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newSchoolWorks.com › VoTechWorks › Transportation › Auto Body & Collision › Welding for Auto Body & Collision Repair › Lesson 7
TRN-AUTO25-04 · Lesson 7 · 18 clock hours · lab

Resistance Spot Welding: Equipment and Technique

Objective. Students will produce resistance spot welds on two-layer 18-gauge steel that meet OEM nugget size and spacing specifications and pass a destructive peel test.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a certified examiner or master technician in the trade or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

What Is Resistance Spot Welding?

Resistance spot welding (RSW) joins two or more overlapping metal sheets by passing a high-amperage electrical current through them while squeezing them between two copper-alloy electrode tips. The electrical resistance of the steel generates heat at the interface, melting a small zone called the weld nugget. When the current stops and pressure is maintained, the nugget solidifies, creating a strong metallurgical bond. Because RSW leaves no visible filler material and can be performed quickly, vehicle manufacturers use it extensively on body panels, structural members, and reinforcements. Auto body technicians must reproduce these factory welds accurately to restore structural integrity and maintain crash-safety performance.

Equipment Overview

A squeeze-type resistance spot welder (STRSW) is the shop-standard machine for collision repair. Key components include:

Portable (gun-style) spot welders are also used where access is limited. Their operating principles are identical; only the physical configuration differs.

Three Critical Settings: Force, Time, and Current

Every acceptable spot weld depends on balancing three variables, sometimes called the welding triangle:

1. Electrode force – holds the sheets in tight contact, preventing surface arcing and ensuring current flows through the interface. Set per the machine manufacturer's recommendation for the total metal thickness being joined.

2. Weld time – the duration current flows. Too short produces a cold, undersized nugget; too long causes excessive expulsion (spatter) and can burn through thin gauge steel. OEM repair procedures specify weld time for each application.

3. Current (heat) – determined by the machine's heat or amperage dial. For 18-gauge mild steel two-layer stacks, consult the machine chart or OEM reference to select the appropriate setting.

Always set the machine on scrap coupons of the same gauge and stack thickness as the repair before welding on the vehicle.

Tip Condition and Dressing

Tip condition is the single most controllable variable in spot welding quality. A worn tip:

Dress tips before each work session and whenever visual inspection shows mushrooming or pitting. Use the tip dresser for the minimum number of passes needed to restore a clean, flat face at the correct diameter. Record tip diameter before and after dressing in your lab log. Overtrimming shortens tip life unnecessarily.

Weld Spacing and the OEM Template

Factory spot welds on structural panels are spaced to distribute load evenly. When replacing or sectioning panels, technicians must match OEM spacing. A typical spacing for 18-gauge steel is 1 to 1.5 inches (25–38 mm) center-to-center, but always follow the specific OEM repair procedure—spacing varies by application. Use the provided spacing template to mark weld locations on the coupon before welding. Welds placed too close together cause current shunting: current takes the path of least resistance through an existing weld instead of the intended interface, producing a weak or absent new nugget.

Performing the Weld

1. Prepare and clamp coupons flat with no gap between layers.

2. Position electrode tips perpendicular to the surface, centered on the marked location.

3. Apply full electrode force before triggering current.

4. Allow the full timed weld cycle to complete; do not release pressure until the timer finishes.

5. Move to the next location; maintain spacing per the template.

Destructive Peel-Back Test

A peel-back test is the accepted method for verifying nugget quality in a training or setup environment. Using a chisel and hammer, peel the top coupon back from one end of the row. A good nugget tears material from one sheet (called button pullout), leaving a hole in one layer and a raised button on the other. Measure the button diameter with digital calipers. The minimum acceptable nugget diameter is defined by the OEM or I-CAR reference for the metal gauge; a common guideline is that the nugget diameter should be approximately 3.5 × the square root of the sheet thickness in millimeters, though always defer to the specific OEM table. If the layers simply peel apart and leave only a smooth surface impression, the nugget was cold or absent—adjust settings and re-test on fresh coupons.

Safety

Key terms8

Weld nuggetThe small, lens-shaped zone of fused and resolidified steel formed at the interface of two sheets during resistance spot welding; its diameter determines joint strength.
Squeeze-type resistance spot welder (STRSW)A shop welding machine that uses pneumatic or mechanical force to clamp electrode tips on either side of a metal stack while passing high-amperage current to form a spot weld.
Electrode tip dresserA rotating cutting tool used to re-face mushroomed or pitted copper-alloy electrode tips to their correct diameter and flat profile without removing them from the machine.
Current shuntingThe condition in which electrical current bypasses the intended weld location and flows through an adjacent completed weld, resulting in a weak or absent nugget at the new location.
Button pulloutThe acceptable result of a destructive peel-back test in which the nugget tears a plug of steel from one sheet, confirming a sound metallurgical bond.
Electrode forceThe squeezing pressure applied by the electrode arms to the metal stack during welding; it prevents arcing, ensures metal-to-metal contact, and consolidates the nugget as it cools.
Weld timeThe duration, measured in cycles or milliseconds, during which current flows through the metal stack; it is set on the machine timer and must match OEM or machine-chart specifications for the gauge being welded.
ExpulsionThe ejection of molten metal from the weld zone as visible spatter, caused by excessive current, insufficient electrode force, or poor fit-up; it indicates a potentially defective weld.

