High-speed holemaking

Manufacturing Engineering, Jul 2000 by Roberts, Randy

There's more to fast cutting than spindles and cutting tools

When most manufacturers hear "fast cutting," they think of high-speed machine tool spindles and cutting tools. But faster spindle rotation isn't the only route to high-speed holemaking. Multispindle systems that can create any where from a few to several thousand holes at a time serve high production goals just as well.

Imagine that you're working with one spindle on a flexible machining center. When increased output is needed, you apply the latest in high-performance tooling technology. You update all your programs. Everything is tuned in; you're meeting your new goals. Then you're hit with a demand to increase production even further. Your machines can't work any faster. Is the only solution buying more machining centers?

Before you decide how many more machines you need, consider another possibility. Simply moving the high-production jobs from your flexible machines to a system dedicated to production holemaking may be a better choice.

Manufacturers turn to dedicated production systems either because they have high production requirements on a newly designed part, or because the flexible machining centers they are using to produce a large quantity of a part or family of parts have reached a production plateau. To produce a large number of parts accurately, effectively, and efficiently, the manufacturer usually needs to create multiple holes in a single pass.

Suppose you need to machine nine holes into a part, and each hole requires three operations (drill/chamfer and tap). An 18-position (two nine-hole patterns) multispindle head will perform all 27 operations in one pass. One of the nine-spindle clusters is equipped with step drills for drilling and chamfering. The second nine-spindle cluster is equipped with the appropriate taps. Through an automated part fixture, the part is drilled and chamfered at the first cluster, then moved to the second for tapping.

On a single-spindle machine, this process takes approximately 5 minutes. With a multispindle head it takes about 15 seconds. While the single-spindle machine is producing one part, the dedicated machine is producing 20. Choosing a solution here is relatively simple because it involves comparing identical operations. Manufacturers may have a variety of requirements, however.

Dedicated equipment can shorten production cycles. At Carrier Corp. (Syracuse, NY), engineers in the crankcase operation were using a single-spindle, radial-arm machine to tap 49 surface holes (in cluster patterns of 12, 19, and 18) on three sides in its four and six-cylinder crankcases. The holes ranged in size from 1/4 to 7/16" (6-11 mm) in diameter, with thread sizes of 14, 16, and 28. Several holes required 1/4-18 and 1/8-27 NPT.

The cast-iron cases, each weighing several hundred pounds, had to be repositioned for every side that required tapping. To simplify this long and often cumbersome process, we developed a machine that can tap all 49 holes in one pass. Each of three gearmotor drives, one for each side of the part, has a multispindle head equipped with individual leadscrews, which permit simultaneous tapping of thread pitches of various sizes. The crankcase tapping operation could now be done in 34 seconds.

There's more to high-speed holemaking than straight drilling and tapping. The milling of shapes precents unique production challenges as well. When Boeing needed to step up an already accelerated production schedule on its Delta IV rocket program, we devised a gearbox that cut milling time 40%.

A five-spindle, gear-driven head with a quick-change machine mounting system and a pulse-mist lubrication system mills triangular pockets into the aluminum skin that forms the rockets' booster cores. In the five-spindle setup, a CAT 50 milling machine mount connects the machine tool's spindle to the main center spindle, which drives the other four. Each of the four comer, or orbital, spindles has HSK 63 sockets for quick change, high-velocity toolholding that can handle a cutting load up to 25 hp (19 kW).

Rotating at 7200 rpm, the center spindle (HSK 100 sockets) cranks the four orbital spindles up to 10,000 rpm. Such high-speed spindle rotation requires special hybrid bearing packs developed for this application.

Indexing and robotic part handling systems that facilitate simultaneous load and unload as a part of the machining cycle help highspeed production machining go smoothly. When automotive component manufacturer Engineered Sintered Components (Troutman, NC) needed to increase production of a powder metal part in which three holes had to be tapped, it chose an automated production tapping system.

At the heart of the new system are a CNC ballscrew-driven feed unit and a multispindle tapping head. The feed unit provides accurate spindle rotation through the tapping head, and a feed rate that matches the thread being tapped (this is known as rigid tapping). The machine can produce a finished part every 5.5 sec.

An on-board robotic parts-handling system loads the part onto an automatic indexing fixture. The part moves to the first station to confirm part presence, then to the second station for tapping, and on to the third station for gaging to check that the tapping has been successful. If the part is OK, it moves to the next production phase. If it is not, the machine stops, and the operator rejects the part and confirms the cause of failure before the production cycle continues.