If the part you see below resembles a human heart, then you have a fair idea of not only the form, but also the function. This is an aircraft fuel control housing. It contains a system for regulating fuel delivery to the engines.
It’s also an expensive part. Machined from a solid block, it requires about 1,000 hours of programming time by a contract shop specializing in aerospace fuel system parts. However, arriving at the five-axis milling tool paths necessary to generate such a complex shape is not one of the factors that make programming this part so challenging.
No, CAM software today is quite effective for milling, says Mark DiLorenzo. He is the president of Tell Tool, Inc. (Westfield, Massachusetts) , the shop that machined this part. CAM packages available today go a long way toward automating the milling-related programming work for a form like this, even in five axes, he says. As a result, arriving at the routines for milling the outer form is relatively easy.
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| This aircraft fuel control housing was machined from solid aluminum. Milling the form isn’t the challenge, says supplier Tell Tool, Inc. The more difficult part is machining the complex array of holes. |
The challenging part is machining all the holes.
That’s because there are so many of them. And so many sizes. And so many different angles. Plus, many of these holes run deep. They intersect with other holes entering the part from far-removed directions. And this intersecting itself leads to still more complication. Holes have to be machined in just the right sequence—generally at the price of extra setups—just to ensure that they locate correctly with respect to other holes.
What all of this means is that hole making accounts for much of the reason why machining these parts requires such expertise.
And at Tell Tool, that expertise is continually refined. A competitive market keeps the shop engaged in an ongoing effort to reduce both lead time and cost. But at the same time, the very work that Tell Tool machines is in the midst of a fundamental change. Where all housings used to begin as castings, an increasing number of the parts are now specified as “hog-outs” machined from solid. Parts that might once have been cast in aluminum are now machined from aluminum billet. And in response to this shift in customer demand, Tell Tool has made itself adept at producing hog-outs.
But the most interesting aspects of Tell Tool’s story lie elsewhere. Achieving the proficiency for hog-outs had a lot to do with buying the right equipment. By contrast, many other process improvements the shop has implemented have more to do with technique—or how the equipment is put to use. And whether the part is a hog-out or a casting, these more subtle process improvements all keep coming back to more effective hole making.
Unlocking Automation
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| Three horizontal machining centers account for much of the housing production. The two shown here are part of an automated cell. |
The machines Tell Tool uses for a large share of its fuel control housing work are three five-axis A77 horizontal machining centers from Makino (Mason, Ohio). Compared to other machines the shop also uses for housing work, these machines are fast. Not only are spindle speed and feed rate higher, but the machines also have control systems equipped with features for maintaining precision through high-feed-rate moves.
These factors let the shop deliver hog-out parts more economically. By definition, a hog-out requires more material to be removed than a comparable casting . . . and by roughing at 16,000 rpm and 400 ipm, Tell Tool cuts through this extra material quickly.
The same capacity for speed and precision also results in more effective hole making—as in the case of holes machined through helical interpolation milling (see below). However, when it comes to more commonplace drilling, high pressure can be just as meaningful as high speed. The horizontal machines feature 1,000-psi through-spindle coolant delivery—a feature particularly important for drilling hog-outs. Unlike the short chips from a casting, the chips from an aluminum hog-out are stringy and continuous. Getting these long chips out of a hole as it’s being drilled often demands an assist from high-pressure coolant pumped through the flutes of the drill.
In fact, this very mechanism has led to cost savings. For deep holes in hog-outs and castings both, the shop has historically used twist drills instead of more rigid gun drills purely for reasons of chip evacuation. Twist drills do move the chips out more effectively . . . but they are also more likely to “bow” during cutting. For this reason, Tell Tool generally machined deep holes using a sequence of progressively longer twist drills, so only the deepest reaches of the hole would see the tool most likely to bend. But now, high-pressure coolant has eliminated the need to make this compromise. The shop gun drills on the machining center instead, allowing the coolant to carry chips away. As a result, holes that might once have required four different drills now can be machined with one.
Other savings come from the way high-pressure coolant helps minimize labor costs.
Two of the three machining centers are part of an automated cell. The remaining machine will eventually be married into this cell also. However, even now, all three machines run lightly attended. Two operators oversee them during the first shift, one operator does the job on the second shift, and during nights and weekends, all three machines often run with no operator present.
