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Dial in for high production machining, accuracy.

The answer to the question of what you get for your half-million dollar investment in a rotary transfer machine is changing. So are the reasons why more and more manufacturers are asking that question.

One reason is that the demands of just-in-time manufacturing are shrinking lot sizes--even for high production parts and components. Another is that CNC has taken hold of the rotary transfer machining process. Along with improvements in workholding devices such as self-centering jaws and quick change tooling, changeover has been speeded from part to part within families of parts and between unrelated parts and components.

This is precisely why rotary transfer machining has become a secret weapon in the machining arsenal of manufacturers of automotive components, electrical components, plumbing parts, and a host of other components that require high-volume multiple machining operations.

Manufacturers who readily admit to making significant gains in their productivity are all too reluctant to give out the details of their rotary transfer machining capability "so that their competitors won't find out how they're doing it."

The volume question

Make no mistake, the key to justification of an investment in rotary transfer machines is still a question of volume. You need large numbers of parts to justify the investment that can range from $500,000 to more than $3 million. Often called rotary dial machines, these machines offer a compact footprint and handle a wide variety of machining jobs in both smaller and larger components for the automotive, appliance, aerospace, and job shop industries.

Volume is really dictated by the amount of machining on the part, the material being machined, and the accuracy required, explains Robert Houdek, director of Machine Tool Div, Custom Products, North American distributor for Hahn & Kolb's variomatic rotary transfer machines.

A very small brass part may require well over a million pieces on an annual basis. A larger cast-iron part--up to a foot cube--might be justified at production levels of a quarter of a million pieces or so. There is a rule of thumb that says that a family of products with 400,000 or 500,000 parts per year is a good threshold for selecting rotary transfer machining.

CNC works

CNC has been a big help in making rotary transfer machining more competitive for high-volume production, says Al Peterson, senior applications engineer, Mikron Corp, Monroe, CT. "Changeovers are getting easier on the machines, but flexibility always costs more at higher levels of production."

Before CNC was introduced to rotary transfer machines, setup of the machines was much more complicated and required skilled setup people who had to adjust each station in a tedious and time-consuming process. "When you were running with trip dogs and positive stops, after changing toolholders, tools, and chuck jaws, the setup person would have to go to each station and set the limit switches and positive stops on a trial and error basis," explains David D'Aoust, vice president of sales, Kaufman Manufacturing Co, Manitowoc, WI.

CNC gave rotary transfer machines memory to store as many as 15 or 20 programs, and using preset tooling facilitated speeding up changeover. After changing chuck jaws and cutting tools, a program is simply called up from memory. Parts that are produced from each station are typically qualified parts on the first run through the machine--as long as the tools have been properly preset, says Mr D'Aoust.

The trend toward CNC has been going on for some time. Kaufman Mfg introduced CNC to its conventional rotary transfer machine and introduced its FLEX:CELL rotary transfer machine to the metalworking market about the same time in the mid-1980s. The FLEX:CELL successfully filled a niche between the single spindle, highly flexible machining center and the very fast but relatively inflexible rotary transfer machine. The FLEX:CELL was different because it replaced the dedicated machining head of the traditional rotary dial index machine with CNC machining units, each of which can have three or more independent axes of travel and a tool changer, if required.

Variety in production

The traditional reason for selecting rotary transfer machining is to take advantage of the high-production rates that can be achieved using multiple machining processes, such as drilling, boring, turning, milling, recessing, threading, tapping, and broaching.

Historically, that high volume has been associated with dedicated machining, with only limited ability to be converted over quickly and easily to machine other parts. These old-style dedicated machines have given way to CNC controlled rotary transfer machines to even a class of hybrids that fall somewhere in between flexible machining centers and conventional rotary transfer machining in terms of volume, complexity of part machined, and ease of changeover.

Mikron's Centromax CNC flexible machining system combines the productivity associated with rotary transfer machining with the flexibility of a machining center, for example. Parts up to 120 lb (60 kg) for such applications as hydraulic and pneumatic valves, pump bodies, secondary operations on die-cast housings, and terminal boxes for electric motors can be machined using four workstations.

Changeover times have even been reduced to a matter of minutes through CNC control of both positioning and individual machining stations and by carefully selecting part layout within part families. As a result, changeover times of five to 50 minutes are not uncommon, for example, compared with changeover times that could take a whole shift to accomplish in the past.

One of the reasons that rotary transfer machining has come a long way from the days in which a part was put on the machine and simply indexed to the next station is found in the workholding. If more than one side required machining, the part was unclamped, turned around, reclamped, reset, and machined. The machines were time-consuming to change over, difficult to adjust, and, certainly by today's standards, not particularly accurate.

