Precision Metrology: Eden-Rolt Comparator and Machining Techniques
The Eden-Millot Comparator (Eden-Rolt Millionth Comparator)
The Eden-Rolt comparator, more commonly known as the Eden-Rolt millionth comparator, is a highly sensitive measuring instrument used in metrology for high-precision dimensional comparison and gauge calibration.
Principle of Operation
This device uses a combination of mechanical and optical magnification to measure minute differences in size. The core principle involves comparing an unknown dimension to a known reference standard (like a slip gauge or master gauge).
Mechanical Amplification
- A lever mechanism is used to translate the slight vertical movement of a measuring anvil into a small angular deflection of a pointer arm.
- The system is supported by parallel steel strips to ensure frictionless movement.
Optical Amplification
- A “web” or opaque pointer at the end of the mechanical lever is illuminated by a lamp.
- Its silhouette (shadow) is projected onto a frosted glass scale through a system of lenses and prisms. The large projection of this small angular movement provides the final, high magnification.
Key Features and Uses
High Magnification and Precision
The Eden-Rolt comparator can achieve magnifications of up to 20,000 times, allowing it to detect dimensional variations as small as 0.00002 mm (two hundred-thousandths of a millimeter).
Measurement Range and Application
- It is primarily used to calibrate gauges and measure small components, typically up to 25mm in size.
- It is a critical tool in quality control and manufacturing settings where extreme accuracy in dimensional measurement is required, such as in precision machine shops, to ensure manufactured parts conform to design specifications.
Advanced Machining Processes
Electro-Chemical Machining (ECM)
Electro-Chemical Machining is a non-traditional machining process in which material is removed from the workpiece by chemical reaction using electricity, not by cutting or rubbing. In ECM, the workpiece is connected to the positive terminal and acts as the anode, while the tool is connected to the negative terminal and acts as the cathode. A conducting liquid called electrolyte, such as sodium chloride or sodium nitrate solution, flows continuously in the small gap between the tool and workpiece.
When DC current is applied, metal atoms from the workpiece lose electrons and dissolve into the electrolyte as metal ions, so material removal takes place without any physical contact. The shape of the tool is copied onto the workpiece. Since there is no heat, force, or tool wear, ECM gives high accuracy, a smooth surface finish, and is used for machining hard materials like turbine blades and dies.
Electro-Chemical Reactions in ECM
In ECM, electro-chemical reactions occur at both electrodes when current flows through the electrolyte.
- At the Anode (Workpiece): Metal atoms lose electrons and convert into positive metal ions, which go into the electrolyte solution, causing material removal. This reaction can be written as: metal → metal ions + electrons.
- At the Cathode (Tool): Water molecules gain electrons and produce hydrogen gas and hydroxyl ions.
These reactions allow continuous and controlled removal of material from the workpiece without any heat generation or mechanical damage.
Material Removal Rate (MRR) in ECM
Material Removal Rate in ECM means the volume of material removed per unit time and depends mainly on the electric current, atomic weight, valency, and composition of the material. MRR is calculated using Faraday’s law, where current and the Faraday constant are used along with the equivalent atomic weight of the alloy. For alloys, the contribution of each element is calculated using its weight percentage, atomic weight, and valency, and all contributions are added together. After substituting values in the MRR formula, the final MRR is obtained in cubic centimeters per minute, which shows how fast material is removed in ECM.
Milling Techniques Comparison
Up Milling
Up milling is a milling process in which the direction of feed is opposite to the direction of cutter rotation. In this process, the chip thickness starts from zero and gradually increases to maximum, due to which cutting forces act upward on the workpiece. Up milling produces more friction, more tool wear, and a comparatively poor surface finish, but it is safer to use when there is backlash in the machine because backlash does not affect this process significantly.
Down Milling
Down milling is a milling process in which the direction of feed is the same as the direction of cutter rotation. Here, the chip thickness starts from maximum and gradually reduces to zero, which results in smoother cutting action and a better surface finish. Cutting forces act downward, which helps in holding the workpiece firmly, and tool wear is less compared to up milling. However, backlash is a serious problem in down milling and must be controlled for safe operation.