VTC Cuts Crane Delivery Times

Manufacturing Engineering, Mar 2005

Delivery time is usually critical when a pedestal crane is needed for construction or repair of a dock, bridge, offshore oil rig, or other structures in coastal waters. However, batch-type manufacturing processes used to control costs tend to prolong lead times.

For example, engineers at Elevating Boats LLC (EBI; Braithwaite, LA) developed processes based on machining batches of 10-12 upper and lower crane pedestals. While reducing machine breakdown and setup costs, batch machining led to long lead times for delivery.

The parts were being machined on older vertical lathes with relatively few tools, which limited the number of tools available for a particular job. Different parts often required loading of new tools, and tool changes included manual loading of tool and fixture offsets, increasing the potential for error. Fewer tools also led to premature tool wear, lack of repeatability, and longer machining cycle times.

That all changed when the company purchased a VTC 60 vertical turning center (VTC) from Giddings & Lewis (Fond du Lac, WI). The machine quickly reduced breakdown, setup, and machining cycle times by up to 57%. With lower setup times, EBI has abandoned batch processing in favor of machining parts as needed or in kits, resulting in faster deliveries and reduced inventories.

"The VTC 60 gives us more flexibility in machining the pedestals for our C10, C20, and C30 cranes," says machine shop manager Ronald Ricouard. "We can machine an upper and lower pedestal and send it off to assembly or painting with minimal breakdown and setup time. We don't have to machine in lots of 10 before sending them to the next process. The VTC allows us to get cranes to our customers faster."

EBI uses the machine to process very large parts, such as a 40 × 38'' (1 × 0.9-m) plate made of ASTM A 588 steel. The part requires facing, boring of an 11'' (279-mm) diam hole, and drilling of a series of holes. Other parts include rings forged from 4130 alloy steel with hardness of 340 HB.

Shop-floor personnel use a 40-ton (36-t) capacity overhead crane to move the large parts into and out of the VTC's 60'' (1.5-m) hydraulic chuck. Modified to accept an additional set of boring mill jaws, the standard three-jaw chuck/table allows EBI to make either three or four-jaw setups-the latter are often useful for rectangular or square parts.

The machine also features a 20-position tool disk, which reduces time spent loading and unloading tools. "On the other machines, we sometimes had to use the same tool for roughing and finishing," Ricouard recalls. "This meant running at lower feed rates to minimize tool wear and maintain smooth surface finishes. With the VTC 60, we can do roughing with one tool and use another tool for the semifinish and finish cuts. This reduces tool wear and saves time changing inserts."

Using an automatic probe to capture tool offsets and load them directly into the CNC is faster than manual data entry and reduces the opportunity for errors, he adds. "In some machines, you have to make a number of calculations," Ricouard says. "With the VTC 60, we put the tool in, it touches the probe, and the machine automatically calculates the tool tip position and location in the disk."

EBI uses the probe for process control and other applications as well. For example, it can be used to level the rail, making sure it is parallel to the table. If a part requires multiple holes, the probe can be used to find the existing holes' position and determine where remaining holes will be drilled. "The probe makes it very easy to locate holes in relation to existing holes," Ricouard says. "We can also determine where the part is located or measure the part for accuracy."

The machine's 40-hp (30-kW) live spindle enables milling, drilling, and tapping to reduce secondary operations and improve accuracy. According to Ricouard, a rectangular plate that is faced, bored, and drilled is machined complete using the live spindle. In the past, the part was faced and bored on a horizontal lathe, then transferred to a mill for perimeter milling and drilling of multiple holes.

An example of the VTC's accuracy and repeatability is a forged 4130 steel ring 10.5'' (267 mm) thick which requires drilling of 64, 17/32'' (13.5-mm) diam holes. "Throughspindle coolant gets the chips out of the way, and we drill the holes half way-to 5.3'' [134 mm] deep," Ricouard explains. "Then we flip the ring over, position it in the fixture, and drill from the other side. The holes are matched perfectly, so we can insert a bolt and it goes all the way through. We did this at 2300 rpm and 18 ipm [455 mm/min]. Operators couldn't believe it."

The machine features a Z-axis ram extension that facilitates deep boring operations, and a programmable rail that accommodates parts of varying heights and minimizes ram extension. "We had our machine built with an extended height of 96'' [2.4 m] under the rail to fit larger parts," Ricouard says. "We can use one program for the upper and lower pedestal of our C30 crane, and automatically program the rail to move up or down when we change from the lower to the upper part. The setup maintains accuracy, and we don't have to adjust offsets to machine the two parts accurately." Circle 308