The irresistible urge to pick up a fellow turner's bowl and check out the bottom resides in all of us. As we casually sneak a peek underneath a friend's newly finished bowl, we want to know if it received the same amount of attention as the rest of the vessel. Does the bottom curve agree with the inside form of the bowl? Is the shape pleasing? Are there sanding scratches? How well do the bottoms of your bowls hold up to such scrutiny?
Available Equipment
Self-centering scroll chucks are available for helping with the task of finishing the bottoms of bowls, and each manufacturer has a different design and variety. In most cases, once you purchase the chuck body, costing anywhere from about $160 to $300, you can accessorize the chuck with a multitude of optional jaws, at relatively modest cost. The type of jaws you would use for holding a turned bowl in order to finish the bottom would be of the sort that are wide, such as jumbo jaws or button jaws.
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| The thickness of the bottom of your bowl can be measured by using a straightedge and a ruler. This will tell you the thickness you have to work with when shaping the inside bottom of your bowl. Remember to subtract the bowl's wall thickness before determining how much wood to remove.
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In addition to scroll chucks, vacuum chucks are an option for holding a bowl on the lathe for re-turning, as well as for many other applications. You might want to consider investing in a vacuum chuck system in the future.
For the purpose of this article, however, I will talk about an inexpensive, relatively easy method for holding a bowl in order to turn its bottom. All you need is either a large-diameter piece of 3/4"-thick plywood or some other type of sound wood, a faceplate, screws and strapping tape. And, of course, a turned bowl.
Design Considerations
This method of finishing bottoms is for bowls of the even-rim type, not natural-edge or uneven-edged bowls. For natural-edge bowls, the most efficient method for attending to the bottoms is to turn them first, before the inside of the bowl is excavated. This is accomplished using a scroll chuck, and I will cover the use of those in a subsequent column.
Assuming you have used the glue-block turning method described in a recent column, you will have turned a bowl with a flat bottom. These are the bowls that are best suited for re-turning in the method described here. Almost any shape or design will work, but some shapes are more challenging than others because of where the tape needs to be placed and/or the shape of the rim.
Be sure to check the thickness of the bottom of your turned bowl. It is helpful to know how much wood you have to work with before you reattach the bowl to the lathe. This is not the time to be creating a doughnut!
A Safety Note
Like most procedures for which we use power equipment, there are safety considerations to keep in mind. When buying the tape for this project, select a good quality strapping tape, not masking or duct tape. It should be 1" wide.
Select plywood or other wood that is sound - something that won't tear apart while it's spinning on the lathe. The tailstock can be used as an added source of pressure to ensure that your bowl doesn't fly off the lathe. And, as always, make sure your tools are sharp! A dull tool requires much more pressure to cut the wood, and that added pressure can dislodge a bowl should you get a catch.
Getting Started
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| Measure the outside diameter of your turned bowl (top photo). Use this measurement to establish the inside diameter of your disc jig. Turn a shallow groove in the jig (middle), making sure the bowl fits in the groove snugly but not tightly (bottom). This centers the bowl. |
The diameter of your bowl will determine the size of plywood you will need. For instance, if your bowl is 10" in diameter, you will need
a piece of plywood that is at least 11" or 12" in diameter.
Cut the plywood into a circle using your band saw. Attach this disc to a faceplate, using the largest diameter screws your faceplate will accept. The length of the screws should be such that they go into the plywood only about 1/2". The larger diameter screws will help make up the holding power lost by shorter screws. If you use solid wood like I show here, the screws can penetrate farther. Use three screws at a minimum, but four or more is much safer.
Attaching to the Lathe
Attach the faceplate/plywood assembly to your lathe and true it up. This should require only a few small cuts. With a pencil, make a mark in the center of the disc, something that's easily accomplished while the lathe is running.
Measure the outside diameter of the rim of your bowl, then transfer that diameter to the plywood. Rotate the disc by hand and scribe a line on the plywood that is approximately the diameter of your bowl. Check the accuracy of the penciled circle against your bowl's rim.
Using the pencil line as a reference, turn a small groove in the plywood. This groove will be what you use to center the bowl onto the plywood. The rim of the bowl will seat in the bottom of this groove. It's best to turn the groove a bit smaller in diameter to begin with, then sneak up on the exact diameter gradually, checking the fit after each cut. If you're lucky, you'll get it right with a cut or two. It's not important how much wood you cut away on the inside of the groove: the key diameter is where the outside rim of the bowl meets the outside rim of the groove.
The fit should be snug but not tight. It's not a jam-fit chuck, only a groove for re-centering your bowl. Sometimes, if too much pressure is applied to a thin-walled bowl when placing it in a jam-fit chuck, that can cause it to crack.
Tape the Bowl
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| Tape the bowl to the jig. Here, the tailstock keeps the bowl in place during taping. Remove the tailstock and shape the bottom. Occasionally measure the depth to make sure you do not turn through the bottom. Sand, and you're finished! |
Set your bowl into the groove and hold it in place using the tailstock. Don't worry if the point of the tailstock creates a small hole - you will be removing a bit of wood here anyway. Apply strapping tape from one side of the plywood disc, over the bowl and onto the other side of the plywood. Place the tape so it does not cover any portion of the bottom you intend to turn.
Make sure the tape is snug and that it is securely affixed to the plywood. I've occasionally had one end of a piece of tape come undone, noisily flapping as the lathe spins. It's not dangerous, just startling!
Use at least three strips of tape, more for larger diameter bowls. The reason for using strapping tape is obvious: the internal fibers make this type of tape much less likely to break or tear. Be aware, however, that if you are careless and cut the tape with your turning tool, the bowl could fly off the lathe. The tape is for holding, not cutting!
I have never had a problem with tape residue on my bowls, but I suppose that could happen if the tape you use is old and beginning to deteriorate. Use good quality, new tape.
Turning the Bottom
Now that your bowl is centered and securely fastened to the lathe, you are ready to turn the bottom. I use a moderate speed, around 1,200 rpm. Take light cuts. Follow the inside curve of your bowl. Check the depth occasionally.
Most of the pressure you are exerting is directed toward the headstock, so it's generally safe to remove the tailstock. But if your bowl design does not allow for securely taping it, keep the tailstock in place. In that case, you will need to remove a small amount of wood with a chisel after the bowl is removed from the lathe.
I like to design the bases of my bowls so the curve of the inside of the base follows the contour of the inside of my bowl. The foot of the base should be wide and thick enough to ensure that it doesn't crack.
After turning the bottom, sand as usual. Apply pressure toward the headstock, not sideways. Carefully remove the tape, holding the bowl with one hand so it doesn't fall to the floor. If there is a small knob left in the middle of the bottom, chisel it off, then sand by hand. Bottoms up!