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Ontario machine shop develops unique approach to machining large propeller blades and waterjet impellers

Imagine a casting weighing 1,600 kg arriving in your 9,000 sq ft facility. Then imagine machining that casting on a simultaneous five axis milling machine to create a propeller blade spanning 2.3 m wide by 1.7 m high with surface roughness better than 63 micro in. roughness with surface precision within +/- 0.012 in. several times better than the ISO Class S tolerances required by the Canadian or US Navy. And imagine doing that in one setup on one machine, without the need for hand grinding and hand polishing.

That’s exactly what Dominis Engineering Ltd., a Gloucester, ON, machine shop is doing.

“The original goal was to machine propellers so there was no hand finishing, to machine to final form and finish,” says Bodo Gospodnetic, who formed Dominis 25 years ago with his father Drasko, a naval architect and mathematician who worked for the ship laboratory of the National Research Council (NRC) in Ottawa, and is now Researcher Emeritus at NRC.

“To do this, we had to develop a system to handle this machining concept. Our objective was to machine all the surfaces on propellers to very tight tolerances in one setup. Everyone thought we were nuts.”

That changed though when father and son developed their manufacturing process called Integrated Propeller Manufacturing System (IPMS) and began to apply the machining concept to hydro turbine blades. In the early 1990s, Bodo Gospodnetic made a presentation to the US Navy and to large US propeller manufacturers.

Dominis Engineering has persevered and today, he says his company is the only one in Canada that meets Navy specifications for propellers and waterjet impellers.

Unique manufacturing system
The Integrated Propeller Manufacturing System (IPMS) has five guiding principles, explains Gospodnetic. All propeller surfaces (face and back profiles, leading and trailing edges, trailing edge chamfers, tip profiles, fillet--pressure and suction sides--and propeller hubs) must be machined to a “final form and finish.” It is an unattended operation that requires cutting tool efficiencies and the elimination of collisions between the tool body, propeller casting and fixtures. And while IPMS has “propeller” in its name, the same concept is applied to waterjet impellers and turbine blades.

“The surface of propellers are defined in a collection of patches. For example, the edges of propeller blades are defined separately from the face, back and hub and they all have to be stitched together, so the CAD program needs to have the ability to create a smooth transition for machining of all these components.”

The right machine for Dominis is a simultaneous five axis CNC milling machine designed with large travels of 2.5 m in the X axis and 1.8 m in the Y axis.

“What I like best about these machines is that they feature fast feed rates of 6 m/min and have a high speed spindle of 8,000 rpm. Another good thing is that the spindle is on a programmable quill so I can move closer to the part; it’s 200 mm in diameter and 450 mm long. It’s like having a tool extension that’s 8 in. in diameter.”

Dominis isn’t done refining its machining process. “We now run mostly unattended because of our process but one of the elements you also need is tool replacement options on the machines capable of detecting when a tool becomes dull and that can then replace the tool with an identical new tool in the tool magazine.” If it’s not identical, it is trouble because you’re machining sculptured surfaces and these surfaces are defined by discrete points and you have to interpolate the surface, which is smooth between defined points.

“We’re not there yet, but tool measurement and tool replacement options are the next step for us when we purchase a new machine.”


Full article written by Mary Scianna for Shop Metalworking Technology

Posted 2015-06-24
Last Modified 2015-06-24 11:25