Can This New CNC Multispindle Work For Your Shop?

A century-old builder believes their new CNC machine extends multispindle processing technology to include any production turning shop.

Article From: 7/1/1996 Modern Machine Shop, ,

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Davenport Machine Company hit on a good multispindle machine design just after the turn of this century. It was good enough, according to Don Firm, company president, that many parts are interchangeable between a 1908 model DA on display at company's Rochester, New York headquarters and the 1996 model B being built up on the plant floor.

However, the world that is home to Davenports, and multispindle automatic screw machines in general, is changing rapidly. Increasingly, market pressures, in the form of lower lot sizes and higher tolerance requirements, added to a scarcity of skilled setup people, are raising questions about augmenting the venerable multispindle automatics.

Mr. Firm is directing an effort by Davenport to create a multispindle machine for the next century. CNC servo technology has evolved to a point of practical application to multispindles.

He sees this new machine, designated as the LS 522, as an answer to screw machine shop problems and as a process alternative for non-screw machine shops. On a recent visit, we were given a preview of this machine before its introduction at IMTS 96.

Multi Basics Multispindle machines make parts quickly because their design allows more than one cutting operation to be performed at a time. In general (because there are many variations), each spindle on a multi is intersected by two independent axes of tool movement.

One is the cross slide, which moves a cutting tool perpendicular to the spindle center line. The second slide carries end working tools and moves on-center from the tailstock end of the machine. Respectively, the tooling on these slides equates to OD and ID tools on a single-point turning center. But, a multispindle machine can use an OD and ID tool on each spindle, at the same time. Davenport machines have five spindles, so they can bring at least ten tools into the cut at once.

Spindles on a multi are mounted about the periphery of an indexible drum. Because all of the cutting operations on a multispindle machine occur simultaneously, each index of the spindle drum produces a finished part. On a five-spindle Davenport, that's five parts for every rotation of the drum.

Quantity Or Quality

As long as most of us can remember, the multispindle automatic has held the high volume production title for small parts of rotation. But it doesn't turn parts faster than other machine designs. Because many multis are bar machines, spindle speeds are conservative compared to chuckers and slow compared to single-spindle CNC turning center capabilities.

The secret to multispindle processing speed is cumulative cycle time versus additive. They do this by breaking the part processing cycle into numerous small operations and then performing those operations simultaneously on multiple spindles.

Machine cycle times on a multi are determined by the longest cutting operation at a station. Here's an example. Station three has an end slide drilling operation that takes 0.75 second (including index). The next longest operation is a 0.65-second cross slide groove at station four. Because the multispindle can use all stations at the same time, cycle time for the job reflects the longest operation performed, which in this case is 0.75 second.

In general, multi-spindle automatics are better known for high speed than for high accuracy. Mechanicals with their cams, shafts, levers and other actuation devices simply stack too much inaccuracy to consistently produce the many close tolerance specs being demanded by customers.

For higher tolerances, many shops look to single-point CNC turning centers. "Their programmability and servocontrol of contouring allow closer tolerances by a decimal point over most automatics," says Mr. Firm.

Because single-point turning is a sequential process, these machines can't produce the volumes of a multispindle machine. A roughing pass is followed by a finishing pass. A facing operation occurs before a drilling or boring operation.

Therefore, cycle time on a turning center is additive. Time required for each operation is added together and the sum becomes the cycle time. Speeds and feeds can reduce overall time by performing each cutting operation faster, but the process is a sequential one.

Automatic Press

Multispindle automatics are among the last areas of purely mechanical motion control. Most of these machines are found in shops that specialize in making "screw machine parts." Among these shops, many are known more for the brand of screw machines they use than for the products they produce. There are Davenport shops, New Britain shops, Acme shops and others.

There is good reason why shops specialize in a given brand of machine. It's a very skilled technician who can set up one of these automatic screw machines. Each machine has specific characteristics.

Setting up and running a multispindle is a hands-on operation. There is much skill and experience required to "tweak" the mechanicals that make these machines operate. Unfortunately, there is an increasing scarcity of personnel who can do the job.

This lack of personnel is exacerbated by an increasing need to setup multispindle machines more frequently. Long run jobs, the traditional lifeblood of the screw machine industry, are trending shorter.

Shops find competition for traditional screw machine contracts fierce because there are fewer such contracts being let. Customers are ordering parts in small lots but on a more frequent basis, responding in part to just-in-time inventory requirements and partly to shorter product life cycles.

Some shops keep a machine set up permanently to run recurring jobs. When an order is complete, the machine sits idle. Shops must decide between the cost of setup and tear-down and not running the machine. With margin pressures increasing, not fully utilizing shop resources is becoming difficult to justify.

What's New

From the base up, Davenport's new LS 522 is a new machine. The base is a single piece cast from an epoxy granite mixture. This composite material has excellent vibration damping characteristics and thermal stability.

Design of the base leaves the tailstock end of the machine open. A chip conveyor with coolant recovery tank can slide into this opening and be positioned directly under the cutting zone of the machine. Chips and coolant are directed to the conveyor by troughs cast in the base.

The enabling technology for this new machine is its motion control system. "Servo technology is now at a point where its application to a multispindle is practical," says Mr. Firm. "Five years ago, just the electronics used on the LS 522 would cost more than the machine's projected base price."

Excluding the main spindle drive motor, there are ten programmable linear axes on the LS 522. Each cross slide and end slide has a dedicated servomotor. Operation of these electric motors is significantly quieter than a mechanical actuation system.

