High speed control slashes machining time.
Sagittarius Mold's latest CNC mill runs circles around the other machines in the shop. The machine's performance is all the more impressive when you consider that it is 13 years old and survived a plant fire. But the older, faster mill has something that the shop's newer, slower machines do not: a high speed CNC control that boasts the industry's fastest servo rate. The high feedrates that the control makes possible have completely changed the shop's approach to 3D part machining and programming.
Sagittarius Mold Inc, a plastic injection mold shop in Greenville, SC, burned to the ground in April 1992, but founder and owner Norm Hervey was not ready to retire. He bought two new vertical machining centers (VMCs) and moved into a leased facility. Three years of long days and hard work later, he moved into a new 22,000-sq-ft plant several miles from the site of the original shop.
The process of converting blocks of steel into molds for plastic injection molding machines involves a mix of 2D and 3D machining, the amount of each varying with the job. When the mix swung too far to the 3D machining side, it created a problem: "Feedrates of our two VMCs - 10 to 30 ipm - were too slow," explains Ellis Jarrell, CNC manager for Sagittarius. "The slow feedrates effectively capped the volume of work we could handle."
Sagittarius needed more 3D milling capacity to handle its growing business. The shop wanted the high feedrates of a brand new machine without having to buy one. It had a 1984 electronic tracer mill, similar to its two newer VMCs, that had been damaged in the fire but kept as a potential candidate for rebuilding.
The shop approached the firm that had made the tracer mill (and its two newer VMCs), hoping to buy a new, state-of-the-art CNC control for the restored mill. The manufacturer offered to sell the shop a control like the one that originally came with the mill, or one like those on the shop's two newer VMCs. However, the firm would sell its newest CNC control only as part of a new-machine purchase.
Sagittarius then turned to independent manufacturers of CNC controls. While exploring the field, the shop learned about the Creative Evolution CNC control made by Creative Technology Corp (CTC), Arlington Heights, IL.
The Creative Evolution CNC is a 3-, 4- and 5-axis control that enables firms to mill 3D contours up to ten times faster than before - on their existing milling machines and machining centers. The control is available for retrofit or OEM applications.
The control has an advanced look-ahead capability that automatically maintains near-constant feedrate and surface speed without compromising cutting accuracy. Its Digital Signal Processor (DSP) measures and updates position commands 30 times faster than other high speed controls. Block transfer times are as short as 220 msec, resulting in throughputs of over 4500 blocks per sec. It can communicate with CAD/CAM systems 1000 times faster than conventional Direct Numerical Control (DNC) systems.
A demonstration of the Creative Evolution control on a machining center at CTC's Arlington Heights headquarters convinced Sagittarius of the validity of the performance claims for the control. Sagittarius bought the control as part of a more comprehensive retrofit package.
The retrofitted machine joined the shop's two newer VMCs and a high speed electrode mill - the shop also operates EDM machines - that was purchased shortly after the firm moved into its new building. From the onset, all 3D milling was scheduled for the retrofitted machine because of its faster machining rates.
"Where our other VMCs would do 3D milling at 10 to 30 ipm, the machine retrofitted with the Creative Evolution CNC can contour mill at 80 to 200 ipm, which has significantly reduced overall machining time," Mr Jarrell continues.
"We can machine 3D mold components in 5 to 6 hours that would take 1 1/2 to 2 days on either of the other machines," added Carl Collins, who operates the retrofitted machine. "On a finishing cut in a 4140 steel cavity, we've been able to achieve cutting rates of 180 ipm with no loss of accuracy.
As the 3D machining backlog eased, concern shifted to a different but related problem: the shop's programming system. Programs for the shop's two VMCs were prepared off the shop floor on a Unix-based workstation and downloaded to the machines via the shop's DNC system.
The arrangement had seemed adequate for the two machines, but problems began to surface when the high speed electrode mill was installed. First, it increased the programming load. Second, programs were downloaded to the electrode mill via the DNC system just as for the other two machines. However, the electrode mill's control processed programming data faster than the DNC could supply it, causing the mill to stop periodically to wait for more data.
Addition of the retrofitted mill, with the still-faster Creative Evolution CNC, aggravated the problems. "I couldn't produce programs for our four machines fast enough with our existing workstation and CAM software," Mr Jarrell recalls. "Parts were backing up in the shop. I was working long days and weekends creating programs to keep the machines running five days per week. We wanted to be able to operate the machines six and seven days per week. We needed to add another workstation and programmer, or find some other way of increasing our programming capacity."
One of the alternatives considered was a 3D CAM software called WorkNC from Sescoi USA Inc, Southfield, MI, installs on a PC-based machine control or shop floor PC for programming right at the machine by the operator. The software is graphics-based. The operator uses the graphic representation of the part on the screen to identify the area to be machined, makes appropriate menu selections and keys in numerical data where required. Using that information, the software automatically generates the desired toolpath. The operator can edit the program and post-process it to run on other CNC mills or VMCs.
Mr Hervey was reluctant to add another programming system because of the investment in hardware, software, and training that had already been made. But reservations about WorkNC quickly disappeared. "Before, when a complex 3D job was set up on the machine, it might run for 10 or 20 hours; the operator would just stand there monitoring the operation," Mr Jarrell explains. "Now, he not only prepares the 3D machining programs for his own machine, but all of the programs for the electrode mill as well.
"WorkNC also provides the most logical division of responsibilities," Mr Collins adds. "Engineers may be strong in design, flow factors, solidification rates...but they usually don't have much hands-on machining experience. WorkNC puts the responsibility for machining the part in the hands of the person who is most knowledgeable about machining and proper tool selection - the operator. It's the ideal arrangement."
Mr Hervey identified the operations benefiting most from the Creative Evolution control: "Before, for a mold with a 4 [inches]-deep, contoured cavity, I would typically quote 110 hours of machining, 70 hours of handwork and 50 hours of spotting. With the much faster Creative Evolution control, I can produce the same mold with 70 hours of machining, 40 hours of handwork and 20 hours of spotting - a savings of 100 hours."
"But the benefits of the Creative Evolution CNC don't end with performance and economy," he adds. "Machine tool control technology is moving rapidly, obsoleting proprietary controls that are just a few years old. For example, our two 5-year-old VMCs are perfectly good mechanically, but their controls are dinosaurs. Nothing can be done to make these original controls run faster.
"The Creative Evolution CNC can also be upgraded to incorporate the latest improvements. Unlike the proprietary controls on our other two VMCs, as CTC develops improved versions of the Creative Evolution control, the company is able to upgrade the control accordingly.
"Good as the control is, addition of programming software makes it even better," Mr Hervey stresses. "With the WorkNC software, instead of the operator just standing around during the 70 hours or so of machining time required for the average mold, he can probably use about 30 of those hours to prepare programs. Thus, in addition to the 100 hours originally saved with the control, we get 30 hours worth of programming as a bonus."
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|Author:||Rakowski, Leo R.|
|Publication:||Tooling & Production|
|Date:||Jun 1, 1997|
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