Machine components developed with the aim of attaining precision of the order of nanometers must be made of stable materials and simple in form.
The design must consider even small second order effects of forces generated at the time of normal device functioning. Environmental conditions need to be identified and considered.
The machine slide described in this article is this kind of a device. Care has been taken that the crucial components are simple shapes, which can be manufactured easily.
The materials used include pearlitic iron and stabilized tool steel; both are materials whose stability and properties are clearly known.
In order to design a machine slide as a stand-alone component, it is mandatory that special attention be given to the slide sub-base rigidity so that mechanism accuracy will not be degraded by stress on the assembly during handling after assembly and testing.
The slide base is machined from iron bar stock that is cast continuously. It is developed so that all longitudinal milling cuts extend through the part completely. The base is quite thick in order to withstand strain during handling.
The position of hydrostatic ways was such that the preload forces generated by the operation are either trapped in a single member or in the case of the ways themselves operate in opposite directions so they reduce the deflections which they cause.
This design is similar to the general design of the Colath lathes produced by Philips. As shown in Figure 1, this design leaves the slide free of stress caused by operation of the ways. The support bearings and slide top are made of pearlitic cast iron.
The guide bearing extends downwards from the slide top into the gap between the guide ways. This component has the recesses forming the hydrostatic guide bearings machined into its opposing surfaces.
The member has an hydrostatic oil restrictor pin on one end, which supplies metered oil to one half of the hydrostatic thrust bearing, which couples the slide to the intermediate slide. The guide bearing’s other end has provisions for mirror mounting or mirrors needed for laser interferometric position feedback.
The guide ways are designed using A6 tool steel. They are stress relieved and stabilized cryogenically. The geometry of these two components is critical to slide performance. It is required that the inner and lower surfaces of the guide ways be as straight and as flat as possible.
Further both the surfaces must have a fine surface finish and be square to each other within 2.5µm. hydrostatic bearings help mask short wavelength disturbances in the straightness of the ways however longer disturbances than 25mm will not e completely eliminated.
This can be overcome by grinding the opposite surfaces of the guide ways at the same time on the way grinder with the mounting surfaces opposite to each other or facing each other.
The ball nut is present in the intermediate slide. The slide also holds a pair of hydrostatic thrust, bearings, which transmit the thrust from the ball nut to the main slide.
Runout between the ball screw bearing and the ball screw pitch diameters can cause straightness errors that are cyclic with the all screw rotation. The intermediate slide prevents these disturbances from impacting slide accuracy.
The cyclic reaction forces due to ball screw errors can cause deformation of the way system itself, hence they must be reduced even though the intermediate slide is used.
This can be achieved by using a ball screw about 50mm longer than needed to obtain a specific stroke. By maintaining a distance between the lead screw support bearings and the ball nut, the forces due to bearing misalignment are reduced. This causes improved slide straightness.
There is an optimum length for the intermediate slide. Figure 2 schematically shows the slide and its related translation mechanism. Figure 3 shows the formula for optimizing the intermediate slide length, and also indicates the general shape of the length versus stiffness curve.
Ball Screw Details
It is important to note that the ball screw thrust bearing are positioned at the screw end away from the servo motor. The ball nut is positioned at the end of the intermediate slide away from the motor. This was done in order to increase slide rigidity when it is farthest from the motor.
The hydrostatic bearings for supporting and guiding this slide are designed for maximum rigidity. They are also designed for consuming the smallest possible oil volume consistent with good design practice.
A low flow rate through bearings implies that a lot of power is not required to pump the working fluid
A low viscosity spindle oil is used as the working fluid. It was selected to reduce hydrodynamic effects that occur during rapid positioning moves and cause changes in the slide position which delay over the first few seconds after the slide has stopped moving.
The bearing includes laminar flow detectors that operate like capillary tubes. Figure 4 shows the restrictor design. For restrictor manufacture, a cylindrical pin is held in a vice while a flat is ground parallel to its axis. The flat length, depth and pin diameter determine the fluid flow resistance.
Operating Pressure and Flow
The pressure at which the bearings operate is 17bar. The recess pressure is 50% of the supply pressure. For all recesses together, oil flows thru the bearings at 152 cc/min (total for all recesses). The bearing clearance is 12µm.
Using an ADE Microsense probe for data acquisition, positioning tests were done. The positioning loop was closed by a laser interferometer with a resolution of 2.54nm. Figure 5 shows the results of a series of moves which include several direction reversals.
Paying close attention to details, a high degree of accuracy can be obtained from modular machine components that can be utilized for the construction of machines having nanometer accuracies. The summary of test slide performance is provided in Table 1.
Table 1. Summary of test slide performance
||0.3 arc sec
||0.35 arc sec
|Mass of moving parts
|Dimensions of slide top, L x W
||460 X 380 mm
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.