The formation of threads on metal blanks in the form of bodies of revolution is one of the most popular and at the same time complex turning operations. The difficulty lies in making basic equipment settings and in preparing an auxiliary tool. In order for the threading on the lathe to meet the technical specifications, you should adhere to the technology of its implementation and do not forget about safety rules.
Machine preparation
Regardless of the type of lathe and tool, the thread will be machined. Through the settings of the machine, the operator determines the angle of the helix of the impact on the surface of the workpiece, which will have a perpendicular position relative to the axis of rotation. Here it should be noted that the machines have different power and, accordingly, the spindle speed - so that the cutting tool can cope with its task qualitatively, it is important to correctly correlate the impact angle and the engine speed from the very beginning.
An important parameter is the step between the cutting lines - it is also taken into account in the equipment settings and make appropriate adjustments in terms of the position of the tool relative to the workpiece. Since threading on a screw-cutting lathe is usually done in several approaches, it is desirable to preserve the primary parameters until the operation is completed. Even if a step is taken along the line of threading, there will remain a risk of violation of the positions of the beginning and end of the deformation sections, which may not coincide. It is important to keep these nuances in mind before starting work.
Types of thread
Turners distinguish between odd and even threads. In the first case, we are talking about slices, which eventually form an even integer number of slicing lines in steps. Accordingly, an odd thread leaves an odd thread. From the point of view of the operation, even threading on a lathe will have its advantages. For example, the operator after each approach can start the sliding nut in the support and quickly return it back with the cutter manually, without stopping the operation of the equipment. Then, with each new pass of the sliding nut, the tool will automatically be sent to the initial cavity, which ensures a certain accuracy of the operation.
In turn, the odd cutting requires, after each pass, the caliper to return to its original position with the cutter, which does not do without starting the reverse stroke.
Multiple and swirl slices can be placed in separate categories. So, in the first case, cutting multiple threads on a lathe will require accurate angular separation of the workpiece during transitions from one groove to another. The initial calculation of the pitch and parameters of the cutting line is important here. As for the vortex thread, it requires additional installation of a rotating cutter head on the support carriage. Several individual incisors can be fixed on it, each of which will cover its own area of work.
Inch and metric
Inch cuts are used for metal pipes. Typically, this thread is obtained by fittings, which are subsequently used to assemble a metal or plastic pipe. The standards for creating such a cut are determined by GOST 6357-81. Based on the technical document, we can conclude that the main parameters of inch cutting are pitch and diameter. Moreover, the second characteristic is estimated as the distance between the extreme points of the thread ridges.
Typical threading of an inch thread on a lathe is carried out by cutters and taps with modifications of the technology as applied to internal processing. Metric cutting is performed taking into account the same parameters, but the shape of the thread crest profile is also added to them. In the case of an inch cut, it is most often sharp - in the shape of a triangle. In addition, as the name implies, the classic pipe thread is calculated in inches, and metric in millimeters.
Thread forming
The cutter is a tool that directly performs cutting. It is made of carbide steel and before work receives a special sharpening in the form corresponding to the requirements of the task. It can be used to thread bolts, nuts, studs and other workpieces. The cutter is installed in the machine chuck or multi-head. The workflow is divided into several passes - at the end of each of them, the tool is moved to the side.
Correction of the thread formation parameters in this case is carried out by adjusting the caliper, which sets the required depth. At the same time, there are standardized settings. In particular, threading on a lathe with a cutter in increments of up to 0.2 cm will suggest that the feed in the transverse movement of the caliper screw will be on average 0.1 mm per one approach.
Simultaneous work with two cutters is allowed. But here it is necessary to take into account a nuance that can affect the quality of the thread line - the chips produced by the foregoing cutter will cling to the waste of the second tool, which will have an insignificant, but still impact on the parameters of the formed edge.
