CourseWiki/Machining/Milling

Climb and Conventional Milling

Climb and conventional milling name the cutter's rotation relative to the feed; that relationship changes chip formation, force direction, chip evacuation and the importance of backlash.

Climb versus Conventional Milling

A milling cutter turns while the work and cutter move past each other. At the part of the cutter that is making the cut, climb milling means the cutting edge moves in the same direction as that relative feed; conventional milling means it moves against the feed. Those two relationships are the whole distinction—not a universal left or right direction on the machine.

Climb milling: Also called down milling. Where a tooth is cutting, it moves the same way the work is fed. Each chip starts thick and ends thin.
Conventional milling: Also called up milling. Where a tooth is cutting, it moves against the feed. Each chip starts thin and ends thick.

Feed per tooth, width of cut (radial engagement) and axial depth of cut are defined together on Feeds and Speeds. This page uses them only to show what changes when the direction changes.

How to read the drawings on this page. Each drawing is a slice straight across the fluted part of an end mill, seen looking down its axis from the spindle; the small side view in the corner marks where the slice is taken, the way a section view does on a drawing. This sketch places the cutter’s axis horizontally, with the table below it; screen-down is toward that table. It is not a top view of a vertical spindle. From the spindle end a standard right-hand cutter turns clockwise. The cutter stays where it is and the work is fed past it, the way a mill table carries the work. Every Work feed arrow is the work’s motion. On a machine that moves the cutter instead, nothing changes: the cutter simply travels the opposite way to that arrow.

The cutter is the same in both modes and spins the same way. What switches climb to conventional is the direction of feed relative to the side of the cutter that is doing the cutting. The down/up force arrows belong to this horizontal arrangement. A top view of a vertical end mill would show forces across the table, not axial force into or away from it.

Climb and Conventional Milling Lab; Interactive

Climb and Conventional Milling Lab

The dashed gold outline is the slice of metal the cutting tooth has taken so far, drawn where it was. The solid gold shape is that metal as a chip, curling off the cutting edge into the flute. The curled chip is scaled to fit the flute at this exaggerated feed; its drawn size is not a thickness measurement. Play it, or drag the tooth through the cut, and use the dashed slice to compare thickness.

Both cutters turn clockwise, and each cutting tooth is about a third of the way through its cut. In climb milling the work feeds left and the tooth has already taken the thick end of its chip. In conventional milling the work feeds right and the tooth is still in the thin end. The graph says the same thing as a line: climb runs down from its peak, conventional rises to it. The feed per tooth is drawn many times larger than it would be on a machine so the chip can be seen at all. The dashed outline is the slice as the tooth cut it; the solid chip is the same metal curled into the flute.

Six things to try in the demo; Checklist

Six things to try in the demo

Choose Compare both and step until each tooth has only just entered the work. Which chip is already thick, and which is barely there?
Drag Tooth position in the cut from 0% to 100%, once in each mode. Watch the uncut-thickness readout: in climb it runs down from the peak, in conventional it runs up to it.
Set the width of cut to 50%. The tooth now cuts through a quarter of a turn and the chip reaches the full feed per tooth. Drop it to 10%: the peak falls to 60% of the feed. At the same feed, a narrower cut makes a thinner chip.
Raise the feed per tooth and watch the dashed uncut slice grow thicker. Its thick-to-thin trend stays the same. The curled chip is scaled to fit the flute and may stop growing on screen; compare thickness using the dashed slice.
Follow one chip from start to finish. Pause just before a tooth leaves the work, then step forward. In which mode is the thick end of the chip at the curled tip, and in which is it still on the edge? Where does each chip go once it is free?
Watch the red-orange arrow, the pull of the cut on the work. In climb it pulls the work the way it is already going and presses it down. In conventional it pushes back against the feed and lifts. Keep that in mind for the card on backlash.

Chip Forming Characteristics

A chip does not stay where the slice was. The cutting edge shears it off the work, it slides along the face in front of the edge and curls into the flute, and the flute carries it round until it is thrown clear. The drawings compare chip paths in the cross-section of this horizontal-cutter setup; they do not predict a universal chip-flight direction.

Swipe the four moments sideways on a narrow screen, or focus the drawing and use the arrow keys to scroll.

One chip from each direction, at four moments. In climb milling the chip is finished at the finished surface, where the tooth leaves the work, and the end that curled furthest is its thick end. It is thrown out behind the cutter, over the surface that has already been cut. In conventional milling the chip is finished at the top of the cut with its thick end still on the edge, and this cross-section carries it toward the uncut side.

