Screw-thread Terminology

Transcription

1 Drafting and Dimensioning of Threads, Screws and Nuts The screws and nuts serves for fastening of details to each other, to assembly parts of machines, devices or equipment s. We do not draw thread as we can see it, because it would take too much work. We draw only straight thick contour lines and thin lines, which shows the depth of thread s grooves. Screw-thread Terminology EXTERNAL THREAD, for example on a SCREW INTERNAL THREAD, for example in a NUT The Screw THREAD is a ridge of uniform section in the form of a helix on the external surface around a screw, or in a internal surface in a nut. CREST OF A THREAD = top of a ridge ROOT OF A THREAD = bottom of a ridge SIDES OF A THREAD between CREST and ROOT DEPTH OF A THREAD distance between the CREST and the ROOT MAJOR DIAMETER the biggest diameter of a thread MINOR DIAMETER the smallest diameter of a thread PITCH DIAMETER the middle diameter of a thread PITCH the distance between the corresponding points on adjacent thread forms LEAD the distance of axial motion, after the one complete revolution 360

2 RIGHT-HAND THREADS receding, while winding, in a clockwise direction most of threads LEFT-HAND THREAD receding while winding counter clockwise. It must be designated by two letters LH left hand. SINGLE THREAD - is a thread, with only one helix on the cylinder LEAD and PITCH are equivalent (most used threads 99% of threads) MULTIPLE THREAD the thread has the same form with two or more helices DOUBLE THREAD the LEAD is two times bigger than the PITCH. TRIPLE THREAD the LEED is three times bigger than the PITCH etc.

3 Standardised Threads MTD - page 180, Tab. 4.1, (third edition page 177). Connection threads serves for removable fastening. (METRIC, WHITWORTH, PIPE, EDISON and ARMOUR THREAD). Motion threads serves for transformation of the rotary motion to the straight-line motion. (TRAPEZOIDAL, BUTTRESS and ROUND THREAD) Metric Thread Metric thread is the most used connection thread all over the world. It is sometimes called INTERNATIONAL STANDARD THREAD. It is generally used for standard screws. Two Forms of Metric Thread Designation Metric thread of basic lead M + major diameter in mm Metric thread of fine lead M + major diameter in mm x lead Example M 12 M 12x1 Metric thread has thread angle 60. Crest is flat and root is rounded. Usual dimensions of metric threads are: M3, M4, M5, M6, M8, M10, M12, M16, M20, M24, M30, etc. Whitworth Thread Whitworth thread is used only exceptionally in Europe. It is used in oversees countries mainly. Designation of Whitworth thread W + major diameter in inches: W ¾ in form of fraction. Whitworth thread has thread angle 55. Crest and root are rounded.

4 Usual dimensions of Whitworth threads are: W1/4, W3/8, W1/2, W3/4, W1, W1 ¼, W1 ½, W2 etc. Pipe Thread Pipe thread is used for connection of water and gas pipes. It is used for big fuse to 60 Ampere in power-electrotechnics as well. The pipe thread and TAPER PIPE THREAD has the same profile as whitworth thread. Several Forms of Pipe Thread Designation Cylindrical pipe thread not sealing G + nominal inside diameter in inches Cylindrical pipe thread internal sealing R p + nominal inside diameter in inches Taper pipe thread internal sealing R c + nominal inside diameter in inches Taper pipe thread external sealing R + nominal inside diameter in inches Example G ½ R p ½ R c ½ R ½ Usual dimensions of pipe threads are: G3/8, G1/2, G3/4, G1, G1 ¼, G1 ½, G2 etc. Edison Thread Edison thread is used for fastening of incandescent lamp. Designation of Edison thread: E + major diameter in mm: E 27. Profile of Edison thread is created from two arcs with the same radiuses. Usual dimensions of Edison threads E10 so called dwarf thread used in electric torch E14 so called mignon thread used in small lamps E27 used on standard incandescent lamps E33 so called goliath thread used in big lamps E40 Armour Thread Armour thread is used for connection of armour electrical pipes.

