Michael Horowitz wrote:
Please see the sketch at http://members.cox.net/mhorowit/cluster/
Using Oxyacetylene, how do I get the base molten without burning the
two pieces that form the 1/8" gap? Are "chill bars" the only choice?
MikeH

If while playing with different heat settings you can't get the base
metal hot enough while not burning away the edges, you'll have to clamp
heat sinks to the upper parts.

However, you don't have to get the base completely molten to start a
weld. You get it just under molten, drop a molten bead on it, then heat
the bead itself until it fuses and blends with the underlying metal on
each surface, then start adding more rod to get the resulting puddle
nice and full with a nice fused feather edge all around, then start
moving. Putting down molten rod just before the base metal is molten
is the best way to weld exposed edges without burn back. You'll have to
carefully observe the edge of the puddle that is on the base material to
be sure it is blending and fusing.

The rod tip itself is key to controlling the amount of heat going into
the puddle. If an edge gets too hot and starts to burn away, don't back
right off, just back off a tiny bit while at the same time putting the
rod tip between the torch and the overheated edge and drop some rod
metal there. That gives a bit of breathing room while you adjust and go
back in. Mastering the technique of backing off just enough but not too
much makes a big difference because now you can stay in control while
doing a really tricky joint that is not tolerant of excess heat, and
when you get burn through's, you can pause to fill the hole and then
keep going without missing a beat.

John