A special spindle design gets old spindles out, new spindles in, and machines back in the cut.
By Patricia L. Smith,
To swap out the RigiDyne quickchange spindle, technicians loosen the bolts, turn three clamps out of the way, and remove just the spindle unit.
They reverse the procedure to install a new spindle unit.
Hydrodynamic fluid bearings give the RigiDyne spindle
6X the stiffness of a mechanical-bearing spindle and 4X the damping.
Somewhere, a jobshop has just crashed a machine spindle, wrecking the bearings and perhaps the motor. If using a conventional high-speed spindle, the shop will spend hours, even days, removing, repairing, and re-installing it. But if the shop is using a new divided spindle, the machine is back to work within five minutes or less.
Unlike conventional high-speed spindles, the RigiDyne 40,000-rpm quick-change system comes in two parts, a spindle unit and a motorized driver. This split design, says manufacturer Ingersoll Milling Machines, Rockford, Ill., ensures that the motor isn't damaged if the spindle crashes. "If you look at spindle failures, you seldom lose the motor," says Peter Mischler, a staff engineer with Ingersoll. "You usually lose the bearings, and that sometimes damages the motor. With the split design, spindle problems aren't communicated back into the motor. And you only have to change out the damaged part, not the motor and everything else."
With a one-piece spindle, shops have to pull the entire unit, disconnecting lubrication and electrical connections. And even if there's a replacement spindle on hand, the process still takes hours. With the split spindle, shops need only a replacement spindle unit to get to work.
Technicians simply loosen three screws, turn three clamps, remove and replace the spindle unit, rotate the clamps back into place, and retighten the fasteners. "We say spindle changes take five minutes or less," remarks Mischler. "And I tell people that's if the spindle is four minutes away from the machine, because the actual exchange time is about a minute."
Even without the spare spindle unit, the repair process is relatively simple and quick. Reportedly, users can rebuild the seven-piece spindle unit in two hours with the right repair kit. But an experienced party could rebuild the unit in an hour or less, says Mischler.
Taking the quick-change concept a step further, Ingersoll has designed the spindle unit to be automatically swapped out if damaged. "The spindle unit has a V-flange like an HSK 125 tool," explains Mischler. "This allows the system to be configured so that shops could exchange the spindle units using a toolchanger."
Another similarity to HSK tooling is the simultaneous fit of the face and steep taper, which ensures a rigid connection between the spindle unit and the motorized driver.
In addition to the quick-change benefits, the RigiDyne quick-change spindle also provides cost-saving advantages. "For the same spare capability, you would have to buy two conventional spindles," comments Mischler. "And when you crash the conventional spindle, you're going to have more downtime costs and a higher cost of repair."
Suit the bearings to the job
The split spindle uses mechanical bearings in the motor section and fluid bearings in the spindle unit. "The mechanical bearings in the motorized portion are isolated from the cutting loads," says Mischler. "And the spindle unit's fluid bearings provide high stiffness and damping, which, ultimately, deliver more power into the cut."
According to Ingersoll, the spindle's high dynamic stiffness is up to 6X greater than a mechanical-bearing spindle and provides 4X the damping. The spindle also has a higher load capacity, which means it will survive crashes conventional spindles won't. Ingersoll touts other benefits of fluid-bearing spindles over mechanicalbearing spindles, including their significantly lower runout. The increased stiffness and damping also increases tool life. And while mechanical bearings require a warmup period, fluid bearings are ready to run almost immediately.
One of Ingersoll's customers has seen the advantages of the RigiDyne firsthand. The customer runs a tricky job at a 60% feedrate (because of chatter) on a machine with a 40,000-rpm mechanical-bearing spindle. Ingersoll brought the job into its own facility, running it on a nearly identical machine — however, this one is equipped with a 40,000-rpm split-design RigiDyne spindle. "We ran it at a 120% feedrate without chatter," reports Mischler. "We could do that because of the damping and the high dynamic stiffness of the fluid bearings." The RigiDyne spindle stabilized the cutting process, allowing Ingersoll to crank up feeds to double the average power delivered into the cut.
"The bottom line for anyone is productivity," remarks Mischler. "And productivity is proportional to average material-removal rate — which is limited by the maximum deliverable power. It doesn't help to have a 50-kW spindle if you can only deliver 10 kW. What shops don't always realize is that spindles can do a lot to limit cutting performance-and productivity. Or a lot to help it," he says.
Ingersoll can generate a cost-analysis spreadsheet comparing the cost of the RigiDyne spindle versus a conventional spindle. In this chart, one of Ingersoll's customers requested a cost analysis specific to a particular application. This information would vary from end user to end user.
In this spreadsheet, Ingersoll prepared information for a specific customer application, where the customer compared the cost of two spare RigiDyne spindle units plus a spare motorized driver versus three spare conventional spindles. Again, the cost analysis would vary from end user to end user.
Hydrodynamic and hydrostatic fluid bearings
Currently, Ingersoll offers the 40,000-rpm, split-design RigiDyne spindle in a hydrodynamic version. But the company is also working on a hydrostatic spindle. Both types use a fluid film as the bearing media, so there's no metal-on-metal contact.
With hydrodynamic spindles, the fluid enters the bearing at a low pressure, minimizing external pumping requirements. Spindles withhydrodynamic bearings have a narrow speed range, so they are generally used in high-speed applications such as profiling of aluminum aircraft components.
Hydrostatic-bearing spindles rely on an external pump to generate high pressures. These spindles have a wide speed range, making them better suited for high-speed machining centers.