screw rotors used in air compressors in applications such as oil rigs, painting, sandblasting, and mining operations. To take advantage of demand, the manufacturer explored ways to streamline the traditional seven-machine process for machining the parts at its facility in Statesville, North Carolina.
Carl Amick, CEO of Machinery Solutions Inc. (MSI), Lexington, South Carolina, and Jim McClellan, production systems manager at DIPP: Statesville, North Carolina, worked through several scenarios of manufacturing helical screw rotor components.
Minimizing Initial Process
The rotor manufacturing process yields helical screw rotors in a male and female mating pair. The male and female screws come together with complex, high-precision geometries and extremely tight tolerances for the best air efficiency. Currently seven rotors pairs or 14 varying components are tooled for production.
“We focused our efforts to develop a machining process for rotor screws as a core competency,” McClellan says. “Since, most of us had our roots in the Ingersoll Rand Co., that was our starting point – to emulate the Ingersoll process of manufacturing rotors.”
Ingersoll’s rotor manufacturing process required seven different machine tools to complete the rotor components. The initial process alone required three machine tools: first a horizontal machining center to face, center, drill, and tap bolt-hole patterns; then a turning center to machine the journals and necessary grooving and threading operations; and last, a vertical milling machining center to cut the key ways. This initial process became the task for MSI to complete a turnkey system in a single machine for DIPP.
From Three to One
McClellan reached out to several companies, seeking a way to use fewer machine tools for the process. That’s where Amick from Doosan Machine Tools distributor, MSI, came into the picture.
“We decided that if we used a multitasking mill-turn machine, we could combine three of the machines into one,” McClellan says.
MSI and DIPP came up with this concept by customizing a Doosan turning center with Y-axis, increasing efficiency and reducing scrap waste and man power.
“Now we have a process in place that has four machine tools making these parts opposed to the original seven,” McClellan says.
First Set of Machining Tools
MSI developed a concept mimicking a B-axis machine to face, center, drill, and tap each end of the work piece from raw material without operator intervention. The machine then completed the turning, threading, and keyway features to produce a qualified component for the rest of the processes. This initial qualifying operation combines three machines into one for final critical operations.
“We didn’t have the capacity to handle the complete range of parts with our Puma MX style turn-mill B-axis machine, so we came up with this concept and we were able to use a special, triple X-axis, programmable steady rest base and develop a clamping device with a programmable tailstock that was also not available on the MX series,” Amick says.
A Y-axis with double-end live tooling was implemented in the machining zone to work raw stock on each end in a clamped state and a customized Kitagawa pullback chuck was used with special work drivers and quick-change jaws engineered for each part in the machine. This collaboration turned the base model Doosan Puma 3100XLY 4-axis turning center into a facing and centering machine with the ability to complete the necessary applications of facing, centering, turning, threading, drilling, tapping, and keyway cutting with minimum operator intervention.
“We’re taking an off-the-shelf machine, and customizing it to meet the application – making a seamless operation.” Amick explains. “All of the tooling is in the turret and no tooling is added or removed except for what tools are needed for each part geometry.”
In addition, the machine has a custom, quick-change jaw system used with the special chuck after the initial outer diameter (OD) cut, to achieve concentricity for jaw clamping.
“The chuck jaws are snapped into place in less than 30 seconds and the machining of a qualified part is completed and ready for helical milling and grinding operations,” Amick says.
MSI wrote custom macros for the general part geometry with operator instructions and part programs to generate the 14 variations of parts.
MSI worked with chuck manufacturer, Kitagawa-NorthTech based in Schaumburg, Illinois, to come up with a quick-change jaw design and work driver system. To accommodate single, double, and triple steady rest arrangements, the collaboration converted traditional, turn-through hydraulic steady rests into vise clamping units. A special bracket was also developed to statically hold the raw material for facing, center drilling, milling, and bolt-hole pattern operations. Doosan Machine Tools engineered a servo-driven steady rest base for the Kitagawa clamping system as a programmable axis to handle part lengths of 13" to 36" and diameters from 5" to 12".
