Spatial mechanism for precise rectilinear displacement application on a multipurpose machine-tool.

Dimache, Ionut ; Craciunoiu, Stefan ; Nita, Raluca 等

1. INTRODUCTION

The slide-guide assembly (Sandu et al., 1967) is a machine part typical for machine-tools construction, highly important in the assurance of dimensional and form precision for the surfaces machined on them. The great and continuously increasing number of trajectory generating machines, comprising the industrial robots and machine-tools, as a result of multiple cutting methods and connective, lead naturally to a great variety of guide types, constructive forms and adjusting methods for them, materials used, computing procedures, manufacturing, lubricating and maintaining methods.

The attainment of generator curves (Sandu, 2008) in surface generating process on machine-tools is deployed in two ways: materialization through tool's cutting edge or by kinematics approach. In kinematical terms, the generator is realised by tool's cutting edge also, but the generator is the tool's movement trajectory itself. But these trajectories have a wide range of forms and their control is one of the most difficult surface generating issues.

To remove those difficulties, mechanisms used on machine-tools (Botez, 1961) are inferior kinematical joints, translational or rotational, made from slide-guide and shaft-bearing elements, respectively.

It is known as fact that the machine-tools retirement, their draw from manufacturing flow is provoked by precision loss due to normal or premature slides wear, and the topical industrial robots have an anthropomorphic structure, using shaft-bearing joints instead of slide-guide.

2. MULTIPURPOSE MACHINE-TOOL

Various precise rectilinear displacement accomplishment ways for a mobile element are known, the simplest are linear translation guides, found especially on machine-tools. The spatial mechanism proposed in this paper assures the precise rectilinear displacement because it's made only by levers with rotation joints. The mechanism assures the kinematical precision, sealing conditions and reduced friction forces in hard environmental conditions--abrasive dust, shocks, reduced lubrication and maintenance--unlike the translation guides classical solutions, which cannot give the precise displacement and low, constant maintaining of friction forces on the entire displacement domain. The spatial mechanism for precise rectilinear displacement is integrated on a NC multipurpose machine-tool for performing special multiple machining applications.

The NC multipurpose machine-tool (***, 2007) is constituted by the bed frame A, composed from vertical longerons (beams) 1, 2 and horizontal superior longeron 3. The cross girders (beams) 4, 5 are gliding between longerons 1, 2 and are driven by two motors each, 6 and 7, respectively 8 and 9, one NC and one follower. Although there are four motors, two axes are taken into consideration on Z-Z direction, one for upper cross girder 4 and one for lower cross girder 5. Those axes can be controlled independently or simultaneous with realizing and maintaining a preload. Two electro spindles are positioned on a Z-Z axis crossing middle of girders 4, 5 each on a girder for tools and/or devices to be mounted on.

Two cross girders12 and 13 are fixed in middle of cross girders 4, 5. Slides 16 and 17 are gliding on them, due to motors 14, 15 and nut-screw system 18, along to X-X axis. Two more slides are mounted on 16, 17, with cross-glide possibility, along Y-Y axis.

For an external cylindrical turning, as shown in fig. 1, the work piece 21 is fixed between girders 4 and 5, actually between electro spindles 10 and 11, through the universal chuck 22 and an a priori known back centre not shown in figure, which has a transmitted rotation on Z-Z axis.

[FIGURE 1 OMITTED]

On any of slides 19 or 20 a turning tool is fixed on a tool holder. On the opposing side a rolling-device support B is mounted for load compensation of cutting force, composed from two rollers and roller-holder frame which can be adjusted up-down by a cursor. After the rollers are locked on work piece, the frame can be bar-locked.

3. ORIGINAL SOLUTION FOR SUBSTITUTING THE SLIDE-GUIDE WITH SHAFT-BEARING JOINT

From the presentation of the multipurpose machine tools results that for actuating the Z-Z axes two engines 6, 7 are necessary for drop-sledge and two engines 8, 9 for actuating the inferior slide. This solution presents several disadvantages:

--either slide has two actuating engines, one of them is with numerical command and the other is for following.