5 Armour electrical pipes are used for protection of conductors in the floor, in explosive surroundings, and in places where the conductors may be mechanically or chemically damaged etc. Designation of armour thread: P + nominal inside diameter in mm:p 16 Armour thread has thread angle 80. Crest and root are rounded. Usual dimensions of armour threads are: P11, P13.5, P16, P21, P29, P36 etc. Trapezoidal Thread Trapezoidal thread is the most used motion thread. In USA it is called ACME thread. This thread is used on 90% of motion screws, which are loaded from both directions. Designation of trapezoidal thread: Tr + major diameter in mm x lead in mm: Tr 24x5 Trapezoidal thread has thread angle 30. Crest is sharp with small fillets on the root. Usual dimensions of trapezoidal thread are: Tr10x3, Tr16x4, Tr20x4, Tr30x6, Tr40x6, Tr60x8 etc. Buttress Thread Buttress thread is similar to the trapezoidal thread, but it is non-symmetrical. It is used for motion screws loaded from one side only. But it must not be loaded from the opposite side. Designation of buttress thread: S + major diameter in mm x lead in mm: S 24x5

6 Buttress thread has angle of one side working one, only 3, Angle of the second side is 30. Crest is sharp. The root is rounded. Usual dimensions of buttress thread are: S25x5, S32x6, S40x6, S50x8, S63x8 etc. Round Thread Round thread is used for very strongly loaded motion screws, which works in dusty, or in corrosion surrounding. Designation of round thread: Rd + major diameter in mm: Rd 40. Round thread has thread angle 30. Crest and root are big fillets. Usual dimensions of round thread are: Rd10, Rd12, Rd16, Rd20, Rd24, Rd30, Rd40, Rd50, etc. Thread for Protective Glass of Lamps There exist many other standardised threads, which are important for electrotechnology, for example thread for protective glass of lamps (ČSN ), etc.

7 Drafting of a Threads In a view and a section view, projected to the projection plane, which is parallel to the axis of a thread, we draw only straight thick contour lines and thin lines, which shows the depth of grooves of the thread. Here is used the rule for simplification of not drawn many times repeated shapes. In a view and a section view, projected to the projection plane, which is perpendicular to the axis of a thread, we draw thick contour circle and approximately ¾ of a thin circle, which shows depth of grooves of a thread. On external thread, on the screw, the outside lines are contour lines we draw them by thick lines, and inside lines are thin. In internal thread, in a nut, the inside lines are contour lines we draw them by thick line, and outside lines are thin. Crosshatching is made to the thick contour line. If we draw standardised threads we do not draw the shape of a thread ridge, because it is precisely done by the standard. There must be chamfered edge on the beginning of the thread. This is necessary for mounting. If we exceptionally draw hidden thread, we draw all lines by thin dashed lines as hidden edges. We do not usually use this we draw internal thread in section The thick line must be drawn at the end of the useful thread. It is drawn from one outside line to the second outside line If the threads are cut, the run-out of the thread is drawn beyond the end line of the thread. The run-out is tool-mark after the tap in the internal thread and after the round screw die on the external thread.

8 Tap Round Screw Die If the internal thread is cut by tap in the blind bole, the drilled hole must be deeper then the thread and run-out on the working detail drawing and on the construction assembly drawing. We may simplify drawing of the internal threads in the blind thread hole and we may draw the thread as deep as the hole but on the assembly drawings for mounting only. If draw external thread in a longitudinal section or in a cross section, we draw it by the same way as in a view. Contour lines are thick, and inside lines are thin. The end of the thread is drawn by thick line, but only to the depth of the thread. If we draw threads on assembly drawing, we draw them in the similar way as on the detail working drawing. If external thread screw is screwed into the internal thread, thread hole, then we draw the external thread as it was alone, and around it we draw internal thread. In the place of external thread the internal thread is not seen it is covered by the external thread. Crosshatching is always made to the thick, contour line, on both details Dimensioning of the Thread Dimension line is always drawn to the major diameter of a thread. We must always write the sign of type of the thread and its diameter major diameter or nominal inside diameter, according to the type of the thread. Length of the thread is usually dimensioned to the thick line of a usable length of a thread. If the run-out of a thread is drawn, we usually dimension its length. If the thread was made on a lathe, it has usually groove at the end and we dimension the length of the thread together

