The present invention relates to a device for rounding off edges of an opening in a work piece, like internal openings in steel plates, which device comprises a belt grinding machine having a continuous grinding belt running over several belt guiding rollers, which are provided on a support frame. 
The demands to maintenance and safety increase both in the marine environment and within land based industrial activity.  Corrosion protection of steel, either it be platforms, ships or steel bridges on shore, are getting increasingly important.  Veritas, oil companies, Norsok, road authorities, etc.  are specifying requirements regarding roundness of sharp edges and removal of temper zones.  Temper zones appear when steel plates are cut with autogenous or plasma equipment.  The temper zones do often have to be removed because the surface is to be sand blasted or metallized. 
Thus it is observed a need to develop a machine for rounding off edges of openings cut out in steel plates.  At first hand one have in mind steel plates for the ship yard industry.  The edges are to be rounded off along the edge of the opening at the top side and under side.  In order to be able to do so, the machine has to be provided with sensors and positioning units such that the grinding head automatically follows the edge of the opening.  The openings may per se be of arbitrary shape. 
The situation at present is that goods of this type are ground with small angle grinders and other hand held tools.  This is performed manually and the process is dusty and noisy in addition to that the working posture is immensely disadvantageous.  The manual operation is time consuming and thus costly. 
By the suggested machine the productivity will be increased, the quality of the ground edge and the working environment will be improved, a more favourable working strain for the operator will be given, and dust and noise be less.  Further, it will be possible to provide an effective dust suction on this equipment.  It will be difficult/impossible to provide an effective suction on present angle grinders. 
As it comes to the grinding process proper, it is natural that the grinding operation has to be adapted to the material that is to be ground.  Thus there is a large difference between grinding of wood work and steel material.  Grinding processes that are developed for wood material can thus not without more ado be used when grinding steel or metals.  To better understand the background for the present invention, some aspects are listed to make a comparison between grinding of wood work and steel material:
Wood Work
Soft material
Easy to work on, i.e. low grinding press.  The force by which the grinding belt is pressed against the work piece is moderate.  This means little belt tension. 
Use of relatively fine grained grinding belt, which thus is also flexible, i.e. easy to bend around an edge. 
Relatively lower speed of the grinding belt, i.e. in relation to grinding of steel. 
Steel Material
Hard material
Heavier to work on, increased grinding press.  This means higher motor rating and thus high tension in the grinding belt. 
Use of coarser grinding belts, which are not flexible, i.e. hard to bend around an edge. 
High speed of the grinding belt, approx.  40 m/sec. 
In grinding steel material the easiest would be to bend the belt around the sharp edge to be ground.  This is possible when wood work is to be ground, but gets in practise impossible on a steel material because:
The grinding belt is not that flexible that it can be bent much around the edge. 
No controls of the grinding press. 
High motor rating will tense the belt which thus is straightening out between the guide rollers. 
To take care of the problems indicated above, is a device of the introductory described type provided, for rounding off edges of openings in a work piece, which device is distinguished in that two of the rollers are arranged on a cradle, which cradle is pivotally supported in the support frame, that the grinding belt between the rollers on the cradle endeavour to run substantially rectilinear for working off the work piece and that means are provided for operating the cradle about the pivotal support during a grinding operation. 
Conveniently is a counter support, or a grinding shoe, provided in back of the grinding belt and between the belt guiding rollers on the cradle.  The cradle can be adjustable in the vertical direction and be arranged symmetrically in respect of a horizontal plane extending through the pivotal axis of the cradle. 
Further, the operating means may be a linear acting motor which in one end thereof is fixed to the support frame and in the other end thereof is pivotally fixed to the cradle.  The linear motor can be in form of a double acting ram, or working cylinder, which actively causes a form of oscillating movement of the cradle by extending respectively retracting the piston rod of the ram. 
The belt grinder may preferably include an upper and lower guiding wheel on the cradle, an upper driving wheel and a lower turning wheel fixed to the support frame. 
The support frame can be provided with a telemeter for continuously measuring of the distance to the plate edge to achieve proper positioning of the cradle in respect of the plate edge. 
It will be of convenience that the belt grinding machine is mounted to a co-ordinate machine having a co-ordinate table for the work piece.  Thus the belt grinding machine in the co-ordinate machine will be displaceable in the x, y and z-direction.  As an option, the co-ordinate table can be displaceable in one or more of the said directions. 
Experiences from tests of such an edge rounding device in the work shop that is based on the principle that the belt oscillate around the edge at the top side and under side of the plate, has shown that the following advantages are achieved:
1.  One may use a stiff grinding belt which do not have to bend around the edge. 
2.  The grinding press may in some extent be controlled dependent of how heavily the pair of oscillating wheels is forced against the piece that is ground.