Turn Mill Machine | CNC CLathe

Using a turn-mill machine - includes related article

Chris Koepfer

It seems every machine tool builder with a lathe in its bag of products offers models with turn-mill capability. This article looks at some of the considerations one shop uses to determine when turn-mill is appropriate for its production mix.

Adding rotary tool capability to a turning center significantly increases the manufacturing capability of the machine tool. Applications that previously required turning and, subsequently, milling, and drilling, can be processed in one setup on the turn-mill machine. However, not every application is a candidate for one-stop processing. Some applications lend themselves well to turn-mill, some do not. Key to successful implementation of a turn-mill machine is recognizing what will work on the machine and what will not. It's not always obvious.

To get some insight into that decision-making process, we visited Task Force Tips (TFT) Inc. (Valparaiso, Indiana)--a manufacturer of fire suppression equipment and Production Dynamic chucks for metalworking applications. Much of the company's product mix is comprised of parts of rotation. Finding better ways to make those parts led the shop to turn-mill machines. Currently, 60 percent of the company's turning equipment has milling capability. They produce over 1500 different parts for their product lines and use turn-mill whenever possible.

Turn-Mill Basics

Fundamental to successful application of turn-mill processing is understanding that it is not the sum of separate turning and milling processes. It is a different way of approaching the manufacture of a workpiece.

For example, using a conventional method of manufacture (a lathe operation followed by a machining center operation) part processing is sequential. First, all the turning operations are performed on the workpiece. On a two-axis machine, rough turn is followed by finish turn of the OD and ID. After step one, the part is either flipped or moved to a second turning machine for step-two processing.

After the part comes off the turning center, it is usually checked by the operator against a tolerance on the part print. If the part is OK, it moves on to the next station.

With the turning complete, the workpiece is moved to a machining center for any milling and drilling operations. Flats are cut, followed by any other milling work. Then holes are drilled, followed by tapping, if specified. Again, when the part comes off the machining center, it is inspected. Depending on the operations performed, deburring may be required, making an additional processing step.

Practitioners of turn-mill processing, like TFT, approach the workpiece more holistically. Rather than plan operations sequentially around two machines, they can program the machine to cut in an operation order that allows optimum manufacturability. Essentially, they can turn some, mill some, turn some more, mill some more, instead of having to turn all, then mill them all. An often overlooked area of savings is in quality assurance. By processing the part complete, only one inspection is necessary for the whole part.

An example of how this processing freedom helped TFT is found in a collar they make for their nozzles. Before it was put on the Mazak Super Quick Turn 18M turn-mill machine, the workpiece required four handlings and had a total cycle time over seven minutes. Not factored into the cycle time was a deburring operation needed after the part came off the machining center because the milling operation damaged the OD threads that were already cut on the turning center.

With turn-mill, the same part is now machined complete in one handling, and takes just three minutes to complete. Because the sequence of operations can be intermixed, the milling of the flats on the OD now comes before the threading operation so the thread deburring step is eliminated.

Consider Complexity

There are many factors that impact the decision to turn-mill a part rather than process it across distinct machine tools. One of those factors is part complexity. But complexity is kind of misleading as a factor because, as Figure 2 shows, complexity sometimes lends itself better to the turn-mill machine.

On the other hand, some simple looking parts just don't work well on a turn-mill machine. One example of a part that looks like a candidate is a simple two-inch-diameter by 3/4-inch-thick convex disk, in aluminum, with two drilled and tapped through-holes on the face. Using a bar feeder and cutoff tool, these seem like they should crank through like salami on a turn-mill machine. In prototype testing for the part, however, it was discovered that the drill going into barstock caused problems making the next part. The drill had to go into the bar far enough for the tap to bottom out. Getting the spindle back to position for the holes to align for the next part was too problematic. So the disk is turned and the holes are done as a second operation. In some situations, additional stock could be faced off the bar to give a fresh start for the holes. The cost of material would be the deciding factor.

Consider Geometric Relationships

Geometric relationships are a better gage of turn-mill processing potential than geometric features. Holding dimensions relative to each other is where a turn-mill machine can really earn its pay.

