Perfect Hollow Form - Part 2
Wednesday 28 October 2009
John Jordan shares his secrets on selecting, orientating and cutting green wood for hollow forms
Refining the inside
11. To prevent possible checks, it is sometimes helpful to dampen the piece with a wet cloth or spray mist from time to time, particularly when using porous woods that tend to dry on the end-grain faster. Lights, fans, heat and low humidity all add to timber drying too fast. I work quick enough not to need to do it on smaller pieces, but nearly always do it on a major, or large, pieces
12. For this piece, I want to make the initial or rough wall thickness about 12mm (1/2in) or about twice the finished thickness. This is plenty thick enough and strong so the piece will keep its shape and still be chatter free for the finish cuts. For this, I start at the top of the piece again and make short cuts to the left, stopping around about where I think the thickness will be correct, then swing the tip toward the centre, advancing the tool a little and moving it to the left again, and so on.
After making a few cuts, the tool is lightly moved up and down over the section that was just cut, smoothing and feeling if it is relatively even. With the lathe stopped I check the thickness with my fingers when they will reach, and callipers when they will not. By completing short sections at a time, there is not much danger of going wrong. Anytime there is any doubt as to where I am cutting, I immediately stop, clean it out and check where I am. A flexible LED light allows me to look inside after blowing it out, but I make no attempt to see inside when turning as I rely on a mental image to know exactly where I am. This process is continued until the hole in the bottom is reached. By getting a relatively smooth and even wall at this point, it is then fairly simple to go down the wall and reduce it by half for a finished thickness of around 6mm (1/4in) in this case
13. A nice fresh burr is always put on the cutter for the finish cuts and I wipe a block of beeswax along the toolrest, to ensure as little drag as possible. I want to feel the tip cut, not the tool dragging on the rest. These finish cuts will be made with the heel of my left hand lightly resting against the toolrest and the expansion and contraction of my fingers controlling the tool movement, as this is very effective in making short, controlled cuts. The tool is moved down the wall using these short cuts, aiming for the finished thickness and leaving a distinct shoulder at the point the cut stops. I check the thickness with my fingers and if satisfactory and smooth, I now have two references: the shoulder lets me know where to start the next cut and the known thickness gives me something with which to compare the next cuts
14. I put the tool right on the shoulder, just lift it away and turn the lathe on – a remote switch is really helpful here. I will then make one, two or three short cuts to the left, stopping where I think the thickness should be and then lightly move the tool back over the previous area to compare and see if the cuts blend in with the known thickness – the rhythm to this is pretty easy to pick up. The shavings are blown out and the thickness is checked. Here, I may take a look with my light, which will allow me to keep the mental picture of where I am. If there is a bump or rough area, I note it relative to the shoulder, making it easy to find and smooth up
Hollowing in stages
Each section is finished before moving on to the next and as the piece gets thinned out, it may go a bit out of the round and be flexible, so I try to avoid backing up. When the bottom is reached, a light pass is made across and either blended into the side or stopped, depending on the shape of the piece. Before removing the piece from the chuck, I check the depth with the drill or a depth gauge and mark the outside bottom. The thickness of the bottom is about the same or slightly more than the wall thickness. The key to a successful hollowing process is to follow this logical, step-by-step approach, finishing each section and then moving on to the next.
The finished thickness can vary, but 6mm (1/4in) to 8mm (5/16in) is a nice thickness for a smooth, sanded or lightly textured piece. That thickness will easily dry without problems and is thick enough to have a mechanical strength that will help it keep its shape and is not so fragile that people are scared of it. Most of the pieces I make are 10mm (3/8in) or even 12mm (1/2in) to accommodate the deep carving I often do. These thicker pieces are more likely to dry unevenly, so I always put them into a cabinet to moderate the drying.
Refining and finishing the bottom
15. The piece can now be sanded, but I am more likely to wait until it is completely dry because usually I will draw and carve the piece. To turn the bottom, a small tenon is turned on a waste block to fit just inside the opening of the piece and the piece is then pinched between centres. Usually, the mark from rough turning between centres is on the bottom of the piece and so it should therefore be accurately centred
16. Having a revolving cup centre with a very small point makes it easy to shift around, if needed. The tailstock is tightened with light pressure and friction will drive the piece for removing the excess wood around the base. A small detail gouge is used, taking fairly light cuts and gradually working toward the finished shape, keeping the bottom thickness in mind
17. I shear scrape with the gouge and shear scraper to blend in to the curve of the vessel
18. Most of the bottoms on my pieces are just a simple concave shape, which I cut with the detail gouge
Removing the nub
19. The nub is turned as small as seems prudent and the remainder pared away with a carving gouge. Later, after the piece is sanded or carved, I flatten the edge of the concave bottom with a sanding block and usually carve the bottom, leaving the tool marks from the gouge. Sometimes, I will hand-sand it smooth and occasionally use a 25mm (1in) sanding pad in the drill press
Final form and finishing
20. To keep the piece from drying too quickly, I place it in a cabinet with the door closed to moderate the airflow. A few days is all it takes for the piece to dry – larger pieces may take a bit longer. The vessel does need to be dry before I do any heavy carving or there is a possibility it may crack along a deeply carved line. If I want a smooth piece, I will put it back on the lathe the same way I turned the foot, but with a flat revolving centre to keep from marking the bottom. The piece will be out of round, so the lathe speed therefore needs to be very slow. Sometimes I use an orbital sander instead of remounting the vessel onto the lathe and I will hand-sand as appropriate – 320 grit followed with ‘0000’ steel wool is typical, but can vary depending on the type of wood. After sanding, I sometimes use an oil finish, but most often use a spray acrylic lacquer/artists fixative. This is a fairly flat finish that can be polished up to any level of sheen