Gluing the two halves together follows the same process
as adding layers. I need to align the
offset between the mating rings then clamp it all together. The one difference is the centering alignment
is more critical here as I want that interior joint to align as close to
perfectly as I can get it.
Once the glue cured I removed the clamps and the small
face place from the top stack. Then it
is time to mount the whole stack on the lathe. That’s easier said than done as the blank is
fairly heavy and pretty awkward. Once
mounted I put a live center with a flat plate end in the tailstock and ran it
up against the MDF disk. The live center
applying pressure against the MDF disk will help kill the vibration while I am
doing the turning. The flat plate is
because as much as I tried the top stack was not exactly aligned with the
bottom stack. It was probably less than
1/32” off but the existing center point of the MDF disk did not exactly line up
with a pointed live center. Here is a
photo of what the tailstock looks like. 
To get a feel for how the full blank turns I started out
at a slow speed, like 50 RPM’s and gradually increased the speed up to about
350 RPM’s. That may not sound like a lot
but remember the blank is about 19” in diameter so that outer edge is whizzing
by at over 1,700 feet per minute. Surprisingly the blank ran very smooth and I had no
problem in truing the outside of the upper stack to match the bottom stack.
With that initial work done I started bringing the rim
and the top 3 layers into shape. After a
little turning I took the MDF disk off and did a check with a pair of calipers
to see how the wall thickness was progressing.
Having just started the outside shaping I had plenty of room and so far,
no surprises. Here the left photo shows
the whole bowl and the right one is a closer look at the initial shaping. Note the sacrificial ring is still attached
although there is only about ¼” material left holding it to the top ring of the
bowl.
As I turn the bowl to its final shape the process is to
cut away some material, take off the MDF disk, measure the wall thickness, mark
the outside with reference marks showing areas still too thick, put the MDF
disk back on, run the tailstock up against the MDF disk then turn away more of
the wood and repeat. Not really hard but
just a little tedious and time-consuming.
Below is where I am not quite half way done working on the outside.
There are two things to keep in mind while doing the
final turning. One is keeping an eye on
maintaining an even wall thickness which I do with calipers. The other is making sure the outer surface is
not only smooth but that the curved surface is just that a continuous curve
with no flat spots, hills or valleys.
That is a lot harder than it sounds as there are no templates, no jigs
to hold the cutting tools and no line to cut to. It is all done freehand by eye. That said I do use a couple of aids – one is
my fingertips. Lightly running your
fingers from top to bottom will let you know a lot about surface irregularities
even when you can’t see them. Another
method is to take something like a credit card or some sort of straight edge
and hold it along the top to bottom axis then rock it back and forth, it should
rock smoothly. If there is an
inconsistency in the curve the rocking motion will not be smooth. Here I am using a card scraper to check.
When I find a spot that is just a hair off and needs help
I turn the lathe to its lowest speed, in my case 49 RPM’s and shave the high
spot off or if it is a low spot work both sides until I get a smooth
transition. I use the card scraper
because it is flexible and I can put a little curve in it which helps even
things out.
With the outside of the top half of the bowl finish
turned I am now close enough to the bottom of the bowl I do not need to keep
the MDF disk attached to the top to kill the vibration while I am turning. As such I can part off the sacrificial top
ring. Here in the left photo you can see
where I have very carefully started the cut.
I continued cutting until I could start to see some gaps then quit and just
pulled the ring off. The right photo
shows the thin sliver, barely 1/32” thick, that was all that was left holding
the ring on.
Getting ready to turn the leftover ridge off and down
flush with the bowl surface it occurred to me that a little raised lip at the
edge might help define the rim. A little
delicate work and I had the transition both inside and outside nearly
done. Here is a closeup of the lip and
the entire rim. I think I will keep it.
Once I had the top done I went back and started working
my way down the last half or so of the bowl.
It’s the same process as the top half except I don’t have to mess with
taking the MDF disk on and off so it goes faster. Here I have the shape roughed out but the
walls are still about an eighth of an inch or so too thick. From here I keep turning and measuring to get
the wall thickness and the shape right until I get to about the last three
bottom layers.
Next up – Final Turning & Finishing
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