Magnetic fixtures such as chucks, tables, pallets of different size form and design are widespread equipment in any metalworking production. Yet all magnetic fixtures have a serious disadvantage: we only use 15% to 25% of their clamping power. This has to do with the fact that magnetic force is applied to the workpiece to be clamped in the direction perpendicular to its base surface. The main component of metal cutting force on the other hand is applied to the workpiece in the direction parallel to the magnetic device working surface. In other words the force opposing cutting forces is in fact the friction force between magnetically attracted workpiece and device working surface.
Depending on several factors this friction force on traditional magnetic devices does not exceed 15% to 25% of magnetic attraction force. In many practical cases this is not enough for efficient metalworking and extra mechanical workpiece support is required for safe and reliable machining. Such approach was first analyzed and described in the book “Magnetic and electromagnetic equipment for metalworking” by Dr. A.Vernikov, “Machinery”, Moscow, 1985. In this article we will describe some advanced methods of combining magnetic and mechanical workpiece clamping means.
Side and positioning stops
Every magnetic fixture has the so called side stops attached to the fixture side surfaces for supporting comparatively large workpieces. These are in fact simple metal strips that allow height adjustment. Widespread mistake here is attracting these strips to the fixture magnetically. This leads to significant decrease of magnetic attracting force on the fixture working surface areas close to side stops and can create unsafe clamping situations.
Positioning stops. These stops are used when we need additional mechanical support for small and medium size workpieces somewhere in the middle of magnetic fixture working surface. They are also often used as the reference base for workpiece precision positioning.
As part of an international R&D project in 2005, Prof. Vernikov introduced the so called magnetic positioning method. This method suggested using the extension of any magnetic pole on the fixture working surface for workpiece positioning. Such positioning stop not only mechanically supports the workpiece, but also magnetically attracts it. This guarantees that a workpiece does not lose positioning base while machining.
Using T-slots in magnetic fixture top plates was first described in the book “Electromagnetic chucks” by Dr. A. Vernikov, “Niimash”, Moscow, 1987. In the same year
T-slots were first used in electromagnetic chucks produced by Walker Magnetics Group Inc. for Voronez heavy press plant in Russia. One of the latest T-slots applications in special magnetic fixtures can be found in precision electro-permanent pallets for high speed milling of molds. This so called “+” technology was developed by DVM in 2007.
Most common T-slot applications in magnetic fixtures up until now have been special top plates such as, for example, Magnaslot system. It is also obvious that the most effective magneto-mechanical clamping solution has to integrate magnetic system into machine tool table with the standard T-slot system.
Magnetic friction systems
The best approach to increasing friction force between magnetically attracted workpiece and magnetic fixture working surface is to increase the friction coefficient between these surfaces. Many magnetic companies have attempted to solve this problem by integrating sharp pieces of special alloys into magnetic fixture working surface, using special paste with diamond powder and abrasive chemical compositions, etc. Most of these attempts have shown to be too expensive or too impractical to find practical application.
In the course of the International Investigation Project in 2008 DVM developed the so called Magnetic friction technology which increases friction coefficient between steel workpiece and switched-on magnetic device working surface by more than 3 times!
This was achieved by special machining of magnetic table working surface in the presence of chemical compositions and final thermo treatment. As a result surface micro-profile of magnetic fixture is changed so that under magnetic force between this surface and the workpiece several micro-penetrations take place into the workpiece base surface.
Depth of these penetrations does not exceed 50 microns which makes them completely invisible on the workpiece base surface, but increases friction coefficient dramatically. While developed for special magnetic fixtures for milling machines, this technology is now widely used in DVM heavy Vertical Handling Systems and DVM lifting magnets for underwater applications.
This method is based on using magneto-rheological compositions which increase their velocity in the presence of static magnetic fields from liquid to almost solid state. In order to perform such clamping a reservoir with magneto-rheological composition is placed on the working surface of magnetic fixture. The workpiece to be fixed is placed into this reservoir submerging sufficient part of its base inside the magneto-rheological composition.
When magnetic fixture is switched-on, the composition becomes solid under the influence of magnetic field and fixes the workpiece. When the fixture is switched-off magnetic field disappears, the rheological composition becomes liquid and workpiece can be taken away. This clamping is in some cases the only way of fixing workpieces from non-magnetic metals of complicated form.
There are several clamping tasks where flat working surface of magnetic fixture does not allow to perform reliable clamping. For example fixing the workpiece with nonflat base surface or performing 5-sides machining of the fixed workpiece. In such cases pre-machined pole extensions can be used. These are steel strips screwed to the poles on the fixture working surface. Such strips can be pre-machined for practically any form of the workpiece base surface, thus combining magnetic and mechanical clamping. Main disadvantage of this method up until 2008 has been a substantial magnetic force decrease on the active surface of pole extensions. This remaining force was in most cases only 30% to 40% of magnetic force on magnetic device working surface.
In 2008 DVM created a new generation of magnetic pallets for high sped milling technology. Neopallet within our Neopower series of advanced magnetic fixtures using specially developed for this purpose by DVM high energy rare-earth magnets. Magnetic system of Neopallet guarantees only 5% magnetic force decrease on the pallet pole extensions by applying high energy magnets with special characteristics. The “Neopallet” pole extensions are very well suited for performing the most efficient and accurate machining tasks.