Perfect Hollow Form - Part 1 archive
Wednesday 28 October 2009
John Jordan shares his secrets on selecting, orientating and cutting green wood for hollow forms
The first thing I do each day before I start turning is a bit of lathe maintenance. I polish up the toolrest to ensure the tools move smoothly, spray the bed and moving parts on the toolrest base with lubricant, followed by the spindle threads, then give it a wipe with a paper towel. Often, because the wood is very acidic and wet, I clean and spray the lathe at the end of the day to take care of rust. The simple act of polishing the toolrest and making the parts work smoothly will immediately improve your turning, with no practice or expense required.
Shaping the hollow form1. I start the piece by mounting it between centres, which gives me the option of shifting the piece to achieve the best orientation. I can shift either end, or both, to adjust the balance, grain patterns and colour, or to avoid defects etc. I will turn a little, then stop and check the piece and adjust it if needed – a continuation of the process that started with laying out and sawing the log. I use a standard four-prong spur drive and a ballbearing cup centre in the tailstock, but I also have an extra large, screw-on spur drive for large pieces.
I roughly shape the piece at a modest speed – I rarely turn faster than 1000rpm and I start the rough shaping at around 300-600rpm. I start the process with what will ultimately form the bottom of the piece toward the tailstock end, set the toolrest at a 45° angle and start turning on the corner.
I use a 12mm (1/2in) side-ground bowl gouge to make fairly heavy cuts and aim to go for the basic shape. I turn from small to large diameter on side-grain pieces and large diameter to small for end-grain work. There is not usually a need to turn the piece into a cylinder first, as that can often be wasted effort. After a few shaping cuts, I turn a waste area and rough tenon. This waste area is important as it gives support to the piece while hollowing and allows enough wood to finish the last part of the unturned shape down to the foot.
Many turners make the mistake of putting a tenon right onto the piece without this waste area, then find there is not enough wood where it is needed to continue the finished curve down to the desired foot/base/bottom, as the illustration demonstrates. The toolrest is then moved to the top of the piece and the wood is shaped in the same way, working ever closer to the desired shape, regularly stopping and checking the work to look for any defects or checks that were missed. I then re-centre the piece if required and I will sometimes discard the piece if I am not happy with it at this stage. This is not as wasteful as it may seem, as at this point I have very little time invested in it and as much as several days work may be put into a carved or textured piece. It is important for me to feel like the piece is going to turn out to be the best I can possibly create
Creating the tenonFor now, the basic shape is all I am looking for. When I feel I am satisfied with the basic shape, I will carefully true the tenon for mounting in the chuck. It is imperative that the tenon/shoulder junction to be cut is clean and square, or even a bit undercut so the shoulder of the chuck jaws will touch the wood shoulder. This shoulder-to-shoulder contact is what provides resistance to flex and chatter – it is not enough simply to grip a tenon.
On larger pieces (12in upwards) I will usually use a 115mm (4 1/2in))or even a 150mm (6in) faceplate with screws. There is no negative effect to using faceplate/screws, but the new generation of chucks works very well on small-to-medium-scale work. The chuck does add several inches to the length of the workpiece, so there may be more flex and chatter when hollowing taller or larger pieces.
It is also important that the shoulder on the chuck or faceplate matches up with the shoulder on the lathe spindle to eliminate chatter and to be sure of this I never use a plastic washer under the faceplate. Instead I put a little lubrication on the threads, and snug up the chuck with the wrenches, allowing easy removal when the time comes.
The work is placed into the chuck, ensuring the shoulders are held tightly together so as to create a snug fit. If the piece runs basically true, I will put the key back in the chuck and tighten securely. It also pays to check the tightness a couple of times during the turning process as green wood tends to compress. If you experience excessive chatter at any time, stop and check the chuck to spindle, and chuck to wood tightness
Refining the shape3. The next stage is the final refinement of the shape. Here, I will make basic bevel-rubbing gouge cuts as needed, then use the long edge of the gouge to shear scrape the surface
4. When it looks about right, I use a double-ended shear scraper of my own design to complete the last bit of subtle shaping/refinement. This tool works just like shear scraping with the gouge, but is more effective
5. A ceramic slipstone is used to raise a burr on both the gouge and shear scraper. This burr edge allows me to make the shape nearly perfect and the surface extremely smooth whilst taking minimal sanding. In most cases, it provides a nice surface to draw my layout lines for carving. I spend as much time as needed at this stage, as the elegance of line and form are very important to the success of the finished piece. Sometimes it comes easily, other times it can be a bit more difficult. Taking the piece off the lathe and standing it upright to view can be helpful and sometimes I need to just take a break and come back to it afresh
6. Use a 10mm (3/8in) drill held with a pair of locking pliers and carefully mark the inside depth on the drill with a piece of tape. It is important that this hole is accurate, as I do not want to leave too much wood in the bottom, or worse, too little. I use the detail gouge to clean up and shape the opening, then make a small dimple for the drill and push the drill in an inch or two at a time. I pull it out frequently to clear the chips. If it is forced or not cleared of chips it could bind up
Hollowing7. The hollowing tools I use are 8mm (3/16in) square high speed steel (HSS) cutters with a full radius on the end. These are simply little scrapers that use the burr edge from the grinder to cut. They are large enough to take a nice sized cut but not so large as to get too much cutting edge engaged when working in the shoulder, or deep in the piece.
Too much cutting edge may result in the tool grabbing and thus being hard to control. Most of the best hollow vessel turners I know use the same type tips, even if their tool configuration is different. I do sometimes use a slightly larger (1/4-3/8in)) tip in the straight tool when turning side-grain, since it is not used in the shoulder or tight areas. I never use large teardrop-style scrapers due to the high probability of a catch. I find I get a nice smooth interior surface by using a light touch with a sharp cutter, and the smooth, polished toolrest also helps me to achieve this.
The hook tool works because the tip is close to the centreline of the tool, meaning there is no twist or torque as long as the toolrest is behind the curved portion of the tool – this geometry makes the tool very easy to control.
The curved section of the tool is very slim and shaped to reach into vessels with small openings, which is the type of work I produce
8. The straight tool is tapered on the end and uses the same, or slightly larger cutter as the hook tool version. There are not any major differences in hollowing end-grain versus side-grain. On end-grain vessels, the hollowing cuts are usually made from the centre to the left, cutting across the end-grain and peeling side-grain, which is the easiest to cut. If the tool is pushed in, it is head-on into the end-grain and can be grabby, particularly as the tool goes deeper. If cutting to the side causes excess flex and chatter, I may need to take light cuts toward the centre as this is along the axis of the lathe.
The piece being worked on here is side-grain and I start cutting toward the centre. Grain direction does not really matter much here and I will cut in any direction that is convenient and feels most natural for me. My first goal is to make some working room inside the piece, and I work to open up the inside 50-75mm (2-3in) with the straight tool, stopping to clear the shavings as needed. Progress is slow at first as there is little room for the shavings, but I can progressively remove more wood as it widens. Shavings are cleared with an air nozzle and compressed air
9. As the inside opens up and if the cutter is nice and sharp, the shavings will be too long to blow out, so they are hooked out with a bent wire. A vacuum can be used but ensure it is cleaned out after use, as the green wood shavings will mould and the unhealthy mould spores will be blown around next time the vacuum is turned on
10. The cuts are then continued, alternating between the straight and hook tools and working my way down the vessel. As soon as the shavings start to interfere with the cut, I stop and clear them out. If one tool is not reaching the area I am working on easily, I simply switch tools. The wall thickness is kept fairly even and plenty thick enough at this point, although I do thin it out just inside the opening so there is room for the tool to move