Monday, July 25, 2011

Making a plinth which is up to the task.

Work on the Art Deco Style Breakfront Bookcase which I am building continues. The three cabinet carcasses are all completed now that the lining board backs have been fitted. They look awesome with the beautiful colours in the recycled jarrah panelling in the backs, and the amazing array of nail holes, bolt holes and other reminders of the former life of those timbers from which panelling and the whole of the structure is made. Nearly all of the adjustable shelving has been made, and last things remaining to make are the plinth and the crown mould.

Planning the plinth face dimensions.
The four front vertical corners of the bookcase each have a 40mm radius, reflecting the wonderful rounded curves that were common in art deco furniture. The three sections of the bookcase are each 1100mm (3' 7")wide, and will be screwed together on site. The middle section protrudes 100mm (4") forward of the sections on either side - hence the term "Breakfront". The plinth, on which the whole cabinet will sits, is 105mm high and will follow the shape of the bookcase front and ends, recessed back 20mm (3/4") all along. This means the four corner curves of the plinth are also set back 20mm, so they needed to be made with an external face radius of 20mm.
You can never have enough clamps. The plinth cramped up with glue drying - all 11 feet of it. 
 The foundational nature of plinths.
Plinths are as essential to the structure of a piece of built-in furniture as the foundations are to a building. Errors in the foundations transfer all the way up through the rest of the building. With built-in furniture, the essential thing is that the plinth is level both along its length and across its width. This cabinet is over 2700mm (9 feet) tall, so an error in the front-to-back level of the plinth will either have the cabinet leaning into the room or leaning into the wall behind. Fully loaded with books, the former is very dangerous and the latter puts stress onto the wall - which my client is concerned about. A high single brick wall, my client specified from day one that she did not want a lot of stress to be placed on this wall behind the bookcase. Hence the objective is to have the plinth correctly level, ensuring the bookcase is plumb when sitting on it. For safety reasons, the top of the bookcase will be attached to the wall, just to ensure the bookcase cannot fall into the room if someone was to climb up the front. A standard requirement. Of course, there are few walls (old or new) which are plumb and flat, and few floors which are perfectly level over a 3.3m (11 foot) length!
Glue dry, clamps off, then curved corners cut with 20mm radius router bit.
 Allowing for anomalies in the wall and floor.
So how do we ensure the top of the plinth is totally level, despite the floor? If the wall leans into the room, or has a bulge which keeps the plumb bookcase forward of the wall at floor level, how do we still ensure the front of the plinth is still 20mm behind the face of the bookcase? There lies the challenge!!

Well, it's all in the planning ... and allowing for a range of as yet unknown possibilities.
There is a 90mm skirting board around the wall, which will be left in place - just in case sometime in the future my client left the house and wanted to take the bookcase with her. So the ends of the plinth will be scribed to fit around and over the skirting. Hence the ends are made "overlong" to give plenty of room to do this scribing once the forward position of the plinth has been determined on site. The back of the plinth has been made to be set 10mm off the skirting, in the event that the wall is plumb or leaning backwards. It will be leveled, packed and fixed in place. The scribing of the base of the plinth to fit the floor is done by setting it up levelled on packers, in position. From the highest point above the floor, a line is then drawn parallel to the floor. Cutting the plinth along the scribed line will then ensure that, on removing the packers, the plinth sits beautifully on the floor (even if it is wildly undulating) while the top remains beautifully level. The plinth is then fixed to the floor and wall to make it rock solid, fixed through packers as required. It's a piece of cake, so long as careful preparation is made to fit this foundational plinth.

The plinth is made heavy duty, in order to be able to withstand considerable weight and transfer this weight to the floor. It is a solid piece of joinery in itself. A couple of coats of oil/varnish polish and its completed, ready for installation day.
Never mind the backdrop... it's the completed plinth we're looking at here!
 Of course, this is the old fashioned way of doing plinths. Very different to those nasty adjustable plastic risers behind plinth facades you see in modern laminate kitchens and built-in furniture these days. Oh well, I am in the heirloom business. There is a lot of wisdom in traditional techniques - and satisfaction too.

Now to get on with making the crown mould...


  1. Hi Greg, great work. inspiring and good lesson too

  2. i was looking for making curves to create coffee table -