Making the Case for B-Axis Machining
As precision part designs become more complex and the economic pressure to complete more operations in a single setup increase, turning center designs are changing. One of the recent innovations to appear on the scene is a B axis.
The case for choosing a B-axis option should be made by assessing the parts that will be, or may be, run on the machine. If the machine will be running a long-term job that has a single angled hole or feature, then perhaps an adjustable angle drill holder may suffice. But if multiple angled features are present on the part, or if an angled feature must be generated by axis interpolation, then a B axis becomes necessary if elimination or reduction of secondary operations is desired.
Additionally, even if angled features are not present on the current parts the customer is running, having the ability to machine more complex parts can help bring more opportunities in the door.
One application that presents challenges for manufacturing is bone screws. With increasingly aggressive helix angles for multiple-start thread whirling, having a true B axis presents a programmable solution to variable thread leads and higher helix angles than most mechanically adjustable thread whirling units. This also reduces the setup time between differing bone screw configurations, since the helix angle, or B-axis angle, is now controlled by the program.
Responding to the needs of the market, Traub has added a B axis to its sliding/fixed headstock automatic 32-mm TNL32-7, a seven-axis lathe. This expands its suitability for handling challenging precision machining tasks. With the additional B axis in the upper tool carrier, which can pivot through 100 degrees, highly complex parts with complicated contour elements can be machined along with additional operations such as milling, drilling, lateral drilling or transversal threading at any preferred angular position. For parallel main spindle operations, it used to be that angled toolholders with typical turret positioning were necessary—now the turret simply pivots, controlled through the CNC.
Additionally, it takes only a few steps for all variants of the TNL32 series to convert the lathe with sliding headstock to fixed headstock turning mode and vice versa. The extremely long Z-axis travel of the headstock ensures proper positioning of the main spindle, either for sliding headstock turning or fixed headstock turning.
As with all multitasking machine tools, simultaneous machining with multiple tools ensures and promotes high productivity levels; as many as three tools are used simultaneously on two spindles.
The TNL32-7B features seven linear axes, a ten-station upper turret with a 100-degree B axis, a nine-station lower turret with an integrated counter spindle, and eight backworking positions, four of which can support driven tools at as high as 12,000 rpm. The TNL32-7B meets the precision machining needs of many industries from the hydraulics industry to medical technology, especially for machining small, geometric and highly complex parts with a maximum bar clearance of 32 mm.
The turrets are indexed through an NC rotary axis that controls movement via an internally meshing planetary gear. This allows the turret to be indexed to any preferred position without a mechanical locking mechanism being necessary.
The free positioning of the turret makes multiple-tool assignments possible on each station so that the top tool carrier can be equipped with as many as 30 tools, and as many as 27 tools on the lower turret. Since mechanical locking is no longer necessary, chip-to-chip times of 0.8 second are typical.
Reliable off-center, parallel axial drilling is accomplished with a swiveling counterspindle fitted onto an XYZ cross-slide that simultaneously also carries the lower tool turret across large axis travel distances.
A total of four stations are provided with an internal coolant supply. Each station can be controlled individually so that cutting oil is supplied selectively. The large travel distances of the counterspindles enable stations to be occupied by more than one tool, such as a dual-drill holder.
Another innovation is a refined drive scheme for the toolholders that save chunks of cycle time. Called “dual drive,” two separate drivetrains in one turret keep cycle times to a minimum. It works like this: While one tool is in use, the tool for the next operation can be accelerated to the desired speed during main machining time, so it is immediately available at full speed after turret indexing. Jerky accelerations and acceleration times are effectively eliminated. In addition, toolholder wear is drastically reduced because of the low acceleration values. Chip-to-chip times with live tools are less than 0.8 second.
Integrated workpiece removal allows the workpiece to be picked up and deposited in a finished-parts bin, or on an available timed parts conveyor for uninterrupted continuous operation.
Quality is the most important consideration when choosing a turn-mill or other multitask machine. When making a major capital investment and then adding 10 to 20 percent to completely tool it, a multitasking machine should provide not only efficient tool positioning options, but should also run with near perfect uptime and provide the mechanical support to quickly apply the cutting tools at optimal speeds and feeds. The machine design should deliver the rigidity to dampen vibration, compensate for thermal growth and provide accurate, precise motion control.
The following advantages of programmable B axis affect setup time and cost optimization:
Setup time optimization: Setting up angle-adjustable toolholders is no longer necessary;
Cost reduction: Expensive angle-adjustable and 90-degree angled toolholders are omitted;
High workpiece quality: The omission of angle-adjustable and angled toolholders creates more stable machining conditions in the machine system;
Complexity gains: Because of additional operations in any angle positions, highly complex workpieces can be produced more flexibly and efficiently.
Precision machining is application-driven. It’s a safe bet that application will become increasingly complex in the future. Machine tool capability, including the use of B axis, helps drive the efficient manufacturing of these workpieces.
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