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Untended machining - a boost to productivity and profits.

Untended machining--A boost to productivity and profits

The machining center is a machine tool that typically includes features making it possible to mill, drill, tap, bore, and contour a workpiece in a single setup. A pallet exchange system allows the operator to fixture the next part while the first is in the cut. An automatic tool changer (ATC) is programmed to supply the spindle with tools in correct sequence by means of sophisticated CNC controls that also control three or more axes of motion.

What more is needed, in the way of control capabilities or mechanical features, to permit the machining center to run untended for an extended period of time, say one shift? This is the question we asked a number of technical experts in the machine-tool industry. We also wanted to find out what sort of benefits can be expected from an untended machining center properly implemented. What does it take and what can you get out of it?

There is no argument that the technology to run untended is available today. Implementation, however, is more difficult than most people imagine; most attempts have met with only limited success. The potential benefits, on the other hand, are extremely attractive. The problem lies in getting users to understand not only the mechanics involved, but also the commitment needed in time and training to make the system work.

It is the great rewards, however, that potentially can be derived from untended operation that make the investment worthwhile. Roger Seifried, manager, Cincinnati Milacron's Contract Technology Services, views benefits derived from untended machining from the standpoint of one machine doing the work of several. He says, "A big savings is the cost of capital equipment needed to make a certain number of parts over a period of a year. It is essential to consider production on a yearly basis and not on an hourly basis. On an hourly basis you only count the hours of actual run time. This doesn't take into consideration the second or third shift when no attempt is made to run. Neither does it consider breaks or lunch time or weekends or holidays when machines are shut down. You've got to focus on the 8760 hours in a year that can be used to make parts.

"A one-shift operation, based on 40 hours for 50 weeks, yields 2000 potential hours of running time. Even at that, most machines will only be running 30 to 50 percent of the time. The rest of the time is used for part loading, unloading, checking, and other noncutting functions while the machine sits idle.

"Because they are designed to do so, untended machines can run all the time. If you are running a one-shift operation, you can potentially get three times the productivity by running untended during the second and third shifts as well as a fourth shift made up of weekend hours and other idle time when an operator is not present.

"The untended machine is going to cost more, perhaps half again as much, but you are buying one machine to do the job of four. On that basis, a two-shift operation will get twice the productivity. It is dependent on the number of shifts.

"The most dramatic productivity improvements, however, stem from factors that really have nothing to do with untended operation. Because the part program must be perfect, because setups must be minimized, because the spindle never is idle due to work in queue and the right tools in the ATC, we see three to one productivity improvements. Just because companies are forced to do their manufacturing engineering right, we see great gains in productivity. This doesn't include redesign of workpieces. People should learn from this and be as careful with their manned operations."

Maximum machine utilization

In almost every case, the people we talked to emphasized maximum machine utilization as the featured benefit of untended machining center operation. Robert W Ellig, president, Fritz Werner Machine Tool Corp, observes, "To effectively use machining centers in low-lot, high-mix situations, it is essential to manage the flow of work to the machines, manage the delivery of tools on a timely basis, deliver the programs when required, and be able to react to the changing needs of the machines. Solutions to these production problems in the past were to simply throw more spindles at the work. That is, as the mix increased and the lots decreased, additional machines were installed to cover for the loss in efficiency. This solution actually compounds the problems of scheduling with decreased output.

"Many users of machining centers have discovered the problems of utilization too late. With basic flaws in an attempt at automating a system, they wind up with a compromise at best. By taking advantage of the organizational capacity that an automated machine can bring to a shop, we have seen utilization increase from less than 50 percent to more than 80 percent on the same machine type with the correct strategies, hardware and software. Final production costs can be reduced by 25 percent or more with this increased efficiency."

The people at Mazak Machine Tool Corp, take a somewhat different approach in dealing with customers investigating untended machining, but it still leads to the question of maximizing spindle utilization. Chuck Daunt, manager, Factory Automation Sales, says, "In the very beginning, we ask the customer to develop a functional specification to clearly define what he expects to get out of the system. He states what parameters he wants to be able to operate within. This could include uptime, throughput, pieceparts--it depends on the customer. They will sometimes tell us the number of hours they want to operate the system, either manned or unmanned. We like to have a document like that so everyone knows what the goal is, a target to shoot for.