According to Stewart McMillan, president of TFT, "Often a part print has handling allowances built into the part tolerance. For example, a 0.001-inch tolerance may be indicated as an allowance for fixturing a workpiece into a machining center after a turning operation. And those tolerances grow as secondary and tertiary operations are needed. It's a compounding effect. With turn-mill machines, that handling allowance is unnecessary since the part is not moved from the original fixture (chuck). Our engineers can design closer tolerances, where warranted, and forego refixturing allowances altogether."

Sometimes the engineers at TFT are able to design parts for turn-mill processing that would not be practical or economical to produce any other way. An example is a proportioning valve, shown in Figure 3. It is machined complete on a turn-mill machine, and would be very complex to make with a machining center. One feature, a semicircular groove on the back side of the major diameter, is readily accomplished with a straight milling tool and C-axis interpolation. "This used to take eight hours to set up and run on a machining center. On the turn-mill machine, it took two hours to program, and runs in minutes," says Mr. McMillan.

Consider Throughput

Any manufacturer, whether making a product of its own or vending for another company, is aware of the need for good throughput in the shop. TFT is no different. According to Mr. McMillan, "We probably run larger batches than we would like, but that tune is called by the finisher who anodizes our aluminum parts. Since most of our products are exposed to water, we anodize all the parts to prevent corrosion. The finisher runs batch quantities that are most economical for him. Generally, those quantities are not ideal for us. But, we accommodate the finisher to keep our costs down."

Another throughput consideration for turn-milling is the relative speed of the turning and milling processes. Generally, turning is about twice the speed of milling or drilling. Depending on the ratio of turning to milling/drilling, it may be better to gang parts on a fixture and machine a bunch at one time, than to do one at a time on a turn-mill machine. "That decision often has to do with part tolerance," says Mr. McMillan. "If the accuracy requirements are less stringent, we'll look to optimize throughput with a second operation."

According to Jim Walker, who operates the prototyping test area for TFT, "Generally speaking, you don't want to do heavy milling with large amounts of metal removal on a turn-mill machine. Separate operations are better suited for that." While geometric complexity is often easier to deal with on a turn-mill machine than on a separate machining center, throughput is a consideration if tolerances for the part are wide enough to allow efficient production off the turn-mill machine. A shop has to weigh the relative gains of single-setup processing against the throughput gains of multiple part setup and machining. In other words, sometimes it's better to make them one at time complete, and other times it's better to turn a batch on the lathe and then second-op them in a multiple-part fixture on a machining center.

Other Considerations

Since implementing turn-mill, TFT has made a concerted effort to look at how each part is processed to see if it can be done complete on one machine. So far there have been many successful transfers. As the capability of turn-mill technology expands, for example, machines with subspindles and dual-turret configurations, the re-evaluation starts over. It's an ongoing process.

"That's how we've done business from the beginning," says Mr. McMillan. "When we started TFT in our basement, we used engine lathes and bench mills. Our first CNC four-axis lathe was purchased in 1980, and within six months, it was doing the work of all of the manual machines. Soon, we bought a second CNC and the growth has continued. Now most of the four-axis lathes have been replaced by turn-mill machines."

Key to successful implementation of turn-mill in the shop was a go-slow attitude, at first. Early turn-mill machines required excessive amounts of time for the main spindle to convert from turning to milling mode and back. As much as 20 seconds might be added to a single cycle. "We would still be in a go-slow mode if this key factor had not changed," says Mr. McMillan. Recent innovations, including integral spindle headstocks, now allow near instantaneous change-over from turning to milling and back. This has changed the equation used to measure times between milling machines and turning machines.

In 1989, TFT bought a used CNC turning center with live tools. Mr. Walker recalls that programming a simple hex on barstock took three hours using the point-to-point method of programming. "Even with that, I could see how this technology could help increase production," says Mr. Walker. "At first, I was not enthusiastic about turn-mill technology. After working with it, the benefits became obvious."

Dedicated Turn-Mill

The system used at TFT to determine what goes across the shop's production turn-mill machines is an off-line turn-mill machine dedicated to production testing and prototype work. Mr. Walker runs the machine. "Not many shops would dedicate a piece of equipment like this to non-production work," says Mr. Walker.

"It serves a couple of purposes for us," says Mr. McMillan. "First, it allows us to prove out a turn-mill candidate without interrupting production. It also allows our engineers to test more than one concept at a time. Prototype turnaround is now a matter of hours instead of days or weeks. The advantages to the company of having a dedicated machine more than justify the costs."

