Quick-change tooling turns the trick.
When Landis, Division of Western Atlas Inc, Waynesboro, PA, decided to expand its turning capacity due to increased demand from the automotive industry for its high-end grinders, it faced the classic choice: add several smaller lathes or buy a single big-ticket CNC turning center.
A second question proved to be equally important: how best to tool the new equipment?
The decision Landis made provides useful insight into how to meet today's increasingly capital-intensive, capacity-pressed metalworking business.
"It was the classic question we all face today," says Jeff Minnick, machining shop supervisor for Landis. "The bigger-ticket machine delivers more capacity per dollar of investment and promises a higher economic return, but brings with it higher risk. More than ever before, we need to monitor not only productivity and profitability, but also our return on capital, as well," he explains. Moreover, the choice would place a higher premium on production planning to keep the machine busy.
Landis decided to purchase a four-axis, dual-turret Giddings & Lewis SmarTTurn lathe equipped with auxiliary spindles to handle the rotating tool operations. The upper turret is a 12-station unit combining both rotating and stationary tools. Ability to run rotating and stationary tooling on the same machine means more time saved and fewer chuckings for secondary operations. The lower turret currently is holding stationary tools only.
The original tooling recommendations from the machine tool builder included individual collets. "We felt we had to go beyond that to fully utilize the technology represented by this new lathe," Mr Minnick states.
Landis tooled all turrets and auxiliary spindles with Sandvik Coromant Capto quick-change tooling. The result: a potential 25% increase in turning capacity, with downtime for tool changes held to 5% to 10%.
Quick-change tooling keeps machine utilization high, says Mr Minnick. "With conventional collet-type and square-shank tooling of the type we've used before, dead time on this machine potentially could have been closer to 25%." The reduction of dead time due to the quick-change tooling is expected to cut the return on investment (ROI) period for the machine by nearly half, enabling Landis to put off the day until it has to add the next machine.
The turning mix
The very nature of the Landis turning operation made it especially prone to high levels of machine dead time. The workload consists of more than 100 different short-run, high-precision spindles, wheels, and cranks that go into the Landis grinders. A typical part may require six different turning operations plus a couple of milling or tapping steps as well.
As a result, the turning center may turn out four or five different jobs every day, each involving job-to-job changeovers on both the stationary and rotating tooling stations. Materials are mostly Nitralloy Cr/Mo/Al steels, tough and very prone to work hardening, as well as cast iron parts.
Landis wanted to do all the machining on a part with a single chucking - thus, the auxiliary spindles for tapping and light milling are right on the turning center. "We focus on minimizing both toolchanging and part handling, each of which attacks uptime," says Mr Minnick.
The critical issue for Landis became selecting the right quick-change tooling from the half dozen or so types available.
Landis' wish list
Available tooling systems were evaluated against criteria that Landis had established for its tooling system:
* Changetime not to exceed 30 sec;
* Ability to withstand high cutting forces of full-depth roughing of tough work-hardening steels at optimum speed;
* Repeatability of 0.000 078[inches] without offsetting or test cuts;
* Fit in tight quarters created by an extra large 24[inches] chuck;
* No interference problems between turrets and chuck;
* Rigidity to handle deep bores with aspect ratios up to 15:1; and
In addition, the quick-change tooling, as well as inserts, would have to be available locally, and the supplier would have to demonstrate that there was, in fact, a large installed base of users, necessary required technical support, and likelihood that its tooling line wouldn't become obsolete.
Narrowing the choice
Several quick-change systems met the criteria for changeover speed, repeatability, and local availability of parts and inserts. According to Mr Minnick, only Coromant Capto met the entire list.
"The high-speed machining requirement was essential, and not every quick-change system out there can withstand the cutting forces generated in full-speed rough turning of steel," he explains.
Another must for Landis was a quick-change system that covered all of its machining needs. "We were making a major commitment to a tooling system that we would be depending on for the long pull. We didn't want an untried system, but we did want a system that would reflect the very latest technology."
