Thread shape dimension measuring device and thread shape dimension measuring method

A thread shape dimension measuring method for measuring a thread shape dimension of a threaded portion in a threaded pipe having the threaded portion formed at an end portion and on which a pair of flank surfaces defining each thread ridge of the threaded portion are both formed in an overhanging manner in a view seen in a cross section including a pipe axis, the thread shape dimension measuring method comprising:

• a first illumination step of illuminating the end portion by emitting parallel light in a first illumination direction inclined in a direction forming an angle larger than a lead angle of the threaded portion with respect to a direction orthogonal to the cross section including the pipe axis;
• a second illumination step of illuminating the end portion by emitting parallel light in a second illumination direction inclined to a side opposite to the lead angle of the threaded portion with respect to a direction orthogonal to the cross section including the pipe axis;
• an imaging step of, in an imaging side facing an emitting source of the parallel light with the end portion interposed therebetween, capturing a first captured image of the end portion by receiving light that has passed between thread ridges of the threaded portion from the parallel light emitted in the first illumination direction, and capturing a second captured image of the end portion by receiving light that has passed between thread ridges of the threaded portion from the parallel light emitted in the second illumination direction;
• and
• an arithmetic processing step of calculating the thread shape dimension of the threaded portion on the basis of the first captured image and the second captured image, wherein
• when one of the pair of flank surfaces whose portion on a cross section including the pipe axis is able to be directly imaged from the imaging side is defined as a first flank surface, and one thereof whose portion on the cross section including the pipe axis is not able to be directly imaged from the imaging side is defined as a second flank surface,
• in the arithmetic processing step,
• a shape dimension of the first flank surface is calculated on the basis of the first captured image captured in a first state in which a focusing position is adjusted to match a position of the first flank surface on the cross section, and
• a shape dimension of the second flank surface is calculated on the basis of the second captured image captured in a second state in which a focusing position is brought closer to the pipe axis by a predetermined distance than that in the first state so that the focusing position matches a position of the second flank surface on the cross section.