Reading the cutter

Cutting edge: the tip of each tooth, the only part that touches the finished surface. The tooth that is cutting is outlined in blue.
Rake face: the face just in front of the edge. The chip slides along it as it forms.
Flute: the curved valley in front of each tooth. The chip curls in it and is carried out of the cut in it, so a flute packed with chips cannot take the next one.
Core: the solid centre, hatched because the drawing cuts through it. In this illustrated family, more flutes leave a thicker core and less room for chips. Actual proportions depend on the tool design.

The chip is drawn shorter and thinner than the slice so it fits the flute at a feed drawn many times too large. A continuous metal chip usually becomes shorter and thicker through deformation; IIT Kharagpur’s chip-formation lesson explains the change. The path it takes is illustrative. Actual chip motion depends on the rotation/feed relationship, engagement, flute helix and chip space, material and chip-breaking geometry, and coolant or air delivery. Helical flutes can also move chips along the cutter axis; this in-plane drawing does not show that axial evacuation. The cutter’s flutes and chip space follow Harvey Performance’s end mill anatomy.

Starting thin means rubbing; starting thick means a bite

A real cutting edge has a rounded tip. When the uncut layer is extremely thin, the edge can push and rub the metal before a chip forms. The peripheral cuts shown here make that difference visible.

Conventional milling starts every chip at zero, so every tooth rubs before it bites. Rubbing makes heat and wears the edge. Look at where the tooth enters, too: on the finished surface, the one you are keeping.
Climb milling starts the chips shown here at their thickest, so the tooth bites at once, with a knock each time a tooth enters.

That is the main reason climb milling is often the preferred direction when the machine can hold the work steady against it. Whether it can is the next card.

When a rough or hard exterior can change the choice

Conventional milling can be useful in a qualified first pass on a rough surface because its gradual entry can reduce the impact on a brittle cutting edge. Tooling guidance also documents specialized exceptions such as machining through titanium alpha case before returning to climb milling. That is not a rule to choose conventional whenever stock has rust or scale: the layer’s hardness and abrasiveness, the work material, tool grade and geometry, engagement, fixturing and the approved process all matter. Use the tooling manufacturer’s data and the method approved for the machine.

Seco Tools’ mechanical-load guidance discusses rough-surfaced material and gradual entry; Kennametal’s titanium guidance gives the material-specific alpha-case exception.

Which way the cut pulls on the work

Climb: the teeth drag the work along the way it is already being fed, and tend to push it down onto the table in the peripheral arrangement shown.
Conventional: the teeth push back against the feed and tend to lift the work in this arrangement, so whatever holds the work has to resist that lift.

Backlash: why climb milling depends on the machine; Watch out

Backlash: why climb milling depends on the machine

A feed screw moves the table by turning inside a nut. Unless the drive is built to take it out, there is some clearance between the screw’s thread and the nut. That clearance is called backlash, and it lets the table shift a little while the screw stands still.

The clearance is drawn far larger than it is, and a real drive engages many turns of thread, not one. What matters is which side of the gap the nut ends up on.

Conventional: the cut pushes the table back, against the feed. That holds the nut against the flank of the thread the screw is driving with, so the gap stays on the far side and the table moves only as fast as the screw turns.
Climb: the cut pulls the table forward, the way it is already moving. If there is clearance, the table can slide across it ahead of the screw, and the cutter takes a bigger bite than the feed was set for, all at once. That sudden, oversized bite is hard on the cutter and on whatever is holding the work.

This is a question to ask about a machine, not a rule. How much clearance a feed drive has, and whether it is built to take it out, differs from machine to machine. This page does not tell you which direction to use on any machine you will meet. Ask the person responsible for that machine.

Why some machines climb mill without trouble

A feed drive can be designed to reduce backlash, for example with a preloaded ball nut. The important question is whether the machine controls that movement, not simply whether it is called CNC. The chip is the same shape either way; what changes is whether the machine can hold the table against the pull of the cut.

Sources: Sandvik Coromant: down milling versus up milling explains chip formation, rubbing and the backlash condition. Harvey Performance: climb versus conventional milling describes the lifting and downward force tendencies in peripheral horizontal milling. The arrows illustrate directions; they do not calculate force magnitudes.

A slot is both at once

When the cutter is buried across its whole width, cutting a slot, each tooth enters at one wall and leaves at the other. Its chip starts thin, is thickest halfway across, and ends thin. The first half of the sweep moves against the feed, like conventional milling, and the second half moves with it, like climb milling, whichever way the work is fed. That is why the demo stops at half the cutter’s diameter: past that point the two can no longer be pulled apart.

Appears in

DET 330 DET Machining Fundamentals