The world’s leading manufacturer of marine transmissions grinds shafts of all lengths,weights, shapes and sizes. The one thing they must have in common is precision,as in very close tolerances and unforgiving surface finishes. Flawed shaft production results in transmission failure. The key is using the best grinding technology available.

RACINE, WI — Twin Disc, a family-owned manufacturer of marine propulsion systems, has been in business for 75+ years making more than 30 different marine transmissions in a variety of configurations ranging from 25 rpm to 3600 rpm. They can be found in luxurious pleasure craft, to stealthy military vessels to hard working tugs, pushboats, heavy-duty fishing boats, ocean-going shrimp boats and crew boats. In fact, Allied forces depended on Twin Disc marine transmissions to drive some 40,000 landing craft loaded with troops and supplies onto beaches during the Normandy invasion and throughout the South Pacific during World War II. Today, Twin Disc marine products can be found propelling all kinds of vessels, on all kinds of water, all around the world. But that’s not all. Twin Disc also makes transmissions for off-road heavy-duct trucks (Freightliner, for example), trucks for the mining industry (think Volvo and Terex) and for the Bradley fighting vehicle and tank retrieval vehicles like the one that pulled down the statue of Saddam Hussein in Iraq.  All of which suggests these transmissions are remarkably rugged and up to any challenge, on or off the water. 

Manufacturing in Cells

“Twin Disc operates in cells,” begins Gary Pope, production manager, “manufacturing cells and non-manufacturing cells (assembly, stator and motor drive, process completion and so forth). There are 21 cells in all, 14 of which are manufacturing cells — milling, turning, grinding, drilling, finishing, etc. Raw stock comes in one end of the plant and the finished product goes out the end.

Critical to the operation of the Twin Disc transmissions are the drive shafts. “We do shafts up to 10 different diameters with a number of different tapers. These weigh from 1 lb to 350 lbs, and there are probably some 1200 different shafts,” Pope says. “They’re from 8” to 48” long and from 0.500” to 10” in diameter. We hold tolerances from 0.0002” to 0.001”, with surface finishes from 10 Ra to 40 Ra. The materials run from 1144, C1144, 4145, 4140, 8620 and some stainless. Order quantities range from a single shaft to 500 pieces.”

Changing methods

Terry Andersen, team leader, programmer and machine operator in the grinding area, explains that Twin Disc had been looking for a process change in the production of the transmission shafts. “We used to have two CNC grinders, which would grind the straight OD’s and an old Cincinnati that would grind the tapers. But the process was operator intensive, and required us to re-fixture the shaft from one process and machine to the next. What we wanted was a machine with a full B-axis where we could combine the processes in a single machine.” Pope says that during their search they actually acquired a European grinder about 10 years ago with a full B-axis, worked with it for about a year, but it turned out to be a
disaster. “It wouldn’t hold diameter size, wouldn’t hold taper tolerance and was just not consistent. We ended up sending it back to the manufacturer and continued with
our old method of shaft production.” He says that when they first started their search for a really flexible grinder for doing shaft work in one shot —meaning with a full B-axis — they were all a little “leery”. There weren’t that many manufacturers out there to choose from, and the dimensions of some of the shafts, especially the upper end on weight (350 lbs) eliminated some right away.

The right solution

If one looks long enough and hard enough and does the right research, the right answer to a problem (or process) usually rises to the surface. In Twin Disc’s case this came in the form of the Kellenberger Kel-Varia cylindrical grinding system with the Heidenhain GRINDplusIT control.

“The Kellenberger not only met all our requirements, but it was a proven machine. It had been in production a while,” says Pope, “so we didn’t have to worry about working the bugs out of a new design. It did exactly what we were told it would do by Iverson & Company, the sales organization we worked with, and more. It’s been very reliable and most important, it holds tolerances and surface finishes with remarkable repeatability.”

The control

Andersen says that the Heidenhain control was, at first, a little different. “The control was unlike anything I’d ever programmed before, there are just so many features, and it does so much, so quickly. I always recommend that they go to Elmira and take the course on the control. It just really lets you take complete advantage of the power of the control and all of what it can do for you.”

The Heindenhain control features a Windows 2000 operating system, a graphics editor, DXF import, form editor, expanded grinding cycles, intermediate dressing at the push of a button, comprehensive tool management, several reference points for each grinding wheel, remaining travel display and more.

Setup times

Andersen reports that since they run as many as 1200 different shafts, changeover and setup are pretty common occurrences. “It’s really remarkably easy,” he says. “Depending on what’s involved, we can changeover from shaft to shaft in minutes. This is not only amazing but crucialfor us when you consider the volume and variety of shaft work we do here.”

It’s a taper-thing

The tipping point in the acquisition of the Kellenberger came down to the taper issue. “We run four or five different tapers, and for a lot of our shafts, the taper is really nothing more than a press fit,” Pope says. “These tapers are absolutely critical and it was critical that we got a
machine that could repeatably and consistently hold very tight tolerances.”

He notes that shaft OD surface requirements range from 10 Ra to 40 Ra. Most of the tapers are 40 Ra as well. But he also notes that there are tapers with an angle of 30 to 1, which is less that a one-degree taper. “So we had to have a machine that could handle that kind of range of demand. It had to be flexible, agile, rigid and above all repeatably consistent in holding very tight tolerances.”

Cells

About a year after the first Kellenberger, Pope bought a second, exactly the same as the first with the exception that the second machine has a hydraulic tailstock to handle even heavier shaft work.

“We used to have two grinding cells,” Pope says, “and it was a lot more labor-intensive. It was slower, with more operator involvement, fixturing and refixturing shafts from the OD grinders to the plung grinder — which was a bottleneck in itself and lowered our ability to improve productivity. Now we’ve pulled the OD machines and the Cincinnati plunge grinder completely.

We now have the two Kellenbergers in a single cell, and a single operator runs both machines, and we’re able to run twice as much product as before. One operator does two shafts at once, and they come off finished, complete. We run these machines six days a week, 24 hours a day, and we’ve already seen improvements in productivity, and the accuracy and repeatability couldn’t be better.”

Pope concludes with these observations regarding the Kellenbergers. More powerful motors and a still faster control system ensure highly dynamic unround and thread-grinding operations. The highly capable new Kellenbergers feature absolute measuring in the B-axis, incremental, distance-coded scales in the X- and Z-axes, ultra-high resolution in the C-axis and expansion options with the addition of a second B-axis, thus ensuring the ultimate in dimensional stability and profile accuracy. “Who can ask for more than that,” he says. “For our needs, we more than made the right choice.”