GB2126137A - Grinding workpieces on internal grinding machine - Google Patents

Abstract

In a method for the optimisation of a grinding process for workpieces with large and considerably varying dimensions, made of materials which are difficult to machine, with extremely long bores and very high quality requirements, the initial conditions are made identical for all workpieces in the grinding process before the finishing stage and simultaneously the dressing speed is optimized in accordance with the specific conditions for the roughing and finishing. The roughing cycle is divided, working backwards from its end point, into predetermined uniform path intervals the end point of each of which is the starting point of a dressing process, and the grinding element is dressed at a high dressing speed for the dressing processes during the roughing cycle and at a low dressing speed for a dressing process relating to the finishing stage. <IMAGE>

Description

SPECIFICATION Process for grinding workpieces on internal grinding machines The invention relates to a method of grinding a workpiece in particular though not exclusively having large and considerably varying dimensions, of materials which are difficult to machine, with extremely long bores and with very high quality requirements using internal grinding machines.

The machining of workpieces with large and considerably varying dimensions, of materials which are difficult to machine, with extremely long bores and with very high quality requirements using internal grinding machines generally requires multiple dressing during the roughing cycle and intermediate dressing after the roughing cycle as a pre-requisite for the finishing.

It is therefore necessary, in order to ensure the quality of the workpiece, that at the beginning of the finishing process the initial conditions such as the mandrel deflection and the grinding element abrasion are as equal as possible for all the workpieces.

In order to achieve this, it is known that the dressing call-up in the case of multiple dressing during the roughing cycle, takes place in accordance with determined distance or time intervals, starting from the beginning of the actual roughing. These intervals begin with the beginning of contact between the grinding element and the workpiece, triggered by a known control. As a result of tolerance variations, shifts in the dressing intervals occur in the entire roughing cycle and lead to deviations in the final roughing stage producing different initial conditions as a result of varying mandrel deflection and grinding element wear during the finishing cycle which lead to variations in quality.

In addition during and after the roughing cycle machining is carried out with a dressing speed and a safety value. The safety value is the amount by which the feed device of the machine must be retracted in order to compensate the wear of the grinding element and the mandrel pretension after the dressing. If the dressing speed is used for the roughing and finishing cycle it is necessary in order to achieve the required workpiece quality, to operate with the dressing speed adapted for the finishing process. This means that machining is carried out during the roughing cycle with fine grinding element surfaces leading simultaneously to a reduced chip rate and therefore reduced time losses. The safety value is, however, dependent on the grinding element wear and the mandrel deflection which arises during the roughing cycle.

If effect, after the dressing between the roughing and the finishing, at comparatively small mandrel deflections, caused by the sparking out after the end of roughing and varying grinding element wear resulting from a deviating part distance before the end of roughing, a so-called air grinding and therefore also a loss in production time. This prior art is known for example from Federal German Republic Patent Specification No.

O/S 1 777133.

An object of the invention is to improve the grinding process in particular for workpieces which are difficult to machine and have large varying dimensions, varying material properties and heat treatment properties and which have extremely long bores (ratio of diameter to bore length of 1:3 and more) and which have high manufacturing quality requirements.

An object of the invention is to make the initial conditions before the finishing stage identical for all workpieces during the grinding process and simultaneously to improve the dressing speed in accordance with the specific conditions for roughing and finishing.

According to the invention there is provided a method of grinding on an automatic grinding machine with computer control, wherein from the time of contact of the grinding element with the workpiece the entire oversize to be machined off is detected by the computer, the finishing oversize is subtracted as a predetermined fixed value from this total oversize, the workpiece-related roughing oversize to be machined off is divided working backwards from the end of the roughing cycle, the end points of these path intervals are used as the starting points for a dressing process, the grinding element is dressed at a high dressing speed for the dressing processes in the roughing cycle and at a low dressing speed for the dressing with respect to the finishing cycle and the safety values during roughing and before finishing may be varied in accordance with the dressing.

As a result of the use of a computer control it is possible to ascertain immediately the actual tolerance with respect to the workpiece with the sparking of the grinding element on the workpiece. The excess amount to be removed by rough grinding is provided by the deduction of a previously determined fixed value for the finishing oversize. This excess amount is in all probability different in relation to different workpieces. The workpiece-related excess amount is broken down in the computer retroactively in predetermined constant path intervals from the end of the required roughing cycle. At the end points of each path interval a dressing process is then carried out.In the second place the roughing cycle may be carried out by means of programmed, fixed dressing call-up points which are subject to the spark control, i.e. the first call-up takes place only after the beginning of grinding. In order to achieve optimum conditions during the roughing and finishing cycle, machining is carried out with different dressing speeds and safety values satisfying the specific contact conditions. The grinding element is therefore roughened at a high dressing speed after the individual roughing intervals in order to achieve a high chip rate. As the feed speeds are varied in a known manner during the individual roughing intervals, it is provided in accordance with the invention to vary the dressing speeds too for these intervals.As a result of the high supply speeds which become possible in this way the grinder mandrel is accordingly strongly pretensioned and the grinding element is used up or deformed to an extreme degree. The extent of the grinding element wear and the grinder mandrel deflection determine the safety value required to carry out the following partial cycle without immediate contact of the grinding element with the workpiece, i.e. the grinding element should not grind with "sparks", but the air grinding should be maintained as low as possible. During the roughing cycle, before the multiple dressing, and after the roughing cycle, before the intermediate dressing, a so-called sparking out with preprogrammable varying time periods may optionally be carried out in order to relieve the grinder mandrel and the entire supply system.In this way corresponding accuracy parameters, in particular with respect to surface quality are achieved during the roughing cycle. As a result of the different dressing speeds during roughing and before finishing it is possible to achieve optimum machining during the roughing cycle and as a result an improved efficiency and a subsequent fine grinding for improving the workpiece quality.

