Nesting should weigh all process elements, inside the punch press and out
May 13, 2008
Nesting for punching must not only consider the part geometry but also the tool boundary. Some nesting software can respect the tool boundary rather than just the part boundary. The software respects the integrity of the punched part, the skeleton, and tooling all at once.
Nesting for the punch press should take into account not only the part boundary, but the tool boundary as well.
Lean principles advocate a flexible manufacturing environment that theoretically can produce a batch quantity of one just as efficiently as a batch of 10,000. Kitting, single-piece part flow, demand-pull, and other lean elements all aim toward this goal. Fabrication shops tackle how raw material moves between processes—from the punch press, to the brake, to welding, and so on. But to become truly lean, a shop also needs to maximize efficiency of the processes themselves, including punching. In a certain light, what happens between the tool and sheet metal is where the rubber hits the road in a lean shop.
In punching, maximizing efficiency takes more than squeezing the most parts onto a sheet. Effective use of available tooling, be it in a turret or rail, also must enter the equation, along with required part accuracy. And all are inextricably linked to efficient part flow and a production schedule that considers due dates together with downstream work-in-process (WIP).
This is where dynamic nesting comes into play.
For years job shops have nested statically, grouping everything for one order on one sheet or batch of sheets. This worked well in the traditional manufacturing environment, with large orders and inventories.
However, when demand becomes more erratic, grouping jobs to maximize material utilization becomes more difficult. Typically, a job of 52 would be nested 100 times on a sheet; 52 of those parts would move downstream while the remaining 48 would sit in WIP inventory. Lean, of course, shuns such WIP. Thats why today more shops have moved to dynamic nesting, grouping dissimilar jobs on the same sheet. By nature, dynamic nesting caters to dynamic demand and varying batch quantities.
Click image to view larger Nesting for the punch press must respect the tool boundaries and ensure skeletal integrity.
Historically, dynamic nesting software has fought an uphill battle against the PCs slow processing speeds: It takes a lot of computation to organize different parts most efficiently on a sheet. Today, however, the computing horsepower constraint has vanished.
In punching, what hasnt vanished is the challenge of ensuring a particular nest accommodates the available tooling. Some nesting software can respect the tool boundary rather than just the part boundary. The software respects the integrity of the punched part, the skeleton, and tooling all at once. It knows what station has which tools, along with the tools size, orientation, how many hits the tool has made, and the tonnage of those hits.
The goal is to minimize tool changeovers. A turret might have 48 tools—but what if the parts on the nest require more? A turret may not have a tool for a particular hole size, for instance, but a smaller punch still may be able to nibble out the shape. This eliminates the need for a tool changeover.
But what if the round punch takes 50 hits to nibble out the larger hole? How does this nibbling affect tool life and productivity? Would it be more efficient to move or delay the specific component so its punched out with the right-sized tool? For efficient nesting under the punch press, all these considerations must enter the equation.
Unlike in laser and plasma cutting, turret punching doesnt have to take kerf quality into account; sharp punches are designed to make a clean cut. But punching programmers do have to worry about punch tools and the space required for operations like nibbling. Punching nests need to be slightly farther apart, when compared with soft-tool processes. For punching, 7⁄8 inch between part boundaries is a good rule of thumb, with the tooling requiring the first third of that space, another third for the skeleton itself, and the final third to account for tooling on the other part edge.
With sufficient space between parts, the operator doesnt have to worry about skeletal integrity during an operation. In laser and plasma cutting, the tool typically moves while the sheet remains stationary; for punching, of course, its the opposite. For this reason, technologies like laser and plasma cutting can cut parts with common cut lines and within extremely close nests, generally with about 1⁄4 in. between parts. As long as the remaining skeleton doesnt become so flimsy it falls apart on its own, part accuracy shouldnt be affected.
In a turret press, a flimsy skeleton can spell disaster. The sheet moves under the turret, so the skeleton must remain robust enough to prevent significant jostling, which can reduce part accuracy or crash the machine.
A fab shop cant avoid tool changeovers, of course, but software can help minimize them. Software can make nesting decisions using currently available tooling. Not only does this minimize changeovers, it also helps avoid a common mishap: an incorrect tool in the machine. If a square tool attempts to punch a round hole, it scraps not only the part its punching, but the surrounding parts in the nest as well. If the software builds nests using only the machines current tools, an operator will make fewer trips to the toolroom.
If a part cant be made with available tooling, the software can alert the programmer, who can save the part for a later run if the schedule permits. In that same vein, the programmer can quickly identify nibbled parts. Excessive nibbling causes tools to wear faster, with each nibble exerting significant side loads to the tool and sleeve. Seeing the nest, a programmer can shift the part to another machine with the right-sized tool, or delay the component to the next tool changeover, again depending on the due date and WIP considerations.
For maximum efficiency, a programmer may choose to punch same-sized holes in various parts on a sheet first, moving the sheet around to accommodate, then index the turret to a different-sized tool, punching those holes in the same manner, and so on. However, because this strategy requires the sheet to move more, accuracy can suffer slightly; the more a sheet moves under a turret, the more chance there is for slippage and, hence, inaccurate punching. Alternatively, a machine can punch all the different-sized holes in a particular part or area. This means that the punch must index more, which is less efficient, but it will ensure higher accuracy.
The tool strategy can change when form tools enter the mix. A few, like offset tools for forming flanged edges for cabinet doors, do affect how the nest is arranged. But for the most part, forms do not complicate the nest itself, since most arent on part edges. Forms do, however, affect punch order and tool arrangement. They are usually made last, after punching, so the form isnt dragged across the table. Some forms—especially high ones—may hit tooling, particularly if a flimsy skeleton bounces slightly before the formed part is finished. Again, nests that take into account the tool boundaries and maintain skeletal integrity eliminate this problem.
Nesting software can "talk" directly with scheduling software, and parts that are past due are given higher priority than those due in a week. But schedules can add complications. Programmers are all too familiar with the last-minute job butting in and, at times, making a nest that doesnt maximize material use. For this reason, software can nest on the horizon, out a day or more. Drawing from a greater number of components, nesting software has more of a chance to arrange parts for maximum material utilization. Here, filler parts, or those demanded frequently, may be able to fill the remainder of a sheet when necessary.
Nesting shouldnt look too far ahead or run too many filler parts, because it could flood the shop floor with WIP. Programmers must balance maximum material utilization with downstream production needs. Getting the most out of the sheet means little if a welder has to sit idle and wait for a component that wasnt nested because it didnt fit neatly onto a sheet. Fitting the component on the nest might produce a little more scrap, but the cost is nil compared with the expense of a highly skilled welder sitting idle and waiting for work.
Nesting has evolved to the point where more punching (and other cutting operations) can become lights-out. Software can dynamically nest parts automatically for maximum material utilization, best tool usage, as well as the best product flow per the schedule. It doesnt eliminate the need for manual intervention at all levels, of course. If a part defect occurs, for instance, an operator must note it in the software, which automatically renests the part in a later operation. But it does change the programmers role.
Instead of concentrating on the best way to nest parts on a single sheet, a programmer now can take a more macro look at the operation. Software isnt perfect, so programmers can manually override some nests and switch out certain part components with others, if needed. They can route parts to different machines to, say, get all the parts of an assembly to a welder faster.
Ultimately, software automates nest generation and intimately ties in the original CAD/CAM file with shop floor operations. It does in seconds what used to be time-consuming work for a programmer, and allows that person to concentrate on making the overall operation more efficient.