Monday, October 23, 2017

Sanding Speed

Don't sand your pieces as fast as you turn them! You get better results at slower speeds.

When many of us head over to the lathe to turn our next and greatest masterpiece, we often heed the advice given by many leaders in our field. That advice is to turn ... as fast as possible.

For turning, that is excellent advice. You get greater control, a more even turning interface (less jitter), and the depth of cut is due more to bevel, sharpness, and tool position ... rather than by the characteristics of the wood itself (although that always forms a part). Yet that same advice is a poor choice for sanding.

When sanding, fast rotational speeds do not equate to faster sanding ... in fact, just the opposite. When sanding, you want to go slow.

There are several reasons for this, the first of which is the same reason why turning fast is good ... the even-ness of the turning interface due to faster speed. As you spin the wood faster and faster, the minute surface irregularities begin to "blend", and the interface between the wood and the tool (the turning interface) becomes less of a thing of "peaks and valleys" (the irregularities), and instead becomes a false interface consisting only of "peaks".

While this may be a hard thing to explain, or even a harder thing to see ... as every turner knows, this is something you can definately FEEL! This is why you get odd sanding patterns and sometimes get a "prickly" feeling when you feel the work being sanded ... in the rotational direction opposite to the rotation of cut. Now while some modern lathes are capable of reversing direction in order to help remove these prickles (errant fibers left by a less than sharp cutting tool, or from scraping) ... this feature is really not needed. Just slow down your sanding speed.

The slower speed allows the sandpaper to "cut" better (remember sandpaper is not a gouge or a plane). It allows the paper to follow the peaks and valleys of the minute surface irregularities, and it keeps the sandpaper cooler. In turning, unless you are going for pyrography, cooler is normally better. Cooler sanding dust also does not bond to the sandpaper as quickly, so clogging is less of a problem. However, "less" does not mean "none". Keep a crepe block handly to clean and "re-charge" your sandpaper. Discard any sandpaper that has lost its grit or has not folded evenly and cleanly. Those malformations and funky little creases will cut more heavily/not at all and cause uneven sanding. It many cases it can even increase damage to the surface.

While this advice is primarily focused on hand sanding, it also proves true for machine and jig sanding. The Sorby Sandmaster is an excellent bad example. The Sandmaster is a sanding device based on sealed bearings and rotation imparted due to a friction interface. Very impressive when first played with (yes, I bought one!) ... but quickly became relegated to the cupboard of unused tools. The rotational speed required to make the Sandmaster perform at its best ... is too fast to make it sand efficiently. The rotating nature of the Sandmaster takes up some of that slack ... but that same nature leaves new swirling sanding patterns that then need to be removed by hand sanding! Hmmmmm. Maybe it would have been better to sand by hand in the first place (probably).

Now, there are some definite advantages to having a high speed machine sander ... don't think that I don't own one or two ... because I do. Bowl bottoms immediately come to mind. But except for very specific applications, I feel that it is better to finish your "tool work" with a sharp cutting tool (skew or skew-like tool) which leaves a smoother surface than scrapers and most gouges. For bowl bottoms try the Bowl Skews designed by Dave Hout and manufactured by Crown Tools ... very impressive, and now on my list of "must have" tools. Then, sand slow using appropriate grits and compositions.

The choice of those grits and compositions is another "science", and needs a seperate article. Much needs to be taken into account including wood type, humidity, water content, grit material, substrate material, surface pre-sanding condition, wood "pores", dust production and clogging, and desired outcome of finish.

You just don't grab a hunk of 60 grit Garnet and spin that baby up as fast as she'll go!

But we all know that now ... right?

Happy Turning!

Jim Byrom

The Travelling Turner