Design of a safety mechanism for machine tools.

Nicodim, Mariana ; Vlasceanu, Daniel ; Baciu, Florin 等

1. INTRODUCTION

Nowadays, the worldwide MT (Machine-Tool) continues to be a domain in which the fundamental or construction solution news is in a continuous evolution. In our country, the concerns of the MT constructors and of the researchers in the field, especially, are framing in the general context of the worldwide effort aimed to introduce the CN and recently the CA in the structure of the most various technological equipments. (Catrina, et al., 1993).

In order to obtain a better level of usage and understanding, this paper aims to describe the construction, the operating principle, the role and the advantages of a self-conception safety mechanism. This mechanism is intended for the RPX-AC400-CNC rocker shaft grinding machine used for grinding the flat surfaces of the "K" and "D" engine rocker shaft. The working piece has five flat surfaces on one side and another five on the other side and it will be simultaneously grinded with five abrasive wheels. The grinding machine has been conceived using the most advanced technologies from all over the world and it is equipped with numerical control (CNC 802D).

2. SAFETY MECHANISM: CONSTRUCTION, OPERATING PRINCIPLE AND ADVANTAGES

The surface grinding machines with vertical axis and linear table type RPV or with rotary table type RPVR for processing plan-parallel parts in automatic cycle are well known. In case of interruption of electrical energy during processing, these grinding machines cannot assure the operator's safety or the safety of the machine.

The article describes a safety mechanism implemented within the kinematical feeding chain of the vertical slide from the RPX-AC400-CNC rocker shaft grinding machine.

The safety mechanism is used for quick withdrawal, during the automatic cycle, of the vertical feeding slide, in case of accidental or forced interruption of electrical energy during processing. In case of electrical energy failure, rechargeable batteries from the hydraulic diagram will simultaneously feed two single-actuating cylinders causing the withdraw of the whole group--vertical slide, trapezoidal threaded screw--screw nut, worm reducer--avoiding the collision between grinding wheel and the working piece.

The purpose of this invention is to assure quick withdrawal, in automatic mode, of the whole group--vertical slide, trapezoidal threaded screw--screw nut, worm reducer avoiding the collision between grinding wheel and the working piece, collision possible due to the clearance of the feeding kinematical chain.( Ispas, et al., 1997)

The safety mechanism implemented in the slide feeder of the feeding kinematical chain from surface grinding machines will eliminate the mentioned disadvantages. In case of accidental or forced interruption of electrical energy during processing, in order to eliminate the chance of collision between grinding wheel and the working piece the safety mechanism will use two single-actuating cylinders which will lift up the whole group: vertical slide feeder, trapezoidal threaded screw--screw-nut, worm reducer--on the slide ways existing on the prop of the grinding machine.

When restarting the automatic mode of the grinding machine, the two hydraulic cylinders are released by actuating the hydraulic distributors and the disk springs of the two hydraulic cylinders will enable the clamping for all the group to its original position with respect to prop of the grinding machine.

Figure 1 presents an example of construction for the safety mechanism implemented to the RPX-AC400-CNC rocker shaft grinding machine.

The safety mechanism implemented in the structure of the kinematical chain in case of vertical axis grinding machines type RPV and RPVR consists of: upper plate 1--provided with two taper holes symmetrically disposed with respect to vertical axis and the mounting and alignment holes with respect to worm reducer 2. On the upper side of the prop 3, there are two single-actuating hydraulic cylinders 4 and 5 mounted with the fixing plates 6 and 7. Each two single-actuating hydraulic cylinders 4 and 5 has a conical face on which the upper plate 1 is supported using disc springs 8 and 9 that are pressing down the pistons 10 and 11 inside the hydraulic cylinders 4 and 5. In order to avoid the collision between the grinding wheel (driven on by the vertical slide 12) and the working piece, using the trapezoidal thread screw 13 the quick feeding with pressured oil from the rechargeable battery A (part of the hydraulic group 16) is assured.

[FIGURE 1 OMITTED]

3. DESCRIPTION OF THE RPX-AC400-CNC

RPX-AC400-CNC rocker shaft grinding machine has three linear axes actuated by the CNC. The feed is being carried out by the alternating current engines, realizing a continuous variation of the number of revolutions by CNC equipment. The engine of the vertical feeding action is an engine with a trammel and on the ball-bolt of the vertical feed further electromagnetic trammel is provided.

