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newSchoolWorks.com › VoTechWorks › Manufacturing & Marine › Shipbuilding / Marine Fabrication › Capstone Project: Integrated Marine Fabrication Build › Lesson 2
MFG-SHIP13-07 · Lesson 2 · 108 clock hours · lab

Structural Fabrication: Fit-Up, Tacking, and Full Welding

Objective. Following the approved fabrication plan, the student will cut, fit, tack, and fully weld all steel and aluminum components of the assigned assembly to dimensional tolerances and weld quality standards acceptable under AWS D1.1 (steel) and AWS D1.2 (aluminum), as verified by instructor visual and dimensional inspection.

Structural Fabrication: Fit-Up, Tacking, and Full Welding

This lab brings together every skill you have built in layout, cutting, fitting, and welding. You will cut steel and aluminum components to dimension, fit them into a fixture, tack-weld the assembly in a controlled sequence, and complete all full welds per a weld map. The finished assembly must meet AWS D1.1 visual acceptance criteria for steel and AWS D1.2 criteria for aluminum, and all critical dimensions must fall within ±1/16 in. of nominal.

Safety First

Before touching any tool or machine, complete your pre-work safety check.

Step 1 – Cutting and Layout

Review the fabrication plan and weld map before picking up any tool. Identify every part number, its material grade, thickness, and finished dimension. Using the layout tools from your Shipfitting Layout course — soapstone or scriber for steel, fine marker for aluminum — transfer all cut lines and reference points to the stock. Center-punch intersection points so your tape-measure hook has a repeatable reference.

For straight cuts on mild-steel plate, the plasma cutter or oxy-fuel torch is fastest. Set your standoff distance, travel speed, and amperage (plasma) or tip size and gas pressures (oxy-fuel) for the stock thickness. After cutting, dress all edges with an angle grinder and flap disc to remove dross, scale, and any hardened heat-affected zone metal that would prevent tight fit-up. Measure each finished piece and record actual versus nominal dimensions on your inspection log before dry-fitting.

Step 2 – Dry Fit-Up and Fixturing

Place all pieces in the welding fixture without applying heat. Use clamps, strong-backs, and wedges to achieve the joint fit-up specified on the weld map: root opening, root face, bevel angle, and land dimension. Check alignment with a combination square and straight-edge. Any gap or mismatch outside tolerance must be corrected now — grinding, re-cutting, or shimming — because tacking locks in error. Record your dry-fit dimensions.

Step 3 – Tack Welding Sequence

Tacking controls distortion. Weld sequence matters: tack the center of long joints first, then work outward in a balanced, alternating pattern. For steel, use SMAW with E7018 or FCAW. For aluminum, switch to your GMAW unit fitted with an aluminum-rated liner and ER4043 or ER5356 filler wire. Tacks must be long enough to hold the joint rigidly — typically 3/4 in. to 1 in. for light plate — but small enough to be consumed by the root pass.

After each tack, perform a visual weld inspection (VWI) before accepting it. Look for cracks, porosity, incomplete fusion at the tack ends, and undercut. A cracked tack must be ground out and replaced; never weld over a cracked tack.

Step 4 – Full Welding Per the Weld Map

The weld map assigns every joint a weld symbol, a process, filler metal, and pass sequence. Follow it exactly. For groove welds requiring full penetration, complete the root pass first, inspect it, then add fill and cap passes. For fillet welds, maintain the specified leg size consistently — use a fillet weld gauge to check each completed bead.

Between passes on steel, chip and grind slag, then wire-brush before depositing the next pass. On aluminum, wire-brush (aluminum-only brush) between passes to break up the oxide layer. Perform self-VWI after each pass using your AWS acceptance criteria reference card.

Step 5 – Self-Inspection and Documentation

AWS D1.1 and D1.2 define visual acceptance criteria for welds: maximum undercut depth, minimum weld size, surface porosity limits, crack prohibition, and overlap. Use a fillet weld gauge to measure leg size and throat. Use a groove weld gauge to check reinforcement height. Document every measurement, whether it passes or fails, in your project inspection record. A nonconformance must have a documented corrective action — grind out and re-weld or blend-grind a high reinforcement — before the instructor's final inspection.

Your inspection log must match the instructor's findings within one minor discrepancy. Any weld that fails VWI must be repaired and re-inspected. The goal is 100% passing welds and at least 80% dimensional conformance before the assembly advances to the coatings phase.

