Contact surface pressure is at best difficult to define and describe. It is not a measurement of force or energy that could be recorded, but, in every wood machining process, we have to seek the best possible conditions for our tools to perform at their best.
The machining processes for wood and wood-based materials are unlike most others in one aspect - the enormous speed. Imagine a 12-inch circular saw blade having 72 teeth, turning at 4,000 rpms. Nothing spectacular, right? As such a tool is put to work, a saw blade tooth makes contact with the workpiece material 4,800 times each second ([72 x 4,000] ÷ 60 = 4,800; i.e., [teeth x RPM] ÷ seconds/minute).
As we examine the operating conditions for other wood cutting tools, most will not come too close to what saw blades endure, but all operate at tremendous speeds. At such speeds, it is easier to visualize that even very small details and geometric features can have dramatic effects. The shape of tool knives, the cutting and relieved features, the angle(s) of incidence, the presence of angular or radiused shear, the relationship of rim speed to feed speed, and of course the physical properties of the material being cut all impact operation.
Contact surface pressure is not a physical force that can be measured and recorded, such as by lbs.-per-square-inch force or foot-lbs. of energy. We have come to call the overall effects of a cutting tool knife or tooth impacting the workpiece material as having an amount, or type, of contact surface pressure, based on the geometric features present and several interrelated operating principles within the ongoing cutting process itself. A list of these principles include:
- the shape of the cutting tool knife or tooth,
- the velocity (rim speed) of the tool knife/tooth,
- the cutting angle of incidence present in the specific process,
- the shear/alignment angle of incidence present in the process,
- the placement of the knives/teeth in the cutting process,
- the number of teeth present in the specific process,
- the relieved features of the cutting tool involved,
- the feed rate of the process involved, and
- the total workload placed on the tool.
Providing the best possible contact surface pressure environment is perhaps the most important element in all wood cutting processes. Because of their unique complexities, this is a critical issue, especially for circular saw blades, requiring that we carefully observe all factors of the physical/mechanical relationships present.
For the purpose of illustrating contact surface pressure, we will use the physical characteristics of circular saw blades to explain its properties and composition, though it is present and a consideration on all cutting tools and machining operations.
Circular saw blades:
- exhibit strength properties only in relation to their momentum;
- have very little lateral strength;
- are capable of enormous workloads in relation to their physical size; and
- have complex peripheral features of tooth/knife numbers, sizes, shapes, and arrangements.
The planing/surfacing operation illustrated in Figure 1 on page 62 shows the contact surface pressure arranged along the leading edge of the tool knife. The tool knife has no shear angle and simply follows the rotational direction of the machining process (i.e., the tool knife is set at 90 degrees to the plane of rotation).
The cutting motion shown is identified as conventional, or abrasive, meaning that the tool is rotating against the direction of feed. The offal chip mass is formed slowly, beginning at the largest diameter of the cutting-process arc. The chip mass builds along the face of the tool knife and has the greatest volume of waste/offal as the tool exits the cutting process arc.
The tooth progression character in the illustration (Sz) indicates the forward movement of the workpiece in relation to the rotational movement of the tool. The Sz character also denotes what is commonly called chip load or tooth progression, though we must be careful to clarify what is being described in every machining operation.
In the instance shown, the massive contact surface pressure can be reduced by any of several means, including setting the knives at a shear angle, using several smaller/shorter knives set in a staggered arrangement, or setting the individual knives in a spiral shape.
The overall effect of shear, whether an angular or radiused feature, has several elements that may not be readily apparent. Shear angle is the arrangement of a cutting tool knife or tooth at any angle other than 90 degrees to the plane of rotation, as illustrated in Figure 2 on page 65.
Perhaps the least apparent and most significant advantage of placing the knives at any incline to the plane of rotation is the fact that we are prolonging the time the knife is in contact with the material being cut. Another advantage is that we are dividing the momentum, and energy, along the leading edge of the tool knife as it progresses into the material.
The angle of incidence is constantly divided along the leading edge of the knife, making the cutting action a more smooth, continuous motion instead of an abrupt single impact when there is no shear angle present. For cutting tools with two or more knives, where the shear feature can be shaped into a spiral, the cutting circle could be completely closed, providing a continuous motion.
The best example of this would be the newest solid carbide spiral router tools. Even on the solid carbide spiral tools, the contact surface pressure can be reduced further by cutting staggered (chip-breaker) sections into the knives, effectively reducing the workload by producing smaller offal waste chips.
The engineering difficulty and manufacturing cost of producing cutting tools having teeth/knives arranged into a shear angle, or the much more complex spiral shapes, is being reduced constantly with the use of refined CNC metalworking machines. At the moment, ongoing research aims to make such technology more readily available and cost effective so as to be more widespread for everyone's use.
In Figure 3 on page 66, there are two clearly evident sets of conditions that will affect the circular saw blade cutting process. In position A:
- there are three teeth present in the cutting process;
- the entrance and exit angles are much more acute;
- the contact surface pressure is directed more forward and less downward;
- the length of time the tool contacts the material is longer; and
- the length of the cutting arc is extended.
In position B:
- there is but one tooth present in the cutting process;
- the entrance and exit angles are less acute;
- the contact surface pressure is directed less forward and more downward;
- the length of contact time is shorter; and
- the arc length is reduced.
All of the elements described will affect the operating conditions in the process simply by altering the tool position in relation to the workpiece material. Finish quality, horsepower consumption, vibration, heat, and tool wear will be different.
For all cutting tool operations, the position of the workpiece in relation to the cutting tool can have dramatic effects. The vertical position of a circular saw blade for instance can alter the entrance and exit angles by as much as 30 degrees and greatly affect the inherent contact surface pressure.
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