Wood products manufacturers often face the dilemma of choosing a straight edge or helical edge cutting tool. Generally, a helical edge cutting tool costs more initially and to maintain; but produces a higher quality surface than a straight edge cutting tool. The helix cutter produces a better surface by redirecting and/or redistributing the cutting forces and stresses, not necessarily by reducing them. The straight cutting edge engages the workpiece along the full width of cut at the same time. A helical cutting edge gradually engages the workpiece to full width and gradually disengages the workpiece. The results are a reduction of stresses along the cutting edge.
The terms helix and spiral have frequently been interchanged but are very different. A helix goes around a cylinder of constant radius and projects as a circle on a plane perpendicular/normal to the cylinder axis; whereas, a spiral goes around a cone with a varying radius and projects as a flat spiral curve on a plane perpendicular/normal to the cone axis. Straight edges project as dots in a circle on a plane perpendicular to their cylinder axis. These definitions are helpful toward understanding the cutting actions of various cutter types.
Shock or gradual engagement
Straight cutting edges generally shock the workpiece with a sudden tooth load, which can cause vibration and might lead to chatter, an uncontrolled vibration condition. The straight edge impact may also cause excessive noise as is usual with planers and moulders.
The gradual engagement of the helical cutting edge reduces the initial shock concentration of stresses. Gradually, a full length of the cutting edge engages the workpiece and then decreases to zero as the edge (tooth) leaves the workpiece. While in the workpiece, each point of the helical edge is at a different depth of cut in the workpiece.
This reduces the possibility of initiating machining defects; particularly when machining against adverse slopes of grain. The direction and magnitude of the cutting forces is reduced, which in turn reduces the stresses that cause uncontrolled wood failure, i.e. wood machining defects. The gradual engagement and disengagement of the helical edge also reduces the noise associated with the initial sudden impact of the straight edge with the workpiece.
Forces action on the workpiece
A straight edge tool has horizontal and vertical force components acting on the workpiece with respect to the feed direction. A helical cutter has an axial component parallel to the cutter axis and perpendicular to the feed direction. Likewise, equal and opposite forces are acting upon the tool along the tool path.
The additional axial component provides the mechanical advantage for a helical cutter. Although the direction and magnitude of the forces vary along the tool path during peripheral milling, as the helix increases; the axial force increases, the radial force remains constant, and the tangential force decreases.
In wood machining, all the advantages are generally not noticeable for helices less than 10-degrees for smaller diameter cutterheads such as router bits and less than 20-degrees for larger diameter cutterheads such as planer heads. Additionally, proper clamping, hold-down, or friction between the feed mechanism and workpiece are required to prevent the workpiece's lateral movement.
The axial component allowed for the development of the compression cutter because opposite helices on the same shaft neutralize (balance) the axial component and minimize the effect on the depth of cut. Consequently, the edge chipping or tearout on laminated or unlaminated panels can be minimized. Further, a single helix that lifts or pushes-down on the workpiece may be desirable. A lifting action may help to clear the chips from the cutting zone; whereas, the pushing down action may help hold the workpiece and reduce upper edge tearout.
Rake angles of helical cutters
The oblique rake angle measured in a plane normal to the cutting edge is smaller than the radical rake angle (a) measured in a plane normal to the cutter axis. The chip flow is approximately normal to the oblique cutting edge; hence, the oblique rake angle (w) in that plane is the one which should be considered most when selecting the tool. The simple trigonometric relation between the oblique (w) and radial (a) rake angle is:
tan w = tan r cos a
w = oblique rake angle in a plane normal to the cutting edge, degrees.
r = radial rake angle in a plane normal to the cutter axis, degrees.
a = helix angle, degrees.
The rake angles (w) and (r) are equal only when the helix angle (a) is zero, i.e. a straight edge. An excellent source for milling is “A Treatise on Milling and Milling Machines,” 1951, Cincinnati Milling Machine Co., 910 pp. Cincinnati, Ohio, which unfortunately is out of print, but may be available in some libraries.
As with all milling processes, moderate tool geometry should be selected and maintained. Helices are generally more expensive initially and to maintain, but generally result in higher quality edges and surfaces.
Sometimes helical cutting edges are restricted to lower rake angles. Likewise, the radial rake angle increases as the helix angle increases and the clearance angle decreases. Providing enough clearance is another critical factor when selecting tooling. Optimal machining parameters such as feed speed, rpm, etc., are also required to suit the milling cutter.
Trying different sources of similar milling cutters and keeping accurate tool wear histories is paramount; a wide unseen variation in tool manufacturing, maintenance, and materials frequently exists. As with any machining situation, they can be optimized.
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