When routing, there are two methods of running the bit along the workpiece (or the workpiece across the bit when using a router table). You can either use the conventional cut or the climb cut. These terms refer to the relative motion between feed direction and bit rotation. Both methods have advantages and disadvantages.

Conventional Cutting

The conventional cut is by far the most common method used and is also referred to as a normal cut. In this type of cut, the workpiece is fed against the rotation of the bit, as shown in Figure 1.

 Figure 1: Table set-up (left) and freehand set-up (right) for conventional cut.
 Arrows show bit rotation and feed direction. (Chips exaggerated for clarity.)
 

In a conventional cut, the bit is cutting on exit from the workpiece. Consequently, the cutting action tends to pull the workpiece in towards the bit. To resist this force, a fence or pilot bearing is used to limit the depth of cut. The inward force also means that the bit is always in contact with the workpiece, resulting in a high-quality cut with minimal chatter.

Cutting on the exit stroke has a disadvantage. For the bit to pull itself into the workpiece, it must be pushing outward on some of the fibers. In porous woods such as oak, this force can exceed the fiber binding strength, resulting in wood failure and tear-out, as shown in Figure 2.

Although tear-out can be alleviated by using a climb cut, you can also minimize tear-out by taking several light conventional cuts. By reducing the cutting depth, the fiber separation force imparted by the bit can be reduced, as shown in Figure 3.


   Figure 2: Wood    failure from    conventional cut.

 Figure 3: Minimizing fiber separation force.
       A = Net force imparted by router bit.
       B = Fiber separation component.
       C = Fiber compression component.
 

Climb Cutting

Feeding a workpiece with the rotation of the bit is known as climb cutting. An example is shown in Figure 4.

Warning: A climb cut on a router table is extremely dangerous and should never be attempted.

 
 
Figure 4: Freehand set-up for climb cut. Arrows show bit rotation and feed direction. (Chips exaggerated for clarity.)   Figure 5: Router bit "climbing" along workpiece.

 

In a climb cut, the bit is cutting on entry and has the effect of pushing the router away from the workpiece. If not resisted by the operator, the bit can exit fully from the cut and "climb" its way along the workpiece, as shown in Figure 5. For a router running at 10,000 rpm, this can mean a sudden loss of control and high risk of injury.

As the bit is cutting on entry, it compresses the fibers before cutting them. This is effective in preventing tear-out. However, climb cutting can create chatter, particularly when a straightfluted bit is used (see Figure 6). A spiral-fluted bit will induce much less chatter.

The chatter is a result of the bit pushing away from the workpiece each time a cutting edge strikes the wood but coming back in when the cutting edge exits. This cycle repeats itself twice per revolution (for a double-fluted bit).

 
Figure 6:
A spiralfluted bit (right) will induce less chatter than a straightfluted bit (left).
B.K.H.
   
 

 
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