Tools and Machinery of the Granite Industry, Part IV

Article excerpt

This article is the last in a series of four on the tools and machinery of granite working. Part I (The Chronicle 59, no. 2) described granite as a material, an industry, and a product and began the description of the granite quarrying process. Part II (The Chronicle 59, no. 3) completed the account of granite quarrying. Part III (The Chronicle 59, no. 4) concerned the most common finishing operations. This final article continues the description of finishing operations, focusing on some of the more exotic but nevertheless important finishing steps and their associated tools and machines including turning, flute cutting, boring, corrugating, lapping and etching. In addition, this article will describe the processing of waste granite including paving-block cutting, paving, and crushing. Finally, the article will conclude with a discussion of power sources, the various finishing job categories, labor unions, and safety and health issues.

Turning

The granite-cutting lathe was similar to a wood- or metal-working lathe, but in its largest size was much larger and heavier than the wood or metal lathe (Figure 1 ). The lathe was very important for architectural and monumental granite in the finishing of columns, balusters, urns, vases, and spheres. Like a common lathe, the granite lathe had a headstock and tailstock between which the stone to be turned was supported and rotated. The headstock was driven by a variable-speed pulley cone or set of gears, and the tailstock was movable along the ways to accommodate stones of different lengths up to thirty-five or so feet. The Woodbury Granite Co. of Hardwick, Vermont, had a lathe that could turn columns up to thirty-five feet long and forty-eight inches in diameter. A slower speed was used for the initial rough turning. Before wire saws were available to cut rolls into an octagonal cross section, the stone block was hand pointed by stonecutters to a rough, circular cross section prior to turning in the lathe. The pointed surface was taken to within 1 to 1 Vi inches of the final surface. A tool carriage was screw-driven along the entire length of the lathe and held a freely turning, 8-inch diameter cutting disc. The disc was tempered steel with its working perimeter beveled to a sharp edge (Figure 2). As the tool carriage moved along the length of the lathe, the disc was forced into the stone, crushing and removing granite rather than cutting as with a wood or metal lathe. For each pass of the tool carriage, the disc was moved incrementally inward until the desired column diameter was achieved. Sometimes a driven Carborundum wheel was mounted on the tool carriage for the final cut. Today, instead of the cutting disc, a plunge saw mounted on the lathe tool carriage is used, which makes a series of shallow cuts close to the final surface. The material between the cuts is broken out and a diamond saw mounted on the tool carriage is then used to cut the final surface.

The grinding lathe was used after the cutting lathe to produce a smooth (but not polished) surface-typically for columns. A long, straight-edged strip of iron was held by weighted rods against the stone, and the operator periodically shoveled abrasive slurry from boxes under the turning stone. Initially, sand was used and later tungsten carbide shot when it became available.

The polishing lathe was similar to the cutting lathe except it was usually smaller and did not require a tool carriage (Figure 3). The head stock was driven by a cone of step pulleys allowing for variable turning speed; polishing was done at a surface speed of 230 to 240 surface-feet per minute. For example, a 12-inch diameter column was turned at about 76 rpm, whereas a 36-inch diameter column was turned at about 25 rpm. Initial grinding was performed by a series of three- to four-inch wide cast iron grinding blocks that rested, tightly spaced, on top of the turning column. The blocks came in contact with about one-quarter of the column circumference and were curved according to the desired finished column surface curvature. …