The stem is an element which transfers the power of the main cylinder on the billet like a column. The stem, which must operate without bending and cracking under a high power needs to bear a pressure of up to 120 kg/mm2 depending on the size of the press and the performed application. Therefore, the centering during the operation must be performed properly.
AISI H13(1.2344) is the most frequent used hot tool steel for the stem. This steel is brought to Rockwell C45-50 hardness by applying heat treatment. Maintaining this hardness depends on the consistency of the operating temperature and on avoiding internal stresses.
The steel needs to be heated prior to using it. The heating should be performed slowly in an appropriate oven up to 150º C. This should never be subjected to direct fire. Thermal shocks and rapid temperature changes will result in cracks of the stem. The pressure tools will cool down at not regularly and continuous operating presses and common failure is that these are subjected to sudden heating. The most common heating method in such cases is to leave the stem inside the heated container until it reaches the appropriate temperature.
The stem must interrupted or continuously operate under high pressure loads. Therefore, the centering must be performed properly during the operation. The centering adjustments must be controlled weekly.
It is obligatory that the pressing is applied properly since high pressures are applied by the stem. An unbalanced pressing may result in cracks and bending of the stem. A variation of the centering may result in serious problems and the possible reasons are; improper mounting, deformed pressing plate, faulty aligned press, jammed butt (causes the relocation of the container) and not proper sheared billet. The most possible care has to be paid in order to avoid such reasons and catastrophic breakages must be prevented. Otherwise, this may harm the press, cause time losses at the production and, more importantly, result in injuries.
Lateral widening may incur at the front part of the stem by time due to the pressure. A ratio of 1/20 is accepted to be normal for widening and recovery. This concentric movement may result in serious cracks. The contact with the dummy block during the repeated pressing will cause hardening on the surface of the stem and fine cracks may incur as a result. These cracks may develop up to the stem body when they are not controlled. In such cases, the stem should be sent to Damatool, examined with an ultrasound test and this part of the stem should be machined and an adaptor should be placed between the dummy block and the stem when necessary.
Relief of the Stress
It is necessary to relief the stress generated in the stem regularly. The stress will vary depending on the size of the press and the pressing quantity.
Stress Relief Frequencies
|
Pressure (kg/mm²) |
Pressing (pc) |
|
|
120-140 |
20,000-30,000 |
|
|
110-120 |
30,000-40,000 |
|
|
90-110 |
40,000-50,000 |
|
|
70-90 |
50,000-60,000 |
|
|
70< |
100000 |
|
The detailed drawings and specifications of the pressing tools should be filed meticulously and updated regularly. The drawings will be necessary at the production of replacement parts and renewals. These drawings are particular for each press.
You should examine the replacement stems as soon as you receive them. Never delay until the moment of usage has come. The stems should be stored at a dry location and at room temperatures.
Maintenance suggestions