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When the chips are flying, you're being productive. But shop owners know that too much of the total time needed to machine a part doesn't include actual metalcutting. Part loading, tool changing and presetting, test cuts and machine indexing can consume as much as 80 percent of the time needed to completely machine a component.
Long before the days of CNC, screw machines offered a way to maximize cutting time. They were the original multi-taskers, with their multiple spindles and tool slides that helped increase productive cutting time and raise parts-per-hour output.
Fast forward to late twentieth-century manufacturing and the rise of JIT supply, lean inventories, and smaller production runs to meet the needs of individual customers. The days of turning on a machine tool and pumping out long runs of parts to stack on the shelves for inventory are gone. Flexible manufacturing and quick changeovers are the buzzwords. Now, when it's time to change a screw machine's setup for another short production run, its multiple tool position assets are a liability.
We talked to Bill Magill, a senior technical service engineer with Kennametal Inc. (Latrobe, Pennsylvania) about these and other trends that are causing changes in the tooling strategy of many screw machine shops. "Changeover is one of the few drawbacks of screw machines," says Mr. Magill. "A shop we know outside Cleveland is a typical example. They were running lots of 100 to 500 pieces. It took 45 minutes to an hour to run some of those lots, and as long as eight hours to set the machine up for the next one."
The Quest For Quick-Change
According to Mr. Magill, one part of the solution to long changeover times is to use quick change tooling on the screw machine. For a screw machine, he says, the maximum benefit from quick-change tooling comes from systems that enable the shop to preset tools off the machine and switch them rapidly when it's time to change the cutting edge or make a new setup. As a start, end slide tooling offers the best and most immediate opportunity to gain the benefits of quick-change tooling.
Quick-change tooling for screw machines does, however, have its own special issues. Space limitation is one. Retrofitting a quick-change tooling system usually extends the dimensions of the tools. Therefore, the system should be as compact as possible. Minimizing the increase of dimensions is also beneficial because it places the load on the insert close to the coupling, maximizing the rigidity and the accuracy of the system.
Kennametal's KM modular quick-change tooling system is one that fits those criteria. It's a compact system that exhibits axial and radial repeatability within ±0.0025 mm (±0.0001 inch). It features face contact between the cutter body and the toolholder, which promotes axial accuracy. The system's self-aligning taper shank promotes radial repeatability. The round, centerline tools are designed for application on lathes, turning centers, multi-tasking centers and machining centers.
Moreover these tools are interchangeable. "A shop owner can put the same quick-change tools on his state-of-the-art CNC lathe or machining center as he uses on his screw machines," says Mr. Magill.
KM tooling units can directly replace the straight-shank collet chucks used in existing tool blocks. They offer the same features as the straight-shank holders including through-tool coolant and use of standard hardware. When changing the tool on through-tool-coolant systems, coolant lines and hookups can stay in place because only the cutter is changed.
"An even bigger big benefit of quick-change tools is that they are presettable off the machine," says Mr. Magill. "For an end-slide tool setup strategy, a shop can have three cutting heads: one in presetting, one ready to go on the machine, and one running. When the drill gets dull, you turn the machine off, open the quick-change coupling, take that tool out, put the new one in, and you're back running in a matter of seconds. Then you can take the used tool away from the machine, put in a new drill, reamer or whatever, reset it, and you're ready to go again. That represents tremendous savings in tool maintenance time. The benefits are similar when it comes time to change setups for a new part. If the next setup uses a 2-inch drill and you're running a 3-inch, you can have the new preset 2-inch drills ready to go in the machine. When it's time for the new setup, you can just take out the cutting unit with the 3-inch drill, pop in the preset one with the two-inch drill, and you're off and running. You can take better maintenance care of your tools because you can take the tool away from the machine, put it in a tightening fixture, and clean it up. It's easier on the operator, too."
Holder Alignment Is Crucial
The KM quick-change system can handle high speed-steel or carbide drills, but Mr. Magill notes that the carbide drills require added care in application. High speed steel drills are more forgiving, he says. "They will bend. So if the alignment is a couple of thousandths off, it will still drill the hole."
But even minor misalignment between a carbide drill and a workpiece can result in damage to the drill or a scrap part. "If a shop uses carbide drills, they have to be sure that each tool block is line bored for the position it's in," Mr. Magill continues. "If you have a number of screw machines and a toolblock gets damaged on one of them, you may take a block from a machine's that's not running. But generally it won't be lined up correctly. To protect the drill and maintain the accuracy of the hole, everything should be on center. This doesn't necessarily mean buying a new tool block; you can sleeve the existing block and rebore it. All tool blocks are marked from the manufacturer, and all were line bored in position originally, but that doesn't mean that 30 or 40 years later they're still the same toolblocks. You can put a sleeve in a block that held a 2-inch shank, possibly and go down to 1 3/4 inch and line bore that. Line boring is simple; put a boring bar in the spindle, get a cam or hand crank it through, and everything's dead center."
