We have a problem and we need fast answer.
If i have a inch dimension picture where is weldin symbol 1/2 at one side of the part what it mean when i convert it for metric system ?

Not seeing the drawing, I cannot say for sure. It could mean:

½ (inches) = 12.7 mm wide weld bead
Sometimes 1 / 2 means “one of two”

One of two would mean a skip weld with 1" (25mm) weld every 2" (50mm).

See here:
http://www.welding.com/weld_symbols_welding_symbols.shtml

I make a raw picture what i mean.

In this case the symbol is for a 1/2" fillet weld which would be 12.7mm. To convert these fractional inches to metric, divide the numerator by the denominator and multiply by 25.4. For instance the 3/8" fillet weld shown would be 3 divided by 8 equals .375 times 25.4 equals 9.525mm.

Looking at your drawing, 1/2 symbols mean weld both sides of the joint using 1/2 inch (13mm) wide weld bead. Both sides because 1/2 is on both sides of the weld specification line. The 3/8 symbols mean weld one side using 3/8 inch (10.0mm) wide weld bead. Put the welds on the joints the symbols point to.

This is clear now.
Thank you all.