Raising the bar with hydro impeller machining

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A state-of-the art CNC machining centre makes a Deming, WA company the country’s leading supplier of large hydropower generating systems west of the Mississippi –- right in the heart of the hydropower market. And some advanced tooling and heavy customer support from Ingersoll Cutting Tools helped make it happen. 

As a result, Canyon Hydro completes huge ten-ton rotors (called runners in the hydropower industry) in one third the time as before, and to much better accuracy and surface finish for higher efficiency over the runners’  projected 30-40 year service life. The runner is the heart of a hydropower turbine, converting water flow to the rotary motion that drives the generators.

Buckets require long-reach contouring

Key machining challenge is to contour-mill the runner buckets, essentially the paddles on a very sophisticated paddle wheel where every machined surface has a hydrodynamically correct curvature. A typical runner, made from a stainless steel casting, weighs ten tons, measures 11 feet across and has 22 carefully contoured double buckets. It can take months of continuous five-axis, long-reach milling to complete, leaving behind a ton and half of stainless steel chips. Virtually all milling involves 19 inch shank lengths. Nevertheless, at the end, as-machined surfaces must be smoother than 32µ and geometrically correct within 0.010 inch, in order to prevent turbulent flow while in service.  

“It’s like a large-scale cavity milling job, with one important difference,” says Ingersoll’s Chris Murray, who devised the tooling solution.  “The contour on each bucket involves undercuts, which you’d never see in an injection mold.”  

Previously, Canyon Hydro finished the buckets by manual grinding, using CAD/CAM templates to check dimensions and contours. Typical cycle times for an 11-foot runner were six months, involving 100 per cent attendance by skilled operators. Typical tolerances were 0.035-0.040 in. The process was more than adequate for its time. It was the era of low oil costs and easier environmental regulations, which left hydropower as a secondary source of power along with solar, wind and nuclear.  

Anticipating greater demand for alternate energy sources including hydropower, the company in 2009 made the strategic decision to automate runner machining. It built a free-standing, lights-out capable CNC center just a few towns away in Sumas. 

The key machine is a FPT five-axis CNC Floor Type Horizontal Mill equipped with, a two axis rotary table, 80-station automatic tool changer and two interchangeable heads: a 360? Universal 3 + 2 Bi-rotational head and a 29 inch extension head. A skeleton crew of 2-4 CNC machinists works 8-hour shifts to handle support functions as the machine runs largely unattended, sometimes overnight as needed to maintain delivery schedules.  “Chipmaking itself is essentially hands-off,” says Mr. Hansen.

As to tooling solutions, manufacturing engineer Mike Hansen invited proposals from all mainline vendors, stressing the long-reach aspect along with the need for lights-out process security, exceptional accuracy and standard tooling. “Cycle time savings were really secondary to repeatable accuracy and smooth finish,” he said, “as we knew the business would become more efficiency-competitive.  Standard tooling was essential to eliminate all the uncertainties and expenses inherent in special tooling.

Most vendors proposed special tools nevertheless, or asked for up-front money for development. Only Ingersoll’s Chris Murray offered full application support for free, and he found a standard tool able to do most of the job and a modified-standard to handle the rest. “It was their up-front helpful attitude that led to our choice,” says Hansen. “Sure we would test each of Chris’s ideas in trials later on, but only to verify recommendations and optimize parameters, not to pick a vendor. We were looking for a marriage, not just a first date.”

The company started up by automating the smaller runners, diameters down to 4 ½ feet, gradually working up to the larger parts. The smaller runners were handled by a standard 1 inch Ingersoll FormMaster Pro specially suited for long-reach roughing and finishing. To defeat harmonic vibration, the three-flute tool features circular, serrated inserts in a “timed” array. Each insert is turned “five minutes” from the other, so its edges engage a different area of the cut and the whole toolpath is covered progressively with every full cutter revolution. Pips in the seat pocket mate with dimples on the backside of the insert to keep it in exact position.  

