§ 7. Types and causes of defects when cutting threads with taps and dies

Defects when cutting threads with cutters and measures to prevent it

Most often, when cutting threads with cutters, defects of the following types are obtained: 1) inaccurate pitch; 2) inaccurate thread diameters; 3) incorrect thread profile; 4) insufficient cleanliness of the thread surface.

1. Inaccurate thread pitch is the result of incorrect selection of replacement gears or incorrect installation of feed handles. You can prevent defects by properly setting up the machine.

2. Inaccurate dimensions are obtained due to insufficient or excessive metal removal when cutting threads; eliminated by frequent measurements, especially during the last passes, or by setting the female stop to depth.

3. An incorrect thread profile results from an incorrect cutter profile and inaccurate installation. Such defects can be prevented by carefully checking the cutter profile and its installation.

4. Insufficient surface cleanliness (scratches, scuff marks on the thread) occurs when the cutter is sharpened incorrectly, the depth of cut is too high, the cutting speed is incorrectly selected, the tool is very dull, the part or tool is not fastened tightly enough, the absence or incorrectly selected cooling, etc. To get rid of this marriage, it is necessary to eliminate the reasons that caused it.

To prevent defects and breakage of the die teeth, it is necessary to ensure that the die is perpendicular to the rod: the die must cut into the rod without distortion.
When cutting with whole dies, a complete thread profile is obtained in one pass. However, to obtain a clean thread, two dies are sometimes used: the first - worn - is used as a rough dies, and the second - of normal diameter - acts as a finishing die to obtain the final dimensions of the thread. The difference in the thread diameters of the rough and finishing dies should be 0.2-0.3 of the thread depth.
Thread cutting with clamps is carried out in the following order. Place half dies in the die and move them apart to a size slightly larger than the diameter of the thread being cut, clamp the workpiece in a vice in vertical position and file a chamfer at the end; put the clamp on the workpiece rod and tightly move the half-dies with a screw so that they cut into the metal by 0.2-0.4 mm. The die with half-dies lubricated with mixed or sulfurized oil is turned 1-1.5 turns clockwise, then 1/4-1/2 turns back. After the first pass, the blanks need to be returned back to the end of the bolt, tighten the half-dies with the adjusting screw and go through the thread again. A complete thread profile is usually obtained in 3-4 passes.
Check the thread with a ring gauge or thread templates, in extreme cases you can also use a nut, repeating the passes until the thread is obtained the right size. At the end of the work, the half-dies are removed from the die, cleaned of chips, thoroughly wiped and lubricated with oil.

Causes of marriage and ways to prevent it

Types of defects when cutting threads with taps and ways to prevent and eliminate them

“Wood structure” - Content of fixed carbon. Fatty acid. Low molecular weight substances. Wood moisture content. Secondary polymeric substances. Animal and bacterial cellulose. Microscopic structure of wood. Components solid fuel. Polioses. Amorphous substance. Wood and its fuel properties. Elemental composition of wood.

"Cutting board" - Example project " Cutting board" Economic and environmental justification. There are so many board options I liked. Now you can find boards various shapes. Historical reference. List of used literature. What's happened creative project. Design work. Conclusions based on the results of the work. Birch bark is a protective cover of a tree that protects the living tissues of the trunk.

"Woodworking machines" - Milling machines on wood. Band saws for wood. Woodworking machines. Wood lathes. Circular saws. This group of equipment includes circular and cross-cut machines. Technology presentation. Planing machines. The copier allows you to make a large number of identical parts.

"Wood" - Light conductivity. Gas permeability. To identify wood defects in plywood production, X-ray testing is used. Knots are an inevitable defect, biologically determined by the growth of the tree. The bark consists of an outer cork tissue-crust and an inner bast. To enhance the electrical insulating properties, wood is impregnated with oil, varnish, and paraffin.

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“Tenon joints” - The choice of the number of tenons on the workpiece depends on the thickness of the parts being connected. Mark tenons and eyes on both sides of the workpiece. Sawing wood along the grain. In relation to the plane of the frame, the saw blade can be rotated by desired angle. The product is secured in the front clamp of the workbench with a backing board.

