Woodturning

Sharpening tools article

This article on sanding lathe tools using a belt sander appeared in Woodturning Design Magazine.




 


Figure 1 The much abused sander made into a sharpening system. I have been using this system for over ten years.


“To reduce sanding wood, sand your tools sharp.”


By John D Williams.


As I have progressed in my woodturning skills the one thing that stands out more than any other is the reduction in the time that I need to spend sanding my turned work. There does not seem to be any one solution to this but rather it is a combination of several small techniques not all of which can be applied willy nilly to any wood or project. After seeing what other turners in magazines and at demonstrations do to get smooth surfaces I realized that there are also various individual approaches which will create smooth surfaces. It is this variety of techniques that make woodturning such an involving craft.
To reduce sanding time on the lathe one must get the smoothest possible tooled surface and in order to accomplish this the tool cutting edges must be super sharp (more about sharpening later) and there are some important tool handling suggestions prior to sanding that should be used.
Just prior to sanding do the following:
 Sharpen the tool.
 Ensure that the tool rest is smooth and nick free.
 Take very light cuts when doing cuts other than push bowl gouge cuts. (When a bowl gouge is pushed into the bowl a fairly heavy cut will still be smooth, as this type of cut involves a curved cutting edge where the rear of that edge leaves the surface and this is what will determine the nature of that surface.)
 Concentrate on making very slow and deliberate movements of the tool to minimize the possibility of leaving ridges caused by uneven tool movement.
There have been many articles on correct tool use which may be summarized into three cardinal rules of cutting:
 Cut with the grain. In spindle turning this means cutting from large diameter to small diameter and in normal bowl turning it means cutting the outside of the bowl from the center to the larger outer diameter and the inside of the bowl from the outer rim to the center.
 Rub the bevel whenever possible. This does not apply to scraping cuts but the cutting edge will be much better at creating smooth surfaces when the bevel is rubbing the wood as this will mean that the cutting edge is presented at the most ideal angle.
 Shear cut. (Also called a “slicing cut”) When the cutting edge is presented at an angle other than 90 degrees the cut is a shear cut. The more acute the angle the greater will be the shear effect. When carving meat the shear effect is accomplished by moving the carving knife back and forth.
If these rules of cutting are adhered to and the tool is really sharp then the surface of the wood left behind the tool should be smooth too. In order to achieve the best possible sharpness the cutting bevels must be smooth and the edge razor sharp.
I know that many turners will hone or whet the tool after it has been ground on the grinding wheel. This whetting process is usually accomplished with a leather or cloth wheel impregnated with fine abrasive or with a diamond whet stick. These techniques will make the edge sharper and will smooth the bevels but they often also round the edge thus actually enlarging the cutting angle. A further disadvantage is that whetting the tool takes extra time and thus time away from actual turning. I have found that using a belt sander to sharpen tools results in smoother bevels and cleaner edges than I can accomplish on a grinder.
By using a belt sander to sharpen lathe tools one can control the amount of metal removed by using sandpaper which is suited to the task. A belt sander equipped with coarse sandpaper can grind the tool edge to shape while the same machine fitted with fine sandpaper will hone the same edges. By using a sharpening jig the tool can be re-sharpened with fine abrasive and with a minimal amount of steel removed. Though a grinding wheel can be obtained in very fine grit it has the disadvantage of needing frequent dressing and the grinding wheel changes its size as it wears. Sandpaper actually seems to hone the tool after it is slightly worn and since the backing plate on the belt sander remains at a constant distance and angle to the jig no matter how worn the sandpaper the settings on the jig never need adjusting due to wear. A belt sander also has the advantage of generating less heat to minimize heat damage to the tool edge. Finally I think that the belt sander is a safer machine, there are no possibilities of exploding cracked grinding wheels and the slower speed will cause less injury if the abrasive surface is inadvertently touched. As a retired teacher I have noted that students find the belt sander less intimidating.
A belt sanding machine is commercially available from Robert Sorby (but not available outside the UK at this writing). I started out using an old, much abused belt sander to sharpen my lathe tools. After acquiring this machine at a garage sale for five dollars and subsequently discovering that the support plate was bent which made it useless for sanding wood surfaces I hung it on a nail in my work shop and found that I could sharpen my lathe tools better on the sander than I could on my grinder. However I was sharpening the tools by eye and though my manual dexterity is good I always ended up having to grind (read sand) several times to ensure that I covered the entire edge. Eventually I decided to build a jig to hold the tool so that I could resharpen tools. The jig had to be able to hold the tool in exactly the same position over and over. I adapted the standard gouge jig to a belt sander. This set up has worked well.


The picture below shows how one can set up a sander to sharpen tools experimentally using a gouge sharpening jig from One Way tools.