A thread gauge is a tool used to measure the pitch or lead of a screw head. They are designed to determine the gradient for external and internal threads. Thread gauges are useful for checking tool angle while cutting threads in metal turning lathes.
This tool is not used as a precision measuring instrument, rather it allows the user to determine the profile of the given thread and quickly categorise the thread by shape and pitch. This device also saves time, in that it removes the need for the user to measure and calculate the thread pitch of the threaded item.
Thread gauges traditionally consist of multiple blades that can be used to measure the pitch. The blades sit within a tool that resembles a pen knife. The blades can then be twisted out of the tool when in use.
To use a thread gauge, first match the type of thread to the gauge. For example, attempting to measure metric threads with an imperial gauge will not return accurate results. To determine the correct gauge, a process of trial and error may be needed if the screw is of unknown origin.
Once the correct gauge is determined, the user should extend one of the leaves of the tool and press it against the threaded portion of a screw. If the teeth (cut into the leaf) match the spacing of the thread, then the user can read off the thread pitch stamped into the leaf. If the fit is not good, the user should try a different leaf.