Low Cost Machine - High Efficiency Machining

This captive machine shop gets extremely high utilization of their vertical machining centers through clever workholding and handling procedures.

Article From: 5/1/1996 Modern Machine Shop, ,

It's a pretty familiar story. A company wants to bring some of their parts machining in-house for cost reasons, quality or control and they have to make an investment. Is the cost justification there? So they start to evaluate alternatives, maybe try to reclaim some old equipment, or maybe do nothing at all. Sometimes it's a Catch 22. As the investment is pared, so too is the return.

What an increasing number of shops are finding, however, is that they really can have their cake and eat it too. For one thing, the initial cost of some very serviceable machining centers are within the range of just about anyone really serious about production machining. You can do more with a $70,000 vertical machining center (VMC) today than its predecessor of ten years ago that sold at twice the price. But equally important, shop managers are getting a lot smarter about the use of low-cost peripherals--such as pallet changers, quick-load fixtures, indexers and other low-key forms of automation that allow them to squeeze much more out of their machining platform. Indeed, some are approaching levels of output that you'd expect to find in highly automated systems, yet maintaining a high degree of simplicity and flexibility all the while.

A good case in point is Watsco Components, Inc. (Hialeah, Florida), manufacturer of a range of components for heating, air conditioning and refrigeration systems. Watsco's management takes a broad view in their justification of technology, considering not just the immediate payback, but also how it fits with the company's larger objectives. "The decision becomes clear," says Dick Spolzino, senior vice president of operations, "when we consider our business objectives in this highly competitive HVAC component marketplace. We constantly review state-of-the-art technology for application to all of our processes. Automated processes are a high priority."

Two years ago the company began to upgrade its machining capability with the purchase of its first VMC, a VF-1 from Haas Automation (Chatsworth, California). A year later, says manufacturing engineer Ozzie Mazzini, demand for CNC machining was already outstripping their capacity, and they were ready to buy another machine. But then they really got smart about how to first get the most out of the spindle they already had before adding another.

Pallets

The Haas machine is representative of what this new class of low cost machining centers is all about. For less than $55,000, Watsco got 20 by 16 by 20 inches of travel, a two-speed gearbox, 20-tool ATC and a 32-bit CNC. Not a lot of frills, but all Mr. Mazzini was after was some basic, dependable machining capacity.

And at that, the machine worked just fine. The shop handled setups in a conventional manner, mounting vises or fixtures directly on the table, dialing them in with the machine's location offsets, and running parts a piece at a time. Because all was going smoothly, it would have been easy simply to add another machine when they recognized the need for additional CNC capacity.

Instead, Mr. Mazzini investigated going to pallets so that workholder setups could remain intact, and different jobs thus could be whisked on and off the machine. Moreover, pallets would provide the opportunity to get more creative with fixturing, opening the door to various approaches to multiple part machining routines that would dramatically cut the amount of time spent loading and unloading workpieces, and thus boost output through higher rates of spindle utilization.

For about another $12,000, Mr. Mazzini achieved this additional capability in the form of a manual pallet system from Midaco Corporation (Elk Grove Village, Illinois). The system includes a powered pallet receiver that is permanently mounted on the table of the machining center. Mating with this particular receiver (model 2216) are 22 by 16-inch pallets on which individual workholders are mounted. In operation, a pallet is rolled over the top of the receiver on a bolster and then, through pneumatic actuation, the pallet is set down over a group of location elements and locked into place.

Mounted on the front of the machine is a two-pallet shuttle system that allows one pallet to be set up or loaded while the other pallet is in the machine. When the machining is finished, the pallet is unlocked, rolled out of the machine and set aside; and the other pallet is rolled into place and locked, with the entire exchange taking less than a minute.

Creative Fixturing

Of course, having a capability is one thing, using it wisely is quite another. It is here in particular, however, that Watsco is showing exceptional promise. A fixture designed for a group of vacuum pump parts illustrates just what level of efficiency is achievable on such a low cost machining platform.

The fixture is designed to hold a kit of four different pieces that are assembled into a portable vacuum pump Watsco makes for evacuating coolant from air conditioning systems. Each kit includes two different stator body pieces and two different endplates, and the fixture holds four sets of the kit. The endplates require machining operations only to one side, and they thus are completed in a single clamping. The stator pieces must be machined on both a side and a face, and so require two clampings, both of which are held in the same fixture. In operation, the stator blanks are first mounted for the side operations. After all of the pallet's workpieces are machined, and the pallet is pulled for reloading, these pieces are moved to the face-machining section of the fixture and re-clamped, and new blanks are loaded in their place. This way, four complete sets of parts come off the fixture after each cycle.

