By Josh Cosford, Contributing Editor
The machine tool is a machine that uses a guided tool for the shaping of materials, typically by cutting, grinding, drilling, boring or forming, etc. As opposed to hand tools, which require the operator to be fully in control of the metal shaping, a machine tool uses one or more fixed or rotating tools to move along an axis to shape metal. The machine may be electrically, hydraulically or manually traversed, but the machine tool uses guides and balls screws to feed the tool along its axis.
Hydraulics improves lathes
Manual lathes of every generation use a chuck to clamp the workpiece down for machining. A manual chuck uses a scroll and pinion to adjust and tighten the jaws. A keyed wrench is used for the tightening, and if the process isn’t done with sufficient force, the workpiece can slip during machining. Machinists must often hammer on the wrench to achieve sufficient clamping force, which can be tiring in a production environment.
The hydraulic (or power) chuck was developed to improve performance in the areas of speed and repeatability. Hydraulic energy can close a hydraulic chuck in a fraction of a second, which obviously improves cycle time compared to hammering on a wrench. And because hydraulic pressure is easy to control with relief or reducing valves, the clamping force can be accurately and repeatably achieved, preventing over- or under-clamping, which can damage the workpiece.
The hydraulic chuck uses an annular actuator to push a tapered collet, forcing the jaws down on the workpiece. Obviously, provisions must be made so that stationary plumbing allows a rotating cylinder to receive flow, although this can be easily overcome with sealing technology allowing for both axial and radial movement.
With a through bore chuck, features can be added so bars of raw material are fed into the machine hydraulically, rather than the operator pulling or pushing the bar through the machine by hand. The draw bar cylinder can clamp and draw the material through the chuck, enabling the operator or machine to part off the material and then begin creating a new finished piece without resetting or restarting the machining process.
Realistically, lathes with hydraulic chucks and draw bars find themselves in CNC machines rather than manual lathes. The speed and power of a CNC machine almost requires the same from the work holding equipment of the lathe. Why machine a complete component in ten minutes, only to spend five minutes setting it up and removing it from the machine?
Simplifying work holding
Work holding is the primary function of most hydraulic components used in the machine tool industry. For milling applications, clamping vices are used for the fixturing of workpieces held onto the work table. Just as with a standard bench vice, those inside a CNC machine must be manually opened, closed and tightened. The same difficulties of lathes exist with mills, in that you risk either loose workpieces with under-clamping or damaged workpieces with over-clamping.
Hydraulic fixturing provides the easily controllable clamping force with speed and repeatability. The advantage to hydraulic fixturing on mills compared to lathes is that mills can be outfitted with the components from the aftermarket quite easily. A lathe must usually be ordered with hydraulic chucks and drawbars, as upgrading after can be difficult. However, hydraulic fixtures for mills can easily be added afterward through third-party suppliers.
Hydraulic clamping for mill tables are offered in many configurations, but the most popular is the swing clamp. Part rotary actuator, part hydraulic cylinder, the swing clamp mounts with rod upwards and pivots at the same time as it retracts. As it bottoms in retraction atop the workpiece, it clamps the piece securely to the bed.
Conversely, the swing clamp extends to release the material as the rod rotates and the clamp bracket moves out of the way, allowing the operator to easily remove the workpiece. Any CNC mill can be upgraded to use hydraulic fixturing and it is usually just a matter of adding a small power unit with control valves. Often, these valves can be operated by the computer controller of the CNC machine, making an upgrade simple. The hydraulic actuators are powered by an external power unit, often a very compact unit which can be tucked out of the way.
Sizing down with the HPU
Hydraulic power units for the machine tool industry are unique to the segment, and they employ techniques not used in other environments. Their miniature size is often the primary consideration, as they must fit into a tiny area of real estate within the machine, especially if the machine is outfitted with a power unit from the factory.
Of all the fluid power topics covered in schools, machine tool hydraulics seemed to be left out more often than not. It’s not that they’re overly complicated or intricate, but it seems that they have just flown under the radar. The world of the CNC machine tool is increasingly competitive, which has resulted in machine pricing dropping drastically over the past decade. Those reduced costs began with less expensive components, of which the power unit price was reduced as well.
The result of this stiff competition led to a handful of compact power unit suppliers being able to produce the components required for a CNC machine. The pump/motor group must be efficient both in dimensions and energy usage, but able to perform well when asked. The pump is often close-coupled to the motor to save space, which means one component must have a custom female shaft. The close-coupled pump/motor is one reason few manufacturers build systems for the machine tool industry.
After the pump/motor group is mounted to the tiny reservoir, the remaining accoutrements must be attached. These components include coolers, filtration, plumbing and sometimes valves. The power unit occasionally has advanced control, like a remote pressure compensator allowing the operator to change pump pressure on the fly. Coolers are often just radiators plumbed into the case drain line of the pump, and if you’re lucky, the motor’s cooling fan will aid in heat dissipation. If valves are mounted to the power unit, they’re almost exclusively compact stack valves on a manifold. Custom compact manifolds with cartridge valves are becoming more popular, especially as suppliers become more flexible in their manufacture.
The room for the independent hydraulic shops in the machine tool hydraulic market is limited, as least from a Tier 1 perspective. The compact hydraulic packages designed for CNC machines are OEM specific, specialized and required in high volumes. You may get lucky to service or replace an old or obsolete power unit, but if anything, a local hydraulic shop will be best suited for custom upgrades, such as with material handling applications which support the primary function of the machine.
However, because of the size of the machine tool industry, the proverbial pie is very large. Even a hose supplier working with some of the big OEMs can secure a significant part of the hose market in this industry alone. If you’re a fluid power component manufacturer, your best bet is to start small and supply ancillary components, such as fixtures or replacement parts. This market will only continue to grow, especially as Industry 4.0 expands to every corner of manufacturing and old, manual forms of manufacturing fall to the wayside.
Filed Under: Fluid Power Basics, Fluid Power World Magazine Articles