The lead screw is an important part that is used as a motion transducer. It changes the rotational motion into linear translational movement. For this, it is equipped with a special nut. In addition, it provides movement with a given accuracy.
Screw Quality Indicators
The screw, as a very important part, must meet many requirements. In order to be able to use it, for example, in a bench vice, it must be suitable for such parameters as: diametric size, profile accuracy and accuracy of the thread pitch, the ratio of the screw thread to its support necks, wear resistance, thread thread thickness. It is also important to note that, depending on the degree of accuracy of the movement provided by the screws, they can be divided into several accuracy classes from 0 to 4. For example, the lead screws of metal cutting machines should correspond to an accuracy class from 0 to 3. 4 the accuracy class is not suitable for use in such equipment.
Propeller material
As a billet for the production of screw using a conventional bar, which is cut from section metal. However, it is important to note here that some requirements are imposed on the material serving as the workpiece. The metal should have good wear resistance, good machinability, and also have a state of stable equilibrium under conditions of internal stress that occurs after processing. This is very important, since this property will help to avoid deformation of the spindle during its further use.
For the production of this part with a middle accuracy class (2nd or 3rd), which will not be required to increase temperature resistance, A40G steel is used, which is medium-carbon, with sulfur and steel 45 additions with lead addition. Such an alloy improves the processing ability of the screw, and also reduces the surface roughness of the material.
Screw profile
There are three screw profiles that are used in the manufacture of the lead screw of a lathe or any other. The profile may be trapezoidal, rectangular or triangular. The most common type is trapezoidal thread. Its advantages include the fact that it is higher in accuracy than rectangular. In addition, using a split nut, you can adjust the axial clearances of the trapezoidal screw, which arise due to wear and tear of the equipment.
It is also important to note here that cutting, like grinding a trapezoidal thread on a screw, is much simpler than rectangular. But it must be understood that the accuracy characteristics of a rectangular thread are higher than that of a trapezoidal one. This means that if the task is to create a screw with the best adjustment for accuracy, you will still have to cut a rectangular thread. Keystone screws are not suitable for very precise operations.
Screw processing
The main parts on which the screw in the machine is based are the supporting journals and flanges. The executive surface of the screw is its thread. The greatest accuracy in a table vise and any other machines having such a screw should be ensured between the executive surface of the part, as well as the main base surface. The technological basis for the production of a lead screw is its center hole. For this reason, in order to avoid deformation, the processing of all these surfaces is carried out using a movable lunette. The use of this part determines the specificity of the processing of the spindle.
It is also important to note here that a screw with a different accuracy class is processed to various values. Parts that will belong to the 0.1 and 2 accuracy classes are processed up to the 5th grade. The screws belonging to the 3rd class of accuracy are processed up to the 6th grade. The screws belonging to the 4th category are also processed up to the 6th grade, but at the same time they have a tolerance field for the outer diameter.
Centering and threading
In order to obtain an acceptable quality screw, it is necessary to carry out several more operations. One of them was the alignment of the part, which takes place on a lathe. The lead screw, or rather, the workpiece for this part, is centered on the specified equipment and here the ends are cut to it. In addition, an operation is carried out to polish the workpiece. To do this, use centerless grinding or circular grinding machines in the centers. It is important to add here that grinding in the centers is carried out only for screws of 0.1 and 2 accuracy classes.
Next, before proceeding with threading, the workpiece must be edited. It should be noted that only screws with the 3rd and 4th accuracy class are subjected to this operation. After that, their surface is additionally ground. A screw-cutting lathe is used as equipment for threading on a lead screw.
Screw Nut Description
The spindle nut is designed to provide precise installation movements. In some rare cases, they can be made from a material such as anti-friction cast iron. This element should ensure constant engagement with the turns of the screw, and also act as a compensating part. You will have to compensate for the gap that will inevitably occur when the screw is worn. For example, the nuts for the lead screws used in lathes are made double. This is necessary in order to remove the gap that may arise either due to the production and assembly of the machine, or as a result of wear of its parts.
The peculiarity of a screw with a double type nut is that it has a fixed and movable part. The moving part, which is right, can move along the axis of the fixed part. It is this movement that will compensate for the gap. The production of the nut is carried out only for screws of zero, 1st and 2nd accuracy class. For their manufacture using tin bronze.
What are nuts and their wear made of?
The most common materials for the production of this type of parts are aluminum-iron bronzes, according to the standards of machine tools MT 31-2. In addition to this material, anti-friction cast iron can also be used as a substitute for non - essential screw gears.
It is important to add here that the nut wears out much faster than the lead screw itself. There are several reasons for this:
- the thread of the nut is poorly protected from any kind of contamination, and it is also quite difficult to clean from these unnecessary elements;
- it often happens that this element is initially poorly lubricated and this greatly affects the service life;
- when the nut and screw engage, it turns out that all turns work simultaneously on the second element, but on the screw only those that are coupled to the nut.
For these reasons, the screws with the nut should be checked more often, since the wear of the nut occurs quite quickly.