One of our main concerns in the woodworking industry is the construction of assembly joints of all types. The concerns are primarily fit and finish issues that relate directly to our use of design criteria, machines and cutting tools.
Perhaps the most common element of wood-joint assemblies involves the use of either a groove, rebate, tenon or dado, (depending on your choice of idiom to describe the particular element involved). In cutting tenons or grooves, the two most common problems to deal with are the loss of groove width and the inside corner detail.
Cutting tools have been made with a distinct spur knife for more than 80 years. The first such tool detail was an odd looking saw blade whose teeth had been bent or hammered, then ground in such a manner to cut only the corner detail. The remnant of the tool is still with us today but called a stack-dado cutter set. These outside cutters have teeth shaped to cut only the lateral wall and corner detail, and none of the groove bottom, which is left of what are called the chippers. These are usually crude two-, three- or four-knife tools with little or no lateral relief. They are much like little propellers with cutting teeth on the ends, and often just as noisy.
The mechanical advantage of groove, rebate or dado cutters having spur knives is that the actual cutting process is divided on two planes of reference. The main knives cut only the groove bottom, while the spur knives, relieved away from their own plane of reference, cut the lateral walls, terminating just beyond the cutting circle of the main knives.
The two sets of cutting knives are remarkably strong and largely unaffected by any particular grain strength and orientation. Further, with such an arrangement, dividing the workload on two planes of reference among the knives elevates feed speeds and workloads are possible with excellent finish quality - with only a minimum number of knives present. By cutting the corner detail with a knife dedicated to the process and on a different plane of reference, we are assured that the inside corner remains an accurate 90 degrees and the width unchanged.
There are two problems of cutting a groove detail without a spur knife:
1. The main knife has to be relieved both away from the corner and bottom, leaving the knife's outside corner with very little strength and prone to impact fracture, the most common failing.
2. The main knife, while being sharpened (even if top ground only), will lose diameter over its useful life. The loss of diameter will admittedly be small, if the tool is provided adequate lateral relief.
The main knife on the tool utilizing spur knives can be manufactured with less lateral relief and very little, if any, lateral relief to the periphery. This leaves more knife mass at the peripheral edge and is stronger in every way.
Within the past 30 years, several large tool manufacturers have developed a wide selection of unique groove and rebate cutting tools for use in an equally diverse selection of machine applications and materials. Everything from tiny tools for use on point-to-point machines, shapers and even hand-held routers to huge groove cutters for deep, complex window and door profiles.
The most unusual collection of such profile shapes is seen in the manufacture of expensive wood doors and windows. These are unusual in the sense of being deep, narrow and complex, unlike those seen in other products. A window grooving tool may have three or more cutting circle and profile depths to control on the same tool, while assuring width dimension and square corners. Spur knives provide the advantage of assuring dimension and finish quality.
Today we can purchase these tools for nearly any needed cutting diameter, width and depth. There is still a wide selection of brazed-tip tools available, but most now use disposable solid carbide insert knives. Of particular usefulness are the adjustable groove tools that can accurately be used over a wide range of applications. Before seeking a unique one-of-a-kind tool, make certain that you look for a more versatile mass-produced cutting solution.
Nearly all of our tool makers are quickly moving to refined computer-controlled design and manufacturing systems. This will serve to reduce the cost of very refined cutting tools, but we will still find considerable cost savings selecting and purchasing items that have been mass produced. Our tool makers stay in constant contact with the machine manufacturers and know the tooling needs of every machine model well before we do.
As cutting tools become more refined and task oriented, it is important to remember that a portion of our tool makers' business is to solve problems that relate to materials and mechanical concepts. They see more of it than we do, and probably have already solved even our most vexing problem.
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