The operation ran smoothly with no chatter, reducing cycle time on average by 50% vs. manual grinding. Tool life was more than enough for lights out operation as needed.

Scaling up brings new tooling challenges

It was February, 2011 before Mike Hansen converted the larger runners over to the automated process, and had to scale up the tool sizes accordingly.  Principal model in this class is the 11-footer, with an average annual volume of 12 pieces.  

Geometry of the runner buckets–six-axis contours, long reaches mostly and some undercuts–cried out for a ball mill for the most part.  Sheer size of the workpieces called out for a tool much larger than most standard models.  “To do a three-foot contoured cavity with a one-inch ballmill or FormMaster would take forever,” says Hansen. “Besides, in stainless the insert would wear out too quickly for secure lights-out operation.”

Chris recommended a completely standard 2-inch Ingersoll ProBall indexable ball nose for the bulk of the work, and a modified standard Form Master button cutter to handle the undercuts. “The button cutter works like a standard contour mill for most of the pass, then like a T-slotter when it reaches the undercut portion along the outer edge of the bucket,” says Murray.

Hansen and Murray worked together, right at machine-side, to establish machining parameters for the ten-ton 410 NM stainless steel casting. With the big ballmill, the standard for mainstream roughing is 15 IPM, 650 RPM, 3/8 in. DOC, ¼ in. stepover. For finishing it is 45 IPM, 825 RPM, 0.050 in. DOC, 0.100 in. stepover.

The undercut portions required the modified button cutter and involved the longest reaches: 20.5 in. from spindle to face. Roughing parameters here are 30 IPM, 700 RPM, 0.375 in. DOC, 0.100 in. stepover. For finishing the settings are 45 IPM, 825 RPM, 0.050 in., DOC, 0.035 in. stepover.

Hansen uses these parameters during the day when the place is manned, backing down the feedrate about 10% as a precaution for lights-out operations. In all cases the inserts last long enough for absolute process security over 12-15 hours; some last 45 hours per edge.  “Impressive, especially in stainless steel, which can be a wildcard in any shop.”

The Cutting Tools

The big two-flute ProBall ballmill features serrated inserts at the ball end plus heavy duty side cutting inserts farther up the active length for larger diameters and deeper cuts. “Visitors to our shop floor are amazed that such a large ballmill comes standard,” adds Hansen.  “Without it we’d be faced with either a standard or some compromise tool.” A screw-on style coupling enables in-spindle tip shuttling to 0.005 in. repeatability so there’s virtually no dead time for tool servicing.

More a modified standard than “special,” the button cutter is needed to access one portion of the cut which involves undercutting much like slotting. The backside of the tool must be effective upon withdrawal, to create the “top” of the slot. The tool uses standard inserts, with the cutter shank extended and insert seats repositioned to present cutting edges on both the front and backsides. Hansen gave Murray the CAD file on the starting and finishing geometry of the outer lip of the bucket, then Ingersoll product specialist Mark Teno and his team back at Rockford took it from there.          

Working together on the shop floor, Hansen, Murray and lead CNC machinist Ken Neal have bumped up the parameters about 15 per cent overall since February, always paying attention to accuracy, surface quality and process security in a very chatter-prone operation.  “It’s as much a matter of listening as anything else,” says Murray.  “I don’t think anybody can truly optimize a long-reach stainless steel milling operation over the phone.”    

Not surprisingly, all Canyon Hydro’s runners are completed much faster and with much less operator attention as a result of the company’s strategic move to CNC automation and advanced tooling. Cycle times average  50% less, labor costs are virtually nil. The mainstream 11 footer runs even better: 500 hours vs. 1500 before. And the tool life is reliably long enough to enable lights out operation as needed.

“What’s most important is the improvement in repeatable accuracy and surface finish quality, which will pay off for hydropower developers whose bottom line hinges on efficiency of these runners. Holding 0.005-0.010 on geometry and near-mirror 32µ finishes at our end will make a measurable difference at theirs,” says Hansen.   

Ingersoll Cutting Tools


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