The longitudinal feed of the kinematical chain (X axis)--the bench has an eigen feed mechanism actuated by the numeric command of the equipment. The mechanism consists of: servo-engine feeder, toothed drive belt and ball-bolt. The screw nut of the ball-bolt is assembled on the bench. The measurement of the bench displacements are carried out with a transducer at the end of the ball-bolt by means of a clutch.

The transversal feed of the kinematical chain (Z axis) actuated by the numeric command equipment consists of: servo-engine feeder, toothed drive belt and ball-bolt. The nut of the ball-bolt is mounted in the prop.(Bruda, et al., 1980)

The vertical feed of the kinematical chain (Y axis) actuated by the numeric command equipment consists of: servo-engine feeder, toothed drive belt and ball-bolt. The nut of the ball-bolt is mounted in the vertical slide. (Kief, H.B., Hasen C., 1999)

The prop (3) is a cast construction guided along the transversal frame by means of rolling slide bars--roller tappet type. This ensures the stone set positioning ("D" and "K" valve-rocker shafts) in respect with mass of the machine on which the alignment and clamping device and the knurl head are assembled, by means of the transversal feed mechanism (Z axis). The alignment accuracy in case of processing along the axis of the piece is assured.

The vertical slide (12) of the RPX-AC400-CNC rocker shaft grinding machine ensures the stone set alignment adequate to the processed mark ("D" and "K" valve-rocker shafts) with respect to mass of the machine ("Y" axis). The radial alignment accuracy in case of processing along the longitudinal axis of the piece is ensured.

The main shaft is mounted within the vertical slide body. A set of stones adequate to the processing mark are mounted on the bevel of the shaft. The knurl head is aligned with respect to bench of the machine, the axis of the knurl head alignment points being perpendicular on the displacement direction X and parallel with the bench mirror. One of the points is fixed, as a reference axis, and the second one orientated in respect with operator is adjustable on the thread. In the knurling process, the 5-stoned set acquires in the radial section the emission profile that will be processed on the specific valve-rocker shafts type "K" and "D".

The alignment and clamping device ensure the piece alignment with respect to mass of the machine: longitudinal (along the X axis), transversal (along the Z axis) and vertical (along the Y axis) of the 2 sets of 10 pieces each. The hydraulic installation (16) is placed behind the longitudinal frame, in the engine, pump, and command and monitoring device for the correct usage of installation. (Nakazawa H., 1994).

[FIGURE 2 OMITTED]

The hydraulic installation ensures operating in the duty cycle in case of piece clamping and loosening machine.

4. CONCLUSIONS

The security mechanism help the functional integration related to the security--for the process that needs to be protected, into an efficient structure.

Beside the fact that it represent a rigid and compact construction, the implementation of this security mechanism on every plane grinding machine type RPV or RPVR assures the adjustment of the geometrical axes of the trapezoidal threaded screw with respect to slide ways of the vertical feeding slide, by means of the conical surfaces existing in the upper plate and single-actuating hydraulic cylinders. The security mechanism also eliminates the chance of collision between the grinding wheel and the working piece in case of accidental or forced interruption of electrical energy.

The security mechanism contains actions to be taken, equipments and also effects: safety and stability. It is the collocation of this moment, passing through the integrated mechanism phase, and going to the future collocation "security system".

5. REFERENCES

Bruda, D., Dobrescu, T.; Chivulescu, C.; Pana, T.; Guta, R., (1980), Special control problems of feed drives of d. c. motors with wide speed variation range, Polytech Inst of Cluj-Napoca, p C. 13-C. 15

Catrina, D., et al., (1993), Machine tools with NC, University POLITECHNICA Bucharest.

Ispas C., Predincea N., Ghionea A., Constantin G. (1997) Machine tools, Ed. Tehnica, Bucharest.

Kief, H.B., Hasen C., (1999) "NC/CNC Handbook", Verlag, Munchen Wien.

Nakazawa H., (1994), Principles of Precision Engineering, Oxford University Press