Results from the dedicated turn-mill cell have been impressive. Last year, product development was able to introduce new products at a rate ten times more than the previous average. And the trend is up, reports Mr. McMillan.

"A big advantage of this CNC machine for R&D purposes is that when an engineer is developing a product and needs prototypes to test, the turn-mill machine can crank out twenty of the same part or twenty slightly different versions of the same part for testing. Our conventional toolroom simply can't produce the quantity and variety that this machine tool can. Plus, the part program serves as a record of what changes were made and what works best. When the design is approved, we can transfer the program directly to a production machine and be assured the production pieces will match the prototypes," says Mr. McMillan.

Programming Turn-Mill

Programming a turn-mill machine is conceptually different than a conventional lathe, but may or may not be more difficult, depending on the capability of the machine's CNC. "We know how to program lathes from our experience with four-axis. That can get fairly complicated and some of the processing techniques, such as simultaneous cuts and operation sequencing, aren't that different from turn-mill," says Mr. Walker. Turn-mill programming is done on the machine. The CNC for these turn-mill machines is powerful, fast and has macros that make the programming job very easy. "The milling portion is as simple as programming a knee-mill, except you're using a high-powered computer to move it around," says Mr. Walker.

"The maximum time for programming a part on one of these machines is approximately two hours," explains Mr. Walker. The part shown in Figure 4 was programmed in one hour and took only about four minutes to cut.

At the prototype machine, Mr. Walker programs and test-cuts potential production parts to prove them out for turn-mill processing. When the program is written and proven, it is duplicated and sent to the machine that will run it. TFT uses DNC to save all programs on a UNIX-based host computer.

Looking Down The Road

TFT firmly believes in turn-mill technology as a viable manufacturing process for many of their workpieces. Where does TFT see itself going in the future? "That's easy," says Mr. McMillan. "More automation and increased capability of the turn-mill concept."

As an example of that future, Mr. McMillan points out a Mazak Super Quick Turn 15MS with a gantry loader. The machine can produce up to 128 parts untended. In operation, the machine is tended about two hours a day, one each on the first two shifts, for part loading. Shift three runs untended until all the blanks are machined.

The machine has a subspindle that picks off the workpiece from the main spindle for back turning operations. Live tooling is available to machine on either spindle. "That's where the technology is going next," says Mr. McMillan. "And as for price, this cell cost only slightly more than our first four-axis turning center back in 1980--actually it cost less in constant dollars."

Is It For You?

Some people argue that adding milling capability on a turning machine is a division of efficiency. Combining distinct metalworking operations on one machine creates a compromise between the operations. One machine can't do two different operations, optimally.

On the other hand, if a workpiece needs turning and milling operations performed, it makes sense to set the part up once and do all operations then and there. This is the rationale for the turn-mill machine. Efficiencies are gained by less workpiece handling, relative tolerances can be more accurately controlled and throughput is faster.

Both of these positions are valid. Turn-mill capability is not applicable across the metalworking universe. However, when it can be applied, results are often dramatic. The technology has advanced to a point where it is a viable machine tool for any shop. With increased pressure on delivery schedules and quality, turn-mill should be added to the must-see equipment shopping list.

A Need And A Napkin

The idea that launched Task Force Tips was sketched out, literally, on the back of a napkin by Mr. Clyde McMillan, the company founder. Mr. McMillan was a volunteer firefighter and an engineer. The original idea was to manufacture a nozzle that automatically controlled the pressure of the water coming out so firemen could always have the correct water pressure to fight a fire.

With automatic pressure control available at the nozzle, it allows the fireman to operate the valve on the nozzle like a throttle. As the valve is opened and closed, the discharge nozzle automatically sizes itself to deliver water in a controlled manner. For the first time, a nozzleman, deep inside a burning building, could control the amount of water delivered on the fire from a dribble to a 250-gallon-per-minute deluge, without having to send a runner out to the pump operator to make a change in pump pressure.

Product innovation continues to come from the company. But those innovative products are possible, in large part, to innovative manufacturing and the use of appropriate technology, regardless of convention.

For more information on turn-mill technology from Mazak, circle 36 on the Postpaid Card.