Key terms10

Weld MapA drawing or marked-up plan that assigns a weld symbol, process, filler metal, and pass sequence to every joint in an assembly, serving as the fabricator's instruction sheet.
Fit-UpThe process of positioning and clamping all parts to the correct joint geometry — root opening, root face, bevel angle, and alignment — before any heat is applied.
Tack WeldA short, temporary weld used to hold parts in position during fit-up; it must be placed in a controlled sequence to minimize distortion and must pass VWI before full welding begins.
Visual Weld Inspection (VWI)An examination of a completed weld using the naked eye and simple gauges to verify that size, profile, and surface condition meet the applicable code's acceptance criteria (AWS D1.1 for steel, AWS D1.2 for aluminum).
AWS D1.1The American Welding Society's Structural Welding Code for steel; it establishes qualification requirements, workmanship standards, and visual and nondestructive examination acceptance criteria for steel weldments.
AWS D1.2The American Welding Society's Structural Welding Code for aluminum; it parallels D1.1 but addresses aluminum's unique properties, including oxide layer management, filler alloy selection, and distortion control.
Fillet Weld GaugeA handheld tool used to measure the leg size and throat of a fillet weld to confirm the weld meets the minimum size called out on the weld map and code.
UndercutA groove melted into the base metal at the weld toe that reduces the cross-sectional area of the base material; codes limit undercut depth because it is a stress concentration and a potential crack initiation site.
Strong-BackA rigid bar or bracket tack-welded or clamped across a joint to resist distortion during welding; it is removed after the weld cools and the joint has gained sufficient strength.
Contamination (Aluminum)The introduction of steel particles, oil, or moisture into an aluminum weld joint or filler wire; it causes porosity and inclusions, leading to rejection under D1.2, and is prevented by using dedicated tools and proper cleaning procedures.

Worked examples4

1. Your weld map calls for a 3/8 in. fillet weld. After welding, you measure the horizontal leg at 7/16 in. and the vertical leg at 5/16 in. Does this fillet weld pass the minimum-size requirement, and is the profile acceptable for a balanced fillet?
  1. Step 1 – Identify the requirement: the weld map and AWS D1.1 require a 3/8 in. (0.375 in.) fillet weld on both legs.
  2. Step 2 – Compare each measured leg to the minimum: Horizontal leg = 7/16 in. = 0.4375 in. ≥ 0.375 in. → meets minimum. Vertical leg = 5/16 in. = 0.3125 in. < 0.375 in. → does NOT meet minimum.
  3. Step 3 – Assess the profile: a balanced fillet has equal legs; the difference here is 7/16 − 5/16 = 2/16 = 1/8 in. This indicates an unbalanced, convex-on-one-side profile, which also concentrates stress at the short leg toe.
  4. Step 4 – Determine disposition: the vertical leg is undersized. The corrective action is to add a weld pass on the short-leg side to bring it to at least 3/8 in., then re-inspect.
Answer: The weld fails because the vertical leg (5/16 in.) is below the required 3/8 in. minimum. A supplemental pass must be added to the short leg, and the weld must be re-inspected.
2. You are cutting a mild-steel plate to a nominal length of 24 in. Your tape measure reads 24 3/32 in. on the finished piece. Is this within the ±1/16 in. tolerance, and if not, by how much does it exceed the limit?
  1. Step 1 – Convert the tolerance to 32nds for easy comparison: ±1/16 in. = ±2/32 in. So the acceptable range is 23 30/32 in. to 24 2/32 in.
  2. Step 2 – Express the measured value in 32nds: 24 3/32 in.
  3. Step 3 – Compare: the upper limit is 24 2/32 in.; the measured value is 24 3/32 in., which exceeds the upper limit by 1/32 in.
  4. Step 4 – Determine action: the piece is 1/32 in. over the maximum allowed length. Dress the end with an angle grinder to remove the excess, re-measure, and record the corrected dimension on the inspection log.
Answer: No, the piece exceeds the tolerance by 1/32 in. Grind the end, re-measure to confirm it falls within 24 ± 1/16 in., and update the inspection log.
3. You need to set a balanced tack sequence for a 36 in. butt joint on 1/4 in. steel plate. How many tacks do you place and in what order to minimize distortion?
  1. Step 1 – Plan tack spacing: a common rule for light plate is tacks spaced 6–8 in. apart. For a 36 in. joint, plan tacks at roughly 0 in. (each end), 18 in. (center), then at 9 in. and 27 in. That gives five tacks.
  2. Step 2 – Set the sequence to balance heat: place Tack 1 at the center (18 in.), Tack 2 at one end, Tack 3 at the opposite end, Tack 4 at the quarter point nearest the first end (9 in.), Tack 5 at the quarter point at the other side (27 in.).
  3. Step 3 – Verify fit-up after each tack: check root opening and alignment after Tacks 1, 2, and 3 before continuing. If the joint has pulled, correct with clamps before adding more tacks.
  4. Step 4 – Perform VWI on each tack for cracks, porosity, and fusion before accepting it and moving to the next tack or to full welding.
Answer: Place five tacks in this order: center → one end → opposite end → quarter point → quarter point. This balanced sequence minimizes angular distortion and bowing.
4. The nominal groove weld reinforcement limit under AWS D1.1 for a plate weld is 1/8 in. maximum above flush. You measure a cap pass that sits 3/16 in. above the base metal surface. By how much does it exceed the limit, and what is the corrective action?
  1. Step 1 – State the limit: AWS D1.1 visual acceptance allows a maximum weld reinforcement of 1/8 in. (0.125 in.) above the base metal surface for most groove welds in plate.
  2. Step 2 – Measure the excess: 3/16 in. − 1/8 in. = 3/16 in. − 2/16 in. = 1/16 in. excess above the code limit.
  3. Step 3 – Determine corrective action: blend-grind the cap to reduce reinforcement to no more than 1/8 in., feathering the grind smoothly into the base metal so no undercut is introduced.
  4. Step 4 – Re-inspect: after grinding, re-measure with a groove weld gauge and confirm reinforcement ≤ 1/8 in. and that no undercut, low spots, or cracks were introduced during grinding. Document on the inspection record.
Answer: The reinforcement exceeds the AWS D1.1 limit by 1/16 in. Blend-grind the cap, re-measure to confirm ≤ 1/8 in. reinforcement, inspect for introduced defects, and document the corrective action.

