Drawn arc vs CD stud welding: which process fits your job
Use drawn arc for studs from about 8 mm upward on plate 2 mm and thicker, where weld strength matters more than appearance. Use capacitor discharge for studs up to about 10 mm on thin sheet where the back face has to stay unmarked. Structural work in the Gulf runs drawn arc. Sheet metal, signage and stainless fabrication run CD.
Both processes weld a stud to steel in under a second. That is where the similarity stops. They use different power sources, different studs, different setup, and they fail in different ways when you put them on the wrong job.
Three questions settle it. How big is the stud, how thick is the base metal, and does anyone care what the back of the panel looks like.
How drawn arc stud welding works
The gun lifts the stud off the plate and draws an arc between them. The arc melts the stud end and a pool on the plate. Then the gun plunges the stud into that pool. A ceramic ferrule sits around the joint throughout, shielding the weld and shaping the fillet that squeezes out at the base.
The cycle runs from around 100 milliseconds on small studs to a full second on 25 mm shear connectors. What you get is a full cross-section weld stronger than the stud itself. Bend test a good drawn arc weld and the stud bends while the weld holds. That is why composite decks, anchor plates and shipyard insulation work all run on this process.
How capacitor discharge stud welding works
CD studs carry a small ignition tip on the weld end. A capacitor bank dumps its charge through that tip, the tip vaporises, an arc flashes across the whole face, and spring pressure closes the joint. The event takes a few milliseconds. No ferrule, almost no heat into the sheet, and on material as thin as 0.5 mm, no mark on the reverse.
That last detail is the entire reason CD exists. Kitchen equipment, electrical enclosures, cladding panels, sign letters: anywhere the back face is visible or painted, CD is the only sensible option. The limit is size. Past roughly 10 mm the capacitor bank cannot deliver the energy and drawn arc takes over.
Drawn arc and CD side by side
| Factor | Drawn arc | Capacitor discharge |
| Stud diameter | 3 to 25 mm | 2 to 10 mm |
| Base metal | 2 mm and up; at least 1/4 of stud diameter | From 0.5 mm sheet |
| Cycle time | 100 to 1,000 ms | 1 to 6 ms |
| Reverse marking | Possible on thinner plate | None on typical sheet |
| Ceramic ferrule | Required | Not used |
| Materials | Mild steel, stainless; aluminium with gas | Mild steel, stainless, aluminium, some brass |
| Power | Generator or heavy three-phase supply | Portable unit, standard socket |
| Typical work | Shear connectors, anchors, structural attachments | Sheet metal, enclosures, signage, catering |
Where each process earns its place
Drawn arc owns structural work. Welding 19 mm shear connectors through metal decking onto beams is a drawn arc job and nothing else will do it. Same for anchor studs on embed plates and the pins that hold insulation on tank shells and ship hulls.
CD owns the fabrication shop. A stainless counter with threaded studs underneath, an aluminium sign with mounting pins, a control panel with earth studs. All CD, because nobody wants to regrind and repolish a show face.
What this means for equipment
A drawn arc rig is a commitment. The power source is heavy, it wants a generator or a serious mains supply, and the cables alone need a vehicle. For a single composite deck project rather than a permanent capability, renting usually beats buying. CD units go the other way. Most are one-person portable and run off a normal socket, which is why small fabricators tend to own them outright.
Mistakes that keep showing up on site
Welding over mill scale, paint or heavy zinc. Both processes want bright metal at the weld spot. Thirty seconds with a grinder saves a failed bend test and an argument with the inspector.
Damp ferrules. Ceramic absorbs moisture overnight and damp ferrules put porosity into the weld. In coastal humidity this is a daily discipline, not an occasional precaution. Store them sealed.
Skipping the shift-start bend test. Settings drift and cables wear. Without the test, the first anyone hears about it is a stud shearing off in service, long after the deck is poured.
Mixing up studs. CD studs in a drawn arc gun, or the reverse. The weld ends are engineered for completely different arc behaviour and the results are unpredictable.
Frequently asked questions
What is the difference between drawn arc and CD stud welding?
Drawn arc lifts the stud and holds an arc for 100 to 1000 milliseconds inside a ferrule. CD discharges a capacitor through an ignition tip in a few milliseconds with no ferrule. One trades heat for strength, the other trades strength for a clean reverse face.
Can CD stud welding be used on galvanized sheet?
Not reliably. Grind the zinc off the weld spot or use drawn arc, which burns through light zinc more consistently.
How thin can the base metal be?
CD works from about 0.5 mm. Drawn arc needs a quarter of the stud diameter, so 2 mm and up in practice.
Do welded studs need testing on site?
Yes. Visual check for a full 360 degree flash on every weld, plus bend tests at shift start and after any settings change.
Can the same studs be used in both processes?
No. The weld ends are built differently and are not interchangeable.