Thread cutting method
Cutting screws is one of the other most important tasks carried out in lathes. A typical thread form is shown in figure. There are a large number of thread forms that can be machined in a lathe such as Whit worth, Acne, ISO metric, etc
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| Simple thread definition |
Thread cutting can be considered as turning only since the path to be travelled by the cutting tool us helical. However, there are some major differences between turning and thread cutting. In turning, the interest is in generating a smooth cylindrical surface, whereas in thread cutting, the interest is in cutting a helical thread of a given form and depth which can be calculated from the formula for different forms of threads as given in Table.
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| Formula for some common thread forms |
The shape of the cutting fool is of the same form as the thread to be generated.
For the purpose of feeding the tool for generating the thread and thread measurement, the feed is given by the lead screw. Feed is the same as the lead of the pitch to be generated. In the normal turning, the thickness of the uncut chip is the same as the feed rate chosen, whereas in the thread - cutting case it is controlled by the depth of cut d in view of the thread form being generated as shown in Figure. The uncut chip thickness 't' can be shown to be
t = 2 × d × tanA
The depth of cut in the case of thread cutting can be given in two ways: plunge cutting as shown in figure or compound cutting as figure.
In the case of plunge cutting, the cutting of the thread takes place along both the flanks of the tool. This would mean that the cutting too! would have to be provided with a zero or negative rake angle. In addition, the relief along the cutting edges cannot be provided in view of the form to be achieved. Cutting is also taking place along a longer length of the tool. This gives rise to difficulties in machining in terms of higher cutting! forces and consequently chattering (violent vibrations). This results in poor surface finish and lower tool file, and thus this method is not generally preferred. With the compound feeding, the tool needs to be moved in both the directions (along the bed as well as a direction perpendicular to it) simultaneously to position the tool tip along one flank of the thread. This configuration helps in more smooth flow of chips as cutting takes place only along a cutting edge. This method, therefore, is much preferred compared to the earlier method. The only problem is to move the tool for giving the depth of cut along the flank of the thread, which can be achieved by the use of compound slide for giving the depth of cut as shown in Figure. while feed is given by the carriage in the traditional manner.
The compound slide is rotated by the half angle of the thread, and the cutting tool is adjusted to make it perpendicular to the workpiece surface. For this purpose, a thread - setting gauge which contains the required form of the thread being cut is kept perpendicular to the surface of the workpiece, and the tool tip is set as shown in Figure.
The next important consideration for thread cutting Dine feeding of the tool along the helical path. For this purpose, the lead screw is normally employed for feeding the tool along the length of the job. In turning the engaging of tool at any point would be of any consequence since the surface to be generated is cylindrical however, in thread cutting, it is essential that the tool tip should always follow the same thread profile generated in the first cut, otherwise no thread would be generated.
One of the methods that can be followed in this case is to reverse the spindle while retaining the engagement between the tool and the workpiece. The spindle reversal would bring the cutting tool to the starting point of the thread following the same path in reverse. After giving a further depth of cut, the spindle is again reversed and the thread cutting is continued in the normal way. This is easy to work and is somewhat more time-consuming due to the idle times involved in the stopping and reversing of the spindle at the end of each stroke.
Another alternative method is to use a chasing dial to help in following the thread. As shown in Figure, the chasing dial consists of a worm gear located inside the carriage in mesh with the lead screw. A vertical shaft connected with the worm gear has a dial with separate markings to separate equal divisions of the circumference Since the worm gear is in continuous contact with the lead screw, which is in continuous engagement with the spindle, markings on its surface being the precise position of the thread being cut on the workpiece. Thus it is possible to engage the workpiece at any desired location.
It is possible to cut threads on a tapered surface by combing the thread cutting concept as explained above along with the taper turning attachment as shown in figure.
For the purpose of feeding the tool for generating the thread and thread measurement, the feed is given by the lead screw. Feed is the same as the lead of the pitch to be generated. In the normal turning, the thickness of the uncut chip is the same as the feed rate chosen, whereas in the thread - cutting case it is controlled by the depth of cut d in view of the thread form being generated as shown in Figure. The uncut chip thickness 't' can be shown to be
t = 2 × d × tanA
The depth of cut in the case of thread cutting can be given in two ways: plunge cutting as shown in figure or compound cutting as figure.
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| Death of cut in screw cutting |
In the case of plunge cutting, the cutting of the thread takes place along both the flanks of the tool. This would mean that the cutting too! would have to be provided with a zero or negative rake angle. In addition, the relief along the cutting edges cannot be provided in view of the form to be achieved. Cutting is also taking place along a longer length of the tool. This gives rise to difficulties in machining in terms of higher cutting! forces and consequently chattering (violent vibrations). This results in poor surface finish and lower tool file, and thus this method is not generally preferred. With the compound feeding, the tool needs to be moved in both the directions (along the bed as well as a direction perpendicular to it) simultaneously to position the tool tip along one flank of the thread. This configuration helps in more smooth flow of chips as cutting takes place only along a cutting edge. This method, therefore, is much preferred compared to the earlier method. The only problem is to move the tool for giving the depth of cut along the flank of the thread, which can be achieved by the use of compound slide for giving the depth of cut as shown in Figure. while feed is given by the carriage in the traditional manner.
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| Thread cutting using compound slide |
The compound slide is rotated by the half angle of the thread, and the cutting tool is adjusted to make it perpendicular to the workpiece surface. For this purpose, a thread - setting gauge which contains the required form of the thread being cut is kept perpendicular to the surface of the workpiece, and the tool tip is set as shown in Figure.
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| Settings the cutting tool for thread cutting in a lathe |
The next important consideration for thread cutting Dine feeding of the tool along the helical path. For this purpose, the lead screw is normally employed for feeding the tool along the length of the job. In turning the engaging of tool at any point would be of any consequence since the surface to be generated is cylindrical however, in thread cutting, it is essential that the tool tip should always follow the same thread profile generated in the first cut, otherwise no thread would be generated.
One of the methods that can be followed in this case is to reverse the spindle while retaining the engagement between the tool and the workpiece. The spindle reversal would bring the cutting tool to the starting point of the thread following the same path in reverse. After giving a further depth of cut, the spindle is again reversed and the thread cutting is continued in the normal way. This is easy to work and is somewhat more time-consuming due to the idle times involved in the stopping and reversing of the spindle at the end of each stroke.
Another alternative method is to use a chasing dial to help in following the thread. As shown in Figure, the chasing dial consists of a worm gear located inside the carriage in mesh with the lead screw. A vertical shaft connected with the worm gear has a dial with separate markings to separate equal divisions of the circumference Since the worm gear is in continuous contact with the lead screw, which is in continuous engagement with the spindle, markings on its surface being the precise position of the thread being cut on the workpiece. Thus it is possible to engage the workpiece at any desired location.
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| Chasing dial principle |
It is possible to cut threads on a tapered surface by combing the thread cutting concept as explained above along with the taper turning attachment as shown in figure.
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| Set up for thread cutting on a taper |