More than 80% of all metals we use have ferromagnetic properties, these are mostly different steel alloys. That is why in practically every metalworking process we use magnetic clamping, positioning and support devices. Magnetic workholding is the most simple, convenient and effective way of clamping steel parts and assemblies during their machining.
Magnetic chucks, tables and fixtures have successfully been used for over a 100 years in grinding, milling, turning, pressing, cutting, welding and many other metalworking applications.
One of the most complete analysis of this field was presented by Dr. A. Vernikov in his book “Magnetic and Electromagnetic Equipment for Metalworking”, Machinery, Moscow, 1984.
Electromagnetic Workholding Devices (EWD)
First electromagnetic chucks for grinding machines were patented at the end of 19-th century and since that time EWD are still the most popular fixtures for grinders.
Main advantages of EWD are simplicity, relatively low costs, high magnetic holding force and deep magnetic field penetration into the object to be clamped.
EWD disadvantages are high level of heat generation which leads to device deformations and decreases machining accuracy; dependance on the external energy supply, which can lead to unsafe situations during machining; high magnetic field level above the fixture which can lead to magnetization of the ferromagnetic cutting instruments.
In the past years DVM together with several partners and as part of several International Investigation Projects has created, produced and supplied the following advanced EHD:
- EWD for highest accuracy machining with maximum working surface deformation of 2 microns and maximum working surface temperature change 2 degrees C
- EWD (rectangular and round tables) for special high power and high speed machining applications with up to 16 kg/sq.cm clamping force
- EWD for high accuracy automatic and robotic applications, including pallets, “hands” of robots and positioning systems
- EWD equipped with: highest capacity back-up batteries with up to 20 minutes back-up time; special radio remote control; continuous monitoring systems of coils heat generation
- EWD based on: recalculated magnetic circuits from steel with high magnetic permeability; solid homogeneous top plates with self-adjusted poles; compact band coils from new alloys with highest current conductivity; new compounds with highest heat conductivity and rigid one piece steel housings
- EWD with special properties for scientific, space and some other specific applications.
Permanent Magnetic Workholding Devices (PWD)
First magnetic workholding devices with permanent magnets appeared on the market in the early of 50s. These were mostly hand switchable magnetic chucks, using ferrite magnets. Later both permanent magnets and switching systems were changed and improved.
Main advantage of PWD is independence of external energy supply. Even if we do need external energy source for switching PHD on and off, this can not lead to unsafe situations, since during workpiece machining the device is using only magnetic flux generated by permanent magnets. Another PWD advantage is no heat generation allowing to achieve higher machining accuracy on PWD compared with EWD.
Disadvantages of PWD are connected with different properties and costs of used permanent magnets. Some permanent magnets cannot withstand high temperatures or humidity produced by themachining process, other permanent magnets lack mechanical strength needed withstand the necessary switching movements. High energy rare-earth permanent magnets are extremely expensive…
In the late 90s DVM established production of special high energy permanent magnets. The aim was creating initial alloys and production technology of high energy rare-earth magnets with special properties for magnetic equipment created and produced by DVM.
In recent years DVM, together with several partners and as part of several International Investigation Projects has created, produced and supplied the following advanced PWD using our own special high energy magnets:
- Series of precision hand switchable PWD, including fine poles, super-fine poles and “invisible, self-adjusted” poles with world highest clamping force for small and thin workpieces
- Series of advanced PWD with pneumatic, hydraulic or electrical switching drives and radio remote control for heavy machining applications with up to 2000t total clamping force
- Series of advanced PWD for: high speed milling; high power grinding; EDM; robotics; automated productions and some other specific applications.
Electro-Permanent Magnetic Workholding Devices (EPWD)
In the early 70s new magnetic devices were patented where permanent magnets were controlled (switched on and off) by electrical coils, placed around permanent magnets.
EPWD combine main advantages of EWD and PWD: no heat generation, no moving parts, no dependance on the external energy source while machining, possibility of remote control. Some scientists have predicted that due to all of these advantages EPWD will soon become the main type of magnetic workholding devices. But after more than 40 years other mentioned magnetic workholding devices are stil on the mrket along with EPWD. This has to do with several disadvantages of EPWD.
Main disadvantages of EPWD are: different properties and costs of used permanent magnets (see disadvantages of PWD); dependence on complicated powerful electronic switching system; crystal structure of some electrically switchable permanent magnets is not able to withstand continuous changing of magnetizing direction, which leads to low longevity for most of such systems.
In 2005 during an International Investigation Project, DVM together with several partners, within developed and started production of special high energy initial alloys and permanent magnets with improved crystal structure, which can withstand up to 20 millions electrical switchings.
In the past years DVM together with several partners and within performing of several International Investigation Projects created, produced and supplied the following energy saving EPWD, using special high energy magnets from our production:
- Series of highest accuracy EPWD, including fine poles, super-fine poles and “invisible, self-adjusted” poles with world highest clamping force for small workpieces, radio remote control and deep demagnetizing systems
- World highest power magneto-mechanical EPWD (rectangular and round tables) for special machining applications with clamping force up to 16kg/sq.cm. Up to 5m diameter round tables and up to 5x10m rectangular tables
- EPWD based on: recalculated magnetic circuits from steel with high magnetic permeability; new electrically controlled rare-earth magnets, solid homogeneous top plates with self-adjusted poles; compact band coils from new alloys with highest current conductivity; new compounds with highest heat conductivity and rigid one piece steel housings
- EPWD for high temperature metal cutting applications: up to 400 degrees C
- Series of EPWD for high accuracy automatic and robotic applications, including self-adjusted pallets, “hands” for robots and positioning systems, controlled from the central technological control systems
- EPWD with special properties for scientific, underwater, radioactive, space and some other specific applications.