Hatz Biplane Association

Hatzers,
I am about ready to tack weld the sides of #136, and I am looking for tips and tricks as far as tack welding is concerned.
I will be using a meco N-Midget torch, which I have used to do practice welding on a bunch of scrap tubing.
Thanks!

Is there anything in particular you are looking for?

I use the Meco midget as well. A few things I can suggest:
1. Make your tubing fit as tightly as you can. While gas is more forgiving of gaps than TIG, a tight fit goes a long way towards minimizing movement of the tubes when you fully weld the joint.

2. Be sure to fully heat your tacks and have them flow into the metal of the tube. If you don't then you'll have a cold lump sitting on top of the tube that isn't really attached. This can easily "pop" off as the tube cools and the stresses change.

3. I generally tack in four places (when possible) @ 90degree intervals. This helps maintain alignment of the tubes. It also keeps things aligned if one of your tacks "pops" or cracks when they cool (see #2).

4. If you tack something and it didn't come out as planned, don't be worried that you are stuck with the result. It is very easy to fix the "problem": heat up the tack(s) and move the tube as needed, heat up the tacks and remove the tube to re-position, grind out the tack, etc.

Hope that helps.

These are good points.
I guess my specific question is with regards to tack-welding the fuselage sides while they lay on the bench.
I have the tubing fitted reasonably well, I am planning on clamping everything so it won't move.
Did you wet the bench in the area of the weld to minimize burning the wood/MDF?
if a weld looks like a "V" (the two diagonal members) laying on a "____" (the longeron), where would you want the tack welds to be?

"_V_"

thanks!

You mention "clamping everything so it won't move". Does that mean you do not have any blocks holding the pieces in place/alignment on the table? The blocking should be sufficient to hold everything in place and provide an easy way to build identical sides.

I did not wet the bench. Using MDF (mistake) was ok, but wetting it would cause it to break down after a while. I didn't worry about scorching the bench. You're only there for a couple seconds. In some spots I used a thin sheet of scrap aluminum as a heat shield which worked well. I slid it under the joint before I tacked it (which I did not do in this photo).

For any joint I tacked anywhere the tubes touched. See photo... When I took the side out of the jig I also turned it over and took the time to tack the other side.

Perfect!
That is exactly what I was looking for.

Buy your stainless sheet for the firewall (buy it a little large). You will use the extra stainless for a hundred things in the future. A small piece of stainless works great at isolating weldments.