Troubleshooting guide › Process troubleshooting › CD stud welding problems

Capacitor-discharge stud welding problems: stud falls off, one-side fusion, splash, backside marks, tilted stud

A CD stud weld takes a few milliseconds: the capacitor bank discharges through the small ignition tip on the stud, the tip vaporises, the arc melts both faces and the spring pushes the stud into the pool. Because it is so fast, the result depends almost entirely on three things being right before the trigger is pressed: the surface, the gun set-up and the charging voltage. This page is for M3-M12 studs on sheet from about 0.5 to 3 mm.

Seen with: Capacitor-discharge stud welders (CDS10M, CDS12M, CDS-series), contact-type guns with three legs, mild steel, stainless, copper-plated and aluminium studs.

Reviewed by the Cruxweld technical team, October 2026. Settings are starting points to confirm on a test piece; acceptance is decided by the code or drawing the job is built to. Checks marked “call” are for our technical team.

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What a good CD stud weld looks like Faults, most likely first, and the fix for each Charging voltage by stud size (steel and stainless studs)Prevention checklist Questions we get asked

Before you start: The capacitor bank stays charged after switch-off. Never point the gun at anyone and never press the trigger unless all three legs are on the work. Isolate before touching cables; the inside is for our technical team only.

What a good CD stud weld looks like

Stud square to the sheet, a small even fillet all round the base, very little splash.
Bend test: bend the stud 10-30 degrees from its axis with a tube; the weld and the sheet stay intact.
Tensile: joint strength above 60 % of the stud’s own tensile strength (an M6 grade 4.6 stud should hold more than about 4.8 kN).
Shift rule: weld and bend one test stud at the start of every shift and after any change of stud, sheet, collet or voltage. If it shears, raise the voltage one step and repeat.

Faults, most likely first, and the fix for each

1Stud breaks off in the bend test, little or no fusion

Voltage too low for the stud size; gun pressure wrong (too high flattens the tip before the arc, too low gives no plunge); ignition tip damaged or missing; sheet oily, painted, rusty or coated; earth clamp poor.

Fix: Set voltage from the table below and raise in steps; set the spring pressure to moderate; check every stud has an intact tip; clean the sheet bright; clamp the earth on bare metal, diagonally opposite the weld.

2Fusion on one side only, crescent-shaped weld

Gun not square to the sheet (one leg off the surface or on a bump), stud not concentric in the collet, legs uneven with spatter, arc blow from the earth position.

Fix: All three legs flat on the sheet, gun pressed straight down; stud tip 2-3 mm above the legs; clean the legs; move the earth clamp to the opposite side or use two earths.

3Excessive splash around the base

Voltage too high; gun pulled back too slowly or pushed too hard; stud tip too long for the stud (wrong stud batch); sheet very dirty.

Fix: Lower voltage one step; press lightly, trigger, lift straight off; check the stud batch; clean the sheet.

4Marks, discolouration or burn-through on the back of thin sheet

Voltage too high for the sheet; sheet below the usable range for that stud (an M8 on 1.2 mm will mark through); no heat sink.

Fix: Lowest voltage that passes the bend test; smaller stud or thicker sheet; a copper or aluminium backing block behind the weld; for appearance-critical panels test on scrap first.

5Stud tilted after welding

Gun tilted; collet worn so the stud rocks; legs of unequal length; stud pushed into the collet past the stop.

Fix: Replace a worn collet; check leg lengths; seat the stud to the positioning stop with the flange 1-2 mm clear of the collet; hold the gun vertical.

6Aluminium studs weld inconsistently

Oxide on the sheet; voltage set as for steel; stud and sheet alloys not matched; sheet not clean.

Fix: Brush the sheet with a stainless brush just before welding; raise voltage by about 10 V over the steel value; aluminium studs only on aluminium sheet.

7Collet will not release the stud, or grips loosely

Collet bore worn oversize or sprung; wrong collet for the stud diameter; spatter inside the collet; lock nut loose.

Fix: Fit the collet for the stud; replace a worn collet; clean; tighten the lock nut.

8Welds fine in the morning, weak in the afternoon

Legs and collet fouled with spatter; sheet batch changed (coated or thicker); mains voltage low so the set voltage is not reached; operator pressing harder as he tires.

Fix: Clean the gun front every hour; check the voltage display reaches the set value; re-do the shift test after any batch change.

Charging voltage by stud size (steel and stainless studs)

Stud	Charging voltage
M3	55-65 V
M4	65-75 V
M5	70-85 V
M6	90-120 V
M8	130-160 V
M10	160-180 V

Typical for Cruxweld CDS-series machines; start at the low end, weld a trial, bend-test, and raise in small steps until the collar is even and the bend test passes. Aluminium: about 10 V higher. Other machines use different energy scales; do not transfer the numbers.

Spring pressure: set by the gun sleeve; moderate to start, lock it, and change only one thing at a time.
Stud tip 2-3 mm proud of the three legs; stud seated to the positioning stop; flange 1-2 mm clear of the collet.
Earth cable tightened with the key, clamp on bare metal diagonally opposite the weld point.
Sheet range for consistent CD welding is roughly 0.5-3 mm; thicker plate is drawn-arc territory.

Prevention checklist

Bend test at the start of every shift and after every change.
Sheet cleaned bright at every stud position; no paint, oil, galvanising or rust.
Collet matched to the stud and replaced when worn; lock nut tight.
Gun legs flat, clean and all three on the sheet.
Earth clamp on bare metal, diagonally opposite.
Voltage display reaches the set value before each weld.
Studs from one batch, tips intact, stored dry.

Questions we get asked

CD or drawn-arc stud welding for my job?

CD for small studs (M3-M10) on thin sheet where the back must stay clean: fast, no ferrule, no marks. Drawn-arc for larger studs (M10 and above, shear connectors) on thick plate, with a ferrule and a visible collar. The machines, guns and settings are different; see the drawn-arc page.

Why does the machine charge and discharge by itself?

Usually the set voltage is too low for the charging circuit to hold, or the gun trigger is sticking. Set a higher voltage and check the trigger; if it continues, it is a control fault for our technical team.

Fix it yourself: your Cruxweld machine

Lamps, fault codes and the checks an operator can do outside the cabinet, for the machines this page applies to.

Stud welding machines and guns →

Does the machine matter?

The machine sets the energy (voltage into a fixed capacitor bank); the gun sets the force and the alignment. A weak-weld problem that survives a voltage increase is a gun, surface or stud problem, not a machine fault. Our CD stud welders cover M3-M12 with stepless voltage from 35 to 200 V.

CDS12M CD stud welder Find the right machine Cost calculator Order spares

Still stuck?

Send a photo of the weld or cut, your machine model, consumable and plate thickness. Our technical team replies on WhatsApp with the likely causes and the next checks to make.

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