What did it take for one aerospace fastener manufacturer to multiply centerless grinding part output without multiplying the number of grinding machines on their floor? In this application, the company needed to make about 4 times as many parts as they were used to. They were happy with the parts they were grinding on their Cincinnati centerless grinding machines, but they would need to buy, install, equip, and man a lot more machines to meet their growing production requirements. Or so most everyone thought. But what if there was a better way? After all, the current process used old machine technology. Staff speculated that maybe the limitations of the older equipment also limited their output in other ways, such as wheel speed, and the ability to automate.
So the company contacted builders of new centerless grinders to find out what it would take to multiply their production rate without duplicating their current process. They learned there were a lot of things they could do differently to up their production while lowering their cost. Some of the enhancements they made included automation, in-process gaging, auto-offsetting in microns, enhanced wheel dressing function, multiple independent position slide adjustability, and incorporating advanced grinding wheel materials that exploited a higher speed grinding spindle on a more rigid and stable machine. Let’s look more closely at a few of the features on the actual machine model installed.
Inherent to any CNC technology is the ability to offset slide movement to compensate for tool wear. Though this CNC function is one of the key advantages over conventional machines, many applications expose its limitations. For instance, when CNC grinding, offsetting is typically limited to the X-, Y-, and/or Z- axes, or any main axis. If the machine is otherwise capable of handling more than one part at a time, most grinding machines are incapable of offsetting for more than one part per cycle. Some multi-part per cycle applications with less critical tolerances might be possible. But because wheel wear is never perfect along an eight inch length, holding extreme tolerances isn’t possible without the ability to offset for each part independently. So why not offset in the wheel dress, transparently? And that’s what was done in this application on a Glebar GT-610 CNC. On this machine, the number of independently adjustable stations was limited only by the width of the grinding wheel and the length of the parts. So if the maximum part length is 1” and the grinding wheel is slight longer than 8”, the machine could be equipped with eight independent adjustable segments and tooling supports that hold the part during grinding, and which can be adjusted in accordance with the post process gauging feedback to accommodate for wheel wear.
OK, now the ability to run and compensate for eight parts at a time is possible, but each part needs to be loaded in position, ground, unloaded, and measured for size and offset, and this all has to happen very quickly, since cycle times might be under 10 seconds total. Clean design, integrated feedback, high-resolution controls and slide repeatability, repeatable gauging to less than a micron, and auto-sensing are key to this aspect of implementation. While all of these elements are feasible, the machine has to be designed in such a way to enable accessibility for the loading and unloading devices. Especially in the case of high volume production, the machine must be capable of handling large amounts of data exchange efficiently. Measuring eight parts to less than a micron tolerance, sending the resulting measurement to the machine control, and offsetting each of eight stations according to the measurement is easy to imagine, but not every gauge to machine control interface can keep pace with cycles of less than ten seconds or faster.
In order to optimize grinding wheel performance and minimize wear, part diameters under one inch require higher grinding spindle speeds then was available on the older technology machines. As grinding wheel materials have evolved to handle aerospace parts, spindle speeds have gradually increased over time. Just using higher spindle speeds can account for a doubling of production rates.
In the end, by replacing the Cincinnati centerless grinders with Glebar GT-610 CNC models, the aerospace company was able to reduce floor space, run multiple machines with one operator, quadruple production per machine, decrease wheel cost per part, improve quality, and introduce greater flexibility to handle a broader range of parts then was possible on the older equipment.