Welding math
Travel Speed
Travel speed is the one welding parameter set entirely by the welder's hand rather than the machine, and it is also the parameter that heat input is most sensitive to, which makes measuring it worth the stopwatch.
The formula
v = distance / time, or v = (V x I x 60) / (1000 x HI)
Travel speed in inches per minute is the length of weld divided by the time in minutes to make it, or, when a heat input target is set, arc volts times amperes times sixty divided by one thousand times that target.
What each symbol means
| Symbol | Meaning | Units |
|---|---|---|
| v | Travel speed of the arc along the joint | in/min |
| L | Length of weld run in a timed observation | in |
| t | Elapsed arc-on time for that length | s or min |
| HI | Heat input the procedure is aiming at | kJ/in |
Why it works
Measuring travel speed is straightforward but rarely done well. Mark a start and stop on the joint, time the arc between them, and divide the length by the time. Convert the time to minutes and the answer is in inches per minute. The discipline is in timing arc-on time only: if the welder stops to reposition mid-run, that pause does not belong in the measurement.
Travel speed also works backwards. If a procedure caps heat input, the volts and amps are already fixed by the wire size and the transfer mode, so the only free variable left is speed. Rearranging the heat input formula gives the slowest speed that still respects the cap, which is a genuinely useful number to hand a welder before a qualification test rather than after.
The two forms answer different questions. The measured form tells you what a welder is actually doing, which is what a procedure qualification record has to capture. The back-solved form tells you what a welder ought to do, which is what a welding procedure specification has to state. Comparing the two is often how a shop discovers its production welds run hotter than its qualified procedure.
Worked examples
Timing a production pass
- Length of weld
- 18 in
- Arc-on time
- 90 s
- Convert the time to minutes: 90 / 60 = 1.5 minutes.
- Divide the length by the time in minutes: 18 / 1.5.
Result: 12 in/min
Twelve inches per minute is a brisk but ordinary semi-automatic speed. Recorded on a procedure qualification record, this is the number that later production has to be judged against.
Back-solving from a heat input cap
- Arc voltage
- 25 V
- Welding current
- 240 A
- Heat input target
- 40 kJ/in
- Compute arc power in joules per minute: 25 x 240 x 60 = 360,000.
- Divide by 1,000 to reach kilojoules per minute: 360 kJ/min.
- Divide by the heat input target to find how many inches that energy must be spread over: 360 / 40.
Result: 9 in/min
At these machine settings the welder must keep moving at nine inches per minute or faster to stay under the cap. Anything slower puts more than forty kilojoules into each inch.
In practice
- Travel speed and wire feed speed are different things and get confused constantly. Wire feed speed is how fast filler leaves the gun; travel speed is how fast the gun moves along the joint.
- On a heat input limited procedure, the back-solved speed is a minimum, not a target. Welders should be given a range with the fast end bounded by fusion quality rather than a single number.
- Mechanised carriages hold travel speed far better than a hand does, which is why automated welds show much tighter heat input distributions even when the average matches.
- If a timed measurement disagrees with the machine's readout on an automatic setup, trust the tape and the stopwatch. Carriage calibration drifts.
Related topics
Practise the arithmetic under pressure
The welding-math domain in the question bank is where these formulas get tested the way they get tested on paper — with distractors built from the mistakes people actually make.