Woodworking: does speed matter with drill presses?

[Paul Alciatore]

While a clean hole is definitely a big concern, I fear you have missed other factors that also are influenced by the speed of a drill bit. I work with metal as well as wood. In fact, I work with many other materials. And all of these will, at one time or another, need to be cut and to have holes drilled in them.

Wood is fibrous: it has fibers. Those fibers will tear apart from each other more readily than the grains of a metal or the homogeneous structure of a plastic. So wood tools are run fast to deny the fibers the opportunity of tearing apart instead of being cut by the tool. This is much like the use of a machete to chop through dense vegetation. A swift stroke with the machete will cut cleanly through the grass and vines while a slower stroke would only nick them, pushing them aside a short distance, perhaps bending them, but leaving them mostly uncut.

On the other hand, when metal is cut there is a region where that metal actually flows. A tool that is cutting metal can cut at a wide range of speeds from slow to faster and the metal will still be cut. The surface finish may vary, but the metal will be separated in any case.

Heat is a factor when cutting something. Wood is a soft material which is easily cut. So a high cutting speed does not generate excessive heat and the use of a coolant is rarely needed. However because metals are generally harder, more energy will need to be extended when cutting them and, like in so many other physical processes, that energy will become heat. That heat will be generated at and near the actual cutting edge. Tools for cutting metal have evolved from simple tool steels to alloys like HSS (high speed steel), tungsten carbide, and even diamond. HSS is a steel alloy that is effected less by the high temperatures associated with the cutting of metals at higher speeds. Tungsten carbide is more so. Coolants are frequently used when cutting metal.

The life of the tool before resharpening is another factor. The heat as well as abrasion acts to wear away the sharp, cutting edge of all cutting tools. The amount of abrasion depends on the material being cut while the heat depends on that and on the linear speed of the cutting edge. Cutting metals will generate a lot of heat at slower speeds than when cutting wood but wood can also generate heat. All of us have seen wood become burned along the cut due to a dull blade.

All of these factors and more have gone into the published tables of speeds to be used when cutting different substances. These tables are ONE person's SUGGESTED cutting speed, not some kind of absolute rule that must be followed. Wood tools generally cut better when used at high speeds. But, a good wood plane which is properly sharpened is used at a very low speed and can produce some of the most perfect surface finishes in all of wood working. Each type of metal (steel, aluminum, brass, bronze, even different steel alloys) will have a different suggested cutting speed. For metal cutting these tables will lean in a compromise between tool life (between sharpenings), time needed for the cut, and factors like surface finish and quantity and size of burrs. Plastics often can be cut at higher speeds but some will generate too much heat and melt if the speed is too fast.

The point is these published tables are only a STARTING POINT for any particular cutting job. The wood worker, the machinist should take them as such and make any adjustments that are needed to obtain the desired outcome. And that outcome is not always just a clean cut.