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newSchoolWorks.com › VoTechWorks › Building & Construction › Welding › Capstone Project: Structural and Pipe Weldment Fabrication › Lesson 2
BLD-WELD01-12 · Lesson 2 · 48 clock hours · lab

Material Preparation, Fit-Up, and Tack Welding

Objective. Students cut, clean, fit, and tack-weld all structural and pipe joints of the capstone assembly to dimensional tolerances specified on the drawing, documenting each fit-up measurement in an inspection log before any full weld pass is deposited.

Why Fit-Up Quality Controls Weld Quality

No welder can produce a sound structural or pressure-containing weld on a poorly prepared joint. Every code that governs your capstone work — AWS D1.1 for structural steel and ASME Section IX for pipe — treats fit-up as a mandatory hold point. A root opening that is too tight traps slag; one that is too wide causes burn-through and can make it impossible to pass a bend test. A hi-lo (pipe misalignment) that is out of tolerance creates a stress riser that can crack under cyclic load. Getting fit-up right before depositing a single full-pass bead is not optional — it is code.

Step 1: Material Requisition, Layout, and Cutting

Begin by reviewing the Bill of Materials and the assembly drawing. Confirm that each piece of A36 plate or A53/A106 pipe matches the specified grade, diameter, wall thickness, and length before picking it from the rack. Mark cut lines with soapstone or a scribe and a straightedge; measure twice before you cut.

Lockout/tagout (LOTO) before touching any power tool. Before starting the plasma cutter, oxy-fuel outfit, or angle grinder, follow the written LOTO procedure posted on each machine: shut off the energy source, isolate it, apply your personal lock, and verify that the machine cannot restart. This is OSHA policy and shop policy — no exceptions.

For oxy-fuel cutting, open cylinder valves slowly, set working pressures per the torch manufacturer's specification, and light with a friction lighter — never a match or open flame. For plasma cutting, confirm that the compressed-air supply pressure meets the machine rating and that all electrical connections are secure before energizing.

After cutting, use an angle grinder with a grinding wheel to bring bevel angles to the dimension shown on the drawing. Finish with a flap disc to smooth the bevel face and remove mill scale from at least 1 in. back from the joint on both sides. Check the bevel angle and root face (land) with a weld-fit-up gauge (such as a Bridgeport-style gauge) before moving to assembly. A typical groove weld on structural plate requires a 22.5° bevel per side (giving a 45° included groove angle) and a root face of 1/8 in., but always defer to your specific drawing.

Step 2: Fit-Up, Measurement, and the Inspection Log

Fit-up is the controlled positioning of parts before welding. Use C-clamps, pipe clamps, and strongbacks to hold members in position without relying on tack welds to pull things into alignment — distortion from pulling is much harder to fix than distortion from releasing a clamp.

Measure each of the following with the appropriate tool and record the reading on your pre-weld inspection log form before tacking:

If any measurement is out of tolerance, flag the joint on the log and call your instructor for review. Do not tack. Correct the fit — re-cut, re-grind, or re-clamp — and re-measure. Document the corrected reading on the same log before proceeding.

Step 3: Tack Welding per the WPS

A tack weld is a short, temporary weld that holds fit-up until full welding begins. Although temporary, it is subject to the same workmanship requirements as a production weld: use the electrode type and size, polarity, and approximate amperage listed in the approved Welding Procedure Specification (WPS). For SMAW tacking, an E7018 electrode is common for A36/A53 joints; confirm with your WPS.

Apply tack welds at the locations, sizes, and spacing specified on the drawing or approved by the instructor. Common guidance: tacks should be long enough to resist the prying forces of fit-up restraint — typically a minimum of four times the base-metal thickness or 1.5 in., whichever is greater, though your drawing governs. Place tacks symmetrically to minimize distortion.

Visually inspect each completed tack for cracks, porosity at the surface, and full fusion to both base-metal faces before releasing clamps or moving to the next joint. A cracked tack must be ground out completely and replaced — a cracked tack will propagate into the final weld.

After all tacks are applied, re-check overall assembly squareness with the framing square and pipe plumb with the torpedo level. If distortion has moved a member out of tolerance, re-clamp and apply a correction tack on the opposite side before the full-pass welding sequence.

PPE for This Lab

Wear your full PPE for every phase of this lab:

The Hold-Point Inspection

When your inspection log is complete and all joints are tacked, call your instructor for the dimensional hold-point inspection. The instructor will verify your recorded measurements with independent readings. Every joint must be within tolerance before you are cleared to begin full-pass welding. A failed dimension means rework — not skipping ahead.