"This also allows us to point out errors in the customer's thinking. Ultimately, however, we try to get maximum spindle utilization time, because that's where the payback is.

Improved spindle operating time is a major benefit of untended operation and helps get a quicker return on investment. Some companies get as much as 20 or 22 hours of uptime by combining untended operating hours with regular shift operation. Another benefit gained with untended operation is higher quality of parts, more consistent quality than they can get with an operator tending the machine."

Some machine tool builders take a slightly different approach to untended machining, preferring to call it limited manpower production (LMP) for example, but the same goals are achieved. One such is Robert Schooley, president, Swedish Machine Group, who says, "LMP does not reduce the machining time of a workpiece. It does, however, require an investment in fixturing and tooling to do the job properly. So, when you discuss the option of LMP, you have to justify it in ways other than the typical return on investment. You've got to look at how much faster you can get a part into and out of production, and how much less you will have invested in in-process inventory. When these factors are taken into consideration, the true advantages become clear, including reduced handling time, reduced setup time, etc. It all depends on how well you plan the system."

Step-by-step approach

Careful planning was one aspect of untended machining mentioned by other machine tool companies. Another dimension to be considered in the decision-making process was added by Mal Sudhakar, product manager, Maho Machine Tool Corp. Consider a company's present machining operations and what it plans to do in the future. For example, if a company is now using conventional machine tools and plans to purchase CNC machining centers in the future, unmanned machining would be a mistake. Says Sudhakar, "Too often, companies want to get right into flexible cells and systems with all kinds of process monitoring functions on the machine. Such a quantum jump in technology from conventional machines is too overwhelming and justification is difficult.

"In this case, the variety of parts run is high and the volume is too low to warrant monitoring functions on the machine. A substantial investment is also required in fixtures, tooling, generation of NC programs and training, which is often overlooked. Rather than focus on untended machining, these companies should be processing jobs efficiently through their shops in the fewest possible setups."

Sudhakar also considers companies using fixed automation, producing high volumes of parts with short cycle times, as poor candidates for untended machining. He says, "These companies have used transfer lines and rotary dial machine technology and are enamored by the promise of flexible automation. With short cycle times and high volumes, feeding a large number of parts reliably through a whole shift without human intervention poses a challenge. The investment in machining centers, sophisticated handling systems, advanced computer systems with a host of monitoring functions, fixturing and tooling is very high and justification difficult.

"As a rule of thumb, a station in a transfer line can cost $200,000 to $300,000 including material transfer systems, fixturing, and tooling. A stand-alone machining center costs as much or more. Integrated into a system, the per spindle cost rises dramatically, up to double the cost for a stand-alone machine. In this situation, companies are finding that loading a number of stand-alone machines using a few operators works reliably, is highly flexible, and can compete successfully with the alternative of fixed automation."

Sudhakar feels that the best case for untended machining lies with companies currently using CNC stand-alone machining centers with plans to eventually advance into cells and systems. Favorable considerations are a good mix of parts with fixtures and tooling already available, cycle times of at least one hour, and medium volume runs.

Sufficient workpiece supply

A basic, stand-alone machining center is merely the starting point for untended operation. There are several considerations beyond the basic machine that must be addressed. To begin with, some way must be provided to queue enough parts in front of the machining center to provide enough work for an extended period of time. There are several ways to do this--pallet pool, carousel, or shuttle--that will keep enough parts ready to be machined during a shift. But, as Gordon Dirksen, DeVlieg-Sundstrand Machine Tool Group, says, "You must look for some way to increase cycle time while the pallet is on the machine. In this way, you can minimize the number of pallets needed to operate for an entire shift." If the machining cycle for each workpiece is 15 minutes, and these are fixtured one to a pallet, then four pallets would be needed per hour or 32 pallets per eight-hour shift. Obviously, the type of workpiece to be machined must be considered.

Milacron's Seifreid says their experience shows that parts with a 15-minute machining cycle time are generally of the size that would fit four to a four-sided tombstone fixture on a single pallet. Parts that take an hour to machine generally fit one to a pallet. Ganging workpieces on fixtures mounted on single pallets is perfectly acceptable in untended machining.

