A method of producing a thread on a workpiece comprising the steps of: providing a CNC machining centre with a tool holder movable in an x,y,z co-ordinate space and rotatable about the z-axis, wherein the tool holder has a CNC datum face and a reference position of the datum face in said x,y,z space and about said z-axis; providing a tapping tool having a thread-producing tip with a thread of pitch P between adjacent turns of the thread, a tool axis and a tool datum face remote from the tip, the tool being fixable in said tool holder with the tool datum face in fixed axial and angular relation to the CNC datum face, and with the tool axis coincident with said z-axis; and disposing said tool in a tool presetter; the method being characterised by including the following steps: in the tool presetter, measuring the length L of the tool along its rotation axis between the tool datum face and a distal end of its thread-producing tip; in the tool presetter, measuring in a first plane containing the tool axis the distance D' between a complete thread-producing peak, being the n th turn of the thread from the tip distal end of the tool, and the tool datum face, and determining therefrom the corresponding distance D in the zero plane containing the reference position when the tool is in the tool holder of the CNC machining centre, based on the relation D = D' ± βP/360, where β is the angle between said first and zero planes; defining a spot face surface on the workpiece at which a thread, to be produced, is to commence at a predetermined start angle with respect to the thread axis; fitting the tool in the tool holder of the CNC machining centre and positioning the datum face of the tool holder at a start position being a distance A from the spot face and angle α degrees between the reference position of the datum face and the start angle, where A = L + α P / 360 + c + L − D − n − 0.5 P where c is a constant equal to an integer multiple of the pitch P; and driving and rotating the tool and tool holder from the start position towards the workpiece at an axial rate of P per revolution of the tool until the thread is produced. providing a CNC machining centre with a tool holder movable in an x,y,z co-ordinate space and rotatable about the z-axis, wherein the tool holder has a CNC datum face and a reference position of the datum face in said x,y,z space and about said z-axis; providing a tapping tool having a thread-producing tip with a thread of pitch P between adjacent turns of the thread, a tool axis and a tool datum face remote from the tip, the tool being fixable in said tool holder with the tool datum face in fixed axial and angular relation to the CNC datum face, and with the tool axis coincident with said z-axis; and disposing said tool in a tool presetter; the method being characterised by including the following steps: in the tool presetter, measuring the length L of the tool along its rotation axis between the tool datum face and a distal end of its thread-producing tip; in the tool presetter, measuring in a first plane containing the tool axis the distance D' between a complete thread-producing peak, being the n th turn of the thread from the tip distal end of the tool, and the tool datum face, and determining therefrom the corresponding distance D in the zero plane containing the reference position when the tool is in the tool holder of the CNC machining centre, based on the relation D = D' ± βP/360, where β is the angle between said first and zero planes; defining a spot face surface on the workpiece at which a thread, to be produced, is to commence at a predetermined start angle with respect to the thread axis; fitting the tool in the tool holder of the CNC machining centre and positioning the datum face of the tool holder at a start position being a distance A from the spot face and angle α degrees between the reference position of the datum face and the start angle, where A = L + α P / 360 + c + L − D − n − 0.5 P where c is a constant equal to an integer multiple of the pitch P; and driving and rotating the tool and tool holder from the start position towards the workpiece at an axial rate of P per revolution of the tool until the thread is produced. in the tool presetter, measuring the length L of the tool along its rotation axis between the tool datum face and a distal end of its thread-producing tip; in the tool presetter, measuring in a first plane containing the tool axis the distance D' between a complete thread-producing peak, being the n th turn of the thread from the tip distal end of the tool, and the tool datum face, and determining therefrom the corresponding distance D in the zero plane containing the reference position when the tool is in the tool holder of the CNC machining centre, based on the relation D = D' ± βP/360, where β is the angle between said first and zero planes; defining a spot face surface on the workpiece at which a thread, to be produced, is to commence at a predetermined start angle with respect to the thread axis; fitting the tool in the tool holder of the CNC machining centre and positioning the datum face of the tool holder at a start position being a distance A from the spot face and angle α degrees between the reference position of the datum face and the start angle, where A = L + α P / 360 + c + L − D − n − 0.5 P where c is a constant equal to an integer multiple of the pitch P; and driving and rotating the tool and tool holder from the start position towards the workpiece at an axial rate of P per revolution of the tool until the thread is produced.