Robotic Welding Crater Fill: 7 Steps to Prevent End Craters

Quick answer: Robotic welding crater fill prevents the hollow depression left when the arc stops before enough weld metal feeds into the shrinking pool. Use the power source’s crater sequence, then tune crater wire feed, voltage or arc length, ramp time, fill time, endpoint motion, and post-flow as one controlled system. Validate every change on a test joint against the approved WPS and inspection requirements.

Written by dxk | JTCLASER

What Is Robotic Welding Crater Fill?

A weld crater is the depression that can remain at the termination of a bead. It forms because the molten pool contracts as it cools while the arc and filler-metal delivery are being removed. If the end sequence stops too abruptly, the pool may freeze before enough metal reaches the center and edges. The result can be a shallow cosmetic depression, severe underfill, a shrinkage cavity, or a crack that begins at the crater and propagates into the weld.

Robotic welding crater fill is the programmed transition from the main weld schedule to a controlled lower-energy termination schedule. Depending on the power source, the adjustable variables may include wire feed speed, voltage, pulse arc length, ramp time, crater time, burnback, and shielding-gas post-flow. The robot program may also control endpoint position, travel speed, dwell, overlap, or a short backstep. FANUC, for example, identifies crater fill and burnback control among the process functions coordinated by its ArcTool welding software.

The goal is not simply to “turn the current down.” In constant-voltage GMAW, welding current is strongly related to wire feed speed, while voltage controls arc length and affects bead shape. A successful automated crater fill sequence keeps the arc stable and continues depositing an appropriate amount of metal while heat input tapers to a clean stop.

Why an End Crater Matters

A robot weld end crater is important for two reasons. First, an obvious depression may fail dimensional, visual, fatigue, or customer acceptance criteria. Second, a concave crater concentrates solidification shrinkage stress. Lincoln Electric’s welding guidance notes that inadequate crater filling can produce solidification cracking and that such cracks may propagate into the rest of the weld. Aluminum is especially sensitive because its rapid heat conduction and solidification behavior make starts and stops demanding; Miller recommends ramping to cooler termination parameters to fill the crater and reduce termination cracking.

Do not confuse a crater with burnback, porosity, end-of-bead undercut, or a robot path error. Burnback is the wire melting back into or near the contact tip. Porosity is trapped gas. Undercut is a groove at the weld toe. A path error places the bead away from the intended joint. More than one problem can occur at the same endpoint, so diagnosis should begin with the actual shape and location of the discontinuity.

Crater Symptoms at a Glance

A reliable robotic welding crater fill diagnosis starts by matching the visible endpoint pattern to the correct group of process, motion, and shielding checks.

Observed symptom	Likely checks	First controlled action
Round depression at every weld end	Crater function, fill time, wire feed, endpoint motion	Confirm that the crater schedule is actually called
Deep crater with a center crack	Abrupt stop, insufficient fill, material sensitivity	Stop production and evaluate under the applicable inspection plan
Raised end button or excessive buildup	Too much fill time, deposition, dwell, or overlap	Reduce one termination variable at a time
Wire freezes into the pool	Burnback, timing, wire stop, torch retraction	Verify the arc-off and burnback sequence
End crater appears only in certain positions	Gravity, travel direction, endpoint angle, joint fit-up	Review the actual robot pose and pool behavior
Porosity at the endpoint	Post-flow, nozzle coverage, drafts, contamination	Restore shielding and cleanliness before retuning heat

Six Common Robotic Welding Crater Fill Failures

1. The Arc Stops Abruptly

If the main welding output drops directly to zero, the pool loses heat and filler delivery at the same moment. It can freeze with insufficient metal at the center. A crater-control function introduces a lower-energy phase before arc extinction. Miller power-source manuals show that this phase can include dedicated crater wire feed, voltage or arc length, ramp time, and fill time rather than one single percentage.

2. The Crater Schedule Is Programmed but Not Triggered

A correct schedule stored in the welder does nothing if the robot never calls it. Check the selected weld schedule, job number, digital I/O handshake, arc-end instruction, and any minimum weld-time or crater-delay condition. Some systems intentionally skip crater fill on short tacks. The robot programmer and welding technician should confirm the live sequence at the controller and power source, not assume that an icon on one screen proves execution.

3. Crater Wire Feed and Voltage Do Not Match

Crater fill wire feed speed controls how much electrode is delivered during the termination phase. Voltage or pulse arc length must support a stable arc at that feed rate. If deposition is too low, the crater remains concave. If deposition or time is excessive, the weld may finish with a raised button, overlap, spatter, or a cold-looking lump. If voltage is mismatched, the wire may stub into the pool or the arc may become long and unstable.

