Most CNC machines are built with high quality components, especially spindles. However, not every motion on a CNC machine is mission critical. CNCs have one or more axes that do not require high degrees of accuracy and precision. These axes can include doors and other auxiliary systems not associated with cutting. By picking an appropriate motion component for these auxiliary axes, not only can you take some cost out of a CNC machine, you can also improve that machine’s reliability in the field.
Doors and auxiliary axes on machine tools need different linear motion components than spindles. Doors tend to be large sheet-metal structures subject to deflection and alignment problems. They also have to work reliably in a contamination-prone environment filled with metal chips and cutting fluids.
Linear bearings are a common choice for these axes, but the size and contamination issues can be especially hard on them. Aligning these bearings can be a difficult process during the machine-building phase. First, the machine surfaces must be mounted to near perfect flatness. Then, the linear bearings are installed using fixtures to ensure that rails are aligned horizontally and vertically. Both of those steps can add significant time and cost to the machine.
In the case of machine tool doors, these steps may be both ineffective and unnecessary. They’re ineffective because the doors tend to be large sheet metal structures that will deflect in use, undoing even the most careful installation techniques. And they’re unnecessary because self-aligning bearings can accommodate deflection without the need for the expensive machining or fussy installation procedures.
The Compact Rail bearing is an alternative to linear bearings for doors and related auxiliary applications. Key features of this bearing include self-alignment and resistance to contamination.
The bearings’ self-alignment capability is due to the interaction with a set of rail profiles. The profiles are designed to allow the rollers an extra degree or two of freedom to offset misalignment between rails.
Combining the Compact Rail’s U and K rail profile supports the slider. The U-style rail has a flat raceway for roller freedom to translate in and out relative to the axial direction of the rail. The K-style rail has geometry for limited angular rotation of the roller, while still offering precise linear guidance. Together, these rails can absorb significant alignment errors in two axes: Parallelism on the horizontal plane and height differences between rails in the vertical plane. For example, at a two-meter span between rails, the system can absorb parallelism errors up to 4 mm in the horizontal plane and differences in rail height of ± 50 mm.
With the Compact Rail system, the source of the alignment errors is not important. Whether the errors are caused by the machine structure, installation or deflection in the field, this system will adjust and do so without affecting the wear characteristics or lifespan of the bearing.
These rails make doors easy to open and close due to a low coefficient of friction that ranges from 0.003 and 0.006 (depending on rail size). A Compact Rail Size 43, for example, would require an opening-closing force of just 1.5 lb when supporting a 200-lb door.
As for contamination, the U and K-style rails keep contamination minimal with seals and wipers. Installing the rails upside down further reduces contamination levels. The overall designs for the small ball bearings are permanently sealed within larger rollers. Relative to typical contaminants, the rollers have a large size with the capacity to roll over many chips that would stop a recirculating ball bearing in its tracks.
Filed Under: Assembly, Design World articles, Motion control • motor controls