Originally Posted by fxstbiluigi
A 'tack' also needs to be strong enough to hold the parts in alignment and withstand the pressure of coaxing the rest of the weldment into alignment. And withstand the stresses of welding once the fit-up is complete.
eh, don't confuse the two. You tack pieces together and give them a smack, maybe too. But a third or fourth time or too much movement and your pieces are going to be laying on the ground.
In other words, tacks hold together only when subjected to a small force. And when the gap isn't right, the fitter will simply break the tack and have the welder start over.