4. Ramp Time or Fill Time Is Wrong

Welding crater ramp time is the transition between the main schedule and the crater schedule. A very fast transition can behave like an abrupt stop; an unnecessarily long ramp can add heat and cycle time. Fill time determines how long the termination parameters remain active. Miller documentation for several power sources treats ramp and crater time as separate variables, showing why one “crater current” number cannot describe the complete process.

5. Robot Endpoint Motion Works Against the Pool

The robot may stop, continue moving, overlap the start, backstep, or execute a short dwell while crater output is active. A dwell that is too long can overfill or overheat the endpoint. Continued travel that is too fast can leave the pool starved. Excessive torch movement can also pull shielding away before the crater solidifies. Effective robot arc-end programming coordinates motion with the power-source timing instead of treating them as independent events.

6. Shielding, Joint, or Material Conditions Change at the End

Stopping near a fixture edge, changing torch angle, extending contact-tip-to-work distance, crossing a gap, or losing gas coverage can make a correct crater schedule appear ineffective. Aluminum, stainless steel, carbon steel, different wire types, joint thicknesses, and welding positions do not respond identically. The schedule must be qualified for the actual application.

Seven-Step Robotic Welding Crater Fill Setup

Step 1: Preserve the Approved Baseline

Record the current robot program, welding job, main wire feed, voltage or trim, travel speed, torch angle, stickout, gas, wire, material, joint, and endpoint position. Back up the program before editing. Confirm that the main weld is acceptable; crater tuning should not hide a broader procedure problem.

Step 2: Verify the Physical Endpoint

Check part location, joint fit-up, torch TCP, wire direction, contact tip, nozzle, and cable condition. If the robot finishes off the seam, solve the geometry problem first. See the related robotic welding system selection and integration guide for the role of sensing, programming, and system design.

Step 3: Confirm the Arc-End Command and Handshake

Single-step or monitor the sequence under the approved safe procedure. Verify that the robot selects the intended robotic welding crater fill schedule, the power source acknowledges it, wire feed changes as expected, the arc stays on for the defined phase, and post-flow continues after extinction. Do not enter an operating robot cell to observe closely.

Step 4: Start From the Manufacturer or Qualified Procedure

The transcript suggests trying one-half to two-thirds of the main current. That may be a practical shop starting range on some systems, but it is not a universal rule. On CV MIG equipment, the operator often sets crater wire feed speed and voltage rather than amperage directly. Some Miller equipment permits crater wire feed as a percentage of main wire feed, while other machines use absolute values or synergic schedules. Begin with the power-source manual, approved WPS, or a qualified schedule for the same wire, gas, material, joint, and position.

For example, reducing a nominal 200 A weld to an indicated 100–130 A crater phase may be reasonable only if the corresponding wire feed, voltage, arc stability, fusion, time, and bead shape are verified. Copying that range to a different transfer mode or material can create stubbing, lack of fusion, excessive convexity, or an unstable arc.

Step 5: Tune Deposition and Arc Energy Together

Adjust MIG crater fill parameters one variable at a time. If the crater is still deep and the arc is stable, additional fill time or deposition may be needed. If the end is overfilled, reduce time, deposition, dwell, or overlap. If the wire repeatedly touches the pool, review voltage or arc length, wire feed, stickout, and timing as a matched set. Record every change and result.

Step 6: Coordinate Ramp, Time, and Robot Motion

Choose whether the robot should pause, continue a short distance, overlap, or backstep during crater fill. The correct choice depends on joint geometry and procedure. Start with the simplest manufacturer-supported sequence. Tune ramp time to avoid a sudden energy collapse, then use only enough crater time and endpoint motion to fill the depression without producing a mound. Keep shielding over the pool through solidification.

Step 7: Validate and Lock the Process

Make repeat test welds on representative material using production fit-up and position. Inspect the crater visually and with any required nondestructive or destructive method. Apply the actual weld crater inspection criteria from the drawing, quality plan, customer specification, or governing code. Once the robotic welding crater fill result is accepted, lock the job where appropriate, document parameter ownership, and include the endpoint in first-piece and change-control checks.

How to Read the Result

• Crater remains deep: verify the function executed, then evaluate fill time, deposition, endpoint travel, and joint volume.
• Crater is shallow but cracked: do not judge appearance alone; review material sensitivity, solidification behavior, heat input, restraint, and qualified repair requirements.
• End is raised: reduce unnecessary dwell, time, overlap, or deposition.
• Wire sticks: review the crater arc balance, burnback, wire-stop timing, and torch retraction.
• Endpoint is porous: confirm gas post-flow, nozzle position, drafts, cleanliness, and crater solidification coverage.
• Result changes between parts: look for fit-up, position, consumable, wire feeding, gas, or schedule-selection variation before retuning.