There are substantial advantages with respect to the prior art if the method of the invention is used: The provision of favourable conditions for a quality improvement as a result of the most optimum adaptation possible of the entire grinding cycle to the machining data, The provision of identical initial conditions for all workpieces at the beginning of the finishing cycle providing the pre-requisite for ensuring uniform quality, Savings in working time as a result of a possible increase of the machining time during the roughing cycle and as a result of the safety value during the finishing cycle.

To help understanding of the invention, a specific embodiment thereof will now be described with reference to the accompanying diagrammatic drawing of which the upper part shows the course of the entire grinding cycle by means of the feed course of the grinding element with time, and the lower part shows the cyclerelated dressing intervals with the dressing speeds of the grinder table.

The feed path from the starting point PO to the end point P3 is plotted on the upper ordinate. The points P1, P1, and P, represent the beginning of the roughing cycle in accordance with the grinding oversize which is actually present. Point P2 represents the end of the roughing cycle programmed in the computer. By means of the sparking of the grinding element on the workpiece, the computer detects the oversize to be machined off for roughing and divides this into predetermined constant path intervals A working backwards from P2. A deviation necessarily takes place in this respect in the path distance before the first dressing. In order to avoid unnecessary dressing, the path distance before the first dressing must be greater than the safety value S, but smaller than or equal to A+S.

The entire temporal development of the grinding cycle for example for four dressing operations during the roughing cycle and one intermediate dressing before the grinding cycle is shown by the heavy line development. The path from the starting point PO to the point P1 acting as the starting point of the roughing cycle is overcome at the spark speed which is smaller than the rapid feed speed. After the sparking the first roughing stage takes place at a high feed speed and in accordance with the subsequent rebound for dressing. In this respect the dressing takes place in accordance with the upper diagram with a high dressing value and in accordance with the lower diagram at a high dressing speed. After dressing the grinding element advances with a high safety value S,.As a result of the high dressing speed there is a roughening of the cutting surface of the grinding element as a prerequisite for achieving a high machining efficiency. The maximum grinding element wear determines the size of the dressing value aAbr, and the wear and the grinder mandrel pretension determine the selection of the safety value S,. The safety value S, is selected such that the grinding element does not travel through the full material at the beginning of the next roughing stage.

Grinding element deformation and grinder mandrel pretension are therefore avoided at the beginning of the roughing stage. After point P2 has been reached in the 5th roughing stage a socalled sparking out takes place for the active grinder tension release and the provision of constant starting conditions for the following finishing stage. There follows a rebound to the intermediate dressing of the cutting surface of the grinding element with a low dressing value and a low dressing speed, in order to achieve a high surface quality for the cutting surface. After this there takes place the forward movement taking into account a reduced safety value S2 in order to avoid as far as possible a time-consuming air grinding.On reaching point P3 the feed is discontinued and a sparking out is carried out in order to relieve the workpiece/tool/feed device system and therefore to achieve the required accuracy parameters, in particular in order to ensure the quality of the machined surface.

As the dashed lines show, sparking out may optionally be provided after each roughing stage in the case of particularly complicated cutting conditions in order to be able to compensate the grinder mandrel deflection. This leads, however as shown in the attached upper diagram, to an increase of the machining time.

The dotted lines show, starting with the points P1, and P", the course of the cycle in the case of varying oversizes. It can be seen, starting from the line of P1,, that only one dressing stage is required during the roughing cycle and one intermediate dressing before finishing on the basis of the small oversize.

As a result of the possibility of varying the dressing speeds, wherein they may also be different during the individual roughing stages, it is ensured that the quality of the working surface of the grinding element may be adapted to the specific cutting conditions, as described above.

The selection of constant path intervals for the roughing cycle, working backwards from the end point of the roughing feed P2 enables a separation of all the different grinder mandrel deflections and grinding element wear conditions in the course of the roughing cycle and provides constant starting conditions for the finishing cycle.

Claims (3)

Claims

1. A method of grinding on an automatic grinding machine with computer control, wherein from the time of contact of the grinding element with the workpiece the entire oversize to be machined off is detected by the computer, the finishing oversize is subtracted as a predetermined fixed value from this total oversize, the workpiecerelated roughing oversize to be machined off is divided working backwards from the end of the roughing cycle, the end points of these path intervals are used as the starting points for a dressing process, the grinding element is dressed at a high dressing speed for the dressing processes in the roughing cycle and at a low dressing speed for the dressing with respect to the finishing cycle and the safety values during roughing and before finishing may be varied in accordance with the dressing.

2. A method as claimed in claim 1, wherein the dressing speeds may be varied for the dressing processes in the roughing cycle.

3. A method of grinding substantially as hereinbefore described with reference to the accompanying drawings.