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Workholding - the key to changeover in turning.

As manufacturing continues its move toward small-lot production, lathe and workholding suppliers are finding new ways to reduce changeover times.

In the short-lot production environment, the lathe has proven to be one of the least flexible pieces of equipment on the factory floor. One of the fundamental reasons for this is the workholding required in the turning process. When you have to hold and spin a cylindrical workpiece at high speeds, the metal tends to bend, deform, and deflect. To control these metallurgical gymnastics, workholding systems need to be somewhat application specific and customized. And it is this specificity that makes changeover of workholding equipment difficult and time consuming. Sometimes only chuck jaws need to be changed. Other times, the difference in part geometries requires changing from a jaw chuck to a collet or diaphragm chuck. The key to being productive in short-lot production is to minimize the time it takes to make these setup changes.

The crux of the changeover issue is that the time spent setting up a machine is out-of-cut or idle time. The machine is not making parts, yet the setup time is a direct labor cost of manufacturing, typically applied at the hourly rate of the lathe. Compromised solutions Anyone who has visited turning contract shops has witnessed a wide variety of jury-rigged solutions to workholding problems. The inventiveness of the small-shop owner is perhaps nowhere better illustrated.

The traditional answer to minimizing workholding changeover has been the use of stepped jaws. With two or three steps in the top jaw, a range of diameters can be gripped without changing the jaws. However, this often entails compromising some aspect of the process, such as having to run the chuck at slower speeds because you are gripping on a less than optimum area. That this compromise would take place at all illustrates the impact and difficulty of changeover for turning operations.

Quick jaw changing

Scott Looney, VP of SMW Systems Inc, explains because cycle times on lathes are so short, the costliness of changeover time is magnified. "The cycle times on a lathe are not long, 31/2 to 4 minutes, and that's a complicated part. Traditional jaw change, depending on the individual, is 15 to 20 minutes."

To reduce the time it takes to change jaws, Mr Looney's company developed chucks with quick jaw-change capability. The system allows three jaws to be changed by an operator in about one minute. Mr Looney believes anyone changing jaws three times a day is a candidate to use a quick jaw-changing system.

"If you're running ten-piece lots, and you have to set up, change jaws, and bore them for second-operation work, that's easily 30 or 40 minutes," he contends. "If I can change jaws in one minute and don't have to bore my soft jaws, I can run all my parts and be done before you get your chuck set up."

Mr Looney points out that many of his customers that are running repetitive work in small lots keep sets of bored softjaws on the shelf. With the SMW system, the company guarantees an accuracy of 0.001" or better when jaws are remounted.

Another method of quick jaw changeover developed by Kennametal, entails modifying existing top jaws to accept a clamping system and jaw insert. "The jaw inserts are held with a locking mechanism that is released by a pushbutton," explains James Goebelbecker, Kenna product manager.

One set of inserts will generally cover a 1/4" diametrical range. For a given chuck size, a range of insert sets are packaged together to provide gripping capability suitable to that size chuck.

Mr Goebelbecker says that the system may reduce the gripping range of a chuck slightly because they are using the mass of the top jaw as a base for the locking mechanism and for support. However, "on a 10" chuck, with our standard product, we cover from 2v4" to 51/2" diameters, which is typically the range people with 10" chucks run."

Depending on the workholding system and machine used, jaw changing can often be automated, either by a robot or device integral to the machine. A number of lathe suppliers offer an automatic jaw-changing option.

Quick-jaw-changing systems can provide tremendous productivity improvements if all the parts being turned require the same basic chuck. An initial investment in s jaws or inserts to handle the parts being machined will be returned quickly in the savings associated with shrinking changeover time. But these systems do not help when you need to change your basic workholding device.

Is changing jaws enough?

"If you can't gracefully change the chuck, you change the jaws," says Bruce White, sales manager of SP Manufacturing Co. "You can change jaws until you are purple, but if you need to go from a three-jaw chuck to a diaprhagm or collet chuck, it ceases to be a jaw change.

