Magnetic fixtures for precision metalworking
Tables, chucks and clamps are essential parts of practically all precision surface machining technologies directly influencing the productivity and accuracy of this technological process. In this article we will concentrate on accuracy aspects, analyze main reasons of fixture inaccuracy and show ways to improve both design and production technology of precision magnetic clamping devices.
Most modern metalworking productions experience the following phenomenon. Expensive high accuracy machines are unable to achieve standard accuracy in the technological process. This problem is caused by wrongly applied or chosen workpiece fixture. Because machine accuracy is mostly tested without such a fixture, this problem can be observed only after all equipment is mounted and production is running smoothly. Most common cause of the above problem are magnetic fixtures which give impression of solid and rigid blocks, but can “eat” up to 80% of machine accuracy.
As part of the International Investigation Project in 2006 we tested more than 100 high accuracy grinders and high speed milling machines equipped with magnetic chucks and pallets in Germany and Japan. It appears that more than 70% of these machines were not able to achieve their passport accuracy due to technological deformations of magnetic clamping devices. Let us start with analyzing these deformations.
Deformations of magnetic fixture
There are two main types of magnetic fixture deformation: mechanical and heat deformation. These types of deformation influence each other during the technological process and directly determine workpiece machining accuracy.
Mechanical deformations are caused by machining forces applied though the workpiece and technological movement forces applied through the machine table. Strength of mechanical fixture deformation is determined by construction rigidity of the fixture in both directions perpendicular and parallel to the working surface.
Heat deformations are caused by machining heat applied through the workpiece and other heat sources such as machine drives heat applied mainly through the machine table and magnetic coils heat (in electromagnetic fixtures) applied through the fixture top plate from energy block. Level of heat fixture deformation is determined by heat conductivity of fixture construction in both directions from the workpiece and from the machine table.
Working surface deformation directly influences workpiece machining accuracy and results from the combination of the above deformation types. Level of this deformation is determined by fixture working surface dynamic flatness in application conditions. Unfortunately reliable methods of direct testing this flatness do not exist and in practice we mostly characterize it by measuring the workpiece machining accuracy.
Since the working surface of any magnetic fixture is the top plate let us analyze main top plate tasks.
Magnetic fixture top plate tasks.
Any top plate has two tasks: magnetic and mechanical.
Magnetically the top plate has to distribute magnetic force along the total working surface ensuring that workpieces can be fixed magnetically to any part of this surface. Every top plate, therefore, consists of magnetic parts of different polarity separated by non-magnetic parts.
Mechanically the top plate forms the base support for a workpiece and a reference surface. It therefore has to be solid, rigid and homogenous.
These magnetic and mechanical tasks set strongly conflicting requirements to the top plate construction. Let us analyze how these conflicts are resolved in traditional magnetic devices.
Precision electromagnetic chucks and tables.
For many years electromagnetic chucks were judged as low accuracy fixtures due to heat deformations produced by their electromagnetic coils. This was based on the wrong electromagnetic chuck design approach, where the goal was achieving the highest possible magnetic force.
Electromagnetic coils as a source of heat deformations. Heat generated by electromagnetic coils in electromagnetic fixtures is conducted to the top plate from the device energy block, heating the bottom of the top plate. In most electromagnetic fixtures this heat source is two to three times stronger than all other heat deformation sources.
In 1994 Prof. Vernikov introduced new electromagnetic chuck design approach, based on minimizing top plate heat deformation. The idea was simple: limit heat generation from fixture coils to values comparable with heat generated by technological heat sources. Resulting heat values are similar to the heat applied to the top plate from outside the chuck and minimize top plate heat deformations. Optimization of magnetic circuits allowed to generate acceptable magnetic clamping forces required by precision machining technology.
This method is aimed to control fixture force and thus meet the requirements of precision machining technologies. Top plate construction was redesigned in order to achieve the best distribution of coils heat over top plate working surface. As a result “Unigrip” electromagnetic chucks were born. Now, after more than 15 years, these chucks are still the world most commonly produced chucks for precision grinding machines.
Further developments in this direction concentrate on improving top plate design and production technology.
Advanced top plate production technologies
Since the early 90th main magnetic fixture top plate design is a laminated plate consisting of steel and non-magnetic metal strips. This requires us to produce solid, rigid and homogeneous assembly out of steel and color metal strips. The better this task is solved, the more accurate machining can be performed on such a top plate.
This technology was created by DVM together with “Paton Welding Institute” (NAS, UA). The technology is based on the phenomenon of mutual micro penetrations within the contact area of steel and color metal under certain vacuum and temperature conditions. As a result, absolutely solid metal laminated block can be produced which behaves as a homogeneous metal both under mechanical load and heat influence. Unfortunately, this technology is expensive and is currently applied only for production of special DVM additional laminated blocks “Lamblock” for extending technological possibilities of highest accuracy magnetic fixtures.
Today most top plates are just pressed and connected by several non-magnetic rods. These rods are inserted through the holes in connected strips and screwed or welded to the side lamination strips. Under mechanical forces and technological heat such structure can behave inaccurately, forming unpredictable boll or convent areas on the fixture working surface.
As part of the International Investigation Project in 2007 Prof. Vernikov developed so called “Lamperss” method which significantly improved top plate pressing technology. Applying special chemicals between laminations, stimulates mutual micro penetrations of different metals under calculated pressure. Using this process we developed precision top plates for our highest accuracy Neomill pallets from DVM Neopower series. Neomill pallets proved to be the most accurate fixtures for advanced high speed milling technology.
In the upcoming parts of this article we will describe “Superfine‘ top plate designs; magnetic fixtures, using magneto-anisotropic materials; precision energy blocks for fixtures with high energy NdFeB magnets; precision electro-permanent fixtures; magnetic positioning methods and some other methods of magnetic fixture accuracy improve.