“The operator never takes any tooling holders out of the turret for any of the parts being made,” Amick says. “The steady rest units allow quick change out and alignment of steady rest clamps.”
MSI developed a crane-loaded V-block system for 85 lb to 1,000 lb. workpieces with a Renishaw probe on each end so the machine can locate the workpiece and determine the amount of milling necessary. Once the Kitagawa steady rest clamp is activated, the machine mills faces, center drills, drills, and taps bolt-hole patterns on each end of the part. Doosan’s servo steady clamp base then automatically repositions the faced and centered part with required end features generated in the clamp state by moving it toward the chuck inserting the workpiece end (female) into the special bolt hole driver (male) and the tail stock advances capturing the part between centers. The machine has a pressure load feature and custom macros that signals the servo to stop moving when the tail stock comes into final position and clamps so the OD turning operation can be performed.
After the first OD turning is completed, the Kitagawa quick-change jaws are snap-locked without the use of bolts. The part is then chucked for heavy metal removal during turning and roughing. MSI also manufactured a chuck jaw safety mechanism with three pockets with proximity switches like an auto-door interlock so operators can’t open the door while the machine is running.
Second Set of Machining Tools
DIPP chose Holroyd EX rotary milling machines to cut the helical screws and the body of the part. The process uses two of these machines for this operation and each machine mills one flute at a time.
“We use the Holroyd because it takes a pretty unique type of machine to do what we need, and Holroyd is best-in-class,” McClellan says. “The Holroyd EX is basically a milling machine and the machine’s head tilts giving it the capability to cut the helical screws.
“The cycle times on this machine are between 11 minutes to 20 minutes, McClellan adds. “We keep the coolant away from the cutting zone because it will dull the cutter.”
Third and Final Set
McClellan uses a pair of Hardinge Group’s Jones & Shipman Ultra Grind cylindrical grinders at their facility as their third set of machining tools to manufacture the rotor piece. These machines are used on the workpiece for grinding bearing journals, faces of the piece’s body, and the OD of the workpiece’s body.
“One of the stipulations we wanted was a dual-head in the machine so it would grind the bearing journals on either end without having to take the part out of the machine and save time,” McClellan says. “We also have in-process gaging on the machine tool.”
The last machine used in the Statesville’s process of manufacturing its helical screw rotors is a pair Holroyd Zenith helical grinding machines.
“Holroyd was just beginning to design and build a new type of grinder that would be cubic boron nitride (CBN) grinding wheel capable, but could also run aluminum oxide, which is on normal grinding wheels,” McClellan says. “We thought we needed that flexibility since we have our own engineering team here.”
Doosan worked closely with Holroyd through this machine development and purchased two of their first completed models.
“We spent a lot of time evaluating that finish grinder because that’s where the ultimate size control is and we’re trying to hold profiles at ±10µm,” McClellan says.
The Holroyd Zenith helical grinding machines are capable of running two wheels in the spindle.
“Basically we’ll have a plated and a vitrified wheel on the spindle,” McClellan explains. “The plated wheel will run two roughing cuts on each flute and the vitrified wheel will be the finish grind and it has about 50µm of stock removal finish grind pass.”
The manufacturing facility in Statesville is working with 3M to develop its own CBN process.
Once the piece goes through the Holroyd Zenith helical grinding machines, the complete rotor piece is finished and sent off to the assembly process of the rotor piece.
“We have a male and female rotor that goes into a cast iron housing and we put all of that together with bearings and so on,” McClellan says. “That then becomes the air-end that is used in our finished products which are air compressors that are also shipped around the world to our sister plants in the Czech Republic and India.”
Process in Motion
DIPP went from seven to four operators for this entire process, and were able to maximize their shop floor footprint by reducing it to a 6,400ft2 operational space. To increase production, DIPP uses a pair of machines for each process. This year, the Statesville facility has begun making its own rotors and building its own air-ends using the helical screw rotors machined in-house.
“We are ramping up to where we will be independent by the end of the year,” McClellan says.
The 435,000ft2 Statesville facility employs roughly 400 individuals.
“The long-term plan is to start selling these air-ends as a stand-alone product to customers that need air-ends and they can put them in their own applications,” McClellan says.