Difference Between Up Milling and Down Milling

Milling is one type of conventional machining process for generating flat or stepped surfaces. In peripheral milling, cutting velocity is imparted by rotating the milling cutter about a fixed horizontal axis; whereas, feed rate is imparted by moving the workpiece (basically worktable) against the stationary milling cutter. Since both cutting velocity and feed rate are vector quantities, so based on their mutual direction, peripheral milling can be classified into two groups—up milling and down milling. Each of them has specific advantages and disadvantages, as illustrated below. Various similarities and differences between up milling and down milling are given in table form in the following sections.

  • Up Milling or Conventional Milling—when feed direction is opposite to the cutter rotation (at the point of engagement). Here uncut chip thickness increases with cutter rotation.
  • Down Milling or Climb Milling—when feed direction is along the direction of cutter rotation (at the point of disengagement). Here uncut chip thickness decreases with cutter rotation.

Similarities between up milling and down milling

  • Irrespective of up or down milling mode, three process parameters (speed, feed and depth of cut) must be provided for machining.
  • In case of full depth end milling operation (slot cutting), both up-milling and down-milling occur one after another in every rotation for each tooth.
  • Same milling cutter can be applied for both the cases; however, the orientation may be different.

Differences between up milling and down milling

Up Milling Down Milling
In up milling, cutter rotates against the direction of table feed. In down milling, cutter rotates along the direction of table feed.
Chip load on teeth (or uncut chip thickness) increases gradually from zero at the point of engagement to maximum at the point of disengagement. Chip load on teeth (or uncut chip thickness) decreases gradually from maximum at the point of engagement to zero at disengagement.
Here, tooth experience gradual loading as contact starts with a line. Here, tooth experience impact loading because of sudden contact with full depth.
Cutting force is directed upward so it tends to lift off the workpiece from the worktable. This necessitates rigid and expensive fixture for mounting the workpiece. Cutting force is directed downward and tends to press the workpiece, which is advantageous. So cheap fixture can be employed.
Thin parts may get distorted due to upward cutting force. It is suitable for milling thin parts.
Due to gradual buildup of chip load, heat generation is also high. So there exists a tendency of chip welding with the teeth. Here tendency of chip welding is less but chip re-deposition on finished surface occurs frequently, which may degrade quality of finished surface.
Burr is formed only on unfinished surface, which is subsequently removed in the same pass. Burr is formed at finished surface, which may degrade cut quality.
At the starting of engagement, teeth rub with lay or feed marks on the machined surface. Thus surface finish improves. No such rubbing action takes place.
However, rubbing during engagement results in heat generation and thus work hardening. Chance of work hardening is minimal.
No chipping action occurs on teeth. So tool wear rate is quite low. Due to impact loading at engagement, chipping occurs and this may cause notch wear. Thus tool wear rate is high. Even there exists a chance of catastrophic failure of the teeth.
No backlash eliminator is required. Backlash eliminator is required, especially if the milling machine is an older one.
If the workpiece or cutter material is brittle (such as ceramic), up milling is always preferred. Down milling is not a good choice if the workpiece or cutter material is brittle.
Surface finish produced in up milling is not so good. Usually down milling produces better surface finish as compared to up milling.

 

References

  • Book: A Textbook of Manufacturing Technology: Manufacturing Processes by R. K. Rajput (Laxmi Publications Private Limited). Buy this book
  • Book: Elements of Manufacturing Processes by B. S. N. Parashar and R. K. Mittal (PHI Learning Private Limited). Buy this book