Machining solid wood presents us with the opportunity to seek more appropriate or more efficient operations within our manufacturing facilities. We can modify any or all of the variables present in the processes to seek improvement.
We can use cutting tools with more or fewer knives, we can turn them at higher or lower RPMs and we can alter the feed rates. Also, we can modify the geometric features of our cutting tools or use tools having larger or smaller overall diameters. All of these will affect our finish quality. In addition, we need to carefully select and monitor our use of raw wood.
We have come to describe the finish quality of solid wood parts that have been profile shaped by method as having a given number of (visible) knife marks per inch. This means we can easily see and actually count the number of cutting tool knife marks left on the surface after a part is machined. The number of marks per inch will vary from a low of 16 to 20 per inch for simple and shallow profiles in softwoods to as many as 50 for fine hardwood detailed profiles.
Mass-produced moulding and trim will have a similar number of knife marks.
The exact number of cutter marks present and visible is the result of several interrelated factors:
- The number of cutting knives on the employed cutting tool.
- How many of the tool knives present on the cutter body are in the same cutting circle.
- The RPM the cutting tool is turned at.
- The feed rate.
- And to a lesser extent, the material being machined.
Another factor that relates to the visibility of tool marks is what we call cutter mark depth. That is a fairly complex relationship between the tool's diameter and the progression of each tool knife.
Cutter mark depth is not often a major concern for cutting tools having diameters of 4 or more inches, unless feed rates are extraordinarily high. However, in the instance where cutter mark depth presents a problem, there will have to be subsequent operations prior to final finishing to remove the visible scallop-shaped knife cut depressions.
The major concerns of machining quality include:
- How well precise profile details can be duplicated.
- The production output of a given machining process, meaning the number of parts actually produced in a given time frame.
- How well the machined surface can be produced perfectly smooth without the need of dramatic subsequent finishing operations.
Feed rate has a more dramatic effect on all elements of the wood machining process compared to all other performance characteristics.
Solving for feed rate is a relatively simple two-step calculation. I explained this on page 110 of the March 1999 issue of FDM.
In the formulas, you must first solve for tooth progression before solving for feed rate. Tooth progression (Sz) is solved by multiplying feed rate (U) by 12, then dividing the product of that by RPM (N) multiplied by 60. It is as simple as that.
Feed rate (U) is solved by multiplying tooth progression (Sz) by the number of teeth/knives (Z) present, by RPM (N), then dividing that product by 12. If you want the answer in inches, omit the division part of the equation.
Finding an ideal feed rate is perhaps the most perplexing of all wood machining problems. Especially in view of the fact that ever faster feed rates have been pursued vigorously since our first powered woodworking machines were developed over 100 years ago.
If we assume that faster production speeds lead directly to increased profitability, then the effort is easily worthwhile. That said, we have to take considerable care to understand all of the elements involved. This includes the precision of all machine movements, and motor spindles that are as vibration free as possible. Also, cutting tools should have correct geometric features, are balanced and have an exact cutting circle.
Even today, we are discovering more about the importance of precise, vibration-free machine and motor movements. Precision motors having pre-tensioned internal bearings are becoming more commonplace, and hydraulic tool clamping systems are being used for many high-feed-rate machining operations.
The importance of tool balance and cutting circle integrity cannot be overstated. Balance and concentricity of cutting tools is now more easily achieved with the use of readily available electronic equipment. Also, the increasing use of CNC metal working machinery by our tool makers are making tools more exact than ever before.
The definition of cutting circle is simply that all knives/teeth present on a tool be in the same circular plane, and all rotating knives be at the largest diameter possible. Whether the cutting tool has from two to as many as 12 knives, to be completely efficient all must be doing the same amount of work in the same circle.
The term one-knife finish defines a multiple knife cutting tool having one knife dictating the finish quality appearance, regardless of how many knives are present. On a four-knife cutting tool, if one knife is dictating the finish quality, the remaining three are doing a lesser part of the work and in a smaller cutting circle. The differences between the knife locations (cutting circles) can measure only in the thousandths of an inch, but will affect the appearance of the machined surface and subsequently the possible feed rate.
Based solely on finish quality, bringing more knives into the same cutting circle increases the possible feed speed in direct proportion. The seeming paradox of bringing the knives into the same cutting circle is that as more come into the same operating tool path, the cutter marks blend together and largely disappear, even though we may be operating the tool at the same feed rate and RPM.
Bringing all of the tool knives into the same cutting circle can involve the use of any of several increasingly common tool practices listed here:
- Precision electric motors
- High-tolerance tool spindles
- Hydraulic tool clamping systems
- Precision knife jointing systems
Achieving increased feed speeds demands attention to details that we may have not even known were there earlier. The full effect of vibration mechanics is still being learned. The machine's mechanical movements are being studied as never before.
A last precaution here - it must be clear that altering feed speeds must be approached carefully and thoughtfully. Feeding too slowly has as many inherent dangers as feeding too fast. For every machining operation, wood material and cutting tool there is an ideal, and they will differ from one situation to another.
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