Key terms10

Root openingThe gap between the two base-metal surfaces at the root of a joint before welding; it controls penetration and burn-through and must be held within drawing tolerance.
Root face (land)The flat, un-beveled portion of the prepared edge at the bottom of a groove; it supports the root pass and prevents burn-through.
Groove angle (included angle)The total angle formed between the two beveled faces of a groove joint, measured in degrees; it governs electrode access and fusion at the root.
Hi-loThe offset or mismatch between the inside surfaces of two pipe or tube ends at a butt joint; excessive hi-lo creates a stress concentration and is a code-rejectable condition.
Tack weldA short, temporary weld used to hold parts in fit-up position; it must meet the same electrode and process requirements as the production weld and must be free of cracks before full welding begins.
Welding Procedure Specification (WPS)A written document that prescribes the variables — process, filler metal, preheat, polarity, amperage range, etc. — required to produce a weld that meets the applicable code.
Hold-point inspectionA mandatory stop in a fabrication sequence at which an authorized inspector or instructor verifies that specified dimensions and conditions are met before work proceeds to the next phase.
Lockout/tagout (LOTO)An OSHA-required procedure in which a machine's energy source is shut off, isolated, and physically locked so that it cannot be restarted while a worker is performing setup, maintenance, or adjustment.
StrongbackA rigid bar or beam clamped across a joint to resist distortion and maintain alignment during fit-up and welding.
Weld-fit-up gaugeA multi-function hand tool used to measure root opening, bevel angle, root face, throat size, and fillet weld leg length; commonly called a Bridgeport gauge or V-WAC gauge in the trade.

Worked examples4

1. Your drawing calls for a root opening of 1/4 in. (0.250 in.) with a tolerance of ±1/16 in. You measure the gap at four locations around a pipe joint and get 0.219 in., 0.281 in., 0.250 in., and 0.312 in. Which measurements are in tolerance and which require correction?
  1. Determine the acceptable range: 1/4 in. minus 1/16 in. = 4/16 − 1/16 = 3/16 in. = 0.1875 in. (minimum); 1/4 in. plus 1/16 in. = 4/16 + 1/16 = 5/16 in. = 0.3125 in. (maximum).
  2. Compare each reading to the range 0.1875 in. – 0.3125 in.: 0.219 in. — within range (pass); 0.281 in. — within range (pass); 0.250 in. — within range (pass); 0.312 in. — within range (pass, since 0.312 < 0.3125).
  3. All four readings fall within 0.1875 in. to 0.3125 in., so the root opening is acceptable at every location measured.
Answer: All four measurements are within tolerance. The joint may proceed to tacking without correction.
2. You are fitting a pipe butt joint and measure the hi-lo with your weld-fit-up gauge. The inside surface of Pipe A sits 3/16 in. higher than the inside surface of Pipe B at the high point of the joint. The drawing allows a maximum hi-lo of 1/8 in. Is this joint acceptable? How much must be corrected?
  1. Convert the measured hi-lo to a common fraction base: 3/16 in. measured; limit is 1/8 in. = 2/16 in.
  2. Compare: 3/16 in. > 2/16 in., so the joint exceeds the allowed hi-lo.
  3. Calculate the amount of excess: 3/16 − 2/16 = 1/16 in. The inside surface mismatch must be reduced by at least 1/16 in. to meet the 1/8 in. limit.
Answer: The joint is out of tolerance by 1/16 in. and must not be tacked. Flag on the inspection log, notify the instructor, and re-fit the pipe (re-position or machine the high pipe end) until hi-lo is 1/8 in. or less, then re-measure and record.
3. Your drawing shows a 45° included groove angle (22.5° per side). You check the bevel on one plate with your weld-fit-up gauge and read 19°. Is it within the ±2.5° tolerance? What is the minimum acceptable included angle for this joint?
  1. The specified bevel per side is 22.5°; acceptable range per side is 22.5° − 2.5° = 20.0° minimum to 22.5° + 2.5° = 25.0° maximum.
  2. Compare the measured bevel (19°) to the minimum (20°): 19° < 20°, so this bevel is out of tolerance.
  3. For reference, the minimum acceptable included angle = 2 × 20° = 40°; the maximum = 2 × 25° = 50°. The current plate would produce an included angle of approximately 2 × 19° = 38°, which is below 40°.
Answer: The 19° bevel is out of tolerance (minimum is 20° per side). The plate must be re-ground to bring the bevel to at least 20° (no more than 25°) before assembly. Record the corrected measurement on the inspection log.
4. You are spacing tack welds on a structural plate T-joint. The web plate is 3/8 in. thick. A general rule used in the shop is that tack weld length must be at least four times the base-metal thickness or 1.5 in., whichever is greater. What is the minimum tack weld length required?
  1. Calculate four times the base-metal thickness: 4 × 3/8 in. = 12/8 in. = 1.5 in.
  2. Compare to the fixed minimum of 1.5 in.: both values are equal at 1.5 in.
  3. The minimum tack weld length is therefore 1.5 in. (the two criteria give the same result in this case).
Answer: The minimum tack weld length is 1.5 in. Any tack shorter than 1.5 in. must be extended or supplemented. Always verify against your specific WPS or drawing, as project requirements may be more stringent.

