Indra’s Drishtikona (Viewpoint)

Manufacturing Technology- Back to Basics, Machining 4

Posted : September 7, 2004 at 8:58 am [IST]

Multi-tasking Turning centers

Multifunction turning centers in single or multiple spindle configurations provide the optimum machining solutions for multiple operations on rotational parts.

60% of turned parts are chucked parts. Rotationally symmetrical-disc type components generally require two/three setups to complete parts on both sides. Turning chuckers with twin spindle configurations, in horizontal or vertical versions today perform all machining on both sides of the parts, which used to take two or more machines. New trends of near-net-shape components are making these machines more popular.

Horizontal front loading chuckers are preferred for components such as hubs, gear blanks, differential cage, etc. up to 200 mm diameter; whereas vertical ones are used for heavier components such as flywheels that provided the benefit of gravity to keep the work in chuck. Gantry loaders assist the loading, unloading as well as turnover functions.

Latest in two-spindle turning technologies:

1. Two-spindle horizontal turning centers: Right side spindle is coaxial with an equally powered left side spindle. Right side spindle directly picks the work piece from the left side spindle. Normally two turrets - one for each spindle- completes the simultaneous machining for both the spindles. Parts with equal or near equal machining at both ends and that involve rotary tooling for milling and cross drilling for operations are better suited for these machines.

2. Sub-spindle turning machines are equipped with a second spindle of a fraction of the capacity of the main spindle. The sub-spindle picks the workpiece from the main spindle. These machines may have one to three tool turrets as well as variety of other tooling arrangements besides turrets. Sub-spindle NC turning machines are the choice if one end of the part is markedly less complex than the other, which is quite often the case. The trend in sub-spindle machine design is towards sub-spindle with nearly half the power of the main spindle. An average improvement of machining time of sub-spindle machines over conventional turning requiring two setups is 20-30%. Setup time improvement over two separate NC lathes is of the order of 30-40%.

Inverted vertical turning chuckers- a new generation has appeared very lately. It is suitable for parts of 200-500 mm diameter such as rotors, hubs, drums and gears for high volume production. The mobile integral motor spindles grip the workpiece from above and the clamping force resists not only the centrifugal and cutting forces, but also gravity. Inverted verticals eliminate a loader and pick up and unload the part from and to the conveyor directly. Work transfer speed from the pickup point to the starting of machining is typically not more than five seconds which is generally 15 to 20 sec. for gantry work-transfer. Chips in inverted verticals fall away from the work and down in chip conveyor.

Center type turning center used for longer shaft type work pieces holds the work-piece between the headstock and the tailstock. Different types of work-holding equipment are in use. For odd shape components, such as knuckle (axle arm) of front suspension system of car/trucks, special work-holding fixtures are mounted on headstock side. Shaft with sufficient bearing face on ends, may be machined in one setup with a holding by face-driver and tailstock center. Four-axis turning centers are becoming increasingly attractive in industry. The machines have two turrets. Each moves independently of the other on its own set of cross slides. Tools on 4-axis CNC twin-turret machine both sides simultaneously and reduce the machining time. CNC center-drive lathe provides another machine configuration for simultaneous machining from both ends for shaft-type parts..

Turning centers today can perform virtually every machining operation including thread rolling, burnishing, back face machining, and even cylindrical grinding. With addition of powered tools in turret and controlled work-spindle rotation axis, secondary operations such as milling, drilling, pocket milling, and key milling etc. can be planned in same setup. The machines can turn, mill, drill and tap parallel to and perpendicular to the spindle axis. Lesser setups automatically improve the quality.

Multitasking turning results in many advantages such as reduced setups, reduced tooling and fixturing, increased work-piece accuracy, and most importantly an even throughput. By moving the machine’s cutting operations around the work-piece, rather than moving the work-piece around the shop to various machines for different operations, crucial work-piece datum is not lost. Reduced work-piece handling also means reduced work-piece marring. As a major technical trend, the turning machines have evolved to a point where they may be called machining center with turning capability. One machine tool does as many operations that formerly required a five-axis machining center and several turning machines.

High speed turning: With excessive centrifugal forces at higher speeds, safety becomes the main consideration. In one case, while machining a small brass part at 4000 rpm, the part flew off a chuck and got embedded into nearby steel structure. Even with well-designed counter-balanced high-speed chucks, machine covers are to be built strong enough to meet eventuality. However, the trend is towards increasing the spindle speed for better productivity. Hard turning, near-net-shape turning, and diamond turning of aluminum components such as pistons are pushing up the turning speeds. High speed turning generates high heat. High-pressure coolant can keep the cutting surface cool and clean from chips. However, high-pressure coolant tends to wash away most of chuck lubrication during machining, and also forces chips and dirt down into the moving parts of the chuck. A completely sealed maintenance-free chuck provides the right answer. Another trend is to incorporate quick-change jaws for easy set up switch over from one part to another.

Accuracy of turning operations is improving because of multiple innovations by machine manufacturers. Different probes at key locations on the machine develop a thermal map and the information is used for compensation. Machine’s stiffness has improved with different materials and designs that provide damping effect and greatly reduce vibration. The present trend is to integrate active balancing system into the spindle itself. The change also improves surface-finish and so also the tool life significantly. Today, a total tolerance of 25 microns and less is easily achievable. In one case, a turning center achieved spindle speed as high as 20,000 rpm with a 4.5-sec acceleration and 3-sec deceleration. The machine’s cycle time was as much as three times faster than those of other lathes. Surface finishes improved by 37% and the tool life increased by 30%. The lathe used liquid-cooled ball screws for 0.00003mm programmable resolution and 0.000005 mm feedback resolution.

With new CNC turning machines, finish turning eliminates down-stream operations like grinding. In one case, the tolerance of 0.003 mm is achieved with a process capability, Cpk at 2. In another case, a manufacturer today finish-turns the faces of the brake discs with very high cutting speed (around 1000 m/min) using high performance tools. Earlier, the brake disc faces were ground after the turning. The switch over has produced significantly improved results. Process capability, Cpk value for surface finish specification (0.4 - 2 Ra) is more than 1.67 and the time cycle varies between 1.50 ~ 1.75 minutes depending on disc size. The grinding of the faces is eliminated completely and so also the high capital requirement for the grinding machine.

- Indra

Category: Machining |