For nearly all solid wood machining processes, finish quality is a constant and ongoing concern that we have to monitor. Even using the same cutting tools and machine process there is a still a considerable list of variables to watch over and control. Among the variables that will affect surface quality are:
- The wood species
- The method of harvesting the lumber
- How and how well the water was removed
- How well moisture content is monitored and controlled after drying
- The condition of the wood grain and its orientation to the involved tool path
- The geometry and condition of the cutting tools involved
- The performance characteristics of the machining operation such as feed speed and peripheral tool speed
- Clamping and feeding pressures exerted on the material
- Accuracy and vibration of all elements involved in the tool path process
- The amount of material removed from any surface during any machining operation
Of all of these, the amount of material removed from the surface seems to be easiest to overlook. And it is often the most difficult to diagnose when a problem arises.
Imagine the task of a single tool knife. It revolves, and during a portion of each revolution a knife comes into contact with the material being cut. As the leading edge of the knife makes contact with the wood material, the peripheral speed and force driving it overcomes the strength of the wood fibers and they are severed and forced onto the face of the knife surface. Progressing forward into the cutting arc, the chip mass is curled and compressed into the gullet, being further compacted and heated.
As the knife exits the workpiece, the pressure and temperature of the accumulated chip-mass diminishes almost immediately as the centrifugal forces now throw the material away from the tool body, completely reducing both the pressure and heat to that of the surrounding air mass.
All of this takes place in considerably less than a millisecond.
It is easy to overlook the fact that the forces driving the tool in one direction and the wood material toward the tool are also affecting the material beneath, forward of the tool and to a lesser extent, the areas adjacent to the immediate tool path.
The material beneath the cutting tool is being slightly compressed and heated by the frictional forces.
The material ahead of the tool knife is likewise being compressed and the forces are telegraphed forward (and as the cutting arc travel progresses, upward as well).
The material on both sides of the cutting tool path is also affected, but to a lesser extent.
Given the nature and magnitude of the forces acting on the material being machined, the amount being removed during any particular machining process can assuredly affect how smooth the appearance will be.
Depth-of-cut is seldom a serious consideration unless the amount of material being removed threatens to overwhelm either the available horsepower or some limitation of the machine or cutting tool involved.
Removing a heavy amount of stock most often does not affect finish quality, assuming all of the related performance criteria are within reason, such as the number of knives present in relation to peripheral and feed speed. The cutting tool knife passing through the material is well supported beneath, ahead of and by the adjacent areas of that being cut away - by the material itself.
Removing only a very shallow or small amount of material poses several problems that are difficult to diagnose, especially after the fact.
The machined surface may have the appearance of being torn or having small portions of the surface seeming to have been plucked away. There are again several possible causes involved:
- Removing only a very small amount of material does not provide sufficient support for the cutting action of the tool knife as it passes through.
- This condition could be further aggravated by a dulled tool, by wood material that is either too wet, too dry or made brittle by the procedure of the drying process itself.
- Incorrect peripheral and/or feed speeds of the machining process.
The plucked or torn appearance described here is most often seen in either shaper or moulder operations where we have calculated our finish dimension cut too shallow. Removing less than approximately 0.090 or 3/32 inch seems to invite this more often than not.
The difficult concept to capture and understand is that cutting tools passing though a wood material have to be supported in some manner as the forces involved are transmitted into the cutting action of severing the wood fibers, compacting them to be transported away.
All of the wood material surrounding the cutting tool's operational path is affected.
In the case of a groove, for instance, the material on both sides of the tool path is also compacted as the tool is passing through. Proof of this is the fact that the completed groove is slightly more narrow than the tool, because the wood rebounds somewhat after the cutting action is complete.
Depth-of-cut is seldom a problem, if we take the time to make sure all of the elements involved are appropriate for the work involved.
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