HMC Provides Advantages Over VMC

U.S. aerospace prime contractors struggling to stay competitive are dictating cost reductions from suppliers.

U.S. aerospace prime contractors struggling to stay competitive are dictating cost reductions from suppliers. As a result, supplier demand is rising for machine tools that deliver shorter cycle times and meet strict quality requirements without increasing costs.
Long-time aerospace parts supplier A and D Precision Manufacturing (Anaheim, California) found a way to decrease cycle times when it purchased a new simultaneous four-axis HMC 400 from Fadal (Chatsworth, California) and became a beta test site. A and D reports a substantial productivity increase compared to close-in-size vertical machining centers (VMCs), yet the cost of Fadal’s new horizontal machining center (HMC) is much less than a high-end HMC.
The HMC400 is said to be built for maximum performance and throughput, enabling a 24/7 duty cycle. Part of the productivity comes from stiffness. A HMC provides a stiffer machine platform than a VMC because of spindle orientation. In a HMC, the spindle is located inside the column, and on a C-frame VMC, it is cantilevered away from the column over the worktable and base.
In addition, the HMC is constructed with flame-hardened box ways with nonmetallic liners and rib-reinforced castings to provide rigidity and vibration damping. “The HMC 400 is more rigid than a comparably sized VMC. We’re cutting to tolerances of plus or minus 0.0015 inch with no trouble,” says Tony Brown, vice president at A and D.

Additional productivity advantages come from the ability to set up more work per cycle and to perform the setup while the machine is cutting. With twin pallets, the machine can almost always be in cut. Setups and part handling can be done on the off-line pallet, while the other is in production. The result is much less downtime for setups and part handling, leading to substantially increased productivity.

The HMC’s 400-milimeter diameter pallets lock onto the simultaneous fourth axis rotary table with 13,000 pounds of holding force. A double enveloping gear on the rotary table and mechanical disk brake with 12 pads ensure accuracy of ±5 arcsecond and repeatability of ±1 arcsecond. The rotary table motor features a ratio of 180:1 and operates at 17 rpm. The machine also offers 20 inch by 24 inch by 20 inch XYZ travels.

The Siemens Sinumerik digital control is standard on the HMC 400. The features A and D finds useful are that the machine provides fully interpolated four-axis capability and program execution speeds for high speed machining. Rapid travel rates are up to 1,200 ipm (30.4 meters per minute) and can hold tolerances when contouring at feed rates up to 400 ipm.

Look-ahead, dynamic feed forward and programmable acceleration help provide comprehensive control for high speed machining of contours, hole position or any program with short axis moves. “The HMC 400 is very smooth when cutting,” Mr. Brown says. “The control moves the axis motors very fast with no jerking, so we get an excellent surface finish because of the acel and decel functions built into the control. Another feature is the ability to embed fixture offsets in the program.”

The company selected an optional 15,000-rpm spindle with air/oil lubrication that also proved to be a good pick. There was some concern that with the lower torque of the 15,000-rpm spindle, hogging operations would increase cycle time. “We’ve seen the HMC 400 rough aluminum at more than 100 cubic inches per minute,” Mr. Brown explains. “One part we make has a 13-minute cycle time, including roughing and finishing, on a VMC with a 10,000-rpm spindle. It’s down to 4 minutes on the HMC with a 15,000 rpm spindle.”

Another characteristic of the machine is that it maintains a constant temperature and ensures positioning accuracy with Fadal’s Cool Power refrigerated cooling system. By circulating a high performance heat transfer agent around the spindle nose and spindle cartridge and through the center of the gundrilled ballscrews, Cool Power reduces thermal growth and repeatability problems caused by heat. The heat transfer agent’s temperature is monitored and chilled as required to keep the temperature of positioning components within ±1.0 degree of the HMC’s ambient temperature. The system is a closed loop, isolating the heat transfer agent from chips and other contaminants.

A beta test without discovering some problems would be very unlikely. At least three problems were identified at A&D and have been corrected on subsequent machines.

“We experienced only a few hiccups during almost 2 months of beta testing,” Mr. Brown says. “We found a seal that was mispositioned and failed, and a switch on the pallet system failed. We weren’t down for very long, and the upside was we gained the opportunity to talk with the Fadal technicians, which gave us some additional insight and allowed us to learn more about maintaining and using the machine. The Fadal HMC 400 is very adaptable to anything we want to do. And it’s opening a lot of possibilities for us.”