February 2012 / Volume 64 / Issue 2|
Two (or More) for One
By Susan Woods, Contributing Editor
| Multiple-operation cylindrical grinding machines enable shops to complete parts in a single setup.
Part manufacturers have traditionally performed grinding in single operations on multiple single-wheel machines. But grinding machines that allow multiple operations in a single setup offer advantages.
The two main benefits of multiple-operation grinders are enhanced repeatability and reduced cycle time. The latter is achievable because work handling between operations is reduced or eliminated. Also, part accuracy improves because dimensions between part features can be better maintained.
Courtesy of United Grinding
“As you reduce the number of setups, you also reduce the chance of making a mistake or having to make offsets or adjustments in the part,” said Shane Farrant, national product manager for grinding machines, Toyoda Machinery USA, Arlington Heights, Ill. “If you flip the part on the machine or have to go to another machine, different variables can be introduced.”
Another benefit is faster throughput. “By grinding in one setup, you eliminate the locating, fixturing and load/unload time going from one grinding machine to another,” said Dennis Peters, account manager for Cinetic Landis Corp., Hagerstown, Md. “This eliminates what we call the ‘parasitic time,’ the nongrind time.”
Also, a single-wheel grinding machine requires more wheel changes. “It can take 15 to 30 minutes for a wheel change, whereas if you can just index to a different wheel it takes seconds,” said Jeff Hilliard, national sales manager for the Grinding Group at Hardinge Inc., Elmira, N.Y.
Courtesy of United Grinding
A fourth benefit is that one universal cylindrical grinder can replace four or five single-wheel grinding machines. Fewer machines, of course, take up less space and require fewer operators.
Traditionally, universal cylindrical grinding machines allow a cylindrical part to be ground externally as well as internally in the same setup. Due to accuracy and quality requirements, typical applications include auto parts, such as those used in engines, transmissions, fuel injectors and pumps, as well as crankshafts and camshafts. Other parts include diesel rod assemblies, oil and natural gas pump components, compressor components, and aerospace components, such as gears, gear shafts, pistons and other aircraft engine components.
Hilliard said the customers for Hardinge’s universal cylindrical grinders include those for which “parts are typically high value and low-production volume. For example, in the die and mold industry, there might be just two pieces per mold but those pieces cost between $10,000 and $20,000 each. Grinding is typically the final operation.”Heads and Spindles
Universal grinding machines can have one wheelhead with several spindles for grinding wheels or multiple wheelheads with multiple spindles for wheels. “You could have a multiple-spindle turret wheelhead able to hold multiple wheels. You could also have a single spindle on your turret, holding two wheels on each end,” said Hans Ueltschi, vice president of sales, Cylindrical Div., United Grinding Technologies Inc., Miamisburg, Ohio.
He added that the machine can do multiple operations simultaneously, such as ID and OD grinding or face and ID grinding. “Or the machine performs the operations in sequence,” Ueltschi said. “Sequential operations take longer but make the grinder really flexible, improving the work envelope and geometry inside the machining area.” United sells both types under its Studer and Schaudt brands.
Toyoda’s universal grinding machines have one wheelhead with two or three spindles and grinding wheels. The machines do not do operations simultaneously, but do perform them in the same setup.
“The typical three-wheel configuration for a universal machine is one plane, one angular and one ID spindle,” Toyoda’s Farrant said. “The plane wheel is used for plunge/traverse grinding; the angular wheel can be used for face, various OD and shoulder work; and the ID wheel is used for internal grinding. There are various ways to configure the grinder, depending on the application. For instance, some customers will use two plane wheels of different grits on the same machine instead of mixing plane and angular wheels. In this case, one wheel is used for roughing and one for cleaning up tight surface finish requirements.”
Cinetic offers universal grinding machines that have two wheelheads with two spindles each. Each wheelhead can have a B-axis. “The B-axis is what pivots about a fixed bearing located in the wheelhead and allows you to apply that second spindle on each of the wheelheads,” Peters said.
The machine can apply two wheels simultaneously, but only one spindle at a time on each wheelhead. Doing two operations depends on where on the part the features are being ground. “For example, say you are grinding the OD and face of the post and the flange on a crankshaft,” Peters said. “You can do those simultaneously because they are on opposite ends of the shaft. You would have a center drive to rotate the workpiece from the central feature of the crankshaft and then you could simultaneously grind the flange and the post on the opposite sides of the shaft.”
Other operations performed on a universal cylindrical grinder include vector grinding (grinding a face and an OD simultaneously) and interpolation. “If you want to generate a radius with the corner of a grinding wheel, you can use interpolation,” Peters said.
He noted that Cinetic is developing a unique application for the ECOFLEX universal grinding machine that finish grinds the part and then polishes it with a polishing head, all in one chucking.
Hardinge’s Kellenberger universal grinding machines do not grind simultaneously. The machine has one wheelhead with up to four wheels on it.
In addition to grinding ODs, IDs and faces, some universal cylindrical grinders feature a C-axis for the workhead that enables form and thread grinding.
Courtesy of Hardinge
“On the Kellenberger line, we can grind a square, a triangle, whatever the shape may be,” Hilliard said. “We also have a new feature that permits CNC jig grinding. The combination of direct-drive C-axis capability and a new hydrostatic B-axis capability allows us to do nonround and what we call off-center, or jig, grinding. In the past, you could always grind a shape but the accuracy may not have been to a quality a grinding machine is normally held. The direct-drive C-axis workhead allows us to achieve positioning and profile accuracy in the 2µm to 3µm range on the part.”
