Climb vs Conventional Milling    (from CNC Cook Book Blog http://blog.cnccookbook.com )

While many CNC'ers have gotten in the habit of always specifying climb milling, there are times to climb mill and there are times where conventional milling is preferred. Before we get into when to use each, let's have a quick definition of the differences.

Climb milling is when the direction of cut and rotation of the cutter combine to try to "suck" the mill up over (hence it's called "climb" milling) or away from the work. It produces the best surface finish. Here is a diagram showing climb versus conventional milling for a number of orientations:

Arrows show workpiece motion, not spindle motion!

Keep in mind that for this illustration, it is the workpiece that moves, not the spindle. On some machines, like a gantry router, the spindle moves, so the labels would reverse. I keep it straight by thinking of the spindle as a pinch roller that can either help move the workpiece in the direction it was already going (climb milling), or that might fight that movement (standard or conventional milling).

Try the experiment on your mill of cutting both ways and you'll see that climb milling is a lot smoother and produces a better surface finish (most of the time, there are times when conventional gives a better finish, see below). Note that depending on which way you are milling, you will need to make sure your workpiece is supported well in that direction.

Characteristics of Conventional Milling:

• The width of the chip starts from zero and increases as the cutter finishes slicing.
  
• The tooth meets the workpiece at the bottom of the cut.
  
• Upward forces are created that tend to lift the workpiece during face milling.
  
• More power is required to conventional mill than climb mill.
  
• Surface finish is worse because chips are carried upward by teeth and dropped in front of cutter. There's a lot of chip recutting. Flood cooling can help!
  
• Tools wear faster than with climb milling.
  
• Conventional milling is preferred for rough surfaces.
  
• Tool deflection during Conventional milling will tend to be parallel to the cut
  

Characteristics of climb milling:

• The width of the chip starts at maximum and decreases.
  
• The tooth meets the workpiece at the top of the cut.
  
• Chips are dropped behind the cutter--less recutting.
  
• Less wear, with tools lasting up to 50% longer.
  
• Improved surface finish because of less recutting.
  
• Less power required.
  
• Climb milling exerts a down force during face milling, which makes workholding and fixtures simpler. The down force may also help reduce chatter in thin floors because it helps brace them against the surface beneath.
  
• Climb milling reduces work hardening.
  
• It can, however, cause chipping when milling hot rolled materials due to the hardened layer on the surface.
  
• Tool deflection during Climb milling will tend to be perpendicular to the cut, so it may increase or decrease the width of cut and affect accuracy.