12 Steps to Segmented Turning Perfection
Below, I describe several steps in the segmented turning process that must be accomplished properly to achieve perfection in
your projects. As I work through my next 2-3 projects, I'll add a link to a short video clip that amplifies each topic.
Form is KING. Establish it first. Compound curves are nice; just be sure to smoothly transition from one to
the next. Your form can be tall and skinny or short and wide. While developing your form/shape, pay no attention to size. Remember, form is
KING. If you need help with proportions, remember the Golden Mean (.618) or the photographer's
1/3, 2/3 rule. Use it for height to width and for positioning the maximum diameter point. The base diameter can be 20% to 30% of the maximum
diameter for artistic pieces and more for utilitarian pieces. Plates/platters need a larger base, 50% or more of the diameter. Click for Video.
Establish your object size. There aren't any rules here. Just make it a size that's appropriate to its intended use or where you'll display it. Click for Video.
Now that you have a good shape/form and its size, establish an even wall thickness throughout the piece. It's OK to be a little heavier down
lower for stability. Click for Video.
The number of segments per ring depends on your preference and the size of the object. The larger the ring, the more segments needed to keep turning
running smooth. 8 sides works well for 6" diameter and smaller. Then use 12 for up to 10" or so. You can step all over this one depending on exactly
what you're doing. Just remember when you turn, 1/2 of each segment is cutting generally, and roughly, up hill and then the last half is a very
comfortable smooth down hill. Commonly, the number of segments is consistent throughout the piece, but you can use a variable number if it makes
sense to the structure of the piece. Click for Video.
In this step, I talk about setting up your table saw so you can get true cuts. Your stock must have vertical sides and a flat, level top. Otherwise,
you're making trouble for yourself in making true circles and flat rings. If you are making sandwiches or multi-generation designs, this becomes
even more important. Click for Video.
When cutting segments, the miter angle needs to be as close to perfect as possible to make rings that are true circles. Cut complete sample rings
out of cheap wood and bind in a rubber band to check for gaps and tune as needed. For all angular cuts, you need to be sure the piece cannot move
in the middle of the cut so that the newly cut face is perfectly flat. Click for Video.
Glue each ring together, as two halves in a single step process, with a thin spacer at the two diameter points. Cheap coffee stir sticks work well.
Clamp top and bottom against cauls to make the ring flat and around with heavy duty stainless steel band clamps turned as tight as possible. The
tight clamp, forget about glue starved joints, ensures minimum width glue joints and maximizes the probability of getting a true circle, even
after truing up the diameter faces. Using the cauls means you will need to remove a minimum of material to flatten the rings. If you're in a hurry,
a mere 45 minutes in the clamps and the rings will be ready for the next step. Click for Video.
Use a flat sanding surface to true up the ring halves for a perfect final joint. I'm using a 10" flat disk from Woodcraft with a 120 grit disk.
Mark where the trimmed halves are jointed for later reference, apply glue, apply clamp (tight!), and clamp from top and bottom at the two joints
to maintain a flat ring. A flat ring requires less removal of materal when flattening the ring prior to gluing. I forgot the first item before
gluing; later I corrected my omission. Click for Video.
At least one face of each ring must be flattened prior to gluing to the project. You can do this with a drum sander or on the lathe. For smaller
rings (< 6" diameter) the drum sander is a bad choice. For lathe flattening, simply affix an MDF circle to a faceplate, flatten as needed,
and mark concentric circles on the MDF surface. Then attach each ring to the MDF/faceplate using double-sided tape and use your circles to get
it centered. Then just a couple minutes with a scraper and a flat sanding board will result in a perfectly flat side.
Click for Video.
Vertical joint alignment is very important to the overall visual effect. Always mark the center of your segments to make alignment with the prior
rings easier. When possible, keep the trimmed diameter joints aligned vertically so that all the error caused by trimming is localized. This
will make the rest of the joints look better and the slight un-evenness at these joints will likely go un-noticed.
Click for Video.
Glue just one or two rings onto the project at one time to minimize how much the chisel needs to hang out beyond the tool rest. This will make the
whole turning job easier and safer. This also prevents accumulation of off-center gluing error. Use a tail stock mounted 45 degree cone to center
each ring as it is glued on to get the best centering possible. Failing this, dry fit and measure around the ring repeatedly to place the next ring
properly and hot-glue blocks to hold ring in place during gluing and clamping. The blocks will turn away easily, or just knock them off after the
glue has dried. Turn the inside of each added ring, using the inside diameter of the next ring as a reference, and fare into the previous layers
and sand. By the time you reach the top of the project, the bottom is long since done.
Click for Video.
While stacking and turning the inside of the project, it will start to sing in the upper layers because of the thin-ness of the walls. So, don't do
any turning of the outside of the project until it's completely stacked. But if the singing gets too annoying, turn the outside of a couple rings
to accommodate a steady rest. If you've frugally sized your rings (no excess material) then turning the outside of the project becomes a "connect
the dots and smooth" affair. If you aren't frugal, then a bit of measuring will be needed to achieve the planned form.
Click for Video.