There are many applications where a machine thread is required in wood. Threaded inserts are very useful because any assembly put together with them can be dismantled. They are therefore perfect for prototypes, jigs and permanent assemblies which are more rarely broken down but which require this feature as a part of the design.  

These stainless steel inserts are suitable for the hard wood, the external threads are standard coarse metric M6x1.0 bite deeply into the wood and keep the insert securely in place. The internal threads are of M4x0.7 which allows numerous fasteners to be used.

They are also suitable for the repair of the broken metric M4 threads, you can enlarge the broken M4 threads to standard M6 threads.

This insert can be only installed with the special tool as per link below: