Sunday, October 21, 2012

Using a Trephine to remove broken screws in Wood

If you do any furniture repair you have probalby come cross a screw or nail that has broken off flush with the wood surface.
Digging it out with prys or pliers generally damages the surrounding wood severely and can compensate the repair.

Enter the Trephine.

Borrowed from the medical and dental field this hollow steel tube sharpened with saw teeth can accurately core out a small section of the surrounding wood leaving a concentric hole that is easily repaired with a dowel plug.
Trephines currently come in 3 sizes and will handle most screws or nails found in common furniture products.
Here's an example of how they are used.
This picture shows a screw broken off flush with the end of a rung. If you have trouble centering the trephine cut a small guide hole in plywood scrap with the trephine and mount that with double sided tape to the top of the rung.
When the surface is cleaned to allow the drill to sit flush against the rung the trephine is gently pressed into the surface with short drill bursts and allowed to clear the debris before continuing.
Once the screw has been tunneled along it's entire length you can use a sharp instrument to pry away the wood between the screw and the kerf until the screw frees up.
Here the screw is out and the hole debrided you can fit a length of doweling in the hole or custom
 turn a matched piece if the repair will show.
 
A narrow chisel will take care of the remaining splinters by twisting it in the channel. If you have a suitable drill bit you can try this as well.


Once the screw is out and the hole debrided you can fit a length of doweling in the hole or custom
turn a matched piece if the repair will show.
Here's the finished repair ready for a new screw or nail.
I purchased mine from Highland woodworking . You'll find them under screw extractors.
 

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