9 with this groove. We dimension internal threads in the same way like external threads. Dimension line must be always drawn to the major diameter of a thread. In the case of the inside thread the extension lines are drawn from the thin outside lines of the thread. Nonstandardised Thread If we exceptionally draw the nonstandardised thread we draw them by the same way as the standardised thread but we must draw detail view, to show the precise shape of nonstandardised thread. We must draw and dimension all its shapes.

10 Drafting of Hexagonal Heads of Screws If it is possible, we must draw hexagonal head of screw and hexagonal nut in that position, where three flats are seen when central flat of the hexagon is parallel to the projection plane. Hexagonal head of a screw and the nut has outer edge chamfered by angle of 30. Intersection between this cone and hexagon are hyperbolas. We simplify the hyperbolic intersection to the arc. The nut has both sides chamfered. The arcs are drawn on both sides. The head of a screw has chamfered the outer edge only. Two Versions of Simplification of Hexagonal Heads and Nuts Drawing If there is place enough around the screw head or the nut, we usually use the simplificated drawing of screws with hexagon heads and nuts. In this case we draw most of sizes approximately according to the standardised empirical formulae, not precise according to the real dimensions of screws and nuts. All dimensions we derive from the diameter of a screw. We draw: Height of the head of screw = 0.7d of a screw diameter Height of the nut = 0.8d of a screw diameter Width of a middle flat = 1d the same size as a screw diameter Circumscribed dimension of the hexagon Big radius R 1 of the middle arc Small radius R 2 of the side arcs = 2d of a screw diameter = 1.6d of a screw diameter = 0.28d of a screw diameter If there is not place enough around the screw head or around the nut, we must use the precise drafting. All sizes of screws and nuts are drawn precisely according to the standards. Only hyperbolas are simplified to the arc. We draw: Big radius R 1 of the middle arc = 0.75e of the Circumscribed dimension of the hexagon, which we must read from the standard. Small radius R 2 of the side arcs = 0.2e of the Circumscribed dimension of the hexagon

11 Drawing Hexagonal Heads and Nuts in the Corner Position If we must exceptionally draw hexagonal head of a screw or a nut in the corner position (it may be done by construction of an assembly): We draw: Radius R 3 of the side arcs = 0.5e of the Circumscribed dimension of the hexagon Drawing of Bolted Connections on the Assembly Drawings There are used bolted connections on all assembly constructions, electromotors, computers etc. Bolted connection by the screw with the hexagon head and the hexagon nut which pass through the free holes, and draw two thin materials together The holes must be bigger than the screw. Between the screw and the holes there must be drawn gaps on both sides. There are drawn two thick lines. Between the head of the screw and the first material, between both materials and between the second material and the nut, there must not be the gap, because in this direction screw and nut draw materials together. There must be drawn only one line. Bolted connection by the fitted screw with the hexagon head and the hexagon castle nut, which pass closely through the holes and draw two thin materials together Between fitted parts of the screw and holes there must not be the gap, and this part is drawn by only one line. But between the part of screw where exists the thread, there must be drawn gaps on both sides two lines. On the castle nut there is cylindrical part with grooves, which serves for fixing of the nut by cotter-pin.

