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°
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.
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.
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
Example
Cylindrical pipe thread – not sealing
G + nominal inside diameter in inches
G ½”
Cylindrical pipe thread internal – sealing
Rp + nominal inside diameter in inches
Rp ½”
Taper pipe thread internal – sealing
Rc + nominal inside diameter in inches
Rc ½”
Taper pipe thread external – sealing
R + nominal inside diameter in inches
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.
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
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 01 4080), etc.
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.
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
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.
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
=
Circumscribed dimension
=
of the hexagon
1d
the same size as a
screw diameter
2d of a screw diameter
Big radius R1
of the middle arc
=
1.6d of a screw diameter
Small radius R2
of the side arcs
=
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 R1
of the middle arc
=
Small radius R2
of the side arcs
=
of the Circumscribed
dimension of the hexagon,
0.75e
which we must read from
the standard.
0.2e
of the Circumscribed
dimension of the hexagon
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 R3
=
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.
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.
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.
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.
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