There are 12 presentations in total

PURPOSE: To study defects that occur during thread cutting and ways to eliminate them.

EXERCISE:

1. Study theoretical material
2. Make a table of defects when cutting threads
3. Identify the thread according to the figure given in the table, and give a description, characteristics and designation.
4. Answer security questions

Task table

CONTROL QUESTIONS

1. Purpose of the round die
2. Safety cartridge, purpose characteristics.
3. List the types of threads.
4. Purpose of the tap

THEORETICAL PART

Preparation of rods and holes of threaded surfaces

When cutting threads with taps and dies (both manually and on metal-cutting machines) or using a special mechanized tool, not only a layer of material is removed from the surface of the workpiece, but also plastic deformation of the outer part of the machined surface occurs. This deformation is accompanied by extrusion of the workpiece material in the thread cavity into its protrusions. This phenomenon must be taken into account when determining the diameter of the rod or hole for threading. Therefore, it is most advisable to determine the dimensions of rods and holes for threading using reference tables, in which these dimensions are given taking into account all the factors that arise during cutting.

In practice when cutting thread diameter The holes are assumed to be equal to the nominal diameter of the thread, reduced by the value of its pitch. For example, when cutting metric thread M10 hole diameter should be respectively 1.0... 1.5 mm, i.e. should be 8.5 mm.

When cutting external threads, the diameter of the rod should be 0.1 ... 0.2 mm less than the nominal diameter of the thread, depending on its size.

When rolling threads, the diameter of the rod is selected based on the average diameter of the thread, which must be specified in the thread processing task, or is determined using special tables. To make it easier to insert the die at the top of the rod, it is necessary to chamfer it with an angle of approximately 60°.

Rules for processing external and internal threaded surfaces

1. Thread cutting must be done with the die or tap generously lubricated with machine oil.

2. When cutting threads, you should periodically cut off the resulting chips by moving the tap or die back 1/2 turn.

3. After cutting a thread on a rod or hole, you need to carry out quality control:

external inspection - avoiding scuffing and torn threads;

threaded gauge (or standard bolt, nut) - the passage part of the gauge (bolt, nut) is screwed on by hand; rolling into the bolt-nut pars is not allowed.

Typical defects when cutting threads, the reasons for their occurrence and methods of prevention

Rules for cutting external threads

1. Before cutting a thread, you should check the diameter of the rod (bot, stud, screw); it should be 0.1 ... 0.2 mm less than the nominal thread diameter.

2. It is necessary to cut off the fence chamfer at the top of the rod (if it is not on the workpiece). When filing a chamfer, you need to ensure its concentricity relative to the axis of the rod, as well as its diameter, which should not exceed internal diameter threads on the end surface. In addition, the angle of inclination of the chamfer relative to the axis of the rod should not exceed 60°.

3. The rod should be secured in the vice firmly and perpendicular to the jaws. The perpendicularity of the rod must be checked using a square.

4. It is necessary to strictly monitor the perpendicularity of the end of the die to the axis of the rod when cutting into the die.

5. Before rolling a thread on a rod, it is necessary to check its diameter; it should be equal to the average diameter of the thread being cut.

6. When cutting threads on gas and water pipes Special attention Care should be taken to maintain the length of the cut part for couplings and moans.

When cutting internal threads, the following rules must be observed.

1. Before cutting a thread, you should check: the diameter of the hole matches the size of the thread being cut.

It must match the thread table data;

hole depth for cutting blind threads. It must correspond to the size indicated on the drawing.

2. When inserting a tap, it is necessary to ensure that its axis is perpendicular to the upper plane of the workpiece in which the thread is being cut.

3. When cutting threads, you should use the entire set of taps: the first - rough; the second is semi-finish; the third is finishing.

4. When cutting threads in a blind hole, it is necessary to periodically clear it of chips.

5. Particular care should be taken when cutting small diameter threads (5 mm or less) to avoid breakage of the tap.

6. When cutting threads with a machine tap on a machine, it is necessary to secure it in a safety chuck.