Worked examples4

1. You are welding 18-gauge mild steel (0.048 in. / 1.22 mm thick per sheet) in a two-layer stack. The OEM table specifies minimum nugget diameter using the formula: minimum diameter (mm) = 3.5 × √(t), where t is single-sheet thickness in millimeters. Calculate the minimum acceptable nugget diameter.
  1. Identify t: single-sheet thickness = 1.22 mm.
  2. Calculate the square root: √1.22 ≈ 1.105.
  3. Multiply: 3.5 × 1.105 = 3.87 mm.
  4. Round to practical caliper resolution: minimum nugget diameter ≈ 3.9 mm.
Answer: The minimum acceptable nugget diameter for 18-gauge (1.22 mm) steel is approximately 3.9 mm. Any measured button diameter at or above this value passes the peel-back test.
2. Your OEM spacing template calls for weld centers spaced 1.25 inches apart. After welding, you measure the distance between weld 3 and weld 4 with calipers and get 1.40 inches. Does this pass the ±1/8 in. tolerance?
  1. Identify the target spacing: 1.25 in.
  2. Calculate the upper tolerance limit: 1.25 + 0.125 = 1.375 in.
  3. Calculate the lower tolerance limit: 1.25 − 0.125 = 1.125 in.
  4. Compare the measured value (1.40 in.) to the limits: 1.40 > 1.375.
Answer: 1.40 inches exceeds the upper limit of 1.375 inches, so this spacing FAILS the ±1/8 in. criterion. The weld is too far from its neighbor; note the deviation in the lab log and adjust technique for the next coupon.
3. You produce 10 spot welds on a coupon. On peel-back, 2 welds show smooth surface impressions with no button pullout, and the remaining 8 show clean button pullout with nugget diameters all above the minimum. Does this coupon meet the lab pass requirement?
  1. Count welds with acceptable nuggets: 8 out of 10.
  2. State the pass threshold: 8 of 10 welds must produce nuggets meeting minimum diameter.
  3. Check spacing criterion separately: assume all spacing measurements are within ±1/8 in. for this example.
  4. Apply both-criteria rule: nugget criterion = 8/10 = meets threshold; spacing criterion = pass. Both required criteria are satisfied.
Answer: The coupon passes. Exactly 8 of 10 welds meet the nugget diameter requirement, which equals the minimum threshold of 8/10, and spacing is within tolerance. However, the 2 cold welds should be analyzed—review electrode force, weld time, and tip condition before the next session.
4. Before tip dressing, your electrode tip measures 6.8 mm in face diameter. The machine specification for 18-gauge two-layer work calls for a tip face diameter of 5.5 mm. After one pass with the tip dresser the tip measures 6.1 mm. How many additional dressing passes are needed, and what is the total reduction achieved so far?
  1. Target diameter: 5.5 mm. Current diameter after one pass: 6.1 mm.
  2. Remaining reduction needed: 6.1 − 5.5 = 0.6 mm.
  3. Reduction achieved so far: 6.8 − 6.1 = 0.7 mm.
  4. Continue dressing in additional passes, checking after each, until the face reads 5.5 mm; exact passes depend on the dresser—check with calipers after every pass to avoid overtrimming.
Answer: Total reduction so far is 0.7 mm; an additional 0.6 mm must be removed to reach the 5.5 mm specification. Take additional dressing passes one at a time, measuring after each, to avoid removing excess copper and shortening tip life.

The math in this lesson1

Learners evaluate 3.5 × √t for 1.22 mm steel to find a 3.9 mm minimum nugget diameter.
A-SSE.1
Brush up in the math halls at newSchoolWorks.com:
In class: Instructor demonstrates resistance spot welder setup: electrode tip dressing, electrode force setting, weld-time selection, and coupon clamping; students dress a set of tips using a tip dresser and record tip condition before and after. · Students produce a row of spot welds on clamped two-layer coupons, spacing welds to match a provided OEM spacing template (typically 1–1.5 in. spacing depending on gauge), then mark each weld location on a diagram. · Destructive peel-back test: students separate the two layers with a chisel and hammer; each nugget is measured with calipers and must meet or exceed the minimum nugget diameter specified on the OEM reference (expressed as a formula or table range) to pass.
Materials: Resistance spot welder (squeeze-type or portable), electrode tip dresser, 18-ga mild steel coupons, OEM spot-weld spacing template, chisel, hammer, digital calipers, lab log forms, I-CAR Resistance Spot Welding reference
Check: Eight of ten spot welds per student must produce nuggets meeting minimum diameter on peel-back test; spacing must be within ±1/8 in. of template—both criteria required for pass.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

I-CAR Resistance Spot Welding / Welding Fundamentals
1. What is the PRIMARY purpose of maintaining correct electrode tip face diameter during resistance spot welding?
ASE B-Series / Welding and Cutting
2. During a peel-back test, both sheets separate with only a smooth surface impression and no material transfer between layers. This result MOST likely indicates:
I-CAR Resistance Spot Welding / Welding Fundamentals
3. Current shunting is MOST likely to occur when:
Shop Safety / OSHA Lockout/Tagout
4. Before servicing or changing electrode tips on a resistance spot welder, the technician MUST first:
I-CAR Resistance Spot Welding / Quality Inspection
5. An 18-gauge two-layer coupon is welded, and the peel-back test shows button pullout on 7 of 10 welds, all with nugget diameters above the minimum. All spacing measurements are within ±1/8 in. of the template. What is the outcome according to lab pass criteria?
ASE B-Series / Welding and Cutting
6. Which variable in the welding triangle directly controls the duration that current flows through the metal stack?
I-CAR Resistance Spot Welding / Quality Inspection
7. Using the formula minimum nugget diameter (mm) = 3.5 × √t, where t is single-sheet thickness in mm, what is the minimum nugget diameter for 18-gauge steel with a single-sheet thickness of 1.22 mm? (Round to one decimal place.)
ASE B-Series / Welding and Cutting
8. Visible spatter ejected from the weld zone during a resistance spot weld cycle is called expulsion. Expulsion MOST commonly results from:
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