One of the basic concepts to understand in any milling operation is Cut Direction. It can be characterized by how the flutes of the cutting tool engage the stock material and form the chip that is removed during cutting. In many of MecSoft CAM’s 2½ & 3 Axis toolpath strategies you will see that Cut Direction is defined by selecting one of three options, Climb, Conventional or Mixed. Let’s take a look at the characteristics of each option.
Climb Milling
Consider each chip being removed as a wedge of material that has a wide end and a narrow end. In Climb milling (also referred to as a down cut) the relationship between Spindle direction and Cut Feed direction combine in such a manner that the wide end of the chip is removed first and the narrow end removed last. This means when the cutter tooth first comes in contact with the workpiece it removes the maximum amount of material (chip width being maximum). As the cut progresses, the amount of material being removed decreases and just before the flute loses contact with the workpiece the chip width is zero. This produces a climbing effect of the cutting tool on the workpiece material as shown in this illustration. The linear distance of the wide end of the chip is referred to as the Feed per Tooth.
Because chips are discarded behind the cutter, the chances of the cutter re-cutting the discarded material is reduced. Climb milling also reduces wear on the cutting tool because rubbing (i.e., contact without cutting) on the workpiece is reduced. Also, the maximum force on the workpiece occurs when the cutter is taking the biggest bite out of the workpiece. The force at this point is directed straight down on the workpiece. This keeps the workpiece more stable with less stringent work fixturing required. However, due to the sudden increase in the force, active management of tool backlash is necessary.
Climb milling is the preferred method of cutting since it results in a better surface finish. However due to the larger forces encountered at the beginning of each cut, machines and spindles have to be more rigid. Conventional Milling
In Conventional milling (referred to as an up cut) the narrow end of the chip at the bottom of the cut is removed first and the wide end removed last. This produces the up cutting effect of the cutting tool on the stock material as shown in this illustration. The Feed per Tooth is not fully realized until the end of the chip removal.
Because chips are discarded upward in front of the cutter, re-cutting of the chips can occur, which can result in a rougher surface finish. Conventional milling can also increase wear on the cutting tool because of increase of rubbing between the tool and the workpiece at the start of the cut where the cutter is not removing any material.
The maximum force on the workpiece occurs when the cutter is taking the biggest bite and then losing contact with the workpiece. At this point the force on the workpiece is directed upwards. This requires the need for more stringent work fixturing than climb milling. However, the cutting forces go from zero to maximum without any sudden increases resulting in less tool deflection.
Even though climb milling is preferred, there are times when conventional milling is used such as when machining materials which have rough or hardened surface finishes. Climb milling on these materials can result in undue forces when the cutter teeth make first contact with the workpiece material. Mixed (Climb/Conventional) Milling
The Mixed option simply means that the CAM software uses a combination of the two milling directions within the same toolpath. This option is typically used in finishing cuts when very small amounts of material are being removed. A Mixed cut direction can reduce tool travel and the number of retracts and transfer motions, thus saving machining time.
Let’s Review:
- Consider each chip being removed as a wedge of workpiece material.
- The distance at the widest end of the chip is the Feed per Tooth.
Climb Milling (Down Cut):
- Less re-cutting of chips, higher quality surface finish.
- Less wear on the cutting tool (tool life is extended).
- More tool deflection encountered but less fixturing needed (cutting force is directed onto the workpiece).
- Machines and spindles need to be more rigid.
Conventional Milling (Up Cut):
- More re-cutting of chips, lower quality surface finish.
- More wear on the cutting tool (tool life is reduced).
- Less tool deflection encountered but more fixturing is needed (cutting force is directed away from the workpiece).
- Machines and spindles can be less rigid.
Mixed (Climb/Conventional) Milling:
- A combination of both methods is used within the same toolpath.
- Typically used in finishing toolpaths.
- Can reduce tool travel and machining time.