The rotary transfer machine industry, however, has progressed dramatically beyond that stage, according to Mr Houdek of Custom Products. Machines are very accurate and, in many respects, can hold tolerances as well as, if not better than, CNC machining centers, especially for machining multiple sides of the part, he says.

"We're holding tolerances that were unheard of 10 or 15 years ago," says Mr Houdek. "We have a total machine accuracy of five-tenths, spread across the entire table."

To appreciate just how much of an accomplishment that is, Mr Houdek explains that machine accuracy involves table index, the index of the chucks, the clamping consistency on a good part, and also the slide and spindle accuracy.

More and more, the user wants to know the tolerance capability of the entire machine, he explains. "A lot of companies are asking for the run off to be of all the chucks within themselves and also as a group. When that happens, you have to be very accurate between the chucks," he says. "We have had a machine in a production situation that has been running positional tolerances of six-tenths for the last six years," he says.

Still more flexibility

CNC continues to transform rotary transfer machining, making it even more responsive to users' machining requirements. Hydromat Inc, St Louis, introduced CNC for both positioning and full control, single-point machining at IMTS and is delivering its first CNC Servo rotary transfer machines. The CNC Servo machine offers single- and dual-axis machining capabilities at every station. The GE Fanuc machine control is running on software written by Hydromat using Microsoft's Windows operating environment.

Motion control programs are processed on a 486 processor and transferred to the servo controllers via the GE Fanuc control. The servo controller executes motion control programs in a multi-tasking environment handling up to 40 independent axes. The servo positioning drive package consists of Bosch proportional valves with Sony Magnescale embedded into the toolspindle units.

The Hydromat rotary transfer machines work in 12- or 16-station configurations using bar stock up to 1 3/4" diameter, fed by magazine or bundle-type bar feeders. Hydromat works in partnership with Witzig & Frank, a German company, to design and build the Turmat rotary transfer machine for machining larger-sized parts such as castings that are typically used for pump housing and the like in the automotive industry. These machines typically have an average value of upwards of $1.5 million.

The newest entry to the US market from Tornos Technologies US Corp is the Tornos Bechler WG C128 Convertible rotary transfer machine. It sports modular design, reducing changeover time from job to job to a matter of a couple of hours. The Convertible is available in six-, eight-, and 12-station models offering complete changeability of any working station, at any position. Also, many types of tools can be combined at any given station. Possible operations include milling, drilling, reaming, tapping, threading, turning, recessing, broaching, slotting, stamping, and assembly, either as primary or as secondary operations. CNC is available in one-, two-, or three-axis control.

What has changed?

Duke Brannum, manufacturing manager, Best Lock Corp, Indianapolis, IN, says that selecting a rotary transfer machine often depends on how many primary and secondary machining operations can be combined effectively into one work center.

"Our experience is that a once in, once out machine setup dramatically reduces the possibility of making bad parts. In one case, we consolidated five different machining operations--and all the part loading, checking, and reloading involved in machining separately--on one rotary transfer machine."

Today, rotary transfer machines' flexibility translates into machines being typically very interchangeable. You can work from almost any axis of the part using rotary fixtures, horizontal units, vertical units, angle head units, tapping units, and riveting units. In fact, just about any kind of machining head can be fastened onto a rotary transfer machine, and that operation can be performed in conjunction with other machining operations.

Basically, the older units did horizontal or center-working unit operations on their parts, explains Mr Brannum. Today's rotary transfer machines feature self-centering jaws so that you aren't referencing against a datum side of the part. Literally, any side of the part can be machined.

"Every time you take a part out and manually relocate and mechanically reload into another fixture, there's a chance of another error. If you can locate it one time and do several operations off that, you can eliminate some of that possibility of error," Mr Brannum says.

Long setups and the difficulty of changeover have also been changed by the introduction of self-centering jaws rather than the old configuration of one moving and one fixed jaw. Changeover time is reduced because, in some cases, multiple setups, which are not eliminated altogether in layout, are speeded up. The secret is designing the layout of a part from a centerpoint that is always going to be the common denominator on that machine.

Changing the jaws on an outside configuration, for example, requires changing the jaws on that pocket. When a new part is loaded onto it, as long as the center point remains the same, you don't have to go back in to find the center and reset the machine, Mr Brannum explains. It's always on the same centerline from one part family to the next whether a round or a square piece is being machined. On machines without self-centering jaws, you would have to relocate your center point and reset the machine, he says.
COPYRIGHT 1993 Nelson Publishing
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Copyright 1993 Gale, Cengage Learning. All rights reserved.

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Author:Lorincz, James A.
Publication:Tooling & Production
Date:Jun 1, 1993
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