin metal is the extreme heat from the arc warps the metal. A factor contributing to this is that you have to dwell longer on one area with the torch in order to melt the metal making a larger heat affected area. If you turn up the amperage to increase your travel rate and decrease the amount of dwell time on any one area - you risk blowing through the metal when the arc starts.

Granted, TIG is the ultimate in welding controllability. You could literally weld a razor blade to an anvil.

If you setup a MIG correctly, you have the minimum amount of heat affected area.

Both MIG and TIG use DCEN (electrode negative) to weld stainless steel. There is no need for high frequency to weld stainless with a TIG after the arc is started. TIG welders with HF start will stop the HF after the arc is struck and stabilized when the machine is set to DC and the electrode is negative.

For reference, I've welded stainless using a lift TIG machine with NO HF at all. It works fine, you just have to get used to the coordination required to start the arc by lightly touching the metal surface with the electrode when you activate the arc. To learn the technique, many people start with a copper plate next to the area to be welded, start the arc on the copper plate and transfer the arc to the weld area because the tungsten electrode won't stick to the copper plate.