High-pressure coolant is one of the factors that allowed the shop to realize this low staffing level. A high-pressure coolant stream applied not only through the drills, but also periodically to rinse the part, effectively eliminates the danger of small chips shortening the life of delicate tools. Thus, the process can be trusted to run more reliably without oversight.
But what about the big chips? Hog-out programs have traditionally included M00 dwell operations just so the operator could pull out nests of stringy chips that had accumulated in the holes. High-pressure coolant would not be enough to do away with this step. Indeed, this very problem threatened Tell Tool’s ability to automate hog-out machining.
Bob Morin, the shop’s VP of engineering and operations, says the key to solving this problem came from an unlikely place. It came from a grocery store’s kitchen utensils aisle, where it cost about a dollar. By buying a simple corkscrew, and mounting this at the end of an extension, Mr. Morin created an effective device for automated chip nest removal.
Now, in place of M00 lines, hog-out programs instead include routines where the corkscrew is slowly rotated inside a machined hole. In this way, the homemade tool takes hold of any nest, and pulls it free when the machine retracts.
Talking To Designers
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| Here’s a simple solution to what used to be an expensive problem. This corkscrew tool overcomes the problem of chip nests accumulating in machined holes. Now, unattended hog-out programs include routines where the corkscrew tool is slowly rotated inside the holes to gather up any nests. |
Using a corkscrew as an automation tool is one example of the kinds of innovations Tell Tool puts to work. However, no matter how much inventiveness the shop applies toward the goal of cost savings, a shop alone can only do so much. Far greater savings result when the shop and the customer can work together, Mr. DiLorenzo says. The key here is communication.
For example, one part the shop was recently asked to machine featured 18 different types of undercuts. This variety served no functional purpose. Instead, it was the result of different designers unknowingly choosing undercuts of different types. And no one was in a better position than the machine shop to see how much cost this variety added to the process in terms of redundant tooling and additional tool changes. Questioning design discrepancies like this is one way the shop can help the customer save cost.
But potentially a more effective route to cost savings is for the shop to communicate its capabilities in as much detail as possible. To Tell Tool, the value of communication like this was recently illustrated by some changes in customer assumptions about “wrap stock.” Defined as the minimum material envelope surrounding a hole, wrap stock is particularly important for an aircraft’s fuel control housing, because wrap stock alone accounts for so much of this part’s shape. The ideal housing is one that comes as close as possible to including no more material than what the various wrap stock specifications require . . . because weight is a critical factor.
However, some housing designs in the past were heavier than they needed to be, because designers chose wrap stock values that assumed too much tool position error. Part of the purpose of wrap stock is to account for potential drill wander. But what some customers failed to appreciate was how much Tell Tool had reduced its drill wander over time, both through new equipment purchases and through improvements in technique. Therefore, to communicate today’s capabilities, the shop drew up its own hole wander chart to replace the one designers were using. As a result, wrap stock requirements on various parts went down . . . and along with them, the weight went down, as well. Communication improved the overall design.
Customizing CAM
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| The office is the focus of process improvement efforts today. To make it easier to generate effective hole-making routines, the company is writing its own CAM software. |
Now, Tell Tool has shifted its process improvement efforts from the shop floor to the office. To begin, the shop has set about ensuring that its own engineering staff follows a standard set of practices. For example, Mr. Morin took on the task of developing a uniform speed and feed rate chart for hole making based on material, hole depth, and other factors. The shop’s various CAM systems will all draw from this chart to set hole-making parameters automatically.
But more significant still is the change that’s coming to CAM itself at this company. To replace off-the-shelf systems programmers use today, the shop is writing its own CAM software for hole-making work.
The goal is to reduce programming time. Defining hole-making routines at Tell Tool today is largely a manual process. No off-the-shelf CAM system the shop has yet evaluated offers the functionality necessary to generate efficient programs for the complex, many-holed housing parts. According to senior NC programmer Jeff Pease, “Program one of our parts without manually editing the hole-making moves, and cycle time would be 30 percent longer.”
