Tack Weld — What It Is and Why It Matters
A tack weld is a small, temporary weld used to hold parts in position during fit-up and assembly before the final production weld is made. Tack welds maintain joint alignment, gap spacing, and overall assembly dimensions while the welder completes the permanent welding.
Good tacking practice is a critical fabrication skill — poorly placed, undersized, or overheated tack welds can cause distortion, cracking, and fit-up problems that cascade through the entire assembly. Tacks should be small enough to be fully consumed by the production weld, placed at strategic locations to resist movement, and made with the same process and filler metal as the production weld on code work.
Tack placement strategy depends on the joint type, material, and welding sequence. Short, evenly spaced tacks prevent gap closure during welding. Bridge tacks (made on small tabs outside the joint) can be ground off after production welding for clean aesthetics. On critical code work, tack welds that will be incorporated into the final weld must meet the same quality requirements as production welds.
Frequently Asked Questions
How big should a tack weld be?
Tack welds should be just large enough to hold the joint in position — typically 1/2" to 1" long for light fabrication, and proportionally larger for heavy plate. The tack must be fully consumed by the subsequent production weld. Excessive tack welds add unnecessary heat and distortion; insufficient tacks allow the joint to move during welding.
Should I grind my tack welds before final welding?
Grind tack welds that have defects (porosity, cracks, high profile) because these defects will be trapped inside the production weld. Clean, properly made tacks that lie flat within the joint can generally be welded over without grinding. On code work, the WPS or contract specification may require grinding all tacks.