How to Turn Square Material to Round Using a Lathe Machine

One of the most frequent questions I’m asked in spindle-turning classes is, “How do I get from square to round without breaking out the corners?” The answer I always give is to use sharp tools and shear-cutting techniques. The actual mechanics of the cut depend on whether you want the transition from square to round to be a square shoulder or a concave or convex radius. Let’s start with the square shoulder.

The best way to cut a square shoulder is to use the toe of a skew chisel. You must hold the tool with the long corner down, its edge on the rest, and the bevel at 90° to the work (see the illustration below). This entails swinging the handle about 21, which is half of the 42° inclusive grind angle. Use as much speed as is safe for the situation-1,100 rpm to 1,700 rpm for standard furniture turnings.

Start by lightly scoring the work, then move to the right (for a shoulder on the left) and widen the score mark. Go back and score the original mark deeper, then move to the right and remove the waste. This process is much like chopping a tree-the cut must always be wider than the inclusive grind angle of the chisel. Do this until the work is scored all the way around, and you are left with a square shoulder on the left and a sloping cut on the right that meets up with the shoulder. Finally, use a spindle gouge to trim away the excess material up to the shoulder.

For this process to work, the tool should be level on the rest and touching at the exact centerline of the work; only the very toe should touch. The tool’s cutting edge must be absolutely vertical because slanting it one way or the other will cause it to walk in that direction. Some care is necessary to use the tool with a light touch and to read the ghost so that the tool returns to the previous cut each time.

Often the transition from square to round is a concave or convex radius. Cutting the radius entails carefully reading the ghost, bringing a spindle gouge into a shear cut, and rolling either a cove or a bead, depending on the shape desired. Again, running the lathe between 1,100 rpm and 1,700 rpm will help. Read the ghost, then bring the heel of the bevel in contact with the corners of the work. This is a highly interrupted cut, and a light but firm touch works best. Trying to force the cut without the bevel rubbing will result in a catch and likely broken corners. Multiple passes are necessary, and restarting requires careful reading of the ghost.

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