by Kip Hanson
Grinding is becoming increasingly high-tech, automated and multi-functional
Cylindrical and centreless grinders have been used for decades to make parts both round and true. Like all other machine tools, however, they’ve grown more capable over the years, to the point that manufacturers who have long sent their grinding work to specialty shops might now consider investing in their own equipment. And for the shops that specialize in grinding, or those that need to grind complex shapes into turned and milled parts, it might be time for an upgrade.
Billy Grobe, aero engine technology manager at Makino Inc., can suggest one such upgrade, especially if your shop is heavy into blade and vane turbine work. That’s because his company’s G5 and G7 model CNC grinders are also quite adept at five axis milling, drilling, and tapping. “Both have the same functionality, but the G5 is built on our A51 machining centre platform, while the G7 uses the larger A61,” Grobe says. “We have hundreds in the field, most of them in aerospace shops making engine components. They’re especially good at that.”
Setting aside its machining capabilities for a moment, what makes the G-series so good at grinding Inconel and nickel alloys, metals notoriously difficult to grind? Grobe says it starts with Viper, a technology first developed in the UK. “It uses a low-cost aluminum oxide wheel that’s around 50 per cent porous, together with high pressure coolant that is forced into the porosity around the periphery of the wheel,” he explains. “This makes it very effective at carrying coolant through the depth of cut wheel engagement and carrying away heat and swarf, two very important considerations when grinding superalloys.”
How effective? Grobe suggests that, depending on the alloy, workholding, and part geometry, this combo machining centre and grinder can take a two millimetres depth of cut at up to two metres-per-minute feed rate—substantially more than with traditional grinding. And since the G-series can also perform milling and drilling operations on the same machine, it offers the same benefits as other multitasking machines, namely fewer machining steps, improved part quality and greater throughput. “We can also hold up to twenty grinding wheels in our 60-tool carousel, filling the adjacent pockets with drills and milling cutters,” he says. “You can’t do that on a traditional CNC grinder, and are forced to use wheel stacks if you need more than one grinding wheel.”
The G-series might be a slam dunk for aerospace applications, but what about more general-purpose grinding? Rob Titus has a few ideas. A grinding product specialist for Okuma America Corp., he notes that many shops hang on to their equipment longer than they should, thereby missing out on the greater usability and features that come with a modern machine.
“In all fairness, a lot of job shops and smaller manufacturers don’t do all that much cylindrical grinding, so there’s little motivation to trade in their old manual grinders,” Titus says. “With true grinding houses or job shops that grind every day, however, it’s a completely different story. They’re already using CNC machines and are now looking at ways to increase throughput; in most cases, the best way to accomplish this is through automation.”
For shops that wish to travel down this road, Titus has a few recommendations. The first, of course, is investment in a high-quality grinder—in Okuma’s case, this might be the GA26W or a similar model. After that, there’s in-process gaging to consider—Titus points to Marposs and Control Gaging as two possible sources—for real-time dimensional feedback during unattended machining. Automated dressing is a must, as well as some type of spindle load monitoring to tell the machine when and how much wheel to remove. And last but not least, there’s workholding, which is significantly different in an automated grinding scenario compared to the drive dog and dead centre approach common with manual processes.
“To achieve the best roundness, cylindrical grinding needs to be done between centres, but drive dogs can’t really be used with robotic part handling,” he says. “A number of companies have developed three-jaw compensating chucks to get around this problem. These are equipped with a centre to locate the part, while the jaws apply enough pressure to drive the part without pulling it off center.”
And what about centreless grinding, a machining technology associated with short cycle times and high production volumes? Surely this is one instance where automation is the norm, right? Not really, says Harry Schellenberg, who along with his brother Dan co-owns and operates Echo Hill Automation Inc., which designs and builds the Tactic 8 automated centerless grinding cell.
“We’ve been in business for 25 years, and the first 15 of that was spent rebuilding and automating other company’s centreless grinders,” he says. “Along the way, we realized that none of them were really designed for automation, and even if you did a good job retrofitting them for unattended operation, the workflow remained a handicap. During the last recession, we found ourselves with extra time on our hands and decided to develop our own grinder with automation and production efficiency in mind. The Tactic 8 was the result.”
Tactic is short for “total automatic compensation through integrated control.” Schellenberg admits it’s a mouthful, but it does a good job of describing Echo Hill’s goal. Starting with a clean sheet of paper, the brothers designed a centreless grinder with inverted guideways to eliminate “the lapping effect” they’d seen on traditional machine designs. With that they included linear drive motors—again, mounted overhead and away from grinding swarf—surrounded by a positive airflow system to remove heat and further reduce the chance of contamination.
Another trend they wanted to address is the increasing lack of skilled machinists. So aside from adding a flexible automation system, they also made the machine easy to operate and suitable for lower production volumes. “I spoke to a lot of shop owners and heard time and again that they need machinery that even an unskilled operator can run,” says Schellenberg. “Having a CNC control makes that possible, but we also just finished an automation package geared for high-mix, low-volume shops. There’s no hopper, so it’s less expensive to tool up and faster to changeover, but still has integrated gaging along with automatic load and unload. It’s a great option for shops that want the speed of a centerless grinder but have lower production quantities.” SMT