Climb vs conventional milling

Every milling cut starts with a choice that is easy to change and matters a great deal: does the cutter turn with the feed or against it? The answer decides whether each tooth enters the work with a thick chip or slides in at zero thickness, and whether the forces press the part onto the table or try to lift it off.

Two directions

• In climb milling (also called down milling), the cutter rotates in the same direction as the feed at the point of contact. Each chip starts thick and ends thin.
• In conventional milling (up milling), the cutter rotates against the feed at the point of contact. Each chip starts at zero thickness and ends thick.

German texts call them Gleichlauffräsen and Gegenlauffräsen. Turkish course notes also use aşağı (tırmanma) frezeleme for climb milling and yukarı (geleneksel) frezeleme for conventional milling.

Method

Climb (down) Conventional (up)

Radial engagement ae/D

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Pause

Engagement angle φe

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Chip at entry h/fz

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Chip at exit h/fz

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Maximum chip hex/fz

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Why the chip at entry matters

In conventional milling the edge meets the work where the chip thickness is zero. It cannot cut at once: it is forced into the material and rubs, which creates friction and heat. It also slides over a surface that the previous tooth has work-hardened. Sandvik Coromant, Mitsubishi, Kennametal, Ingersoll and Harvey Performance all describe this rubbing at entry and its result: shorter tool life. At the exit, the chip is at its thickest and hottest, which Sandvik notes causes high tensile stress in the edge and can make the chip stick to it.

In climb milling the edge enters at full chip thickness and leaves at zero, so it cuts from the first moment. Kennametal and Harvey note that more of the heat then goes into the chip. Sandvik Coromant sums it up as its “golden rule” of milling: thick to thin.

The price is a heavier impact at entry. Seco notes that the full-thickness entry puts a heavy mechanical load on the edge, though common tool materials usually withstand it.

Forces and backlash

The two directions push the work in different directions:

• Conventional milling tends to lift the workpiece from the table, and its resultant acts against the feed. The fixture must resist lifting.
• Climb milling pulls the workpiece in the direction of the feed and presses it toward the table or fixture. The fixture must resist pulling.

The pull is why climb milling needs a feed drive without play. If the lead screw and nut have backlash, the cutter can jerk the workpiece forward by the amount of the play, as Mitsubishi and the German Wikipedia describe; Harvey Performance warns that breakage can result. Conventional milling pushes the table against the screw, which is why it was the traditional method on older machines. The German Wikipedia, citing the textbook Fachkunde Metall, names ball screws, free of backlash and stiff, as the drive that suits climb milling.

Surface finish

Harvey Performance, ISCAR, DMG MORI and the German Wikipedia agree that climb milling usually gives the better surface. In conventional milling chips tend to fall in front of the cutter and can be recut, marring the finish.

The recommendation, and its exceptions

Every tool maker consulted recommends climb milling as the default. Sandvik Coromant calls it “always the preferred method whenever the machine tool, fixture and workpiece will allow for it”; Seco, Kennametal, ISCAR, Ingersoll and Harvey say the same in their own words.

Conventional milling remains the better choice in a few cases:

Face milling: where to put the cutter

In face milling the cutter usually engages on both sides of its centre, so part of each pass is climb and part conventional. Its position decides the chip thickness at entry and exit.

• Diameter. Sandvik Coromant recommends a cutter diameter 20 to 50 % larger than the width of cut. Its video puts the best position at about 70 % radial engagement; ISCAR, quoted in Canadian Metalworking, gives 70 to 80 % of the diameter; Mitsubishi recommends D = 1.2 to 1.3 times the width of the workpiece.
• Off centre. Sandvik Coromant and Seco advise against placing the cutter on the centre line of the work: a central position mixes climb and conventional forces and gives a thick chip at exit, with a higher risk of vibration. Mitsubishi, by contrast, describes a centre near the middle of the workpiece as most suitable. The makers disagree here.
• Roll into the cut. Sandvik Coromant recommends entering with a rolling (arc) toolpath so that the chip is thick at entry and thin at exit. It reports up to seven times longer tool life in Inconel 718 at 46 HRC with this technique, a figure from Sandvik alone.

The formulas

For a cutter of diameter D at radial engagement ae, the engagement angle, measured from the point where the cutting path is tangent to the finished surface, is

$$\varphi_e = \arccos\!\left(1 - \frac{2\,a_e}{D}\right)$$

and the chip thickness at angle φ, with feed per tooth fz and entering angle κr, is

$$h(\varphi) = f_z \cdot \sin\varphi \cdot \sin\kappa_r$$

The maximum chip thickness for ae < D/2 is therefore

$$h_{ex} = f_z \cdot \sin\kappa_r \cdot 2\sqrt{\frac{a_e}{D}\left(1 - \frac{a_e}{D}\right)}$$

and equals fz · sin κr when ae ≥ D/2.

Example. At ae = 0.25 D, φe = arccos(0.5) = 60° and hex = 2√(0.25 · 0.75) · fz · sin κr ≈ 0.87 · fz · sin κr: the chip never reaches the feed per tooth.

The direction of rotation does not change these numbers; it decides whether the edge meets the thick end of the chip first or last. The milling calculator applies the same formulas to set the feed for a target chip thickness.