Chapter 4 Section (10) Ensuring Accurately Sized Parts, Joint Preparation & Fit-Up
Flame Cutting Rules and Thermal Expansion
The workpiece must stay connected to the large plate until the final severance cut is made to prevent displacement from thermal expansion.
Movement during numerical or computer-controlled flame cutting alters circular paths into ovals, preventing the torch from returning to its starting point.
Starting points and paths require preplanning; for a circular part near a corner (Figure 4.32), cutting proceeds from point A counterclockwise or from point B clockwise.
Ring Flange Cutting Sequence
Ring flange production utilizes a sequential three-cut procedure (Figure 4.33):
Cut 1: Proceeds counterclockwise through to define the outer diameter.
Cut 2: Proceeds counterclockwise to establish the inner diameter and detach the centerpiece.
Cut 3: Bridges the endpoints of the initial cut to release the finished flange ring.
Internal piercing initiates cuts inside the plate rather than along the edge, using the cutting kerf width to provide expansion clearance.
The centerpiece scrap must never be removed prior to the outer cuts; premature removal pulls the inner diameter inward and produces uneven ring width.
Edge Preparation and Joint Fit-Up
Irregular or excessively wide joints create localized heat concentrations, resulting in high residual stresses.
Excessive root face paired with tight joint fit-up prevents complete fusion and restricts weld shrinkage, inducing severe shrinkage stresses and cracking hazards.
Double-sided joints (Figure 4.34) balance weld metal and shrinkage forces around the neutral axis, minimizing distortion from eccentric loading while reducing joint volume.