The math in this lesson1

Learners compare fillet legs, cut lengths and reinforcement to ±1/16 and 1/8-inch limits.
N-Q.3
Brush up in the math halls at newSchoolWorks.com:
In class: Set up and operate plasma cutter or oxy-fuel torch and angle grinder to cut mild-steel plate and structural shapes to layout dimensions within ±1/16 in. tolerance; mark and punch all reference lines using layout tools practiced in the Shipfitting Layout course, then dry-fit all pieces in the fixture and record actual versus nominal dimensions on the inspection log. · Tack all steel joints in the approved sequence using SMAW or FCAW; perform visual weld inspection (VWI) on each tack for porosity, cracks, and proper fusion before accepting and logging each joint, then repeat the cut-fit-tack process for aluminum components using GMAW (MIG) with the correct filler alloy per AWS D1.2 requirements, using dedicated aluminum tools to prevent contamination. · Complete full-penetration and fillet welds on all joints per the weld map; after each weld pass, grind slag (steel) or wire-brush (aluminum) and perform self-VWI using a weld gauge and the AWS visual acceptance criteria checklist, documenting all measurements and any nonconformances with corrective actions taken on the project inspection record.
Materials: Mild-steel plate and structural shapes (instructor-specified grade), aluminum plate/extrusions (6061-T6 or equivalent), SMAW power source with E7018 electrodes, FCAW wire feeder, GMAW unit with aluminum-rated liner and ER4043 or ER5356 filler wire, plasma cutter or oxy-fuel cutting outfit, angle grinders with flap discs and grinding wheels, welding table/fixture, clamps and strong-backs, layout tools (soapstone, scribers, combination square, tape measure), weld gauges (fillet and groove), AWS D1.1 and D1.2 visual acceptance criteria reference cards, full PPE (welding helmet auto-darkening min. shade 10, leather gloves, FR clothing, steel-toed boots, hearing protection, respirator rated for welding fume)
Check: Instructor performs dimensional inspection (all critical dimensions within ±1/16 in.) and VWI of all welds against AWS D1.1 and D1.2 visual acceptance criteria; student's own inspection log must match instructor findings within one minor discrepancy. Any weld failing VWI must be repaired and re-inspected; student must achieve 100% VWI-passing welds and ≥80% dimensional conformance score before moving to coatings phase.

Practice check8

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

AWS D1.1 Certified Welder – Workmanship and Visual Acceptance
1. During tack welding of a steel butt joint, you discover a small crack running through one of the tacks. What is the correct action?
AWS D1.2 Aluminum – Workmanship
2. Which of the following best describes why dedicated tools must be used for aluminum and never shared with steel fabrication?
AWS D1.2 Aluminum – Filler Metal and Procedure
3. A weld map specifies ER5356 filler wire for a 6061-T6 aluminum joint. In the shop you find only ER4043. What should you do?
NCCER Shipfitting – Safety
4. When using a plasma cutter, which safety precaution is MOST critical for protecting bystanders in the adjacent work area?
AWS D1.1 Certified Welder – Visual Acceptance Criteria
5. According to AWS D1.1 visual acceptance criteria, cracks of any length in a completed weld are:
NCCER Shipfitting – Distortion Control
6. A balanced tack sequence on a long butt joint starts at the center and works outward alternately toward each end. The PRIMARY reason for this sequence is to:
ABS/USCG Weld Test – Quality and Documentation
7. When completing the self-inspection portion of the project inspection record, a student finds a fillet weld leg that measures 1/4 in. where 5/16 in. is required. The CORRECT entry on the inspection record is:
NCCER Shipfitting / NCCER Maritime Welding – Safety and LOTO
8. Before changing a grinding wheel on an angle grinder, the FIRST action required is:
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