To emulate the stroke motion generated by a cam, these compact servos use an integral gearbox that changes the servomotor's rotary motion into linear motion that creates stroke on an output shaft. The linear motor couples directly to its respective tool slide, eliminating the need for ballscrews. The stroke of these motors is fully programmable.

The gearbox provides a mechanical advantage that gives the output shaft sufficient thrustrated at 800 pounds per slide--to feed form tools into most materials.

Index positioning of the spindle carrier is critical to workpiece dimensional accuracy. A Hurth multitooth coupling is used to seat the spindle carrier. Index time is 0.4 second. Bar capacities are 0.875 inch for round stock, 0.625 inch for square and 0.75 inch for hex.

Main spindle drive is a 14-hp AC motor that drives the five spindles through a planetary gear system. Speeds are programmable through the CNC and range from 250 to 6,000 rpm. Individual spindles can be programmed to stop for straddle milling, cross drilling or cross tapping operations.

Control for the new Davenport is a 486 PC. To actuate the axis slides, a motion control board is used to communicate between the computer and the servos. A SERCOS loop feedback, which uses fiber optic cables to link the servos with the motion control board, carries positioning data to and from the servomotors.

Accuracy?

"Dimensional accuracy capabilities for Davenport multispindle mechanicals run out when tolerances over 0.002 inch are needed," says Mr. Firm. "They simply were not designed to deliver any better tolerances."

That's another motivation for creating the LS 522. Application of servomotors in place of cams and the use of a Hurth coupling on the spindle drum increase accuracy of the new machine to ±0.0002 inch.

Setup Speed Up

"Traditional mechanical multi-spindles, Davenports included, can take from 12 to 48 hours to set up the machine for a new job," says Mr. Firm. "Complete change for this new multispindle will be around two hours and, much less than that for running a family of parts. With setups this short, there is no reason to limit resource use by dedicating machines to specific jobs."

Key to reducing the time involved in changeover is the programmability of the servodrives. The motion of the cross slide and end slides on a mechanical machine is created by a cam. Fine tuning the stroke (axis compensation) is a time-consuming process of trial and error with mechanical adjustments usually required on each of the ten moving slides.

Servo technology, in place of cams, dramatically modifies this process. With programmable stroke, rapid movement and feeds are set with key strokes. Tool offset compensation is accomplished on the control.

Two reference points are used for each spindle. Longitudinal zero is the starting point for the cross slide. The spindle center line is the radial zero point from the endworking slide. All axis moves are positive numbers from these starting points. Like cam-type machines, each axis move is a plunge. Standard form tooling is used on the new Davenport so shops won't need to discard stock tooling.

An operator/programmer simply inputs a starting and finish dimension. A resident database calculates the feed rates between 0.0002 and 0.050 inch per revolution. Input is repeated for each slide. Operation assignment and sequence is done using a process planning sheet.

The forces that are shaping production turning operations demand that shops look at workpiece processing in a different light. Cycle time has been almost exclusively the measure of productivity and an important benchmark in successful bidding for jobs.

Customers and shops are still very cognizant of cycle time. Its importance should not be discounted. However, the total cycle time for a run should not be ignored either. With the shortening of production runs and the need to quickly respond to JIT scheduling, total cycle time--what happens before and after the part is cut--is an increasingly important indicator of efficiency. How fast a shop can change jobs is key to wringing more margin out of a production run. And, a shop's overall throughput ability is more often than not the key factor in winning or losing a contract.

Conquering New Worlds

It's virtually a given that the new Davenport LS 522 will find a welcome market in screw machine shops. The machine's servo actuation system faithfully duplicates the functionality and speed of mechanical machines.

Job processing experience honed on mechanical multispindles is directly transferable to the CNC machine. Tooling too is applicable to the new machine without alteration.

"The efficiency of simultaneous processing is undisputed," says Mr. Firm. "Freeing shops of the burden of long changeover, allowing production of higher tolerances, simplifying operation all while maintaining part processing speeds as good or better than the mechanicals is the promise of this new machine."

In the non-screw machine production turning area, Davenport expects the LS 522 to provide a processing alternative to single-spindle, single-point turning centers. These shops know about programming but many are less familiar with form tools and process planning for simultaneous operation.

If these shops will make the transition from two-axis, single-point, contour turning to single-axis, form-tool turning, Mr. Firm sees medium to high production turning shops applying these new multispindle machines to dramatically increase their throughput.

Is It For You?

There are essentially two kinds of shops that might consider this new multispindle machine. These are screw machine shops and shops with medium to high volume, high mix turning applications currently done using single-point turning centers.

Screw machine shops should recognize reduced setup requirements without losing mechanical-like functionality as a production boon. Expanding the shop's capability to economically include shorter runs and closer tolerances makes sense in today's manufacturing environment.

For shops using single-spindle, single-point turning, a transition to multiple spindles, and form tooling, represents a significant change in processing technique. "Changing how you process parts is never easy," says Mr. Firm. "Consideration must be given to the costs of form tooling over single-point tools. It's a different way to turn workpieces that must be learned."

But it may be worth the change if quality requirements can be maintained while a significant increase in production is realized. More throughput per machine means more jobs can be put across each machine.

So, if turning a high mix of precision parts in medium to high volume is your business, shops that haven't considered CNC multispindle machines before may find this new machine tool worth a look as a processing complement to single-point turning centers or an alternative to automatics.

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