how to find zero error in screwgauge.................................?

Zero error in Screw gauge can be found by the following method :

first take the screw gauge and move it i decreasing order along the index line until it gets tight.At that point if there is any error in the Screw gauge,we can find there because if there is any error the zero on the head (rotating ) scale does not coincide with the index line when the head scale edge is sharply at the zero of the pitch scale .If the zero of the head scale is above the index line then the error is negative ,if the zero is below the index line then the error is positive .

then by the formula

Corrected Head Scale Reading ( CHSR ) = Head Scale Reading ( HSR ) – Zero Error(along with their positiveor negative sign)

 

                            I hope You understood.........!

Zero Error&Zero Correction:
The screw is rotated until the flat ends s 1 and s 2 just come into contact .If the head scale zero exactly coincides with index line of the pitch scale then the screw gauge has no zero error . So zero correction is zero. If the head scale zero is just above the index line of the pitch scale then the screw gauge has negative error and correction is positive head scale zero is just below the index line of the pitch scale , then the screw gauge has positive error and zero correction is negative .The correction must applied to the observed reading to get a fraction .So we get more accurate value.

          i hope you understood.....................................................!