Ball Screws vs. Helical Rack and Pinion
The Techno Atlas, Titan, and HDS Series CNC Routers use precision helical rack and pinion drives on the X and Y axes. This has proven to be the most accurate, repeatable and rigid drive system so far. Techno used to use ball screw drives but, over the years, we found that this drive system can be difficult to maintain and produces unwanted vibration for 4 foot, or longer, travel lengths. Ball screws can whip and vibrate at the longer lengths because they are suspended between the ends. Helical racks are mounted directly on the precision machined steel frame/resting on a machined ledge, and therefore, do not vibrate. Helical rack and pinion drives give our customers the best of both worlds—high accuracy, with minimal wear for long term reliability, no vibration, high speeds, and easy maintenance. In addition, Techno uses a direct drive from the X and Y axis motors to the pinion, eliminating the need for a belt and pulley which can introduce play and backlash into the drive system.
Let’s look at ball screw and helical rack and pinion drives more closely. There are many different grades of ball screws (C0 though C10) which determine accuracy and repeatability (see chart to the left). C0 through C5 are ground ball screws, where the screw is ground for precise channel depth for the ball nut ball bearings to ride in. The balls for the ball nut are precisely measured and fit into the ball nut so there is a high level of accuracy. These type of ball screw assemblies are typically used on precision measuring machines and not with CNC cutting systems. Then there are less precise ball screws, known as rolled (C7 through C10) ball screws. These are the cheapest ball screws because very little measuring is required and they have cumulative inaccuracies. An example of this is the average rolled ball screw has a position accuracy of +/-.002” over 12”, which means at 4 feet this accuracy could be out +/- .008”. This is typically okay for CNC routers since they are cutting materials such as wood, plastic, foam or signage materials not requiring high precision cuts, but it becomes more of an issue at lengths over 4 feet.
Here’s where a ball screw can be a maintenance concern– as a ball nut rotates and traverses along a ball screw, any dirt, dust, and debris on the screw will collect inside the ball nut, affecting the recirculating ball bearings (see example picture to the right). If this is not cleaned regularly and properly, the ball bearings begin to grind and bind, causing problems with your machine operation.
Techno recommends helical rack and pinion drives for better long term performance and easier maintenance. If a pinion wears out it is much easier, and less costly, to replace than an expensive ball nut.
This is why most of the industry, building large format CNC Routers, use helical rack and pinion. Helical rack and pinion is the choice of Techno CNC Systems large format machines. These racks are mounted to the precision machined surfaces of the steel frames. The helical racks used hold a positioning tolerance of +/-.002” per meter. In addition to the greater accuracy, this allows for greater speeds and accelerations. The two pictures above show how the THK linear rails and helical rack and pinion are laser aligned using state of the art software. This technology assures highest level of precision alignment for the system drives
The above illustration shows the longer the ball screw length, the more sag the screw will have. Under high speed motions, it’s the sag or deflection that causes vibrations, which will resonate throughout the machines mechanics, ultimately resulting in poor cut quality.