Workpieces designed

for automation

In addition to fixturing considerations, there are other workpiece-related pointers to keep in mind. De-Vlieg-Sundstrand's Dirksen says, "Parts that have good prior processing history are needed. If, during a manned shift, a part can be processed with no manual intervention, that becomes a good candidate for untended operation. Certain parts are better candidates than others for untended machining. For example, good candidates are parts that from a fixturing standpoint don't need something changed midway through the process. Parts that cause excessive tool wear needing attention during the process are poor candidates."

Another viewpoint on workpieces is shared with us by Mazak's Daunt. He says, "Parts with very close tolerances are not good candidates for untended machining. For example, a transmission case with a 0.0002" tolerance bore might cause a problem. You want to avoid tools with long reaches and castings with thin walls. We recommend careful visual inspection of castings before they are fixtured. You can do probing routines on the machine for, say, a core shift in a casting, but if you've got a very thin wall and have a core shift without an operator watching, you are not going to know that you have a problem until it is too late."

There was general agreement among companies we talked to that workpieces can be designed with automated machining processes in mind. Milacron's Seifried points out, "In untended operation of a machining center you need a big tool chain. You might get into 20 tools per part or per pallet. With ten pallets in queue, that could add up to 200 tools or more, which is expensive on a machining center.

"The right solution is to redesign the parts. The right solution to a lot of manufacturing problems is to redesign the parts. You can then use set of tools common to many workpieces."

Many companies have done just that. Mazak's Daunt says, "The decision to go to flexible manufacturing in our company was made more than ten years ago. Our president decided that, starting then, every product component would be designed to be capable of being made on a flexible manufacturing module, cell, or system. This forced our engineers to make parts only with a certain list of tools, keeping in mind that if the part can't be machined with these tools, it won't go through the system. We can put together a system to track 2000 tools; the only question is money. If you only have to manage 80 tools, it is much more cost effective."

Tooling and cell management

Tool capacity is one consideration, but you also have to have a means to monitor tool life in untended operations. Along with this, matching tool selection, broken tool detection, and a set of tool compensations--the ability to measure tools and automatically compensate for both length and diameter offsets--are important. Most agree that only the end user knows which tools need to be redundant.

Each machine tool manufacturer has ways to accomplish these tooling needs, but there are similarities between systems. Jim Hellwig, Giddings & Lewis Machine Tool Co, describes their approach: "As part of our tool changer, we have a tool-length verifier. This measures tool length before machining begins and again after machining is completed. It then compares the two values. The programmer sets the tolerance value he wants, for example, within 0.002" or 0.020". If the system finds the tool is not within limits set by the programmer, one of several things happens. Usually the program will call for an automatic tool exchange. The new tool will then go back and recut the part.

"We also have another software program called retract and reentry. This is used with adaptive control. If the horsepower limit is exceeded or horsepower drops to zero (meaning a broken tool), this condition will be sensed and automatically trigger the software to bring in a backup tool.

"We can also trigger tool exchange with a spindle-mounted probe as an in-process gaging operation. After the part is cut, the probe measures the part to see whether or not it is withing tolerance and makes a decision as to whether or not remachining is needed. The software automatically updates tool length or diameter compensation values according to the size requirements of the program."

In addition to tool monitoring, untended operation requires a means to monitor operation of the workpiece exchange and machining functions. Hellwig describes the management of these functions through a cell manager program. He explains, "It manages the workhandling and allows the programmer to assign a part number to a pallet as well as assigning a priority or pallet machining sequence. When the machine is ready to run a particular pallet, it will pull the appropriate NC program out of memory, make sure the tooling needed is there, and--if all is ready--shuttle the pallet into machining position. If everything fails--all backup tools are used up, for example--and there are no alternatives for machining that particular part, the software will shuttle that part out of the machining position and select another part for which there is sufficient tooling. It will keep machining parts untended. If all else fails, it will shut down the machine."

