Screw threads

Screw threads are complex helical forms and their detailed characteristics in terms of such things as angles, root diameter, pitch circle diameter and radii are closely defined by ISO standards. Thus, if the designation 'M8' appears on a drawing it would appear at first sight to be very loosely defined but this is far from the case. Screw threads are closely defined in the standard ISO 6410, parts 1, 2 and 3:1993. The 'M8' designation automatically refers to the ISO 68-1:1998, ISO 6410-1, 2 and 3:1993 standards in which things like the thread helix angle, the vee angles and the critical diameters are fully defined. Thus, as far as screw threads are concerned, there is no need to do a full drawing of a screw thread to show that it is a screw thread. This takes time and costs money. The convention for drawing an engineering thread is shown using a combination of ISO type A and B lines as shown in the drawings in Figures 3.1, 3.2 and 3.3. A screw thread is represented by two sets of lines, one referring to the crest of the thread (type A line) and the other referring to the roots of the thread (type B line). These can be seen for a bolt and a hole in Figures 3.5 and 3.6. This representation can be used irrespective of the exact screw thread. For example, on the vice assembly drawing in Figure 3.1, the screw thread on the bush screw (part number 5) and the jaw clamp screw (part number 6) are very different. In the real vice, the former is a standard vee-type thread whereas the latter is a square thread. Line thicknesses become complicated when a male-threaded bolt is assembled in a female-threaded hole. The thread crest lines of the bolt become the root lines of the hole and vice versa. This means that in an assembly, lines change from being thick to thin and vice versa. This is shown in the vice assembly drawing in Figure 3.1, with respect to the bush screw (part number 5)/jaw clamp screw (part number 6) assembly.

Engineering Drawing for Manufacture
by Brian Griffiths
Publisher: Elsevier Science & Technology Books
 

THEORETICAL BACKGROUND BLOW MOLDING

Blow Molding (BM) process makes it possible to manufacture molded products economically, in unlimited quantities, with virtually no finishing required. The basic process of blow molding involves a softened thermoplastic hollow form which is inflated against the cooled surface of a closed mold. The expanded plastic form solidifies into a hollow product. Blow molded components are now seen all over the markets and industries for traditional materials, particularly in liquid packaging applications. The last few decades saw the introduction of polyethylene (PE) squeeze bottles for washing liquids, polyvinyl chloride (PVC) for cooking oil and fruits squash bottles, and polyethylene terephthalate (PET) for carbonated beverage bottles. Nowadays, it is also used for the production of toys, automobile parts, accessories and many engineering components. Blow Molding Process is intended also for manufacturing of most automotive parts and accessories. Below are some of the car plastics parts that are being produced by blow molding process. The use of plastics parts make our car more light weight and helps our car run faster. Extrusion Blow Molding Machine Parts and Functions • Extruder Motor—Drives the screw in the barrel to rotate and push the melted material into the die head. • Gearbox—Reduces the speed of the extruder motor into a required speed enough to push the material into the die head. • Hopper—A feed reservoir into which the material is loaded. • Extruder—A part of the machine that accepts solid resin material, conveys it in a surrounding barrel by means of a rotating screw, melts the material by means of heaters, and pumps it under pressure into the die head. • Cooling Fans—Cools down the barrel during machine shut down to prevent the material from degradation. • Heating Bands—Device attached on the barrel and the die head used to melt the solid material at a required set temperature. • Die Head—Used to form the melted resin into a parison and also used for adjusting the characteristics of molten resin to create a stable parison. • Die & Pin—Used to align the flow of parison to get a good and centered parison. • Hot Cutter—Cuts the parison after the mold is closed for the blowing process. • Blow Pin—Used to blow compressed air into the parison to inflate it after the mold has been closed and form the desired design of the mold. • Mold—A hollow form or a cavity into which a molten plastic material, called parison, is introduced to give the shape of the required component. • Deflasher—Used to cut the excess material on the bottle which is called a flash material (top and bottom). • Post Cooling—A part of the machine that is used to cool down the inside of the bottle, to lessen the cooling time required inside the mold. • Article Discharge—A part of the machine used to take the bottle out.

Higher Institute for Plastics Fabrication
WORKBOOK for Extrusion Blow Molding
Practical Course
Prepared by
Extrusion Blow Molding Department

Flats on cylindrical or shaped surfaces

It is not always obvious that surfaces are flat when they are on otherwise curved, cylindrical or spherical surfaces. In this case, flat surfaces such as squares, tapered squares and other flat surfaces may be indicated by thin 'St Andrew' cross type diagonal lines. An example of this is shown in the entirely fictitious gear shaft in Figure 3.17. The extreme right-hand end of the shaft has a reduced diameter and approximately half of this cylindrical length has been flat milled to produce a square cross-section. The fact that the crosssectional shape of this region is square and not cylindrical is seen in the end view as a square and in the right-hand side elevation by the crosses. 

Engineering Drawing for Manufacture
by Brian Griffiths
Publisher: Elsevier Science & Technology Books
 

The headstock - The back gear LATHE

The headstock The headstock is secured permanently on the inner ways as the left hand end of the lathe bed, and it provides mechanical means of rotating the work at multiple speeds. It comprises essentially a hollow spindle and mechanism for driving and alternating the spindle speed. All parts are housed within the head stock casting. The spindle of headstock is made of carbon or nickel-chrome steel. The back gear The back gear is an additional feature of a belt driven lathe and is used to obtain wider range of spindle speeds, for the number of speeds obtained from “direct speeds” is limited to number of steps only. When the back gear is engaged, the spindle is speed is reduced considerably. So it is also used when it is necessary to have a slow speed of the spindle that cannot other wise be obtained by direct speed. A slow speed is necessary in the following cases. 1.In turning jobs of large diameter within the available cutting speed of the material 2.In turning jobs tough or hard material when the material is hard it becomes necessary to apply greater cutting force by the tool to shear out the metal. This increase in cutting force will require greater turning torque necessitating slower spindle speed. 3.In operations like thread cutting, reaming, e.t.c. 4.In taking deep cut as rough turning.
fr. NTTF ( NETTUR TECHNICAL TRAINING FOUNDATION)