The math in this lesson1

Learners check decimal root gaps against ±1/16 in, bevel angles against ±2.5°, and tack length as 4 × thickness.
N-Q.3
Brush up in the math halls at newSchoolWorks.com:
In class: Perform all material requisition, layout, and cutting using oxy-fuel or plasma cutting equipment; verify lockout/tagout procedure on each machine before startup; deburr and grind all cut edges to the bevel angle and root face dimension shown on the drawing, measuring with a weld gauge to confirm compliance before assembly. · Fit and clamp each joint, measure root opening, root face, groove angle, and pipe alignment (hi-lo) with a combination square and weld-fit-up gauge, record all measurements on a pre-weld inspection log form, and flag any out-of-tolerance condition for instructor review before tacking — correction is required prior to proceeding. · Apply tack welds in accordance with the approved WPS (size, spacing, and location as specified or as approved by instructor); inspect each tack visually for cracks and fusion before moving to the next joint; check overall assembly squareness and pipe plumb with a framing square and level, re-clamping and re-tacking if distortion exceeds drawing tolerance.
Materials: Structural steel plate and pipe stock per Bill of Materials (typically A36 plate and A53/A106 pipe, grades per drawing); oxy-fuel cutting outfit or plasma cutting system; angle grinder with grinding and flap disc wheels; C-clamps, pipe clamps, and strongbacks; combination square, framing square, torpedo level, weld-fit-up gauge (Bridgeport or equivalent), tape measure; pre-weld inspection log forms; SMAW machine for tacking; PPE: welding helmet (minimum shade 10 for SMAW tacking), leather gloves, leather jacket, safety glasses, hearing protection, steel-toed boots; fire watch equipment
Check: Instructor performs a dimensional hold-point inspection at fit-up completion using the student's recorded inspection log and independent measurements; all critical dimensions (root opening ±1/16 in., groove angle within ±2.5°, hi-lo within 1/8 in. for pipe) must be within tolerance — any out-of-tolerance joint must be corrected and re-inspected; 100% of joints must pass before full welding begins.

Practice check8

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

Safety and Health
1. Before starting the plasma cutting machine to prepare parts for the capstone assembly, which action is required by OSHA and shop policy?
Measurement and Quality Control
2. A drawing specifies a root opening of 3/8 in. with a tolerance of ±1/16 in. Which of the following measured root openings would require the joint to be flagged and corrected before tacking?
Welding Inspection and Quality
3. When inspecting a tack weld visually, what condition requires that the tack be completely ground out before a replacement tack is applied?
Fit-Up and Dimensional Control
4. What does 'hi-lo' describe when fitting up a pipe butt joint?
Measurement and Quality Control
5. A student measures the groove angle on a beveled plate joint and records 48°. The drawing calls for a 45° included groove angle with a ±2.5° tolerance. What should the student do?
Code Compliance and Procedure
6. Which document specifies the electrode type, polarity, and amperage range that must be used when applying tack welds during the capstone fabrication?
Fit-Up and Dimensional Control
7. After applying tack welds to the capstone assembly, you check overall squareness with a framing square and find that one plate has distorted 3/16 in. out of square, which exceeds the drawing tolerance. What is the correct next step?
Measurement and Quality Control
8. A web plate is 1/2 in. thick. Using the rule that tack weld length must be at least four times the base-metal thickness or 1.5 in. — whichever is greater — what is the minimum required tack weld length?
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