Difficult problems and solutions in machining the

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Difficult problems and solutions in thread machining

in machining, there are many kinds of threads, which are mainly used as connectors and transmission parts. Common threads have national standards, and standard threads have good interchangeability and versatility. There are also a few non-standard threads, such as rectangular threads. The types of threads can be divided into connection threads and transmission threads according to their applications; According to the tooth type, it can be divided into triangle, rectangle, trapezoid, sawtooth and circle, etc; According to the direction of helix, it can be divided into right rotation and left rotation; According to the number of helix, it can be divided into single thread and multi thread thread; According to the shape of thread matrix, it can be divided into cylindrical thread and conical thread. Now there are variable pitch threads

key words: surface roughness plastic brittleness multi thread random tooth lead random tooth pricking knife

I. background

with the development of the machinery industry, thread processing is an important part of cutting, whether it is ordinary lathe thread processing or NC processing. For small-diameter threads, you can tap them with a tap, but for large-diameter threads with large lead, you have to turn them with a thread turning tool. It is inevitable to encounter many problems in the process of thread processing. How to solve them in order to do it well

II. Difficult problems encountered in thread machining

in the daily teaching process, turning triangular threads with small pitch usually adopts high-speed steel thread turning tools, which are processed at low speed, with slow speed and efficiency. For mass-produced parts, such efficiency is too slow. It can be calculated according to the formula, using cemented carbide cutting tools, using medium high speed 500r/min, three knife processing is completed

(I) when machining threads, carbide turning tools are used, and micro feeding cannot be carried out, and the surface roughness cannot meet the requirements. How to solve it

when finishing turning threads, especially when finishing turning larger lead threads, high-speed steel turning tools are usually used. This is mainly because high-speed steel carbides have fine grains and good grindability, which can form sharp edges. When finishing turning, the cutting force is small, and the surface roughness can be reduced through the reasonable selection of cutting fluid. On the contrary, due to the coarse grain of cemented carbide tools, it is not easy to form sharp edges during grinding. Generally, cemented carbide is less used to finish turning threads except for turning small pitch threads at high speed. However, the hardness and wear resistance of cemented carbide are better than that of high-speed steel, and cutting fluid can not be used. If cemented carbide can be used to turn threads, it will obviously bring certain economic benefits

(II) why should the hole diameter before turning the internal thread not be equal to or less than the small diameter of the internal thread

because the diameter of the inner hole will be reduced due to the squeezing effect of the turning tool when turning the internal thread (it is more obvious when turning plastic materials), the hole diameter of the horizontal workpiece before and after adjusting the dynamometer on the front of the swing rod before turning the internal thread should be slightly larger than the small diameter of the internal thread, and should not be equal to or smaller than the small diameter of the internal thread. Otherwise, the small diameter of the processed internal thread will be smaller than the size specified in the standard, and the surface quality of the thread will not meet the requirements. In addition, since the actual diameter of the internal thread after machining is allowed to be equal to or slightly larger than the basic size of the internal thread diameter, in order to facilitate calculation in actual production, the aperture size can be determined by the following approximate formula. Generally, there are only a few MV

when turning the internal thread of plastic materials, D ≈ D-P

when turning the internal thread of brittle materials, D ≈ d-1.05p

where: D - nominal diameter of thread (major diameter)

p - pitch

(III) what is multi thread thread? What are the first problems to be solved and paid attention to in turning multi thread thread

the thread with only one spiral groove on the cylinder is called single thread. Threads with two or more spiral grooves are called multi thread threads. The advance distance of the helix is called the lead after each rotation of the multi thread thread. The lead is equal to the product of the pitch and the number of threads

the first problem to be solved in turning multi thread thread is the method of line separation and correct line separation. If the line dividing method is not selected properly, the line dividing operation is incorrect, and the line dividing error occurs carelessly, resulting in the unequal pitch of the multi line thread. It will seriously affect the screwing accuracy of internal and external threads and reduce the service life. Therefore, we must pay attention to the selection of multi thread parting method, and pay attention to the correct operation of the parting process in the turning process

when turning multi thread threads, we should also pay attention to the following three problems:

