The carriages or slides utilized to provide highly accurate translational motion for determining machines or optical quality machine tools present unique challenges to design engineers. All decisions involved in developing precision slides are interdependent to some extent. Hence, any description of the design process is an oversimplification.
The customary starting point is to measure the size of the slide top, and the travel or stroke needed. It is vital to keep the slide’s natural frequency as high as possible, and that means the moving component should be extremely light. Large slides are more sensitive to thermal growth issues when compared to their smaller counterparts. They are also more costly to produce.
Weight Trade-Offs
The aim of having a slide which is extremely light so as to achieve a high natural frequency is in variance with the next requirement, which is to make a slide which is as stiff as possible. This issue has plagued engineers for a long time. Providentially, some materials are available which can prove useful.
Ceramics can be utilized if the slide is small; certain ceramics have superior mechanical properties, but the cost may be excessive for any but the smallest structures. A more useful approach would be to use composite construction using light materials in low-stress areas. The aim now is to design the lightest structure possible which will serve like a rigid body under the predicted operating loads.
Aspect Ratio
Slides which are wider than they are long should not be designed. Long, narrow slides can be sensitive to errors of roll, showing up looking like errors in straightness of travel. This straightness of travel is usually one of the design goals.
Long slides are associated with weight penalty and hence should be avoided. Lastly, long slides also increase the size of the support structure needed, which often has an adverse effect on cost.
Choosing the Bearing System
After the shape and size of the slide have been chosen, the next step is to select the bearings to be utilized. A number of different types of bearings are generally employed and each comes with its own advantages and disadvantages.
Rolling Element Bearings
Roller or ball bearings can be utilized to support machine slides. One key benefit of this type of bearing is the low coefficient of friction, which reduces the force needed to shift the slide. Both dynamic and static coefficients of friction are almost similar. This reduces the effects of stick-slip, which will impact the positioning precision of the slide and its control system.
A number of disadvantages exist with respect to the use of rolling element ways. Extreme care must be taken to ensure that all the rollers or balls which operate on a given surface are the same size within close tolerances. Even 5 or 10 microinches variation in the size of the elements can create issues with straightness.
Another issue inherent to the use of rolling element ways is the guidance of the rolling elements. The balls or rollers are often restricted to move parallel to the slide travel at certain predetermined spacing by a retainer. This retainer, particularly in case of rollers, must be highly accurate.
In case the rollers are allowed to place any considerable force on the retainer due to diameter mismatch or skewed pockets, the slide will show variations in attitude when its direction of travel is reversed and the forces applied on the retainers are mitigated.
In brief, rolling elements appear to work best on large, heavy slides where the weight of the slide can be utilized to increase the preload on the rollers or balls. The rolling elements must be extremely small sans getting into strength of material issues, and must be as numerous as possible. If rollers are used, they should be short and should not be more than three or four diameters in length.
Non-Metallic Friction
Non-metallic friction types are used in highly accurate mechanisms- There are a number of commercially available materials that are Teflon-based composites and designed for use in sliding contact applications. It is possible to apply these materials and virgin Teflon itself on machine slides. In case these materials are used, they should be lubricated with common way oils.
A certain amount of stick-slip would be there even in the lubricated condition which will impact positioning precision. In addition, at low speed a shearing action occurs between the way surface and material which results in the generation of debris and this in turn will affect the precision of motion of the slide. This debris is significantly reduced when virgin Teflon is utilized in place of a composite material.
Air Hydrostatic Bearings
A common choice for ultra precise slides, air hydrostatic bearings can be easily designed and have a zero coefficient of static friction. They tend to average the short term imperfections on the way surface. However, air bearings offer almost no damping to the machine system.
As a result, structural resonant frequencies can become a serious issue. In most slides, the lowest natural frequency is a mode where the slide vibrates along the ways with the ball screw or the spring.
The frequency of this mode usually falls within the range rotating machine members, thus rendering certain spindle speeds unusable. Efforts to reduce the slide mass can really pay off by increasing this vibration mode out of the range where spindle speed will excite it.
Vibration can also pose a challenge in the design of the bearings themselves because the bearing will vibrate once it is pressurized. One way to prevent this is not to design bearings with recesses around the inlet, unless absolutely required. If recesses need to be utilized, they must be very small but even then instability may restrict the air pressure which can be utilized.
Sometimes, porous bearings are employed in this application to enhance the stability of the air film and also to minimize the effects of local imperfections and scratches on the mating surface.
However, these bearings can display the same instability as the rare common orifice controlled bearings. Attention must be paid to the cleanliness of the air delivered to air bearing ways. Also, the pressure of the air delivered to the bearings should be precisely controlled so that the thickness of the air film remains stable.