"Top jaws are regularly a compromise," he continues. "You feel since you have a chuck on the machine, you'll make jaws and you'll be damned if you won't contrive a set of jaws to do a second operation or another part. You go a long way in contriving jaws."

The prospect of changing chucks between short-lot runs may seem preposterous at first. All those bolts to unscrew, and actuators to be threaded on. What a mess. Scott Looney estimates that the time it takes to change over a conventional chuck is about three to four hours.

Recently, however, a number of chuck-changing systems have appeared on the market. And what they provide is a custom solution by providing the correct workholding device for a given application.

Most of these systems use an adaptor plate that is mounted to the spindle of the lathe. In essence, this adaptor becomes the new spindle nose, and will accept different chucks that contain backplates that are configured to mate with the adaptor on the spindle.

The key to maintaining accuracy between chucks is the method used to mate the two plates. Some systems use a coupling to locate the plates, and a series of wedge-like clamps to pull the backplate back and lock it to the spindle adaptor.

Another method uses a tapered shank on the backplate, that locates the chuck when inserted into a tapered bore on the adaptor plate. This system is similar to tapered toolholders used in automatic toolchangers. Once the chuck is located by the taper, a series of locking pins are engaged to hold the chuck in place.

While changing chucks allows you to get just the right workholding for an application, there are some downsides, one of which is weight.

Sometimes the combination of adaptors, chucks, and workpiece can weigh almost 500lb" says Bruce White "so you need a gantry to load it." Weight will essentially be determined by the chuck and part sizes, but when you get up to the 8" to 10" range or greater, it ceases to be a job that an operator can handle without some type of load assist. Another area that poses problems for some chuck changing systems is bar work. Sometimes the configuration and combinations of plates, adaptors, and chucks prohibits the ability to swallow parts inside the spindle.

Another downside is that the spindle is not running while you are changing over between workholding devices. And after the change you still have to grip the part before you can begin cutting, unless you can grip the part off-line, before the chuck is loaded into the machine. Pallets or chucks

With the exception of rotation, the issues are the same as they are on a machining center," Bruce White explains. "But nobody complains about having to change fixtures on them because they don't rotate the part."

Because a lathe rotates the workpiece, anytime the spindle isn't rotating, the machine is not working. That is why some chuck-changing systems allow you to grip the part while the chuck is off the machine. This requires either manually operated chucks or chucks with a self-contained actuator.

The benefits of this approach, though, are best demonstrated in a cell environment, when parts can be shuttled directly from a turning machine to a machining center without ever changing the workholding systems in between.

And at the high end of this spectrum is a machine such as the Tsugami 10APC System. This combination machining center and turning machine features an automatic work-changer station that accepts a wide variety of workholding devices.

The system uses a square pallet that can hold fixtures or chucks. The back of the pallet locates onto the spindle using coupling teeth. If a chuck is mounted on the pallet, the entire setup (chuck and pallet) rotates in a turning operation.

"At the operator station there is a device that is an exact replica of what is on the spindle nose," says James MacGregor, VP of technical sales at REM Sales. "This allows the operator to do all locating of the workpiece off-line, before the pallet is loaded onto the work-changer queue station."

This highly automated sophisticated system goes a long way in reducing setup time because it keeps the spindle of the machine running while parts are being loaded outside of the machine. However, the machine configured with the pallet changer and an automatic toolchanger runs about $500,000.

Cutting setup cost

The key to justifying any investment in setup-saving devices is the frequency of changeover in your operation. If all you are running is 2" bar work, you only need a permanent collet chuck. (Of course an automatic barfeeder will help reduce load time!)

However, setup reduction doesn't necessarily require added investment. It may simply require better organization of flow of materials through the work area-doing things smarter.
COPYRIGHT 1991 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991 Gale, Cengage Learning. All rights reserved.

Article Details
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Title Annotation:includes related information on Kennametal Inc.; small-lot production
Author:Arter, Richard K.
Publication:Tooling & Production
Date:Feb 1, 1991
Words:1619
Previous Article:Automation swings to cells.
Next Article:Simultaneous engineering.
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