Using CNC On Cam Multis

A continuing debate among precision parts manufacturers is flexibility versus dedicated setups. Most multi-spindle shops built their business models based on high volume part orders without a need for very much flexibility.

Classic multispindle machine tool design reflects this traditional view of the business. However, as most shops will attest, the traditional model has changed dramatically in recent years.

No longer can shops afford to dedicate capital resources to one or two jobs. Not only has the volume of work across those resources changed, but customer demands have driven shops to reevaluate multispindle production also. These demands include tighter tolerances, interrupted ship lots from JIT, lean and other inventory control requirements. Moreover, traditional responses to these demands are exacerbated by a general loss of skilled machinists.

Part production is migrating, from processing philosophies that involved multiple steps to complete a workpiece, to integration of what were formerly secondary processing technologies onto the multispindle machine itself. Much of this incorporation of secondary operations on multis is accomplished by the use of attachments.

As the "all singing, all dancing" multi-spindle screw machine evolves, the basic questions isn't should CNC be applied—it is required—but to what degree is it effective and efficient? At the same time, development of cam actuated multi-spindles has not stood still, and its effectiveness in today's manufacturing environment is more competitive than ever.

To find out how to best apply CNC and cam actuated multi-spindles, we sat down with John Ross, national sales manager at Euroturn (Dayton, Ohio) to discuss how the company determines the appropriate level of CNC technology when tooling up for a job on its line of automatic six- and eight-spindle multis. He also shared some specific applications of CNC attachments for processing actual customer workpieces.

Industry Flip-Flop

"Let's face the sad but true facts," says Mr. Ross, "the geometrically simple, low tolerance, super long running jobs are not being made in the United States. That work has picked up and gone overseas. What we have left is high tolerance, lower volume, complex individual and families of parts that we must figure out how to make money on."

In the last 2 years, Euroturn has seen a reversal or flip-flop in the types of application requirements that screw machine shops are asking for. Two years ago, says Mr. Ross, 80 to 90% of the business was for traditional screw machines—cam actuated, straight-stick machines that basically replaced older traditional makes and models. In the past two years he's seen demand for these traditional machines become a trickle in the United States market. "Even as business turns up," says Mr. Ross, "we're not seeing the traditional jobs that ran in screw machine shops come back."

That same 80 to 90% of the traditional screw machine business has now been replaced by highly engineered, more flexible and accessory-laden multispindles. Part of the change is perhaps attributable to the economy, but much of it is the need by shops to do more value added work to their customers' parts.

To remain competitive, these shops need to do more, faster and cheaper. That means, in many cases, incorporating secondary and tertiary operations onto the multispindle machine. It also often means the use of CNC for actuation of some of these attachments. It is the process engineering required to blend traditional cam machines and incorporate appropriate CNC attachments and accessories that has kept Euroturn busy, in spite of the recession.

Appropriate Technology

Many look at the use of cam machines and CNC machines as an either/or proposition. Perhaps a third alternative that blends the two actuation systems on a single machine, so each is applied where it's appropriate, is a way to go. This is the tack that Euroturn has chosen.

In its basic function, the multispindle machine tool, or any metalcutting machine tool, is designed to deliver a cutting edge, with precision and repeatability, to a workpiece feature. It is selecting the best actuation method for creating the motion that moves the tool, which is at the core of process engineering decisions in many of today's multispindle applications.

What makes the multispindle so productive is the machine's ability to simultaneously perform numerous operations. Of course that makes it very complicated from an actuation perspective.

On a cam-actuated machine, all slides, cross and end working, must be synchronized to make good parts. The exact travel of each tool must be set mechanically. Tool wear and other compensation must likewise be set mechanically. This all takes time and skill. But once a cam machine is set, stand back and watch it run. Its productivity is second to none.

CNC brings electro-mechanical actuation to slide motion. Generally the system works using a precision ballscrew, attached to a tool slide. A servomotor spins the ballscrew. Each revolution of the servo moves the slide a known distance. Resolvers in the servo keep track of the revolutions and hence the position of the slide.

For setup and compensation, the CNC servo is very simple and quick to prepare. Stroke distances are entered into the machine control, and that's about it. The servo knows how far the distance is in revolutions, and the calculations are automatically computed in the CNC.

The flexibility of CNC actuation is offset, slightly, by operation that is generally not as fast as mechanical. Better CNC processing power and improved servo feedback looping make the speed issue increasingly a wash, but most builders agree the mechanicals are overall still faster. But each technology can play an appropriate role in complex part production.

"The skillful part of the exercise," says Mr. Ross, is picking where and when CNC is the better choice. That's where application experience comes in."

Rules Of Thumb

In general, it is difficult to apply hard and fast rules to when CNC is better than cams or vice versa. It's always about the part that needs to be produced—its features, finish requirements, volumes, material and a myriad of other specifications that must be evaluated.

However, there are a few general rules of thumb that can be used in specifying a CNC attachment on a traditional multispindle screw machine. For example, a CNC attachment is better for double hits. If a workpiece feature needs to be approached twice or more, programming a CNC unit is the way to go. Also if the process needs a dwell time, this too is better done with a servo.

Often angular holes or asymmetric milled surfaces are easier to machine with a CNC attachment than a cam driven one. CNC is very good for high accuracy and high surface finish requirements. Moreover, if burrs are a concern, a second pass is simply a matter of programming.

On one application, a form tool is driven by a combination ballscrew and hydraulic cylinder to generate sufficient thrust to turn the workpiece. "Thrust forces on form tools can be 1,500 pounds," says Mr. Ross. "We use a standard unit that combines the power of a hydraulic ram with the accuracy of a ballscrew. The package is compact enough to fit easily into the crowded interior of a multispindle."

Still, the best way to understand how CNC can be used to solve production problems is to look at some actual parts. The accompanying boxes will give examples of specific parts and the rationale and justification for applying CNC to their production.

Imagination Is The Limit

New manufacturing problems call for innovative solutions. These examples of applying CNC attachments on mechanical multispindles represent just a few of many possible ways of making the parts.

Metalworking manufacturing is a blend of art and science. The science is in the technology that is available through the merging of electromechanical tools that can do more things better than one or the other alone. The art is taking these technologies and making them perform the tasks required. It's the challenge and the reward of manufacturing.