What is a modular workpiece clamping system?

 A modular clamping system, also known as a clamping system kit, comprises standardized clamping elements or components which can be joined together by means of uniquely identifiable joint forms (threads, T-slots etc.) that are standardized throughout the system. The standard components include:

-  fixture bases,
            -  building blocks,
            -  support elements,
            -  stops, retainers and other positioning elements,
            -  clamping elements and
            -  adapter elements.

sing a set of clamping components, i.e. supports, stops and clamping elements arranged in a tree structure, a workpiece is fixed onto a base plate lying flat on the TopFix reference plane / assembly bench.

The standard joint forms between the clamping elements correspond to industry standards wherever these are available, e. g. bolted connections and T-slots of specified dimensions, but in many cases the connections are only specified by the clamping system manufacturer.

TopFix as a supplement to TopMill

If TopFix is used as a supplement to TopMill NC programming and simulation software, initial access to TopFix is via the set-up dialog of the workpiece clamping selection in which the modular clamping system is selected as the means of clamping the workpiece followed by the call of TopFix.

The modular workpiece clamping solution generated with the aid of TopFix is entered as the clamping element list in the TopMill set-up sheet. If TopMill is then run in automatic mode, the respective workpiece clamping assembly is set up on the machine table of the active TopMill machine tool when the program is started. The TopFix clamping elements are taken into consideration in the TopMill collision tests.

The machine table of the active TopMill machine tool is used as the TopFix reference plane in the respective TopFix application.

Initial considerations concerning the TopFix product

It is a generally known fact that experience is far more important for selecting methods of clamping pre-formed milling workpieces (e.g. castings) than for the actual CNC milling of such parts. 

But in CAM systems and CNC shop floor programming systems, this fact is hardly taken into account and in CNC simulation systems, workpiece clamping is largely neglected or completely absent.

Due to the many recent requests of our CNC training customers for inclusion of more complicated workpiece clamping, we started to do some research on the internet and enquired at various manufacturers of modular clamping systems in order to learn more about graphical interactive systems for determining workpiece fixation methods.

In the meantime, all clamping system manufacturers offer 3D CAD data of their products for downloading or on a CD, free of charge, so that these components can be virtually assembled on a 3D CAD system to model the clamping of workpieces for which the respective CAD data are available. However, this means that a 3D CAD system must be available and that someone has to be able to operate it. We were not able to find any easy-to-operate software specially intended for designing and determining clamping assemblies.

In some cases, clamping device manufacturers asked us whether such software systems were available or could be developed, since they too envisaged increasing demand for such systems in operational use (increasing deployment of 5-axis machining centers in production, smaller production lots).

Two years ago, we set out to fill this gap by developing the modular TopFix clamping system, modelling it on industrial modular clamping systems.

Basing our work on experiences with clamping algorithms in our standard software modules TopMill, TopTurn and, in particular, on development of the TopCAT modular tool management system, in 2008 we started to design and develop an interactive virtual modular 3D workpiece clamping system named TopFix.

We oriented the clamping components of our modular system on original clamping elements made by some leading manufacturers. However, we did not take over their CAD data, but used our own software tools to reconstruct the elements in the form of templates which the user can easily modify by changing parameter values, therefore also enabling the virtual models to be adapted to describe real clamping elements made by different manufacturers.

This functionality allows the user to build up its own clamping device management system, which includes only the available clamping elements.

Objectives of TopFix

TopFix makes it possible to create a workpiece clamping assembly quickly and easily in an interactive virtual 3D graphics environment for any given initial workpiece or premachined raw part, so that no mounting faults or clamping element assembly collisions result. Even complex clamping situations can be easily planned and realized.

 If the points of the workpiece where the clamps are to act are also specified graphically, the interactive design procedure makes verification of the feasibility of a suitable workpiece clamping assembly easier. At the same time, this procedure empowers trainees to plan complicated clamping assemblies using standard clamping elements and to implement these assemblies.

 Something which must not be forgotten is to test whether the workpiece can be easily removed from the clamping system after the Clamping claws and clamps have been slackened off without having to dismantle the entire clamping assembly.

 This means that the trainee becomes familiar with the possibilities offered by modular clamping systems and learns to use these in a hands-on manner.

What makes the arrangement of workpiece clamping assemblies so complicated?

 The first step for a user is to decide on the basis of 3D models of the pre-formed raw part or workpiece and optional the finished workpiece which machining processes should be applied first while the workpiece is clamped in the assembly to be designed, and whether other machining operations should be carried out subsequently in another clamping assembly.

 If multi-face machining also has to be taken into account in the clamping system design, the workpiece will have to be clamped at a minimum distance from the machine table in order to allow the tool spindle to approach the workpiece without collisions when machining the workpiece sides. 

 Image: CNC-Simulation in TopMill with a workpiece clamping assembly made by TopFix

 Once these initial decisions have been made, the bottom, lateral and top fixation points or surfaces of the workpiece are defined by assessing the workpiece graphically, unless these have already been pre-defined in preceding machining steps. The geometry of the intended finished part can be taken into account in this procedure provided that the 3D data file has already been entered as the reference workpiece in workpiece selection dialog. This makes it easy to check that the clamping positions do not coincide with machining faces. For this purpose the graphic representations of the workpiece and of the finished part can be turned on and off independently of each other and, what is more important, can be superimposed over each other. If the workpiece has already been machined appropriately, stopper edges and supporting faces can also be selected.

 The normal clamping criteria:

three supporting points,
            three stopper points and
            clamping points (ideally but not necessarily located above the supporting points)

do not automatically guarantee that the workpiece is clamped in the most suitable manner for the machining process.  Several other aspects for which no clear or specific solutions exist also have to be taken into account. Finding suitable solutions to cover the following aspects depends essentially on the machine tool setter’s and/or the programmer’s experience: 

- dimensional stability and strength of the workpiece,
            - oscillation behavior of the workpiece when clamped (natural harmonics),
            - retention forces of the clamping assembly,
            - possible stresses on the workpiece which might even lead to deformation and
            - effect of the machining forces at different points on the workpiece.

To date, there are no satisfactory universal solutions to these general and complex issues, and TopFix does not automatically provide such solutions. However, TopFix does make it possible to verify the feasibility of clamping at the required clamping points and to design a corresponding clamping assembly without assembly faults while checking conformity to the joint standards and avoiding assembly conflicts (overlaps) by collision tests.