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CNC Router – Ball Screws vs Rack and Pinion

the CNC Router StoreThe New FMT Patriot CNC Router sold through Plastimach and the CNC Router Store features Ball Screw Drives on the XY and Z axes. We often get about the pros and cons of the two methods.

Ball-Screw vs. Rack-N-Pinion

Rack-n-Pinion Explained – Most machines in this price category use strait rack-n-pinion to drive the axes. It is a design where the teeth of a steel pinion gear are pushed into the teeth of a steel rack. The connection is metal on metal with grease applied, and sometimes the pinion will be a little softer so it will wear, since it is the easier component to change. The pinion spins in a bearing housing, is usually belt driven by the drive motor, and typically has no precision adjustment for tightening the belt or meshing the pinion into the rack. When tightening the belt or replacing a pinion a lot is left to your imagination. In some cases helical rack-n-pinion is used, and because the teeth are on an angle, backlash is reduced. It is a metal on metal, medium-to-high wear design, that has the potential to wear un-evenly. Over time, replacement of the rack, pinion or both is likely. It is hard to keep lash out of this system design. Pushing the pinion harder into the rack will usually result in stress on the drive motor, uneven wear, and quicker deterioration. The only way to truly eliminate backlash in a rack-n-pinion design is to use two drive motors with two pinions on the same rack with the second motor applying gentle motion in the opposite direction of the axis travel.

Ball-Screw Explained – A ball-screw design uses hardened steel rod, precision ground or cut with circular leads or grooves. A matching steel ball-nut is machined with the same lead/groove pitch as the screw and includes recirculating tubes. As the screw or ball-nut spins, steel ball bearings run down the matching leads of the ball-nut and screw and back to the top again through the recirculating tubes to start the process over. It is a low/even wear, precision ball bearing design that is sealed to keep out debris and easy to lubricate. Ball-screws are easy to dismantle and clean. As leads wear, a larger ball bearing can be applied to eliminate backlash, or two ball-nuts can be opposed and spring loaded to eliminate lash. Many ball-screws are cut with two sets of leads, so if one set wears the ball-nut can be spun onto the other lead set. It is not unusual in high demand applications for ball-screws to run problem free for 10 plus years.

 

Rack-n-Pinion Applied On a moving gantry machine, a single drive motor design to drive the bridge is complicated. In this arrangement the drive motor spins a long shaft mounted in end bearing blocks with pinions on both ends meshed into racks on each side of the machine. Most machines use a dual drive motor, rack-n-pinion configuration to drive the gantry, one on each side. In either case, keeping the rack-n-pinion mechanics adjusted on both sides for optimum performance and wear can be tricky. Dual motor systems can also get out of sync, and tune, causing unpredictable results and machining of parts that aren’t square. Applying single rack-n-pinion drive systems on the cross bridge and vertical axes is strait forward. If you choose rack-n-pinion you better plan to become a fairly solid technician.

 

Ball-Screws Applied – Most machines in this category, including ours use a single, center ball-screw design to drive the moving gantry. The bridge frameworks is an elongated “O” design, connected to profiles rails on both side of the machine, which wraps around below the table. The ball-screw and ball-nut connect to the center of this framework under the table. It is a simple single drive design with no tuning issues, and the screw is below the table so debris buildup is virtually non-existent. If you want problem free, ball-screws are the way to go.

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