Plane Articles

London-Pattern/Sheffield - List Carving Tool Sweeps
By Don McConnell

It is generally understood that # 3 through 9 sweep carving gouges, made according to the London Pattern, are portions of true arcs. It is also understood that the gouges of any given sweep are proportional - that is, the depth of each gouge of a given sweep has the same ratio to its width as every other gouge of that sweep.

While this can be intuitively grasped by studying a chart of London Pattern sweeps, the method by which the arc of each gouge of a given sweep was determined is not immediately apparent. So, I decided to see if I could reconstruct a practical method which would produce consistent results.

The first factor I wanted to account for was the traditional method for manufacturing carving tools. After the bolster was formed and the working end "plated" on the anvil, the curvature was hot forged in a pair of dies. The lower die formed the outside of the gouge, while the critical upper die determined the arc/curvature of the gouge.

In studying my carving gouges, I discovered that the arc remains constant up the length of the blade, despite becoming shallower and narrower toward the bolster. In other words, the critical portion of the upper die is, essentially, a portion of a cylinder, which, by definition, is defined by a single radius. This led me to hypothesize that the curvature of each gouge of a given sweep was determined by calculating the radius of this cylindrical upper die. The question was how to calculate the radius?

I decided to start with the #9 sweep gouges because they are true half circles. In this range of sweeps, the radius of each gouge is very easy to calculate since it is one half the width of the gouge (which is equal to the diameter of the circle). Thus, the one inch #9 gouge has a radius of 1/2", while the 1/2" #9 gouge has a radius of 1/4".

I reasoned that this same approach would work for the other sweeps as well. So, turning my attention to the next shallower gouge, the #8, I decided to see if a slightly larger radius (multiplier) would produce the desired results. It did, and that value is 9/16". Thus, the arc/curvature of a one inch #8 gouge is produced by a radius of 9/16". This produces a slightly shallower/slower gouge because the one inch arc is part of a larger circle.

The appended chart (below) contains the values arrived at by comparing calculated radii with the early sweep charts I had at my disposal. I call the radius of the one inch gouges the "multiplier," because it is that value which produces the radius of the arc of each gouge in that sweep range. Simply multiply the width of the gouge by the "multiplier," and the result is the radius which defines the arc of that gouge. Or, to return to the method of manufacture, the radius of the partial cylinder of the upper die is arrived at by this simple act of multiplication.

On a practical level, there is some variation in the actual sweeps of carving tools from different manufactures. Wear of the dies, as well as subsequent grinding and glazing, results in some variance from the ideal. But, in comparing my various older gouges with the values arrived at by this method, I find they conform fairly closely to my chart.