Making Difficult Angle Calculations for Turning
Q: I’d like to make a hollow cylinder with an inside diameter of 6" or 6½", glued up from ¾" stock. I’ve figured that the circumference is 18.85". By making the boards 1" on the inside it would take 19 boards, but I can’t figure the angle to cut them for glueup. I’ve asked several people (including one woodshop teacher who said he’d have to go home and get his calculator) and I’ve gotten three different answers, all with the phrase “ ... I think” attached. I’d love to have a formula/equation with an explanation, but I’m not much of a mathematician, so the simpler, the better.
Ray Lanham responds:
Don’t feel alone in your inability to figure out the various angles required for your hollow cylinder. Many woodworkers have been stumped by such calculations. Throw in wedge-shaped staves for compound angles on the pieces and it’s enough to make even a math-savvy woodworker go cross-eyed.
Fortunately, the problem has been solved by some woodworking nerds among us; Lloyd Johnson (woodturnerpro.com) and Kevin Neelley (turnedwood.com) are just two of many who have worked out inexpensive software to do the calculations for you.
If you want the cylinder to have straight sides, I’d suggest using an even number of pieces which will allow for an easy common angle. (In this case, a 20" circumference comes out to a 6.37" diameter, just what you’re looking for.) Here’s a simple way to make the calculations: 20 pieces divided into 360 degrees is 18 degrees. Since each piece has two sides, 18 degrees divided by 2 equals a 9-degree angle on each side. The more pieces you have, the smaller the angle will be, but the formula remains the same.
However, if you wanted that cylinder to be larger at the top than at the bottom, you’ve created a need for compound angles on each side. Any of the software I’ve mentioned will do the calculations for you. A compound stave cylinder having 12 pieces with a 12" outside diameter at the top and a 4" outside diameter at the bottom requires an 11.6-degree angle on each side which is then cut at a 9.57-degree miter angle.
That’s way too much calculating for my pea-sized brain, so I simply turn to the software. I decided long ago that I don’t need to understand all the math, as long as someone can help me with the numbers. Good luck with your cylinder!
Ray Lanham is woodturner who specializes in segmented turning. His Web site is coeur-de-larbre.com.
Choosing the Right Bandsaw Blade
Q: How do I know when to change my bandsaw’s blade – besides when it’s dull?
Mark Duginske responds:
Obviously, when the blade is too dull to saw properly, or has been damaged, causing a kink, it should be replaced with a new blade. Otherwise, you should change out your blade according to the task at hand.
Even though bandsaw blades all have the same basic design, each configuration has particular cutting characteristics. To get the best possible cut, use a blade with characteristics suited for the given task, such as cutting thick material or resawing. For 90 percent of my work I use the following three blades.
Cutting thick material requires the “beam strength” of a wide blade to prevent blade deflection. For a 14" bandsaw the widest practical blade is 5/8", but I would put 1/2" blades in this category too. A fairly coarse blade, such as one with only three or four teeth per inch (tpi) and an aggressive hook form, is best suited for cutting thick material. Wide blades are perfect for straight cuts such as ripping or cutting tapers.
By using a curved resawing guide, you can still get good use from a wide blade that is dulling, because the curved fence helps compensate for blade lead, which is one of the characteristics of a dulling blade. This blade will cut a circle the size of a coffee cup, making it the blade of choice for woodturners. When tuning up your saw, use the wide blade to check wheel alignment.
The best all-around medium-sized blade is a 1/4" width with either four or six tpi. This blade is fairly efficient at cutting thick material and can cut a circle about the size of a quarter. This blade does a lot of tasks well, such as cutting a cabriole leg where it is required to make a tight turn in thick material. Because of its versatility, this style of blade is the one that many woodworkers keep on their saws most of the time.
A 1/8" blade with 14 tpi can cut a circle the size of a pencil eraser and excels at crosscutting, which makes it good for removing waste material in joints such as dovetails.
Mark Duginske is the author of “The Bandsaw Handbook” (1989, Sterling Publishing) and is featured in the Fine Woodworking DVD “Mastering Your Bandsaw.”
Getting Truly Round Dowels
Q: I’ve noticed a lot of the dowels I buy for my projects are slightly over- or undersized, and many of them aren’t even round. Is there a place to get a really good dowel?
Thomas Lie-Nielsen responds:
Commercial dowels are cheap, but seldom perfectly round. If you want an accurately sized, round dowel, you have to make it yourself. But if you do, you can use any wood you choose. With just the right wood, you can get a perfect match, and the dowel will be virtually invisible. Or you can choose a contrasting wood for interesting color and pattern combinations.
The easiest way to make your own dowels is to use a dowel plate. Just whittle or plane a piece of wood to the approximate size (an octagonal blank works well) and drive the wood through the plate with a hammer or mallet. It will help to chamfer the leading edge to get it started. This works best for short dowels or the sort commonly used in furniture making. You can also size the end of a larger piece, such as making the tenon on the end of a chair rung.
Dowel plates can be shop-made with a piece of steel, a drill and a countersink. They are also available commercially from a number of manufacturers.
Dowel plates don’t need to be sharpened very often, but the way to do it is simply to stone the entire top of the plate to an even flatness, which hones the edges of the dowel holes.
Thomas Lie-Nielsen is a toolmaker and author of Taunton’s “Complete Illustrated Guide to Sharpening.”