He says the reasons why have to do with rapid moves outside the cut. On five-axis machines such as Tell Tool’s that pivot the workpiece and not the spindle, CAM software often is unable to calculate the proper tool clearance plane for indexing the part to a hole at another angle. Therefore, the software has the tool retract completely— an overly conservative move that wastes time.
Similar problems result when programmers try to generate routines involving a sequence of drilling moves. In many cases, the software is unable to calculate a rapid move to bring the machine to within 0.050 inch of where the last cut left off. Saving time by getting the machine to rapid this close often requires manual intervention on the programmer’s part.
To reduce the need for manual work like this, Tell Tool has hired a programmer of a different sort. A computer programmer instead of the NC variety, she aims to create the next system Tell Tool’s NC programmers will use. The project before her is to capture the company’s various techniques for machining holes . . . learn what hole-making functionality the programmers wish they had . . . and apply this knowledge toward creating a CAM program custom-designed for Tell Tool.
The work of this programmer is ongoing right now. But when this project is finished, the company hopes to have effective hole-making equipment in the office to complement the equipment on the shop floor. MMS
| Why Hog-Outs? Today’s faster machining centers make it practical to cut complex housing parts entirely out of solid aluminum, instead of using a casting to achieve the basic form. And in Tell Tool’s market, customers are increasingly asking for the all-machining approach.
Lead time is the most important reason why. The two-step sequence of casting plus machining requires coordination between two different processes and two different suppliers. By contrast, when cutting can begin without waiting for castings to arrive, this leaves the machine shop free to deliver completed parts with much less advance notice. Replace a cast part with a hog-out, says Tell Tool’s Mark DiLorenzo, and the manufacturing lead time is generally cut in half. A small additional benefit comes from scrap rate reduction. With cast material, porosity variations occasionally result in defective parts that aren’t airtight under pressure. Hog-outs do away with this cause of scrap. Finally, machining the part from solid permits a more flexible process. This is true for manufacturer and designer both. The machine shop no longer has to deal with core shift or pattern wear variations that can change how the part locates from one casting to the next. And as for the designers, machining from solid means that they can change a design after the job has been sent to production without having to worry about reworking or reinventing expensive tooling. With hog-outs, the change can be made in the program instead of the pattern. |
Helical interpolation milling is a hole-making technique Tell Tool finds essential for certain applications. However, the ongoing move from castings to hog-outs is gradually doing away with the most important reason why the shop started milling in circles in the first place.
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| To machine threads in holes this small, the shop uses a helical milling tool designed and ground in-house. |
That reason is to finish cored holes. Holes initially formed during casting are rounded and straightened at the machine tool by cutting away the unevenly distributed material envelope that remains. This is a job that often required five tool changes when straight-feeding tools like drills and reamers were used. Switching to helical milling let the shop consolidate all of this work into a single milling cutter.
But drilling offers the higher metal removal rate. So for holes that are not cast, and therefore permit a more even cut, the shop prefers to drill. As a result, the passing of the cored hole is taking the need for helical interpolation away with it . . . but only to a point.
According to the staff at Tell Tool, here are the cases where this technique will continue to be applied:
• Machining harder materials. Metals other than aluminum make a stronger case for helical interpolation as opposed to drilling, because the former approach simplifies chip removal. When the shop machines housing parts in steel or titanium, it relies on helical milling much more.
• Undercutting. Where key cutters are needed to machine some feature around the exit of a through-hole, helical interpolation can save on the number of tools the part requires. An alternative to using a different key cutter for each diameter, helical interpolation can allow one tool to make undercuts of various diameters.
• Thread milling. After cored holes, the shop sees thread milling as the most important helical milling application. For very small holes, the shop grinds its own custom thread mills according to a proprietary design.
In many shallow holes, helical interpolation offers the only practical way to machine the threads. But the shop also uses the technique for some through-holes, because milling can place the threads more accurately than tapping.
For through-holes, anodizing is often the factor that dictates which thread machining choice is used. Thread tolerances are tight to begin with, and uncertainty in the anodizing layer can eat up much of the tolerance window. Synchronized tapping at the machining center does take less time than thread milling, but sometimes the need to machine the threads as accurately as possible justifies a small added cost in cycle time.
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