What factors affect the precision of High Precision Linear Rails?
High Precision Linear Rails (usually used in semiconductor manufacturing, precision testing equipment, high-end machine tools and other scenarios, the accuracy level of C5 and above, repeat positioning accuracy of ≤ ± 0.005mm) of the accuracy of the strict constraints by the factors of multiple links, and its impact on the factors than the ordinary linear guide is more complex and sensitive, can be divided into the following categories:
First, the core of the manufacturing chain of influencing factors
1. Material and heat treatment process
Material uniformity:
guide body mostly using high-carbon chrome bearing steel (such as SUJ2) or ceramic materials, if the material internal presence of loose, segregation or non-metallic inclusions (size > 5μm that may affect), will lead to uneven distribution of heat-treated stress, which in turn, in the long-term use of stress release due to the generation of small deformations (such as curvature, micro-crack) The straightness of the product will be damaged.
Heat treatment accuracy:
If the temperature is not properly controlled during the hardening process (such as heating rate fluctuations ± 5 ℃), it will lead to uneven hardness of the guide rail surface (HRC58-62 and the deviation ≤ 1HRC), the local wear resistance difference will lead to uneven abrasion and cumulative accuracy error after long-term operation; and tempering is not enough will be residual internal stresses, resulting in a small amount of deformation of the guide rail over time (such as the deformation of the length of each meter of the year＞0.002mm that affects the accuracy). 0.002mm of annual deformation per meter length will affect the accuracy).
2. Precision machining process
guide rail cross-section micro-precision:
guide rail raceway radius of curvature tolerance needs to be controlled at ± 0.001mm (ordinary guide for ± 0.01mm), if the deviation is too large will lead to the ball and the raceway contact point offset, resulting in additional torque, affecting the operation of the straightness; guide side of the datum plane requires ≤ 0.002mm/100mm (ordinary guide for ≤ 0.01mm/100mm), otherwise the installation can not be installed, the guide can not be used. 100mm), otherwise the installation can not guarantee the positioning accuracy through the reference surface.
Grinding process stability:
The final processing of precision guideway relies on ultra-precision grinding (grinding wheel speed ≥ 15000r/min), if the grinding wheel wear is not uniform (such as wear per hour ＞0.002mm), the fluctuation of the feed (± 0.001mm or less is qualified), it will lead to the appearance of the surface of the guideway with the degree of corrugation (requirement ≤ 0.1μm/25mm), which directly affects the smoothness of the operation of the slider.
3. Assembly and pairing accuracy
precision guide rail is usually used "rail - slider" pair grinding process, if the pairing of steel ball diameter tolerance (requirements ≤ ± 0.0005mm) is inconsistent, the positional deviation of the circulating channel in the slider (≤ ± 0.002mm), will lead to uneven force on the steel ball during operation, resulting in cyclic vibration (amplitude of ≤ 0.001mm). (amplitude ≤ 0.001mm), reducing the repeatability of positioning accuracy.
Second, the installation and commissioning factors
1. Installation datum precision:
Installation of the base of the guide rail flatness needs to be ≤ 0.005mm/100mm (ordinary scene is ≤ 0.02mm/100mm), if there is a bump or depression, will lead to deformation of the guide rail deformation of the force (deformation of the amount of per meter > 0.003mm that affects the accuracy); base surface roughness needs to be ≤ Ra0.8μm, otherwise the bolt will be tightened due to uneven contact with the local stresses. Otherwise, when the bolts are tightened, localized stress will be generated due to uneven contact.
When splicing multiple guide rails, the height difference (≤0.002mm) and misalignment (≤0.003mm) at the joints need to be strictly controlled, otherwise the slider will have an impact when it passes through the joints, resulting in instantaneous fluctuations in accuracy.
2. Installation fastening method:
Uneven bolt preload (recommended torque wrench control, error ≤ ± 5%) will lead to local bending of the guide rail; if the bolt spacing is too large (more than 300mm), it may be loosened due to vibration after a long period of operation, and the positioning error will be accumulated.
Guide and base of the fixed way (such as adhesive assisted fastening) if the thickness of the adhesive layer is not uniform (deviation > 0.002mm), will lead to trace tilt guide, affecting the straightness.
3. Parallelism and coaxiality adjustment:
When the double guide is installed, the parallelism error needs to be ≤ 0.003mm/500mm (common scene is ≤ 0.01mm/500mm), otherwise the force on both ends of the slider is unbalanced, generating additional friction torque, resulting in running track offset.
Coaxiality deviation (≤0.005mm) with the drive system (e.g. ball screw), if exceeded, will generate lateral force due to the direction offset of the drive force, destroying the motion accuracy of the guideway.
Third, the use of environmental factors
1. Temperature fluctuations:
Precision guide is extremely sensitive to temperature changes, ambient temperature fluctuations of 1 ℃, the guide (steel) will be due to thermal expansion and contraction of the length of 0.011mm / m changes (coefficient of linear expansion of 11 × 10-⁶ / ℃), if the equipment does not have a temperature compensation system, it will directly lead to positioning errors. For example, in the semiconductor lithography, the ambient temperature needs to be controlled at 23 ± 0.1 ℃ to avoid thermal deformation of the guide.
2. vibration and shock:
external vibration (such as ground vibration acceleration > 0.01g) will lead to guide and slider between the micro-sliding (amplitude > 0.0005mm), reducing the static positioning accuracy; frequent shocks (such as cutting force fluctuations in machining equipment) will accelerate the wear of the steel ball and the raceway, so that the accuracy of the use of time with a rapid decline.
3. cleanliness and lubrication:
dust in the environment (particle size > 0.5μm) into the raceway, will lead to steel ball and raceway grinding wear, short-term decline in precision; grease purity (impurity particles ≤ 0.001mm) and filling (every 100km of operation to replenish the amount of time) is not enough, will exacerbate friction, resulting in cyclical precision deviation.
Fourth, the material and the use of loss factors
1. Material stability:
Insufficient aging treatment of guideway material (e.g. residual stress >50MPa) will produce slow deformation (annual deformation >0.002mm/m) due to stress release in long-term use, resulting in precision drift.
2. Wear and fatigue:
Even under ideal working conditions, the contact stress between steel ball and raceway (up to 3000MPa or more) will lead to tiny wear, and after accumulative operation of 100km, the repeat positioning accuracy may decrease by 0.001-0.002mm; if there is off-loading (load offset center＞5mm), the wear will be aggravated, and the speed of precision decline will increase by 3-5 times.
In summary, the accuracy of High Precision Linear Rails is the result of the synergy of the whole chain of "manufacturing accuracy - installation control - environmental management - use and maintenance", and any small deviation in any link may magnify the final accuracy error, so it is necessary to carry out the whole process of control in design and application.
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