Method of detecting root gap and arc welding method using the former

An arc welding method, comprising the steps of:

• controlling a weld seam tracking and a welding torch height;
• detecting a waveform of a welding voltage or a welding current of an arc;
• storing the waveform of said welding voltage or said welding current under an appropriate welding condition to a reference gap G.sub.ref as a reference waveform;
• obtaining an area .vertline..DELTA.SG.vertline. of a deviation part of the detected waveform to said reference waveform;
• obtaining .DELTA.SG by judging a positive and a negative of .vertline..DELTA.SG.vertline.;
• experimentally determining the relationship between the value of .DELTA.SG and a root gap G in advance;
• calculating the root gap G from the value of .DELTA.SG;
• determining allowable ranges +.DELTA.g1, -.DELTA.g2 of the root gap G in achieving the welding under an appropriate welding condition to the reference gap G.sub.ref ;
• obtaining allowable ranges +.DELTA.S1, -.DELTA.S2 of the differential waveform .DELTA.SG from the relationship between said .DELTA.SG and G;
• and
• changing the welding condition to a preset appropriate welding condition for the gap of G=G.sub.ref +.DELTA.g1 or G=G.sub.ref -.DELTA.g2 when said .DELTA.SG is deviated from said allowable range.