1 the change gear of the machine tool and the adjustment of the machine tool should be determined and adjusted according to the lead. Never use the pitch to calculate and determine the position of the adjusting handle of the change gear

2 the lead of multi thread is generally large, and special attention should be paid to the influence of the lead on the back angle of the two cutting edges of the turning tool. The spiral angle should be calculated according to the lead. When sharpening the tool, the back angle of the cutting edge on the side along the cutting direction should be increased, which is suitable for the construction of special-shaped parts with many nodes; Environmental protection is odorless, and the back angle of the cutting edge on the other side can be reduced by one helix angle

3 fine turning of thread shall be carried out together after rough turning of all spiral grooves. Otherwise, the consistency of each tooth type cannot be guaranteed

(IV) why are there disordered teeth during thread processing

generally, the thread can only be processed to the required tooth profile and dimensional accuracy after several times of cutting. If the tip of the turning tool is not in the spiral groove of the previous cutting, but on the left, right or on the crown of the tooth in the first cutting, and the tooth shape is broken, this phenomenon is called disordered tooth

the causes of disordered teeth are:

1 the screw pitch of the lathe lead screw is not an integral multiple of the workpiece screw pitch. During machining, the reverse turning is not used for turning, but the lifting and closing nut is used for turning. In this case, it is basically impossible to ensure that the later cutter is in the spiral groove of the previous cutter carriage, which will inevitably lead to disordered teeth

2 the lathe pitch is an integral multiple of the workpiece pitch, which can be turned by lifting and closing the opening and closing nut. If the opening and closing nuts are not completely closed in the subsequent tool feeding, this will cause the workpiece and the turning tool to be misaligned, resulting in disordered teeth

3 the gap between the opening and closing nuts is too large and loose, and the opening and closing nuts are automatically lifted during the turning process, which will also cause the dislocation of the workpiece and the turning tool, resulting in disordered teeth

4 the axial clearance of the lead screw is too large, resulting in axial movement during turning

5 in the customized fibers in the processing process, the turning tool was not recalibrated when sharpening or changing the tool

6 the workpiece is not clamped, and the workpiece rotates or moves axially in the chuck during processing; The turning tool failed to fasten, and the turning tool moved in the axial direction during processing. These will change the original relative position of the turning tool and the workpiece, resulting in disordered teeth

(V) what to do if there is a "stabbing" phenomenon when rough turning trapezoidal threads or worms

due to the large rough turning allowance, in order to make the cutting light, the rough turning screw cutter or Worm rough turning tool generally grinds out the radial front angle. If the radial forward angle is too large, the clearance between the horizontal feed screw and nut is too large, which is prone to "stabbing". In order to avoid "stabbing" during rough turning, the following problems should be paid attention to:

1 the radial forward angle of the thread turning tool should not be too large, generally 3 ° ~ 5 °

2 just cross feed the lead screw gap, hand feed is light, and push and pull the tool holder by hand without moving

3 do not use the straight cutting method for turning, because the straight cutting method cuts three cutting edges at the same time, and the cutting force is very large, which is easy to cause "stabbing"

4 in some cases, in order to avoid "stabbing", the method of turning with negative rake angle and high turning tool can also be adopted. In this way, the direction of radial force is changed, and the purpose of preventing "stabbing" is achieved. This method can ensure that the rake face still passes through the center of the workpiece by properly adjusting the height of the cutter, so it has little effect on the tooth angle error

III. summary

thread machining is an important part of machining, involving dimensional accuracy and surface roughness; And the feeding route in the processing process; For single line and multi line thread processing, their parting method; And produce disorderly buckles, stabbing knives and so on. A little mistake will cause disordered teeth, unqualified tooth shape, problems in coordination, scrap or affect the service life

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