High Feed Machining – Science of high metal removal rate

By Bhalchandra Chorghade, Editor, CNCTimes.com

We are amid the fourth industrial revolution (Industry 4.0). Manufacturing industry is bracing new technologies, helping them in improving machine utilization. CNC mills and CNC lathes are being built to perform metal cutting operations at never before speeds. As a cutting tool partner, Tungaloy has enjoyed developing equally good metal cutting tools for these new generation CNC mills and lathes.

The three most important parameters to form a chip out of a parent material are:

Cutting speed (Vc – meters/min or SFM)

Feed (fz – mm/rev or Inch / rev)

Depth of cut (DOC – mm or inches).

All three when applied at the right proportion, based on the material being cut, produce the most desirable chip. Optimizing the three to gain the optimal Metal Removal Rates (MRR – cc/min or cf/min), has been the endeavor of every tool engineer. This is the way to improve utilization of a complex CNC mill or a CNC lathe.

In the middle of the last century HSM (High speed machining) evolved as a metal cutting process, which suggests beyond certain cutting speed the heat generation starts declining. HSM employs high spindle speeds and high table feeds, taking shallow cuts. This was a big change in the way metals were processed into complex parts. With the development of more complex CNC milling machines and CNC lathes, the cost of such machines also escalated.

By utilizing HSM, smaller diameter tools started being used in place of large diameter tools.

However, HSM became and still remains popular for finishing operations of tough to machine materials and hardened steels. HSM yielded better results with surface quality and profile machining operations, which required blending of fine curves to improve the aesthetics of the machined part, however, it has not been popular as a process for roughing to semi finishing operations. HSM also remained popular for use with smaller diameter tools and end mills, mostly on hardened steels, especially carbide ball and bull nose end mills.

What is High Feed Machining?

High feed machining is associated with higher metal removal rates (MRR). Working with low depth of cuts and high feed per revolution and at applicable cutting speeds for a given material, high feed machining helps in –

Keeping the chip load low

Optimizing the heat generation and dissipation of heat thru the chips formed.

High feed machining relies on the chip thinning effect. The below images of a 90° shoulder mill, a 45° face mill and a 15° – 17° high feed cutter, provides two distinct points: 

To produce the same thickness of chip we need to increase the feed as the approach angle reduces. This means, for the same amount of heat disposal thru the chip, we can employ a high feed cutter at higher feed rate (almost 3-4 times of a 90° shoulder mill).

The resultant cutting force always acts perpendicular to the cutting edge. Therefore, in a 90° shoulder mill, the resultant cutting force is perpendicular to the tool axis. This leads to tool deflection and chatter. In a face mill, the radial and axial forces are equal, as the resultant force is at 45° to the tool axis. And in a high feed cutter, the axial force is higher than the radial force.

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