United’s Studer brand universal grinding machines also feature a C-axis for form and thread grinding.
Some universal cylindrical grinders can perform peel grinding in one setup as well as OD grinding. In peel grinding, a narrow grinding wheel moves in a traverse path across a workpiece. Examples of parts peel grinding is used for include shafts and carbide cutting tools.
“With peel grinding, you are utilizing much higher surface speeds on the wheel,” Hilliard said. “And instead of using a conventional grinding wheel, you typically use a superabrasive wheel.”Don’t Interfere
The main limitation with performing multiple grinding operations in one setup is fixturing. “You have to determine how the workpiece can be clamped, and if it is suitable for multiple-operation processing in one clamping,” Ueltschi said. “In some scenarios, it’s not possible to do multiple operations in one setup due to workholding interference. For example, if the geometry is such that you have to hold the workpiece between centers, you could do multiple operations on the external side but maybe not on the internal side because you don’t have access to the bore.”
Some manufacturers minimize interference issues by using grinding simulation software to see, among other things, the position of the workhead and the workholding area. Simulation software helps to determine ahead of time if a given workpiece can be machined in one clamping doing multiple operations and if the grinding wheels can reach the workpiece surfaces and the positions for dressing. It also reduces setup time.
Courtesy of United Grinding
“You usually know ahead of time if there’s going to be interference,” said Toyoda’s Farrant. “Anyone who does their due diligence looking at the part will know where the interference issues are. To determine if there is interference, check the workhead and footstock clearance with the wheelhead—also known as throat interference. This usually occurs when grinding smaller diameters with smaller wheels, such as some diamond or CBN wheels, because it requires the wheelhead to advance closer to centerline, which can cause interference.”
Interference can also occur when smaller diameters are adjacent to larger diameters that need to be ground. Additionally, fixtures can occasionally be large, so that also needs to be considered during processing. “Taking measurements ahead of time to account for wheel diameter and part diameter will let you know where the problems, if any, will show up,” Farrant said. “You just have to make sure that you have enough clearance to hit all of your grind diameters.”In Position
Grinding machines with multiple wheels require complex CNC software and a coordinate system to keep track of the wheels and coordinates in relation to the workpiece and diamond dressing tool positions. Ueltschi said: “To keep track of one diamond dressing tool with one grinding wheel is pretty simple, but if you have multiple dressing tools and wheels, it becomes more challenging. If you have multiple wheels, you may need multiple dressing tools. And you have the dressing tools set up at different angles so they can dress the wheel geometry correctly. You may need multiple dressing tools for each wheel if both sides of a wheel need to be dressed.”
Courtesy of Cinetic Landis
United designed a quick-set measuring system to make it easier to set up multiple-wheel grinding machines and to maintain the coordinates of the wheels, even if they are indexed to different angles. “The Quick-Set ties the wheel reference points, diamond reference points and workpiece centerline together to the turret wheelhead center point,” Ueltschi said. “It does this by the use of a calibration probe mounted to the turret head. After an initial calibration, all wheel points are known in polar coordinates. The same is true for the workpiece and dresser points.” This can translate into significant workpiece-change savings with a multiple-wheel turret machine, he added.
Courtesy of United Grinding
“Generally speaking, shops operating multiple-wheel machines gravitate toward using all aluminum-oxide wheels or all superabrasive wheels,” Cinetic’s Peters said. “But some might want to use CBN and aluminum oxide in the same setup due to ‘historical processing,’ such as if a part is already being processed with aluminum oxide for some features and the manufacturer would like this to continue. In that case, you would probably have different diamond dressers for different wheels.”
The most common dressing tool for multiple-wheel grinders is a single-point, blade or rotary diamond mounted on the workhead, tailstock, table or subtable.
Diamond or CBN wheels require much higher surface speeds than conventional abrasive wheels. Before applying superabrasive wheels, shops must determine if their grinding machines are capable of running at those speeds.
One concern when using multiple-wheel machines might be the grinding load on a workpiece being ground simultaneously. But deflection is overcome in two ways: proper workholder design and feed rate control.
Courtesy of Toyoda
“Typically, we use CAD software to make a 3-D model of the part and apply the grinding loads to that model,” Peters said. “That will determine how many work rests should be applied to that part given the grinding load. We can predict how much the part will deflect and then compare that deflection to the required part tolerances.”
Also, the grinding loads may be high at the beginning of a cycle but not at the end. “You have a step-down in the cycle where you start with a rough grind feed rate, go to a lower grind feed rate, then a still lower fine feed rate and then to a dwell,” he said.
Peters concluded that the part features and the volume of parts required are the key factors in determining how flexible a multiple-wheel grinding machine must be. “If I was looking at making a large family of parts where I needed flexibility to grind several different parts in the family, I would strongly consider doing multiple operations in one setup. If I was grinding a high volume of the same part, the advantage of multiple-wheel grinding would be more from a quality standpoint, a tolerance- stackup standpoint.”
Combining operations eliminates multiple fixturing in different machines and ensures each operation is referenced to the same datum point. “As a manufacturing engineer, I was schooled into finding the right datum and making sure that datum is available when you actually chuck up or locate the part in the machine,” Peters said. “And once you’ve done that, try not to move that part.” CTEAbout the Author: Susan Woods is a contributing editor for CTE. Contact her at (224) 225-6120 or by e-mail at email@example.com.
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