Chip removal methods

One consideration when a machining center is expected to run untended for a long period of time is chip control. It may be necessary, depending on the type of material being machined, to use a special chip conveyor. It may seem like an insignificant thing, but there is no operator present to remove chips from workpieces or from tools. This could cause difficulty with probing routines or tool-setting routines. some machine tool makers provide a system that floods parts to insure chip removal. Mazak, for example, describes their's as a Niagara coolant system, supplying thorough coolant to insure clean parts. They also have a special tap coolant system for customers who need a tapping fluid different from regular coolant. When the program calls for a tap, separate fluid is fed from a tank to the tap. For flushing out drilled holes, an undersized drill with through-the-tool coolant can be programmed to cycle through holes and flush out trapped chips, or it can function as a wand to reach other areas of workpieces. One way to remove stringy chips from twist drills is to rotate them in reverse after drilling and before returning them to the tool chain.

One aspect of untended machining that may not be obvious is how to cope with getting the machining center back under operator control after an extended untended run. As Seifried puts it, "After running untended, the operator comes in and must know whether the machine skipped a part, or missed a machining sequence. How does he know? He wants to load a part and start making chips as soon as he begins his shift. But the program that ran untended is no longer where it was. It has not stopped at some sequence that he can identify. He may spend hours figuring out how to restart that part.

"You need to save all the information so he can put the part back in and recreate the situation that existed on the untended shift. without that, he should really measure all the parts that were run untended to see if the process was in control. You may lose much of what you've gained in untended operation if the operator has to waste time trying to figure out what happened on the untended shift."

One of the factors that many experts mentioned to us concerning untended operation is that it forces people to think in terms of doing more machining per setup. This gets into fixturing the part where you can get to all sides and requires more thought up front. Users find they are putting more work on the manufacturing engineering function in a general sense, and specifically on the NC programmer, planner, fixture designer, etc. Some customers complain about doubling engineering staffs in order to run untended. But there is strong agreement that the benefits are considerable.

Ron Ewald, product manager, K&T's Modular Machine Products Div, says, "Because we are not comparing apples to apples, it is hard for a company to justify the purchase of an untended machining center. It would be easy to justify on the basis of reduced cycle time using traditional cost justification methods. But today, with the advent of just-in-time philosophies, most companies work with smaller lot sizes. This emphasizes the need to reduce changeover time, improve delivery, and strive to eliminate rejects and rework. This all points to having a more efficient, more accurate method of machining.

"A firm cannot automate by just buying the technology. You must learn the technology, use it and make it work for you. In order to reap the full benefits of automation, you must make the attempt to understand the system and learn how to continually improve it. And that takes a considerable investment in time and training."

PHOTO : Control of untended workpiece changing is demonstrated in the MC4000 six-pallet machining cell made by Giddings & Lewis Machine Tool. It uses flow-thru pallet stands placed strategically around the machining center. A workcharger controller allows choice of sequencing moves to be made for automatic pallet changes.

PHOTO : Sufficient coolant flow for positive chip removal is essential in untended machining operations. A case in point is the Mazatech H-400 horizontal machining center shown here with a six-pallet workpiece charger. Both tombstone and pallet-mounted workpieces occupy the carousel system on this machine built by Mazak Machine Tool Corp. The inset shows the company's Niagara coolant system with large-volume discharge to flush away chips, facilitating untended operation.

PHOTO : Shown is a large workpiece mounted in machining position on a Swedish Machine Group (SMG) machining center that typifies parts ideal for untended machining, i.e. multiple machining operations requiring long cycle times in a single set-up in order to increase machine-tool utilization. A spindle-mounted probe is being used to check the workpiece.

PHOTO : A variety of workpiece types and fixturing possibilities in untended machining are shown in queue before machining centers in a cell of the type made by Fritz Werner Machine Tool Corp. This illustrates the ability for untended machining to handle low lot, high product mix situations. To do this, however, programs must operate the machine and monitor it's functions automatically.

PHOTO : Several manufacturers of machining centers offer simultaneous, tilting table fifth-axis features, such as this one by Toyoda Machinery USA Inc. Their FH60/80/100 machines can be equipped with this feature that enhances productivity by saving refixturing time.

PHOTO : Seen in photo (a) is the MC50 5-axis machining center by Maho Machine Tool Corp, with one type of pallet arrangement. In this case, it is an automatic 10-pallet system. The inset shows the load/unload station with a robotic pallet transfer system delivering workpieces to the machining area.
COPYRIGHT 1989 Nelson Publishing
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Copyright 1989 Gale, Cengage Learning. All rights reserved.

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Author:Green, Richard G.
Publication:Tooling & Production
Date:Apr 1, 1989
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