Machine Setup, WPS Interpretation, and Bead-on-Plate (Flat)

Gas Metal Arc Welding: Machine Setup, WPS Interpretation, and Bead-on-Plate (Flat)

What This Lab Covers

Before you strike your first arc, you must be able to read a Welding Procedure Specification (WPS) and translate it into correct machine settings. This lesson walks you through that process, explains the physics behind each control on a GMAW machine, and sets the standard your beads must meet under AWS D1.1 visual acceptance criteria.

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Safety First

Personal Protective Equipment (PPE): Wear dry leather gloves, a flame-resistant welding jacket or sleeves, leather boots, and an auto-darkening helmet set to at least shade 10 for GMAW on steel. Flying spatter and ultraviolet/infrared arc radiation are present the instant the arc strikes.

Ventilation: GMAW produces fumes containing iron oxides and, depending on base metal coatings, other hazardous compounds. Work at a ventilated station or use local exhaust ventilation; never weld in a confined space without an air-supplied respirator and confined-space entry permit.

Fire and burn prevention: Clear combustibles from the weld area before starting. Hot plate coupons look identical to cold ones — use pliers or marking chalk, and allow adequate cool-down time before handling with bare hands.

Electrical safety: A GMAW machine is energized at the contact tip the moment the gun trigger is pressed. Keep the gun pointed away from yourself and others when the machine is on. Before changing wire or contact tips, release tension on the wire feeder and confirm power is off.

Grinder safety: When using an angle grinder with a flap disc to clean plate, wear a face shield over safety glasses, not just a welding helmet.

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Understanding the WPS

A Welding Procedure Specification is a written document that tells a welder exactly how to produce a weld that meets a given code. AWS D1.1 requires that every production weld be made in accordance with a qualified WPS. The key information you must extract before touching the machine includes:

• Base metal (type and thickness — today: A36 carbon steel, ¼")
• Filler metal classification (today: ER70S-6, 0.035" diameter)
• Shielding gas and flow rate (today: C25 — 75% Argon / 25% CO₂ — typically 35–45 CFH for indoor work)
• Welding position (today: 1G / flat)
• Polarity (GMAW solid wire uses DCEP — Direct Current Electrode Positive)
• Voltage range (the WPS will give a minimum and maximum; you must stay within that window)
• Wire feed speed (WFS) range (expressed in inches per minute; amperage is controlled primarily by WFS on a constant-voltage machine)
• Preheat requirements (for ¼" A36, preheat is generally not required at room temperature, but always confirm on the WPS)
• Interpass temperature limits (maximum temperature allowed between passes)

The Constant-Voltage Machine and Its Controls

GMAW uses a constant-voltage (CV) power source. On a CV machine, the voltage you set holds approximately steady while the machine automatically adjusts amperage to maintain the arc length as the wire melts off. This self-regulating behavior means:

• Voltage controls arc length and overall heat input appearance. Too low → stubbing, cold laps, porosity. Too high → wide, flat, spatter-heavy bead.
• Wire feed speed (WFS) controls deposition rate and, indirectly, amperage. Higher WFS = more wire = more current = more heat.
• Travel speed controls bead width and reinforcement height. Faster travel = narrower, taller bead; slower travel = wider, flatter bead.

Set polarity first (DCEP for solid wire), then match voltage and WFS to the WPS ranges before you strike an arc.

Gun Technique for Flat Position

For flat bead-on-plate work, hold the gun at a 10–15° drag angle (pushing is called forehand; dragging is called backhand — with GMAW on steel, a slight drag/backhand angle gives better penetration and cleaner bead profile). Maintain a contact-tip-to-work distance (CTWD) of approximately ¾" for 0.035" wire, which keeps the electrical stickout in the range specified by most procedures. Keep your travel speed steady and uniform; inconsistent travel is the most common cause of width variation.

AWS D1.1 Visual Acceptance Criteria — What You Are Judged Against

For visual inspection of welds, AWS D1.1 sets specific limits. The criteria you will apply today include:

• Porosity: No visible porosity is acceptable for statically loaded structures in the most stringent categories; your beads must be free of surface-breaking pores.
• Undercut: Undercut at the toe of the weld must not exceed limits stated in the code (generally no more than 1/32" deep for most structural applications).
• Overlap/cold lap: Weld metal must fuse smoothly into the base metal at the toes with no unfused overlap.
• Bead width consistency: For this lab, your instructor applies a practical tolerance of ±1/16" across the length of each bead.
• Surface irregularity and cracks: No cracks are acceptable; the surface must be free of unacceptable roughness.

The Process Log

Record every bead you run: voltage set, WFS set, travel speed observation (fast/medium/slow), and your visual result. This log builds the habit required in production environments where WPS compliance documentation is mandatory and is also the foundation for troubleshooting — you cannot improve what you have not measured.

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Tying It Together

Reading a WPS, setting the machine correctly, and running a bead that meets code criteria are the core competencies that appear on AWS SENSE and NCCER practical evaluations. Every adjustment you make today — one variable at a time — mirrors the systematic approach a coded welder uses throughout a career.