Mr. Mazzini has designed a number of features into the fixture to add both convenience and consistency to the process. For example, location pins and tracks assure simple and foolproof workpiece positioning. The clamping throughout the fixture is carefully uniform. All the hold-down bolts are set with a pneumatic torque wrench. The end clamp--which holds the stator bodies for the initial side-machining operations--is also pneumatically actuated. The pressure is regulated such that they can get a firm and consistent grip, but not so heavy that they inadvertently mar the surface of the parts. Given that these pieces fit together to form a vacuum chamber, surface imperfections are flatly unacceptable.

The side-machining station on the fixture is also interesting for several other reasons. The stator pieces are double-disc ground before they are machined. While this preliminary operation is primarily motivated by the need for high tolerance flatness and surface finish on both faces of the parts, it also provides the added benefit of extremely consistent thickness from piece to piece. This allows the eight stator pieces simply to be stacked, albeit on their sides, for the side machining operations while still maintaining an acceptable location tolerance for each piece. So to load the pieces, all the operator has to do is pull a pin on the side rail, lay in eight pieces, and re-pin the rail.

The end clamp will not be actuated until the pallet is set in the machine. Rather than having to deal with a lot of piping and awkward connectors to get air to the pneumatic actuator, Mr. Mazzini made his own connection that is as simple as it is convenient. The air supply is run through the back of the machine to a connector permanently mounted on the machine table. A mating connector, which is mounted on the pallet, runs to the pneumatic actuator on the fixture. The two connectors are positioned such that, with a simple O-ring seal, the pneumatic connection is made when the pallet is dropped into place on the receiver.

Also very important for the sake of productivity, the fixture makes the very most of the machining center's physical capacity, crowding as many workpieces as the table will allow into a single cycle, further reducing part-loading downtime. The fixture is actually larger than the machine's travel range; however, the workpieces are situated such that each machined feature is accessible.

Time And Location

Even without the pallet changer, it would have made sense to build the fixture for these parts. Besides all the benefits associated with machining multiple pieces in a single setup, machining them in kits this way assures that Watsco maintains a balanced inventory for assembly.

But with the pallet changer, they dramatically increase the efficiency as well as the capacity of the machine. The cycle time to produce a pallet full of parts is 28 minutes. Then it takes about 15 minutes to unload the pallet, blow it free of chips, and then reload it with new parts. But because Mr. Mazzini has built two identical fixtures, part loading time is virtually nil. As far as the machining process is concerned, the only internal loading time is the 40 to 50 seconds the operator takes to roll out one pallet and roll in another. Mr. Mazzini figures that increases his daily output for this process by some 65 percent.

And that's not to mention the reduction to initial setup, which has become negligible, or the impact on quality. Workpiece position repeatability across the entire setup, Mr. Mazzini believes, runs somewhere between two and five "tenths" (0.0002 to 0.0005 inch), which is well within the tolerance range of the job in terms of general feature position. That means that this or any other similarly toleranced and proved-out job can be rerun on the machine without the need to re-establish workpiece locations, or to take slow trial cuts.

The most challenging feature is the bore of the stators. The overall bore is done with a fine pitch, adjustable KPT Kaiser boring head, and held within one thousandth of an inch. However, within the bore, on one side, there is a smaller undercut arc. This cut is extremely critical from a functional standpoint, as it provides the pathway by which the vacuum is created. Though the spec is somewhat more generous, Mr. Mazzini has challenged himself to hold five "tenths" on this feature, relative to the larger bore, and so far has consistently done just that. A regular sampling of in-process parts is checked on a Sheffield Cordax CMM, so Mr. Mazzini has hard data to verify they are achieving that objective on a regular basis.

Systems Thinking

Watsco did buy another machining center after all, not to handle the overflow, but to apply similar process strategies to other kinds of parts. For example, the new Haas VF-3 has a 40-inch table for which Mr. Mazzini has some interesting plans. The pallet receiver is already mounted, but next to it will be an indexing unit integrated with the machining center's CNC. To go with it, Watsco is currently building swiveling three-part box fixtures to hold investment cast housings that must be drilled and tapped. The swivel mechanism includes a gear that meshes with another mounted on the indexer when the pallet is set into place. This way, the repositioning of the entire box fixture is controlled within the part program, allowing the machining center to get at three sides of these workpieces in a single setup.

When you walk into Watsco's CNC shop, what you see are just two relatively compact machining centers with manual pallet switchers out front. But when you think about it, what they are really doing here is applying many of the same basic principles that underpin highly automated systems: Extremely high rates of spindle utilization. Low labor content and minimal operator intervention. Quality and consistency engineered into the process.

But unlike an awful lot of automation, they are also keeping their "system" simple, with the flexibility of a stand-alone machine...and a price tag that forced no compromises to their original objectives.

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