• Milling tools can have multiple inserts per flute which is costly, while the aptly named twin cutter only requires two. • A consistent inline feed requires less programming than the stepped removal required of an end mill or multiple boring bars. • The two-edge effective tool enables faster feed rates and reduces cycle time. • Perfect roundness is easier to achieve more consistently in preparation for finish boring. Twin cutters are most often used for rotationally symmetric roughing where the edges are balanced in both height and diameter. This is an ideal arrangement for light to medium cuts, because the strength and rigidity of the direct balance makes very high spindle speed and feed rates possible. Sounds good, right? It gets better. The real power of twin cutter is realized when it’s used outside of its balanced applications, most notably in stepped cutting. Also known as double offset roughing, this is when the insert holders are not balanced in height and/or diameter – a lead insert is set to one diameter and the other is set slightly back, to the wider, desired final diameter. Stepped cutting effectively cuts two larger diameters with two different inserts in one operation. This does affect feed rate, but time is made up in the increased material removal, elimination of tool changes, presetting and programming. When it comes to depth of cut, stepped cutting offers a 60 percent to 100 percent increase in the stock allowance of the twin cutter in its balanced configuration. The setup is best for through-bores, where a stepped profile is no concern. A twin cutter boring head took a Ø5.38" bored hole to Ø6.23" on one pass and reduced the cycle time to less than 40 min. compared to the previous
CUTTING TIME COMPARISON
140 120 100
MILLING BORING
MILLING 2" Face Mill, 5 Flute Feed/Tooth: 005"
MATERIAL: 4140 STEEL STARTING HOLE: 3" FINAL HOLE: 4"
80 60 40 20 0
ROUGH BORING 4" 319 SW, Step Cutting Feed/Tooth: .014"
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12 15
BORE DEPTH (IN)
As bore depth increases, the cycle time advantage of a twin cutter becomes exponentially more advantageous compared to circular interpolation.
these scenarios there’s a constant single-side load on the tooling and spindle as the axes are changing and the tool is fed in – a potentially harmful force for even the most robust machine tool and its bearings. That brings us to boring as an option. Most often, this involves a starter hole created by a drill and a series of boring bars set to increasing diameters for stock removal one pass at a time. This will yield a straight and accurate hole, but it’s a pricey and relatively expensive cycle with so much tooling and time involved. All of these aforementioned approaches may be suitable for one-offs or small batches, but it would be difficult for a shop to rely on any of them for consistency and efficiency in a production environment. Fortunately, another technique comes to us from the boring world that I firmly believe is best for holemaking production. It doesn’t require elaborate setups or creative cycles, but rather a relatively simple and affordable tool that can be used on most any machine, CNC or not. The addition of a twin cutter boring head to a shop’s tooling arsenal opens the door to multiple holemaking options. Once thought of only as light-duty, semi-finishing tools, today’s high-performance twin cutter boring heads have proven they can deliver in holemaking operations. Quite simply, once a starter hole is established with a drill, a twin cutter is plunged axially to remove the rest of the stock. This offers several natural advantages over helical interpolation and other similar processes: • The plunging motion results in axial load as opposed to radial, preserving the machine and spindle components. • Superior load balance in the cut makes for reliable and consistent performance, especially noticeable in long-reach applications.
interpolation method, which took four hours.
Stepped cutting is especially effective for long- chipping materials because the staggered cutting reduces chip length. With such a high volume of stock being removed, regardless of material type, chip removal is a key consideration in stepped cutting; high pressure coolant may be necessary. It’s also important
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