With proper alignment on centers, a speeder can be used to magnify the productivity advantages of carbide drills. If drilling time is a controlling factor in the overall machining cycle time, changing to a carbide drill and employing a speeder can significantly cut drilling time and thereby reduce the overall time necessary to complete a part. The savings could well justify the use of carbide drills.
A Quick Step Toward Quick-Change
There is a simple quick-change upgrade for screw machines of which many shops simply aren't aware, says Mr. Magill. "Many shops don't realize that they can replace several tools in the screw machine cross slide with standard square shank lathe tools that use indexable inserts. In many ways, a screw machine is simply a multi-spindle lathe. You can go through a catalog of lathe toolholders and attachments and pick out any number of tools that will plug right into a screw machine.
"Standard lathe tooling includes an extremely wide selection of insert materials and geometries," Mr. Magill continues. "Squares, triangles, diamonds and rounds are available in a variety of grades and chip-control geometries that enable a shop to tailor the tool to its specific job. That can be a big help in dealing with the speed and feed restrictions that are characteristic of screw machine operations. We're not looking for maximum performance out of the carbide; we're looking for maximum productivity out of the machine. Cutoff, especially, is an area where indexable insert tools can make a big contribution to productivity.
"When you put square shank holders on the cross slides, indexing an insert becomes a basic form of quick-change, especially from a tool maintenance aspect. It's easier to index an insert than to change a whole tool, regrind it and reset it. There are gages for resetting a reground tool, but there can be problems. When you're indexing an insert, you're always within a thousandth or two of being there, and for rough chamfering, facing, turning and breakdown that's fine," says Mr. Magill.
Dwelling On Carbide Over HSS
Carbide tooling still has to gain full acceptance in many screw machine shops. "A lot of shops are using brazed carbide tools, and many are using inserts," says Mr. Magill. "They'll just spot them here and there. One of the big arguments has been ‘I tried carbide once and it doesn't work.' Over the years there have been tremendous advances in carbide tooling, and today it's much more forgiving when subjected to dwell. All screw machines feature dwell in the cutting cycle, because if you simply go in and then back out with a tool on the cross slide you'll have an egg-shaped part. There must be a couple revolutions of dwell in the stopped position to assure that the part is round. Dwell also produces better part finish in some operations."
Carbide tools can enable users to increase feeds and speeds, and perhaps speed up the slower operations in a part sequence that determines the overall cycle time needed to make the part. Carbide form tools to replace high-speed steel are available from a number of specialty carbide manufacturers. These dovetail tools are easy to set. "You just set them flush with the end of the holders," says Mr. Magill.
Another Carbide Advantage
In addition to enabling reductions in machining time, use of carbide tooling can also help reduce setup times. "Assume you are running a family of different-sized bushings made of an inexpensive workpiece material such as 1030 steel," says Mr. Magill. "Every time you change from 3/4, to 7/8, to 1 inch, and so on up to 1 1/2 inches in the part family, you normally would change the bar stock. But the setup time may cost you more than if you went from a 3/4 to a 7/8, used the same bar stock, and machined away the excess. Faster cutting carbide can rough the extra material without a problem, while high speed steel would be pressed to cut as fast. Of course, with more expensive exotic workpiece materials this idea would not be worthwhile. It would be more cost-effective to change the bar stock."
Mr. Magill adds that carbide tools are usually more forgiving than high speed steel at higher feed rates. "Sometimes if you feed a high speed-steel tool another thousandth it will blow up. A carbide tool may provide another ten thousandths inch per revolution that you don't have with high speed steel."
Taking Quick-Change To The Max
The ultimate in quick-change tooling for screw machines requires custom engineering of tool blocks, cutting units, specially ground inserts and workholding systems. As an example, one valve manufacturer in Illinois was running a family of valve bodies, in a range of diameters and workpiece materials, over a pair of screw machines (chuckers) with a third machine as a backup. Using quick change tooling for the end slide, special quick change facing and grooving tools on the cross slide along with quick-change chuck jaws, the shop managed to cut setup time from more than 18 hours to only 4 hours.
"The results were so good that the shop was able to completely shut down a machine they had previously need for backup, and they brought in all the work they were getting done on the outside. All the work was run over those two machines with the custom quick-change tooling," says Mr. Magill.
Not By Tooling Alone
It's important to note that the efforts to develop quick-change technology for screw machines are not limited to tooling alone. Machine tool makers and their accessory suppliers are producing quick-change collets and pusher fingers, as well as other items, to deal with setup issues.
Machines with CNC controls can be programmed to employ tooling in ways that eliminate some tool change and setup operations. "It's not just the cutting tool industry that's been trying to address this. It's everybody, and the beneficiaries of these efforts are the screw machine shops," says Mr. Magill.