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Choosing & Preparing the Timber for a Wooden Coffee Table Top.
Traditionally
timber used to be planked using the quarter sawn method. This meant
that the planks to some degree all radiated outwards from the core of
the trunk as opposed to the present through & through method where
only the middle planks do.
If possible, it is best to choose planks
that are quarter sawn, (look at
the end of the plank, the grain lines should run roughly vertically).
This not
only reduces the risk of cupping, but maximises the figure
(medullary rays) on your coffee table top.
One of the woods I use is English oak which is cut through &
through, and I consequently usually have to sift through a whole stack
of planks to select the central (quarter sawn)
planks. (See figs 1
& 2 below.) If you cannot obtain quarter sawn timber you should
use opposing grain directions on alternate planks, this allows any
slight tendency to cup to be balanced out over the width of the coffee
table instead of the whole top trying to bend in one direction. (See
fig
3.)
Once you have selected your timber cross cut the slabs of oak to 50mm longer than your finished coffee table top length, rip off the sapwood and splits to produce sawn planks, (I generally use between 175mm / 200mm wide), or if your timber is already planked simply cross cut them to 50mm longer than your required finished coffee table top size. Make sure that the total combined widths of your planks is about a minimum 100mm wider than the finished width of your table top, remember that you will loose width when you plane the edges of the boards and if any of them have bows in you may loose quite a lot.
Now plane the
faces and
edges of the boards on a planer thicknesser, if you find any board with
wind (twist) in it discard it and cut yourself a new board, if you
attempt to plane out the wind you will loose a lot of your required
thickness.
Lay the boards out on a flat surface edge to edge. If you
are using quarter sawn boards you will be able to select all the best
faces to be upmost, otherwise you will have to lay them out as in fig 3
which may mean some of the planks having best face down.
Mark the top face of each board and number the planks so you know which go next to each other. Look at how well the board edges fit to each other and replane the edges of any that need it with the infeed table set for shallow cut, making sure that your fence is set at exactly 90 degrees to the outfeed table. Check each plank with a square to make sure the face & edges are at 90 degrees.
If your planer is
properly
set up, you should be able to get a perfectly straight joint between
each board which is the ideal situation.
Some cabinet makers
advocate
planing a very slight inward bow on each plank so that when the table
top is cramped up the ends are under positive inwards pressure. This
helps to avoid splitting which is most likely at the ends. (see fig 4).
The one thing you must avoid is an outward bow on the planks, leaving a slight gap at the ends. Although you may be able to close this through cramping it will be constantly under pressure to spring apart and will cause the ends of the planks to split. (see fig 5).
Cramping & Gluing Up the Coffee Table Top.
Always cramp up on a flat and level surface.
I use p.v.a. adhesive which starts to set in about 20 minutes and for that reason I make sure all the below preparations are done before I start.
Set out the
bottom
end cramps first, (the cramps are alternated one underneath and one on
top), lay
the planks face up on the cramps and set the cramps to a
width to take the boards plus a thin strip of waste wood on either side
to prevent the edges of the top being dented.
Set the rest of the
cramps to the
right length & put aside. You are now ready for gluing up your
coffee table top.
Stand the planks on their sides on the bottom cramps and coat the edges with p.v.a. with a glue brush, lay the planks flat in the right order and do up the cramps one at a time starting from one end, making sure that the top face of the boards are level at the joints across the line of the cramp, (even though the boards have been planed flat they will sometimes take on a slight bow overnight especially if you have taken more off one side than the other whilst thicknessing). You can achieve this at the ends by using G cramps and blocks of wood at each joint but make sure you take them off immediately after cramping or the glue oozing out of the joint will stick them to the surface of the table. To level the joints not at the ends you may have to use a heavy weight on a plank, hand pressure, or even, if there is anyone to help, get some one to stand on a plank during cramping to force it down level.
If you have access to a biscuit jointer using it for the joints between planks will not only strengthen the joints, but will help keep the top faces level and cut down on the amount of time taken for the flattening.
Do not over tighten the cramps, they
need to
be done up firmly enough to pull the joints together but over
tightening will only pull the cramp and the top into a bow.
Wipe off the excess glue from the joints with a damp cloth and leave
for a couple of hours before removing the cramps. The coffee table top
is now ready
for flattening.
This can be done in a variety of ways. There are different types of panel sanders which can be used to sand and flatten a coffee table top or in a smaller workshop it may have to be done by hand either using a belt sander or a smoothing plane. ( You can get some information about this at this site: Flattening the top ).
Once flattened and coarse sanded the top can be cut to final size, the ends and sides coarse sanded, a moulding routed on if desired, and then finished as described earlier in the information sheet.
Researched & written by Nick at TheCoffeeTable.co.uk - Copyright © TheCoffeeTable.co.uk - Telephone: 01420 474862