Advanced Feed Forward Increases Speed and Accuracy of Contouring Operations

It used to be that a CNC control simplified simultaneous interpolated X, Y and Z axes moves and provided a minimum of features that enabled G code to be entered manually. But VMC manufacturers, competing to deliver the most productive machine to their customers, now pack CNC controls with features that reduce setup time, increase accuracy and, most of all, reduce cycle time.

Today's CNC control is loaded with a variety of capabilities and features, such as one-button operations for loading tool offsets, third party PC software, conversational programming, tool load compensation and many others that increase the functionality of the VMC. Some of these new features, such as Fadal Engineering's Advance Feed Forward, can increase speed as well as accuracy and ultimately reduce cycle time by 20 or 30 percent or more during high-speed contouring operations.

High speed contouring operations are characteristic of mold shops that cut mold cavities, cores and graphite electrodes according to Jim Snow, president of James Tool, a mold shop that makes plastic injection tooling for pharmaceutical, automotive, telecommunication, housewares, office products and beverage applications. A moldmaker for 30 years, Mr. Snow has seen the speeds and feeds of VMCs increase dramatically, but found that high speed machining, more often than not, is incompatible with geometry and tolerance requirements. That is until Fadal Engineering offered Advanced Feed Forward on its VMCs.

"We tried a number of methods to cut at higher speeds but they only produced marginal results under optimum conditions," said Mr. Snow. "Every time we cut at fast feed rates, we would end up with gouges and violations all over. The ability to servo tune the axis drives on-the-fly is an amazing opportunity to reduce cycle time."

Advanced Feed Forward reduces cycle time by as much as 30 percent or more without sacrificing accuracy by servo tuning axis drive motors on-the-fly. Most servo motors are tuned at the factory to achieve balanced results over a wide range of machining operations. With Advanced Feed Forward, the machinist does the adjusting on-the-fly, which enables him to achieve optimum speeds, feeds, accuracy and surface finish. Four adjustments, including gain, ramp, detail and feedrate are utilized to tune the axis servo drives. By automatically calculating optimum feed rates according to distance and direction changes Advanced Feed Forward increases accuracy and enables high-speed contouring operations.

"Few, if any, moldmakers know their geometry well enough to know how the VMC will react to a new tool path," said Mr. Snow. "One mold gouges at 70 inches per minute and a similar mold can be machined at 120 inches per minute with a mirror finish. We found that Advanced Feed Forward is a process that helps us get to the optimum speeds, feeds and accuracy without editing the program."

Feed rate is a dynamic adjustment that allows the machinist to adjust the programmed feed rate for the remaining moves, however any feed rate subsequently set in the program will override this setting. An increase in feed rate speeds up the cutting process, but a feed rate that is too high makes the VMC susceptible to sudden starts and stops, i.e., jerky motion, and increases the wear on the VMC components.

For example, a graphite electrode that James Tool machined was set for 90 inches per minute. At this speed, the geometry and surface finish were satisfactory, and it became apparent that it could be machined at a much higher feedrate. But how high?

"Advance Feed Forward is a process that allows the machinist to observe the cutting operations while on-the-fly adjustments are made," said Mr. Snow. "If the adjustment is too much, it can quickly be reduced."

The feedrate was then increased to 110 inches per minute. The geometry is holding and no gouges. Even with a lot of accel/decel points the speed is still too slow. The speed can be increased by another parameter, Detail (corner tolerance), which is a parameter that specifies how closely corner portions of the workpiece are to be machined. Detail, originally set at 0.0003" is reduced to 0.001". The speed is now 120 inches per minute and no violations and the geometry is still accurate.

When the Detail parameter is set at a large value that increases the speed of the cut. If a finishing cut is needed, the Detail parameter is set at a small value, which improves the accuracy of the cut. In conventional VMCs, the Detail is fixed at one value and either speed or precision, or both are compromised.

"Some contours are so complex that you need to run them slower, but mainstream parts are not," said Mr. Snow.

Again geometry is within tolerance and surface finish is smooth. So the feedrate is moved up to 150 inches per minute. At that speed some variation appears from the cutter slightly overstepping its bounds. Adjusting Ramp Time eliminates gouges by reducing the time required for accel/decel. The gouges disappear and the VMC is still running at 150 inches per minute.

