Monday 9 July 2018
Screw threads have long been used as a way of joining wood, bone and ivory. Thread chasing on boxes and puzzles is becoming a popular turning discipline thanks to the work of people like John Berkeley and the late Bill Jones, but these tend to be fine engineered threads. The screw threads found on antique furniture that hold drawer pulls on to drawers or finials in place are more coarse, with the male threads originally being cut by hand and the female part cut using often unique, homemade taps.
Restoration work is amongst my favourite, the challenge of matching a hand cut thread as well as copy turning the shape, staining and polishing, always has me looking forward to doing the job.
The tools I use for this are 60° and 45° 'V' shaped carving tools, a triangular file for refining the shape and a pair of dividers for setting the pitch of the thread.
Before making any cuts I draw some rough guide lines to help with the cut. With experience it is possible to cut the thread by eye but the guide lines do help. To set out the thread draw four horizontal lines to form quarters, set your dividers to the width of the thread and make the necessary number of marks along the first line at the set spacing. One rotation of the lathe will move the thread across one mark so this should allow you to roughly sketch around the wood and join up the marks
As you cut the thread compare it to the original to try and get the best match you possibly can. By fitting the threads of the original into the groove you are cutting, you can see how you are progressing and make any necessary alterations. Because this is hand cut it is nowhere near as precise as an engineered thread. It is surprising what you can get away with but pride in your work says that you should try to achieve the best possible match and fit. You can now rotate the original thread against your new one to ensure it matches all the way around; you can then adjust to fit
Once you feel that you have achieved the best results you can with the 'V' tools, use a triangular file to refine the threads as much as required. I find that the fine carving tools often try to follow the grain of the wood rather than the path I choose so it can be more effective, if a little slower, to use the file for the later stages. By changing the angle that you present the file it is possible to widen the thread in one direction or the other, or by tilting it you can remove wood from an area of thread that is too thick. Regularly check the thread against the original as you proceed to ensure the best fit
With the thread cut, matching the original and the knob crisply turned and sanded, your new knob is now ready to be colour matched and polished. In my experience of this type of work, most of the jobs have been in mahogany (Khaya ivorensis), but if you don't have access to such material, sapele (Entandrophragma cylindricum) is a good alternative in this instance. It can be difficult to identify the type of wood originally used for these items, the sample having years of handling and polishing, known as patina, developed on its surface. I have come across sycamore (Acer pseudoplatanus) and beech (Fagus sylvatica) used for these types of jobs, sometimes solid and sometimes laminated