Crater Fill, Burnback, and Post-Flow Are Different

Robotic welding crater fill deposits metal while transitioning to a controlled finish. Burnback timing controls the relationship between wire stopping and arc extinction so the electrode does not freeze into the pool or melt into the contact tip. Post-flow keeps shielding gas over the hot endpoint after the arc stops. These functions interact but solve different problems. A complete robotic welding crater prevention strategy checks all three.

Modern robot welding power source integration can coordinate job selection, arc start, main welding, crater parameters, burnback, and gas timing through the robot interface. This improves repeatability, but only when schedule numbers, communication, and program revisions are controlled. For broader defect diagnosis, see the welding undercut troubleshooting guide.

Common Mistakes

Most robotic welding crater fill problems become harder to solve when several settings are changed at once or when an endpoint defect is treated as purely cosmetic.

• Applying “50% current” to every material, transfer mode, wire, and joint without verification.
• Changing crater wire feed, voltage, time, robot dwell, and endpoint position simultaneously.
• Programming a crater schedule without confirming the robot calls it.
• Using a long dwell to hide poor fit-up or an incorrect seam endpoint.
• Ignoring burnback and post-flow because the visible crater appears filled.
• Accepting one good test while production results remain variable.
• Grinding away evidence before measuring and documenting a suspected crack.

Frequently Asked Questions

What causes a crater at the end of a robotic weld?

The arc and filler delivery may stop before the shrinking pool receives enough metal. A disabled or mismatched crater schedule, short fill time, unsuitable endpoint motion, poor shielding, joint variation, or material sensitivity can contribute.

Should crater current always be half of welding current?

No. One-half to two-thirds can be a trial concept on some applications, but the correct settings depend on the power source, process, wire feed, voltage or arc length, transfer mode, material, position, joint, WPS, and fill time. Use approved starting data and test systematically.

How long should crater fill last?

There is no universal duration. Manufacturer equipment may offer broad adjustable ranges, but the correct value is the shortest validated time that produces an acceptable termination without excess buildup, overheating, or cycle-time waste.

Can a robot dwell at the endpoint to fill the crater?

Yes, if the qualified procedure supports it, but dwell must be coordinated with deposition and energy. Too little dwell may not fill the crater; too much can create a raised end, excessive heat, or burn-through.

Can a crater crack simply be welded over?

Do not assume so. Stop and evaluate the discontinuity using the applicable inspection and repair procedure. A crack may need removal, examination, approved rewelding, and reinspection. Covering it can leave the crack underneath.

Safety and Technical Limits

Important: Robot cells contain hazardous automatic motion, stored energy, electricity, fumes, hot metal, radiation, and pinch or crush hazards. OSHA has documented fatal incidents involving robots that were not properly locked out. Only trained and authorized personnel should enter, service, teach, or test a welding cell. Follow the risk assessment, safeguarding and lockout/tagout procedures, equipment manuals, approved WPS, and applicable regulations.

This article provides a diagnostic method, not a qualified welding procedure or universal acceptance limit. The responsible welding engineer or qualified authority must approve production changes. Perform testing on suitable coupons and use the required inspection method before releasing the process.

Conclusion

Robotic welding crater fill is a coordinated termination process, not a single current percentage. Confirm the robot calls the correct crater schedule, then tune wire feed, voltage or arc length, ramp time, fill time, endpoint motion, burnback, and shielding as a system. Use manufacturer data and an approved procedure as the baseline, change one variable at a time, and validate repeatability under production conditions. A properly controlled finish protects weld quality while reducing repair, inspection failures, and unplanned downtime.

Technical Review Note

Prepared by dxk for JTCLASER from field-oriented troubleshooting notes and the primary references below. The article corrects the idea that one current ratio is universally valid and distinguishes observable symptoms from qualified acceptance decisions.

Authoritative Sources

• Miller — Crater Fill Sequence Manual: crater wire feed, voltage or arc length, ramp time, crater time, and schedule behavior.
• Miller — Pulsed MIG Welding on Aluminum: ramping to cooler parameters to fill the crater and reduce termination cracking.
• Lincoln Electric — Weld Discontinuities: crater cracking mechanism and propagation risk.
• FANUC — Arc Welding Robots and ArcTool: coordination of welding parameters, crater fill, burnback control, and robotic arc functions.
• OSHA — Control of Hazardous Energy: lockout/tagout principles for preventing hazardous energy release.