Submarine-propellers: big challenge for the "machining twins"

Philadelphia (USA) - A highly skilled and versatile workforce of engineers, operators and support personnel is working for the Naval Foundry and Propeller Center (NFPC) in the Philadelphia Naval Business Center (formerly Philadelphia Naval Base). The NFPC manufactures submarine propellers for the United States Navy. NFPC has very large machining capability in order to manufacture propellers. Because of the time required to build these parts, the challenge the engineers were faced was finding a way to cut manufacturing times. About eight years ago, NFPC started a research program to test high speed machining on the propeller Copper based alloy. Successful testing led to a search for high speed machines; large enough to accommodate the largest submarine propellers of up to 24 ft. in diameter.

“When we started to survey the machine tool market, we looked at the traditional companies in the US, France and Japan. We did not know Starrag existed, especially their capabilities. After the technical specifications were published, Starrag and another company were selected for the final evaluation based on the technical merits of the offers. At that stage, both companies being technically acceptable. Starrag won the contract with the best price and the excellent Droop+Rein FOGS machine which they offered.” says Faris Ibrahim, Mechanical Engineer at the NFPC. “From that day in 2005, we started an excellent relationship with Starrag which led to another Droop+Rein FOGS purchased in 2012 and three more in 2019 we will need many more in the future.”

Starrag successfully delivered in 2007 and 2012 two high-speed machining centers of their product range Droop+Rein. The machines are Droop+Rein FOGS 70 68 TT M30 C, which per the Navy specifications had the following dimensions: (X-axis: 267 inches, Y-axis: 275 inches, Z-axis: 90 inches, rotary table 275 inches with main power drive 2x134 hp). The twins are the biggest machine tools in the history of FOGS-machines. They are turning, drilling and contour milling the US Navy’s submarine propellers.

"When we started to survey the machine tool market, we looked at the traditional companies in the US, France and Japan. We did not know Starrag existed, especially their capabilities"

Speaking about the future, the next three Droop+Rein machines were successfully delivered in 2019. These machines are FOGS 70 68 M 30 K, FOGS 75 75 TT M30 C and FOGS 85 85 TT M30 C.

Ibrahim: “The Naval Foundry and Propeller Center is the US Navy’s only submarine propeller producing facility. The Droop+Rein high speed machines are doing all of the finish machining work. The machines are performing extremely well. They’ve become the workhorses of our shop. Our investment is paying off with improved surface finish and accuracy.”

The machining requirements are very high, the propellers are made of a Copper alloy that is difficult to machine. Additionally, the blades have a complex geometry with local accuracies on the order of 0.010 inches or 254 µm. 

The propeller has very complex curvatures modelled in a CAM system to produce very complex cutting motion which the Droop+Rein FOGS machine must reproduce extremely accurately with their six axes. Ibrahim: “The capability of the machine control combined with high speed cutting, allows us to produce almost mirror finish sculpted surfaces.”

These excellent results are also based on special machine tool characteristics.

Ibrahim: “When we had our first meeting after contract award, we presented the Starrag team a difficult task. We needed a special head in order to machine between the tightly stacked blades. After many discussions and simulation studies, the Starrag team presented us with a unique head design that eventually became the now famous “Hammer Head”. This head gave us the ability to easily reach and contour every part of the propeller. It was a brilliant design that really showcased their technical ability.”

For the Mechanical Engineer and Project Manager Ibrahim, the Droop+Rein FOGS machines were a good example of how turnkey machine tool projects should be handled. Starrag handled everything from the design and construction of a very large foundation, shipping, unpacking, installing and testing the machine tool at the Navy site in Philadelphia with the help of local subcontractors.

Ibrahim: “We are very satisfied with these machines, they are performing per the specifications, Starrag’s support has been phenomenal and we hope to continue our cooperation for our future machine tool needs.”

((Caption))

The Droop+Rein high speed machines are doing all of the finish machining work. The machines are performing extremely well.

Company profile Starrag

High-precision machine tools for greater productivity

Starrag Group is a global technology leader in manufacturing high-precision machine tools for milling, turning, boring and grinding workpieces of metallic, composite and ceramic materials. Principle customers are internationally active companies in the Aerospace, Energy, Transportation and Industrial sectors (Industrial Components, Luxury Goods, Med Tech). In addition to its portfolio of machine tools, Starrag Group provides integrated technology and maintenance services that significantly enhance customer quality and productivity.

The umbrella brand Starrag unites the product ranges Berthiez, Bumotec, Dörries, Droop+Rein, Ecospeed, Heckert, Scharmann, SIP, Starrag, TTL and WMW. Headquartered in Rorschach/Switzerland, the Starrag Group operates manufacturing plants in Switzerland, Germany, France, the UK and India and has established a network of sales and services subsidiaries in the most important customer countries.

The shares of Starrag Group Holding AG are listed on the SIX Swiss Exchange.