12 Bolted connection by the screw with the cylindrical slottedhead, which draw the thin material to the thick material with the blind thread hole The hole is bigger then the screw in the thin material (upper part of a picture) because there is no thread. There must be drawn gaps on both sides. There is screw screwed into the thread hole in the thick material (lower part of a picture) with the blind thread hole. In the engineering construction assembly drawings we may not simplify drawing of the thread at the end of the blind thread hole. If it is cut by the tap, we must draw run-out and the blind thread hole must be deeper then end of the thread with run-out. By the some way the thread with run-out may be not drawn near the head of the screw it is cut with round screw die. Simplified drawing of threads we may use in the assembly drawings for mounting only. Because we must know engineering s job perfectly, we will use completely drafting of a blind thread holes in our subject only in the second test and during the examination. Bolted connection by the screw with the hexagonal head, which draw the thin material to the thick material with the blind thread hole The hole is bigger then the screw in the thin material (upper part of a picture) because there is no thread. There must be drawn gaps on both sides. There is screw screwed into the thread hole in the thick material (lower part of a picture) with the blind thread hole. Bolted connection by the screw with the cylindrical head with inner hexagon hexagon socket head, which draw the thin material to the thick material with a blind thread hole The hole where is not the thread, is bigger than the screw. The hole in which is head of a screw is bigger than the screw s head as well. Between the screw head and this hole must be drawn gaps on both sides.

13 Bolted connection by the screw with the cone head countersink screw, which draw thin material to the thick material with the blind thread hole Under the cone head of the screw and cone part of the hole there must not be any gap. This part is drawn by only one line. But under the cone, where is not the thread, is the hole bigger than the screw, there must be drawn gaps on both sides. The stud with the hexagon nut, which draw the thin material to the thick material with the blind thread hole The upper part of the connection we draw by the same way as the Bolted connection by the screw with the hexagon head and the hexagon nut which pass through the free holes. The hole must be bigger than the screw. The washer need not be drawn. The lower part (screwed in the blind thread hole) we draw by similar way as on the other pictures, but the lower thread is screwed in the blind hole to the end of the run-out. On the construction assembly drawing we draw threads with the run-out of the thread on the screw and in the thread hole as well. SLOTTED HEAD In a view projected to the projection plane, which is parallel to the axis of the thread, we must always draw slotted head in a position in which is the slot seen in a real shape. In a view projected to the projection plane, which is perpendicular to the axis of the thread, we must draw head in a position of a slot of 45 to the main axes of a drawing.

14 Simplification of Drawing Screws and Nuts Very small screws on very large objects, we need not to draw precisely. In this case standards allow us to simplify their drawing and not to draw the chamfering on the end of the thread and not to draw the chamfering hyperbolas on heads of screws and on nuts. We are also allowed to simplify the slotted heads of bolts. The slots must be always visible, and drawn with the slope of 45, we may draw it only as single line. Wood screws, self-tapping screws or screws to plastics we draw by the similar manner as the ordinary screws. A thread is represented by a thick contour line and depth of the threads is drawn by a thin line. Small wood screw we may draw simplified. The heads are drawn in the way we just discussed. The shank is drawn simplified only as a triangle. The thread is indicated by the single oblique line only. Schematic Drawing of Screws and Nuts We draw screws and nuts by schematic symbols in the kinematics schemes. Screws with flat head is drawn by symbol like capital letter T. Symbol for screws with cone head, looks like a capital letter Y. Symbol for nuts is a cross as capital letter X. Schematic symbols for washer is only a single line. Schematic symbols for wood screws and self-tapping screws are the same, but there is an arrow on the end of the shank.

15 Dictionary English armour thread bolted connection brass buttress thread carbon steel cast iron casting steel castle nut crest of thread fitted screw framework construction grey cast iron hardened fabric hardened paper hexagon socket head lead malleabliying cast iron nodular cast iron nut pitch pitch diameter root of thread round screw die round thread run-out screw self-tapping screw slotted head steel stud bolt tap tensile strength thread wood screw wrought Czech pancéřový závit závitový spoj mosaz lichoběžníkový - pilovitý závit uhlíková ocel litina litá ocel korunová matice závitový hřbet lícovaný šroub prutová konstrukce šedá litina tvrzená tkanina tvrzený papír - pertinax hlava s vnitřním šestihranem (imbus) stoupání temperovaná litina tvárná litina matice rozteč roztečný průměr závitové dno závitové očku (nástroj) oblý závit závitový výběh šroub závitořezný šroub hlava s drážkou (pro šroubovák) ocel závrtný šroub (bez hlavy - se závity na obou stranách) závitník mez pevnosti v tahu závit vrut tvářený