Oil Hydrostatic Bearings
Oil hydrostatic bearings offer one of the best options for ultra precise slides. With a zero static coefficient of friction, these bearings can be designed to have high rigidity and enable damping to be designed into the system to control vibration. However, oil bearings create a mess where oil comes out from everywhere.
Nevertheless, this problem can be prevented by applying certain techniques. For instance, the slide can be designed is such a way that the drainage of oil occurs away from the bearing surfaces and back to the reservoir.
This can be achieved by having large holes and adequate amount of clearance through passageways. The drain gutters can be made larger than necessary and capillary action will pull oil into tiny gaps in dust shields.
Hydrostatic bearings need an external source of pressure, which usually involves a type of oil pump. Since oil pumps consume power and thus become warm, they turn the oil warm. One benefit of designing around low oil pressure is that the forces applied on the slide structure by the bearings are lower, which results in less distortion of the structure.
Low-viscosity oil can be used, although this elevates the flow through the bearings and boosts the power required. Liquid bearings have a considerable load capacity because of the relative motion of their surfaces. This effect causes the slide to shift away from the way surfaces when it moves.
Low viscosity oil reduces this problem. In addition, the oil pressure delivered to the bearings must be carefully controlled. Cleanliness is also vital to ensure the proper operation of oil hydrostatic bearings. One advantage of oil bearings is that it provides vibration control.
Designing the Way System
Once the type of bearings has been selected and the configuration of the slide has been decided, the configuration of the ways themselves must be decided. Thanks to machine designers, different types of ways are being used today. However, certain basic rules apply to all designs.
The first question is whether to use just three points of support or support the slide with a bearing at each corner. This does not often apply to rolling element bearings where the retainers tend to be longer than the slide. Three points can be used in the case of oil bearings with control valves for improved rigidity.
Overconstraining of the slide must be avoided as this leads to high forces which are locked up in the ways and in the slide. These forces cause deflections, which could degrade the accuracy of the system instead of improving it.
Symmetrical ways can be designed if possible as this reduces the chance of yaw errors at slide reversal. When selecting the configuration of the ways, the number of guide surfaces must be reduced as required.
All preloaded ways create forces that are locked up in the slide and way system. To some extent, these forces deform both the ways and the slide and can impact the straightness of the slide travel if they pass via areas of changing rigidity as the slide travels along the ways.
In order to reduce this distortion, preload forces should be used which are no larger than required. Moreover, the way support structure should be designed to have the same rigidity at all points along the travel of the slide.
Lastly, when designing the ways, it is important to have someone who can build the design. This is because superior designs are poorly executed in terms of manufactureility and serviceability. During the design process, the handling and fixturing of parts should be taken into account. This will result in component parts that are more precisely machined. These precise components will contribute considerably to the precision of the end slide.
Peripheral Concerns
Besides the design of the slide and the ways, a number of other factors should also be considered if the precision of the end product needs to be improved. In case vacuum, air, electrical, or hydraulic lines coupled to the slide they must be joined so that their impact on the straightness of travel of the slide is minimized significantly.
Limit switches must be mounted externally and must be of the non-contact variety. Position feedback devices should be placed in a clean, dry area so that they are exposed to oil or other impurities and must be located as close as possible to the line of action of the slide to reduce Abbe offset errors. Way covers must be used to protect the ways against damage from dropped objects or tools.
Conclusion
In order to design an effective, highly accurate machine slides, careful attention to detail and an unwillingness to accept anything less than the very best solution to the problem at hand must be practiced. Inexorably, compromises have to be made, but they must be as small in effect and as few in number as possible.
About Precitech
Precitech began operations in 1992, but continues the rich history of ultra-precision machine tool building dating back to 1962, when Pneumo Precision was founded. In October of 1997, the Pneumo ultra-precision machine tool division of Taylor Hobson (formerly Rank Taylor Hobson / Rank Pneumo) was merged with Precitech. The Precitech name was retained for this corporate entity and all offices and manufacturing facilities are now located at 44 Blackbrook Road in Keene, New Hampshire.
Our facility staffs approximately 100 talented individuals in a recently designed 60,000 Sq. Ft. building.
Precitech is a member of AMT (The Association of Manufacturing Technology) and has corporate affiliations with several professional societies and academic institutions such as Germany’s Research Community for Ultra Precision Technology at the Fraunhofer Institute, ASPE the American Society for Precision Engineering, and EUSPEN the European Society for Precision Engineering and Nanotechnology.
This information has been sourced, reviewed and adapted from materials provided by Precitech.
For more information on this source, please visit Precitech.