Ramp Time is used by the control to specify the linear rate of change between different programmed feed rates. For example, if the programmed feed rate changes from 5 in/sec to 10 in/sec between moves, the control increases the programmed feed rate gradually from 5 in/sec to 10 in/sea in the amount of time specified by the ramp time. A decrease in the ramp time speeds up the cutting process, but a ramp time that is too small makes the machine susceptible to sudden starts and stops (i.e., jerky motion).

But the sound of the VMC is that it's working very hard. It's buffering. The buffer is controlled with the gain adjustment. As the gain is adjusted the VMC becomes more rigid. All the time that adjustments are being made, geometry must be watched so it doesn't overstep its bounds or slowing the feedrate.

"A lot of time is saved because you don't edit the program which can take an hour just to upload before you can get to the editing," said Mr. Snow. "We started this program at 90 inches per minute and wound up at 150 inches per minute. You watch the graphite being cut as you make the changes. All along the way you watching for deviation, if it happens, 90 percent of the time it's insignificant."

The graphite is still accurate and surface finish is smooth, but the VMC is working very hard by the sounds that it is making. There is an adjustment for this, too. Gain allows servo stiffness to be tuned. Gain is representative of the responsiveness of the drive assembly to the difference between the desired and current positions. An increase in the gain speeds up the cutting process, by reducing the buffer between the desired and current positions.

"Now the VMC is more fluid and still cutting accurately at 150 inches per minute." said Mr. Snow.

"Before we bought our Fadal VMC, we bought another VMC with a special buffering system that would allow us to cut 30+ on complex geometry," said Mr. Snow. "It was a workaround. On real long and flowing geometry it would work fine, we could cut 30 and 50 inches per minute. Any time we exceeded that, we'd round off the edges at the parting line. Or we'd do have to do extra programming. When we would push the speeds on molds that required a lot of accel/decel, we'd clip the top and have run much slower. The same electrode that we've been talking about took 3 hours to run. But the Fadal VMC cut the electrode in 42 minutes and with higher accuracy."

The VMC maintains a table of control parameters where preferred control parameter values for different types of tools are stored for future use. When a particular tool is selected by the CNC machine, the control parameters table is accessed and the CNC machine is initialized with the preferred values of Gain, Ramp Time, Detail and Feed Rate that are stored in the table.

The VMC is programmed to provide the machinist with an interactive control of the parameters that control the speed and accuracy of machining. With this feature, the operator is able to observe the effects of changing the control parameters on the speed and accuracy of machining and to customize the control parameters to a particular type of tool or application based on the observations.

"Our productivity is up three to four times, so we've reduced cycle time to about 25% of what it was," said Mr. Snow. "Faster speeds means smoother surface finishes, so there's less hand finishing and cycle time. And we're able to schedule more work.

For more information contact:

SWM
South Texas
Paul Cody
Chaparral Machinery
419 Century Plaza Drive #230
Houston, TX 77073
281-821-6791
Fax 281-821-6795

Texas, Okla.,Mexico
Dave Stilley or
Rusty Hendrix
Chaparral Machinery
3435 Roy Orr Blvd. #100
Grand Prairie, TX 75050
972-313-1987
Fax 972-313-1984

Arkansas
Steve Wherry
Cardinal Machinery Inc.
7535 Appling Center Drive
Memphis, TN 38133
901-377-3107
Fax 901-377-4855

SEM
Florida
Jeff Thomas
Magna Machinery Co., Inc.
11811 31st Court North
St. Petersburg, FL 33716
Tel: 813-572-7777
Fax 813-573-2203

Florida Panhandle/Georgia
Steven R. Flint
Flint Machine Tools, Inc.
3710 Hewatt Court
Snellville, GA 30039
770-985-2626
Fax 770-985-2706

Virginia
Barbara Jan
W.C. Burroughs & Assoc., Inc.
7146 Montevideo Road
Jessup, MD 20794
410-799-5900
Fax 410-799-1705

Alabama/Mississippi/ Tennessee
Steve Wherry
Cardinal Machinery, Inc.
7535 Appling Center Drive
Memphis, TN 38133
Tel: 901-377-3107
Fax: 901-377-4855

North Carolina/ South Carolina
Weldon Childress
Dixie Machine Tools
Div.Dixie Tool Co.
1805 East Main Street
Easley, SC 29641
Tel: 864-859-7576
Fax: 864-859-7684