The Sandvik Coromant system is installed in more than 5000 machines worldwide and includes more than 2000 components - covering both stationary and rotating tool applications - with 600 more to be added this year, Landis learned.
Landis' experience with both Sandvik and its distributor also carried weight in the decision. "We had invested in Varilock tooling over the previous decade and were well satisfied; however, the old tooling could not meet our criteria, including the clearance issue."
Because of its short overhangs, the Coromant Capto tooling system fit easily into the restricted tooling space created by the extra-large chuck. "The 24[inches] chuck was selected as part of our strategy to enable the single machine to handle all our turning requirements, which includes some pretty big parts. With other quick-change systems, we would have had to go with a smaller chuck or give up some quick-change latitude because of interference between chuck and tool. Not so with the Coromant Capto systems," he says.
Parts range in diameter from 2[inches] to 16[inches] and length from 2[inches] to 60[inches]. A 14[inches] dia bolt circle on one part accounts for the extra large chuck. Deep, small diameter bores requiring long tool reaches are common to many parts. Some of the bore depths reach 10 to 15 times dia. The Coromant Capto boring tools handle these aspect ratios with very little easing back required on removal rate and no chatter, either," says Mr Minnick.
When the new machine started up, things actually went better than expected. Dead machine time for toolchange has averaged less than 10%, and tool changes are averaging about 20 sec per station. Throughput is running 30% to 50% ahead of projections.
"By and large, we've cut turning time for all parts by half and are doing virtually all operations with a single chucking on a single machine," Mr Minnick reports.
To keep the new turning center busy, Landis upgraded its production planning and procurement practices. Because of the unit's capacity, Landis handles virtually all its turning operations in-house, using the SmarTTurn and about 100 Cormorant Capto quick-change cutting units. The mainstay insert for the steel turning is Sandvik P line grade 4025 MNMG, which handles both roughing and finishing.
"By doing both operations with the same insert, Landis saves tooling stations, reduces tooling inventory, and eliminates another source of dead time for turret indexing," explains Curtis Scott of Sandvik distributor, Petry and Morrow, who suggested the idea.
Typical machine settings are 600 sfpm speed, feeds from 0.015 to 0.025 ipr, and depths of cut between 0.15[inches] to 0.25[inches]. Tolerances are [+ or -]0.001[inches], 50 to 60 rms, using a soluble cutting fluid.
To change to a new job, the Landis operator simply swaps pre-measured cutting units in accordance with the operations sheet, switches the machine control program over to the new part, loads the workpiece, and pushes "start." Average tool changeover time per station is 20 sec. There is no need for test cuts or offsetting to compensate for variations in cutting edge location. Worn cutting edges are indexed or replaced off the machine, so there are no stoppages because of worn edges.
Coromant Capto quick-change toolholders consist of a cutting unit, which holds the cutting tool, and a clamping unit, fixed to the machine's turret. A drawbar mechanism, activated by an Allen wrench, makes it possible to switch cutting units in 20 to 30 sec.
The mating surface between cutting unit and clamping unit is a unique tapered polygon, structurally much stronger and more rigid than conventional, key-slotted, round-tapered shafts. The coupling is self-positioning, self-aligning, and self-centering, achieving the 0.000 078[inches] repeatability in all directions. Its design also eliminates the backlash found in key-slot systems and makes it impossible for the operator to improperly install the cutting unit. Landis claims to get far greater positioning accuracy and quick changeover than was possible with square shank holders.
Length of taper and the polygon geometry counteract bending tendencies and enable the system to handle large torques, especially on the larger parts. Torque from cutting forces is transferred back to the clamping unit with 80% surface contact and full face contact.
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|Title Annotation:||computer numerical control turning center|
|Publication:||Tooling & Production|
|Date:||Oct 1, 1995|
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