1982 Volume 48 Issue 7 Pages 925-932
In measurement of pitch diameter of screw thread by the three-wire method, it is necessary to keep the condition for single contact between the measuring cylinder and the thread flank of the groove. But in the case of the screw thread with small included angle and large lead angle, it becomes impossible to measure the pitch diameter, because of double contact between the cylinder and the flank. So a full and detailed consideration of the condition for single contact between the cylinder and the flank is necessary. This report deals with the limiting condition for single contact between the cylinder and the flank on the milled helicoid. A precise formula which determines whether the contact is single or double is derived by an analytical method. And the way to calculate the limit measuring wire diameter which avoids double contact is given. It is clarified that in most cases the danger of double contact on the milled helicoid does not exist through theory, numerical examples and experiments.
