In the design of machine tools, particularly in custom machines, it is often necessary to create dedicated mechanisms for a specific application. These mechanisms are usually designed as tailored equipment for the particular machine tool. It turns out that some mechanisms of this type could also be used in other machines and for slightly different purposes. An example of such invention can be a “floating screw mechanism”, designed to drive the extension of the milling sleeve and at the same time to lock it in the working position.
This mechanism was created in a special machine tool factory as a development result of the design for milling headstocks used in special machine tools – milling machines. In older solutions of this type, the sleeve was extended by means of a hydraulic actuator, where the sleeve was clamped, after being extended to the hard stop, with two jaw mechanisms at the front and rear of the sleeve, driven by separate actuators mounted transversely to the spindle axis (also hydraulic). This solution, although it met the requirements – providing accurate axial extension and clamping with the appropriate force, was large (clamping cylinders protruded significantly beyond the headstock body) and “inelegant”. At the same time it required the use of machine tool hydraulics, sometimes used in the machine tool only for this purpose. Thus it was rather expensive and most often troublesome to use. Working on a universal milling headstock solution used in the design of special machine tools, it was necessary to limit very different solutions and find the most optimal one, without protruding elements in various places, such as: hydraulic cylinders, hydraulic power pack, pipes, wires, etc.
![floating screw mechanism diagram](https://about-engineering.com/wp-content/uploads/2021/07/floating-screw.gif)
As a result of various analyzes and concepts, three sizes of headstock with geared motors of a new design, a new quill extension and a clamping mechanism were created. These three sizes basically satisfied the needs of milling units in any machine tools built at the time, with some minor exceptions, such as large dimensions, which were then specially constructed. The so-called electromechanical wrench, consisting of an electric motor and a planetary gear, with the possibility of mechanical adjustment of the screwing and unscrewing torque (less torque is needed to tighten than to unscrew). Any other unit of this type with similar size can be used for the drive, e.g. an commonly available, almost mass-produced electric motor with a planetary gear and electronically regulated torque, or even with a hydraulic or pneumatic motor, if these media are already present in the machine tool under construction.
The figure shows only the output shaft of this unit (item 7) with a diameter of Ø20, and the entire mechanism of the floating screw is drawn in the working – starting position. Next, the torque from the drive unit is transmitted through the bevel gear to the screw (item 3), which rotates through the nut (item 5) and moves the quill forward or backward (depending on the direction of rotation). After reaching the hard stop (item 4), the nut stops the quill (item 2), and the bolt performs further rotations causing the bolt to move back by about 3 mm, with simultaneous deflection of the disc spring package. This movement results in clamping of the quill through the ring mechanism – small and large (item 6) together with the appropriate angular cut and a free gap 0.25 mm (0.098 in). The package of disc springs is selected in such a way that by bending, it provides the appropriate axial force enabling the quill to extend into the working position, press against the hard stop and retract to the starting position. The screw with the bevel gear is connected by a spline involute. The nut connected to the quill by means of a special body, cut and clamped elastically with a screw, has the option of adjusting the starting and ending position of the quill, and thus setting it in relation to the machine tool and the workpiece. The figure also shows the end of the milling spindle (item 1).
The milling spindle, although interesting in itself as a design solution, is not shown here so as not to obscure the drawing. View A shows a system of microswitches which provides adequate control depending on the position of the mechanism parts. In the semi-automatic and automatic control of the moving units of the machine tool, each such unit uses complex microswitches or a set of microswitches, which is designed to signal and stop the drive at the appropriate positions of the assembly. These are most often: a starting position, rapid motion slowdown (an end of rapid infeed or retraction) and transition to work motion or slow infeed motion, an end position and emergency position – if the “limit switch” is not activated.
The presented mechanism, although it caused some problems for the assemblers during the assembly and adjustment process, worked very well. The milling headstock itself was significantly simplified, with no hydraulic cylinders or hydraulic power lines sticking out. There was no need to install a hydraulic power pack in the machine tool. This mechanism, as I mentioned, can be used, after some modifications, in other machines – where it is necessary to extend a part or assembly axially and clamp it in the working or end position and then lock it. Instead of the clamping ring (item 6), a transversely extendable rectangular, cylindrical, conical or wedge bolt can be used.
Aleksander Łukomski