The machine MA 10/E presented in 2010 offers several grinding methods like continuous grinding, automatic downfeed grinding and integrated spring length control device.

Photos: OMD

The pioneers of spring manufacturing: the “Officina Meccanica” founded by the swiss Eng. Emil Kunz presents at an exhibition the spring grinding machine M5.

All depends of a good end

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In 1950s the spring end grinding machines were working only with throughfeed grinding. With this procedure the springs are loaded into a continuously rotating loading plate which takes them between two opposed grinding wheels mounted on not movable spindle groups, where the spring ends are ground plane parallel. In this case the spring is not blocked, but it is inside the bore of the loading plate. The grinding pressure is given by the spring itself since it is distributed on both terminals, which will be plane ground on both ends.
The grinding wheels remove a certain quantity of material according to wire and coil diameter and according to the cutting capacity of the tools, at a defined grinding pressure corresponds a certain grinding time. During continuous grinding the spring is ground in only one passage, therefore a certain grinding time or a certain distance between the grinding wheels is needed. This grinding travel determines the diameter of the grinding wheels and, consequently, the machine size. Ideally, a grinding travel which permits to ground the spring in only one passage with the utmost loading capacity of a machine operator or of an automatic loading device should be choosen. To obtain this, according to quality requests and spring dimensions, large grinding wheels diameter and, consequently, large machine sizes are needed.
The downfeed grinding machines developed in 1960s have removed this cost increasing factor; infact, these machines have enabled the grinding of larger wire diameters using grinding wheels with lower diameters. These downfeed grinding machines take the springs not only once and slowly between the grinding wheels, but they take the springs several times between the grinding wheels and with a speed of 20 min-1 up to 50 min-1. Thus, the grinding travel will increase. While the springs are passed several times through the grinding wheels, one spindle feeds towards the springs until the preset Lo value is reached.
Spring loading and unloading is made during slow rotation. The grinding feed is preset on a linear speed value, limited by a minimal and a maximal current absorption of the spindle motor. When the maximum admitted value (ampère) is reached, the spindle stops until the current absorption decreases on the minimal value. At this point, it continues the feed until the maximum value (ampère) has been reached. This step-by-step downfeed with contstant speed implies an unsteady grinding pressure. Since a grinding wheel has an optimal cutting capacity only with a certain grinding pressure, this step-by-step downfeed grinding may not be defined an ideal method. Even not in the case that the basic time is subdivided in levels with different speeds.
The successively developed cam control was near enough to the ideal solution. To keep the grinding pressure constant, the downfeed speed must be changed from fast to slow. It’s possible to obtain this by means of a certain form of the control cam. According to the wire and spring diameter, the cam rotation speed varies. With this electro-mechanical control it has been possible to increase considerably the machine output. The disadvantage of this system was the fact that applying an universal cam it’s not possible to make optimum use of the cutting capacity of the grinding wheels. Moreover, different spring diameters having the same wire diameter result in differences of material quantity which has to be removed.

This problem has been solved by introducing in 1980s the electronic current absorption control of the spindle motors.
The spindle feeds towards the springs loaded into the loading plate which rotates at a preset basic speed until the programmed spindle current absorption value has been reached. At this point – and this is the substantial difference respect to the above mentioned system – the spindle does not stop, but it continues constant downfeeding (ampère value) until the spring Lo value has been reached. The downfeed speed will be reduced automatically and exactly related to the increased grinding surface. This type of downfeed grinding, realized by OMD, can be considered an optimal method.
With this electronic control it has been possible to increase productivity from 25% up to 30% and to optimize the tolerances on the spring length.
These machines have been successively improved for what concerns running, control and materials.
At present the electronic controlled OMD double-spindle machines offer: automatic grinding wheels compensation, automatic grinding wheels dressing system and frequency inverter for changing the grinding wheels rotation speed.
Servomotors for spindle movement, motorized loading plate, standard and special cooling system, spring length control system, PC with touch screen monitor and USB interface. Possibility of downfeed grinding and continuous grinding. On request it is possible to equip the machine with automatic loading devices.
These new generation machines offer from 30% up to 40% more productivity and Lo tolerances of 0,05 mm – also due to the use of new abrasives and dust extractors. Beyond downfeed and continuous grinding machines, OMD production range includes also horizontal wet grinding machines, spring deburring and chamfering machines and automatic loading devices.