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Making a dedicated machine tool flexible.

The dial index machine has been around for over 50 years. Its fundamental design and operating principles establish it as an efficient means for performing sequential, single-axis machining on a variety of relatively small parts at fairly high production rates. Workpieces machined on a dial index machine generally include plumbing and electrical conduit fittings, electrical housings and fixtures, hydraulic fittings and manifolds, small automotive and agricultural equipment parts, pump casings, etc.

The majore feature of a rotary transfer or dial index machine is its ability to precisely move fixtured workpieces into and out of machining stations at a rapid rate. Indexing of the dial in precise increments to transfer workpieces from one station to the next can take as little as two seconds. Generally, one station is left open to allow loading and unloading of workpiece fixtures.

High productivity of these machines is derived from the inherent ability to make chips at all active stations at the same time. Although there is a practical limit to the table diameter, machines have been built with up to 16 stations around the dial.

To further increase the productivity of these machines, various devices have been used to load workpieces into the fixtures. Finished parts can be discharged automatically into chutes.

Machining center alternative

Dial index machines are, above all, dedicated machines. Once tooled and set up, they are designed to turn out volumes of the same parts. In addition to the fixed location of the tool slide at each machining station--with a single axis of motion--part-holding fixtures generally are designed to accommodate a specific workpiece.

Although changeover from one size workpiece to another within the same family of parts is possible, tool and fixture changeover is typically a time-consuming process. Control of machine functions is electromechanical, with mechanically set stops, limit switches, and cams controlling tool actuation, feed, and operation sequencing, thus making setups and changeover a trial-and-error process.

The alternative to achieving flexibility needed to machine different workpieces on one machine, or to modify machining patterns from one workpiece to another, is by using a single-spindle CNC machining center. Equipped with a toolchanger and CNC program, machining centers offer a great deal of versatility in machining workpieces of varying sizes and shapes. They also can be programmed for a variety of machining pattern using as many as the tool magazine will allow.

However, when compared to a dial index machine, there can be a great difference in production output. A machining center makes chips with one tool at a time, and even tool changing, although automatically programmed, takes away from chip-making time. On the other hand, changeover from one workpiece to the next certainly is slower than on a dial index machine.

Flexible automation

To fill the gap between these two options, one company has devised a flexible dial index machine combining the features of a machining center with the relatively high output of a transfer machine. Called the Flex:Cell manufacturing center, the builder, Kaufman Mfg Co, Manitowoc, WI, proposes that versatility can be added to a previously dedicated machine tool by substituting small one-, two-, or three axis CNC machining units, spaced around the dial, in place of the dedicated machining stations. Additionally, each of the multi-axis machining units can be equipped with a toolchanger. See figure 1.

As one example of what this means, consider that the tool changer on the Flex:Cell might allow a workpiece to be spotted, drilled, and tapped without moving the part from a station. The same could be said if that part were spotted, drilled, and tapped on a machining centr. But, because of multiple stations on the flexible dial index machine, other machining operations are taking place simultaneously on other workpieces, thus increasing its productivity several-fold. On a traditional, or dedicated, dial index machine, spotting, drilling, and tapping would be done on successive stations, with an index made between each.

Another way that a flexible machine can show its versatility is by machining more than one side of a part at a time. An arrangement called a three-sided machine uses a cube-like fixture in the center of the dial, to which workpieces are fixtured, Figure 2. The term three-sided refers to simultaneously machining three sides of the workpiece in one setup.

By proper positioning of the spindle slides, both horizontally and vertically, it is possible to machine all six sides of a box-like workpiece in two passes around the dial, with one repositioning of the workpiece at the load/unload station. Should machining paths or hole patterns change, the concept of a flexible dial index machine allows repositioning of the necessary tooling through simple program changes, as would be done on a CNC machining center.

Two ways to achieve flexibility on this type of machine are by eliminating setup time through programming of tool axis positions, and by using toolchangers to bring more than one tool to the workpiece at a given stop on the dial. Another option noted by the manufacturer is to group two or three Flex:Cells and position a robot loader to feed parts to the workholding fixtures of each.

According to Kaufman executives, the Flex:Cell is designed for families of parts within a reasonable size range, none of which should exceed a 10" cube. It is intended for the market where repeat orders for parts are frequent and lot size is moderate to large. This precludes use in, say, a job shop, where every part is special and repeat orders are seldom called for.

To precisely determine the cost benefits that would accrue when using a flexible dial index machine, a case study is shown in the accompanying box. The workpiece is a malleable iron enclosure requiring drilling and tapping of eight holes around its flanged edge. Machining options include, (1) an eight-station conventional drilling and tapping machine, (2) a conventional CNC machining center, or (3) the Flex:Cell concept.

As the recap chart for machining rates for each case shows, the flexible dial index machine is more productive for 100- and 500-pc lot sizes, however, for this particular workpiece, the eight-station drill and tap machine is be the choice for longer production runs, even with a much longer setup time.

Personnel at Kaufman Mfg are careful to point out that each case must be studied and final choice of machining options must be based on individual merits. The Flex:Cell concept isn't the answer to all production machining situations.

Another consideration, one often overlooked, is the cost of carrying inventory of finished workpieces. Conventional wisdom states that cost savings can be gained by making long production runs using the most cost-effective method, such as a dedicated machine, where cycle times are short. Such extended production runs may result in a several-month inventory of finished parts.

More recently, however, manufacturers are realizing that the costs of storage and carrying inventory are high. It may be more cost effective to produce shorter runs of parts on a flexible production machine--say, a week's worth on inventory--and make production runs more often. This is possible when using a flexible machining concept that minimizes setup and allows quick changeover between parts by the use of CNC.

For more information from Kaufman Mfg Co, circle E19.
COPYRIGHT 1985 Nelson Publishing
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Copyright 1985 Gale, Cengage Learning. All rights reserved.

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Author:Green, Richard G.
Publication:Tooling & Production
Date:Sep 1, 1985
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