Dice application
Dies are purposefully used to work with the same hardware in the form of bolts and studs, but only when creating an external thread. The zone, which is planned for cutting, is pre-processed and cleaned. Also, the diameter of the threaded surface is calculated so that it is less than the external diameter of the applied thread. In the case of metric technology, this difference may be 0.2 mm for small threads. To form a thread lead, a chamfer corresponding to the height of the threaded profile is first removed at the end of the workpiece.
Although the process itself can be carried out in a manual way, the most common is machine threading with a die on a lathe using a special die holder. The cartridge itself is fixed in the rear quill or in the head socket with several mounting niches. The average speed of such cutting will be 5-6 m / min.
Taps application
Taps are used specifically for working with metric threads applied from the inside. In this case, the diameter should be no more than 50 mm. On lathes, machine taps are usually used , allowing the operation to be performed in one approach.
This opportunity is largely due to the single use of several tools that are installed in multi-purpose universal cartridges. If in the case of a conventional cutter, the process is divided into several stages along the aisles, then threading with a tap on a lathe can be segmented by the quality of processing by different types of tools. Again, they work simultaneously, following one after another, which compensates for the load that falls on one cutter. Allocate taps for rough and fine threads. Moreover, the first tool removes about 75% of excess metal from the target ditch.
Tapping heads
To use several cutters at the same time, special heads with cartridges are also used. This is a turret equipment into which the same screw-cutting nozzles can be integrated - tangential, radial and round. After completing the cutting, their combs diverge on the return stroke and no longer come into contact with the thread made. It’s the most common round cutters of this type, since they are able to carry out several regrindings, have increased resistance and are easier to maintain.
However, internal threading on a turret lathe is made only with prismatic combs having a special entry cone. To work with "worms" and long screws, tool heads are used that are integrated directly into the machine support. They allow the formation of both external and internal threads.
Features of internal threading
Different types of screw cutting tools can work on such threads. Unlike surface external processing, such operations require the initial creation of a hole by drilling. In some cases, an appropriate bore is required, which will subsequently allow the use of threads for interfacing with parts of a specific diameter.
But if the diameter is incorrectly calculated, cutting the internal thread on the lathe can ruin the workpiece. This happens in cases where the initial diameter itself corresponds to the dimensions of the target part, which should be integrated into the target cavity. This can be avoided by allowing technological thickening of the inner walls before drilling. This tolerance should correspond to the height of the thread crest.
Thread line end
The quality of the created thread will be determined not only by the accuracy of the edges and the convergence of individual sections, which were performed with different passes. It is also determined by how thread lines end. The technology requires that the contour end with a special waste groove - it will provide the possibility of free entry of the screw part.
In addition, threading on a lathe in the final stage should lower the height of the crest. That is, the working tool when leaving forms a runaway groove, as well as its reduction. Sometimes, for a better design of the final line, a special passage is provided that allows you to correctly correct or even cut the already created groove run.
Quality control slicing
After the operation is completed, the operator checks the thread for compliance with the required parameters. For this, a template is used, on the surface of which fingerprints of the thread crests are printed. There are different patterns for metric and inch threads, while in both groups, strips are also distinguished to check the pitch, depth and angle of the grooves.
A comprehensive assessment is already given by calibers - these are devices that allow you to evaluate the correctness of the profile. If, for example, threading on a lathe with a cutter was performed with a critical slope, this will be fixed by a suitable gauge in size. Also for such tasks, micrometers and inserts are used. Accurate testimony is given only in the case of preliminary cleaning of the threads.
Conclusion
Creating threads on a lathe requires the use of a quality cutting tool and the care of the operator. First of all, a device for threading on a lathe is being prepared in the form of a cutter, tap, die or turret. As for the user, his responsibility is to properly configure the equipment.
The least hassle for the operator is the preparation of programmable CNC machines. They physically fit and position the snap-in in accordance with the specific data that is entered through a special input panel. The risk of errors in working with such models is much lower than with conventional lathes.