PURPOSE: To facilitate dimension control for a work by measuring dimensional change due to heat contraction of a work after the end of grinding in process gauge, at the time of grinding end, in arbitrary time later when it reaches a fixed value, obtaining a heat outflow coefficient and a heat inflow coefficient according to designated calculation procedure and sequentially computing thermal expansion by a specific first-order lag element relational expression.

CONSTITUTION: The dimensions of a work W at the time of working end, and the dimensions DM when it is contracted to a fixed value are measured by an in-process gauge 8 to obtain thermal expansion δL=DL-DM of the work W. After the end of working, the dimensions DX at an arbitrary time are measured to calculate the thermal expansion δX=DX-DM and dimensional change speed dDX/dt at that time. A thermal outflow coefficient β is obtained by an expression I, and further a thermal inflow coefficient α which satisfies δ=P.DO.Θ=δL is obtained. Using the coefficients α, β, a thermal expansion computing part 5a sequentially computes a thermal expansion by an expression II. In the expression, θ indicates a work temperature rise, P indicates grinding power, ρ indicates work linear expansion coefficient, t indicates time, and DO indicates work reference diameter.