Thursday, June 7, 2007
Chatter, The Tools
There are a number of factors that can create vibration and the telltale chatter on a workpiece, but most of the time, I turn to my tools and technique for the answers. I always use a shearing cut when turning, as opposed to a scraping. Scraping in spindle turning leaves a coarser surface, takes longer and encourages vibration. When scraping, the only part of the tool that makes contact is the cutting edge, unlike shear cutting where the bevel of the tool is in constant contact.
When shear cutting, the bevel rubs against the surface behind the cutting edge. This offers several advantages. The pressure applied can actually help steady the workpiece and reduce vibration. The freshly cut surface is smooth and as the bevel rides on it, the cutting edge can pare away any surface chatter, much like a regular chisel pares. This paring action also helps to create fluid shapes because it relates the already cut surface to the surface being cut. Bevel contact also reduces the chance of any nasty catches. Catches happen when the cutting edge is unsupported and is overcome by the force of the spinning piece.
In order to keep the bevel in constant contact, I've found that the shape of the metal behind the cutting edge is crucial. The hollow grind and a small flat help keep a low clearance angle, and the bevel in easy contact. If the metal behind the edge gets rounded, from repeated buffing etc... the bevel must be lifted off of the workpiece to get the edge to cut, and CATCH!!!
So anytime that I am getting chatter that I cannot seem to get rid of, I sharpen up my skew and take a light cut with solid contact. (The other thing to remember with the skew is to keep moving forward, any attempts to back up can lead to a catch.) I rub the bevel on the smoothly cut surface while "paring" away all the nasty chatter. I will often use my hand behind the workpiece to steady it as I cut with the skew, always keeping my thumb on the tool. It does take some practice and a light touch, if your hand gets too hot, you're holding too tight. After a while, you'll sense the trouble arising, adjust accordingly, and the chatter won't get there in the first place. The difficulties in learning to turn should not be underestimated. Early on I set the goal of not sanding my turnings, this led to many a botched leg as I worked to master the more difficult techniques involved. Luckily I heat my house with wood and hard maple burns like coal! The reward of this effort is the ease and speed that I now enjoy, though I never forget that I am always a tiny bevel away from disaster.