--the cross rail 4 for the superior slide and the 5 for the inferior slide filled large space in the x axes lengthwise from the machine jig, which is unjustified because the axes 10 and 11 are always staying on X axes.

--the structure is over measure and it has an unjustified supplementary weight for its utilization, for example the utilization of an only slide on the Z axes, which could need a supplementary space as in the superior side so as in the underside of the machine.

To throw-away these shortcomings a solution (***, 2002) in which joint-slide guide is replaced by many shaft-bearing joints is proposed. Fig. 2 shows for comparison the spindle-bearing joint constructive solution for the superior and inferior slides, with the objective of keeping the same vertical dimension.

In addition to removing the above-mentioned disadvantages, the proposed solution has the advantage that aside of Z axis, along the X axis, more space will be cleared due to shortening of the cross rails 4 and 5, space in which there can be set up handling systems for parts and tools. Another possibility to get an exact or approximate rectilinear motion is achieved by guiding mechanism consisting of levers and rotation joints.

For these mechanisms the rectilinear trajectory of the led element is not carried out by special rectilinear guides, but by choosing the appropriate structure of the mechanism and the length of its elements.

The main advantage of these mechanisms is that the manufacturing requirements, less demanding than those imposed by the translation guides, allow to realize simpler, better sealing, in constructive, efficient and safe terms for the rotation joints, also exhibiting much smaller moving masses than those of the mechanisms with rectilinear translation guides.

Shaft-bearing solution description: For the proposed solution, the girders will be diminished to a dimension which correspond to a 1/3 of the initial dimension and will form a frame with square section, on which shackles will be executed. On the upper girder shackles symmetrical to the column will be also mounted.

The link between upper cross girder and column is made by two levers articulated with three bolts for each pair of shackles. This way a translation movement along the Z axis will be realized with a driving screw-nut mechanism on each joint. When applying this solution some advantages are obtained as follows:

* ensuring the driven element displacement on a rigorous rectilinear path defined by the intersection line of the mobility planes of the jointed arms,

* ensuring optimal sealing and lubrication by usage of rotation joints only,

* achieving a high stiffness due to the use of a spatial structure,

* the possibility of full eliminating of clearances by the right choose of constructive solution for rotation joints of the articulated levers,

* the possibility of getting a complete mechanism equilibration for the mobile element's horizontal displacement.

[FIGURE 2 OMITTED]

4. CONCLUSIONS

The original solution presented in this article is a mechanism for the precise rectilinear displacement formed only by levers, articulated by rotation joints, mechanism that ensures the kinematical precision, and also the precision conditions for safe sealing and reduced friction forces in hard working conditions (abrasive dust, beats, reduce lubrication and maintenance), on the entire moving range.

The device for the precise rectilinear displacement is a particular case of Stewart table, formed by two pairs of articulated arms with mobility in concurrent planes, the pairs being cinematically coupled through rotation joints to a fixed element and a mobile element. The last one has a precise rectilinear displacement along the intersection line between the two concurrent planes in which the pairs of articulated arms are moving.

5. REFERENCES

Botez, E. (1961). Cinematica masinilor-unelte (Kinematics of machine tools), Ed. tehnica, Bucuresti,

Sandu Gh.; Moraru V.; Minciu C. (1967). Ghidajele masinilorunelte (Guides machine tools), Ed. Tehnica, Bucuresti,

Sandu I. Gh. (2008). Generarea suprafefelor: tratat (Generating Surface: Treaty), Ed. Acad. Romane, ISBN 978-973-27-1730-1, Bucuresti

***(2002). SC ICTCM SA, Mecanism pentru deplasare rectilinie precisa (Mechanism for precise linear motion), Cerere pentru brevet de inventie A/00894/2002, Bucuresti,

***(2007) SC ICTCM SA, Masina multifuncfionala CNC pentru prelucrari prin strunjire, frezare, gaurire, alezare, mortezare, danturare si rectificare plana, cilindrica exterioara si interioara si helicoidala(Multifunctional CNC machine for machining by turning, milling, drilling, boring, mortezare, grinding flat, cylindrical outside and inside) Contract: INOVARE 45/2007, Bucuresti