Woodturning/Cutting logs for bowl turning
WARNING - Chainsaws are extremely dangerous. Be sure to read, understand and carry out the safety instructions that come with your saw.
Most large bowls are made from logs. The pith will almost certainly crack as the wood dries, so it is usually not included in bowls that are intended for functional use. This article suggests ways to cut large logs into bowl blanks ready to go on the lathe.
Approach 1 
In this method, two bowl blanks are cut from one short section of a log.
Marking two parallel planes.
Cuts made along the lines with a chainsaw.
Cardboard circle templates.
Trimmed and ready to go on the lathe.
A flat is cut either side of the log to show mark lines better and to stabilize the block during sawing. The lines are laid out carefully to ensure that blocks of uniform thickness are made from the center section. A piece of card is used to mark the lines parallel to the floor. If the log is tapered, consider wedging-up the narrow end to keep the pith in the center of the waste block and away from the bowl blanks.
A chainsaw is used to cut the block. First, shallow cuts are made straight along the marked lines. These then help guide the saw as deeper diagonal cuts are made. [note 1]
The piece of wood in the middle that contains the pith will be cut up and used for smaller projects. The pith will be cut out and discarded, since it is certain to split, leaving two blocks that will make small bowls or be cut up further for spindle turning.
The cardboard templates are used to mark the circumference of the bowls on the log halves. The chainsaw is used to cut off the corners and excess material before the blank is mounted on the lathe. This makes the block of wood more balanced so the lathe can run faster, which can save a lot of time.
Approach 2 
A bandsaw mill is used to cut whole logs into big thick slabs. Again, the wood containing the pith is cut up into boards a couple of inches thick, and the pith discarded.
Bandsaw mill in operation
Some of the large pieces for bowl turning
The advantage of this technique is that it saves having to make ripping cuts with the chainsaw, which is hard work for both the saw and the operator.
- Most chainsaws are sharpened for cutting across the grain, i.e. straight through the trunk. Cuts across endgrain produce nothing but dust and the saw tends to kick and buck. Cuts along the grain produce big long shaving that tend to clog the saw. To slab-up a log as shown here, it is best to cut on a diagonal whenever possible, somewhere between these two extremes.