In high-speed milling applications, axial chip thinning is often used to increase feed rate with a small end mill. This shop uses that same effect to increase metal removal rate with a standard-size end mill run on a moderate-speed machine.

By Peter Zelinski

As the very name of the shop makes clear, Robertson EDM doesn’t specialize in machining center work. This job shop in Edgerton, Ohio, has four wire EDM machines and one sinker EDM machine, but at present only one CNC machining center—a reliable Mazak VTC-16B vertical that the shop purchased used.

However, as the shop continues to succeed and continues to draw more business just by word of mouth, both the range of work and the range of opportunities continue to grow more broad. To take on large-diameter turning work, for example, the shop recently invested in a Johnford ST-40A slant-bed turning center with 31 inches of swing over the bed from Absolute Machine Tools. The volume of work for its machining center is also increasing, and the shop has met this challenge by implementing high speed machining on this VMC—sort of.
The profile of the inserts
The profile of the inserts blends from a 0.106-inch radius to a 0.5-inch radius. Chip thinning occurs because a light depth of cut meets the insert along this curve, allowing feed rate to increase.

The term “high speed machining,” particularly in applications involving steel, often refers to small tools run at high spindle speeds and feed rates with light depths of cut. Despite the light cuts, machining in this way achieves a high metal removal rate by taking the passes very rapidly. The passes can then become even more rapid when using a ballnose end mill, because a ballnose tool making a light cut takes advantage of axial chip thinning. By virtue of this effect, the chip thickness is smaller than the advance per tooth. This means that the advance per tooth can be increased, and correspondingly the linear feed rate in inches per minute can be increased even beyond what the high spindle speed already allows.

But that’s high speed machining. By contrast, Robertson EDM does not have particularly high spindle speed, nor does it have the complex 3D milling work that could benefit from a small-diameter ballnose tool. However, the chip thinning that increases feed rate in high speed machining is still a real and valuable option for this shop, even with a tool that has a larger diameter. To increase its own metal removal rate, the shop employs a tool design supplied by Iscar that realizes the chip thinning effect. The insert design, called “Feedmill,” features a curved profile that makes this possible. Chip thinning, in other words, does not need a ballnose tool and does not need a small tool diameter. At least this one aspect of high speed machining can be applied in a more standard roughing application on a moderate-speed machine.

This part, which was slightly oversize
This part, which was slightly oversize relative to the work zone, was machined in two setups using two different types of inserted end mills. The table captures the difference in performance the shop realized using the chip-thinning insert.




Inserted tool with insert taking advantage of chip
thinning




Shop’s previous inserted roughing end mill
Tool diameter (inch)
0.75 0.75
Number of teeth 1 2
Coolant Air Air
Overhang (inch) 1.5 1.5
Speed (rpm) 3000 2000
Depth of cut (inch) 0.04 0.05
Width of cut (inch) 0.5 0.5
Feed rate (ipt) 0.04 0.0088
Feed rate (ipm) 120 35
Tool life (pieces per edge) 20 15
Cutting time (seconds) 584 977
This part, which was slightly oversize relative to the work zone, was machined in two setups using two different types of inserted end mills. The table captures the difference in performance the shop realized using the chip-thinning insert.

Chip Thinning In Action
One of the shop’s general partners is Jeffrey Robertson. The local sales and applications representative for Iscar is Greg Mallett. Mr. Mallett was meeting with Mr. Robertson to tool up the new turning center when he saw an application for the axial-chip-thinning insert. Robertson EDM was roughing a fixture component out of cold-rolled steel, and the way the shop was machining this part actually provided an excellent basis for a before-and-after comparison. The part was too big for the machining center’s travels, so the shop was machining it in halves.

After the first half was finished with a 0.75-inch inserted tool, the shop agreed to run the second half with a 0.75-inch tool body using the chip-thinning insert design. The tool with the latter insert cut with one tooth instead of two, and it also took a lighter depth of cut. However, because of the higher feed rate resulting from chip thinning, productivity increased. Tool life increased, too. (See table.)

The curved profile of the insert begins as a 0.106-inch radius, blending into a 0.5-inch radius. A depth of cut light enough to fall along this curve features an advance per tooth that can “cheat” its way higher, because the chip thickness is lighter than the advance per tooth instead of being equal to it.

transmission cover
The transmission cover is another part machined using the high-feed-rate tool. This is the part being machined in the photo at the beginning of the article.

The greater tool life the shop saw can be attributed to the curved profile, too, says Mr. Mallett. The tool life increase that Robertson EDM observed was likely the result of a more stable cut, he says. As is the case with a ballnose tool, the material meeting the curve of the tool at a point below the tool’s full radius exerts a cutting force that is not entirely lateral, but instead pushes up toward the centerpoint of the ball. (See p. 98.) In other words, only some of the cutting force is directed along X and Y. The remainder is directed along Z, the direction of the spindle, which is the most rigid of the three axes. With correspondingly less force directed along the side of the tool, there is that much less opportunity for tool deflection, and the cut is that much more stable.

As a result, capacity has increased on the shop’s machining center. The shop now routinely uses this tool for its steel-roughing applications—the transmission cover on p. 98 is another example of a part for which the tool was suited. As the shop continues to grow, no doubt its number of machining centers will grow as well—perhaps one day to include a high speed machine. For now, however, the additional capacity provided by faster roughing feed rates is enough to meet the need.



What Is Axial Chip Thinning?
ballnose tool

On a ballnose tool, any cut that meets the tool at less than the full radius of curvature will behave differently than a more standard milling cut. The chip thickness and the advance per tooth will not be the same—the chip thickness will be less. The diagram shows geometrically why this is. As a result of the thinner chip, the advance per tooth can be increased to achieve a higher linear feed rate, resulting in a higher metal removal rate for an application that was already employing a relatively light depth of cut.

The effect is most commonly associated with ballnose tools in high speed machining. However, it can also benefit milling cutters using circular inserts, as well as cutters using curved profiles designed specifically to realize a chip-thinning effect (such as the Feedmill tool mentioned in this article).

The diagram also shows the direction of force. The force on the tool shown here is not directed laterally, but instead is directed along the diagonal dashed line from the material up to the centerpoint of the tool’s curve. In other words, instead of being directed all along X and Y, some of the cutting force is directed up into Z, a more rigid axis of the machine, resulting in a more stable cut.

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