unit55 threads and thread cutting
TRANSCRIPT
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Copyright The McGraw-Hill Companies, Inc.Permission required for reproduction or display.
PowerPoint to accompany
Krar Gill Smid
Technology of Machine Tools6thEdition
Threads and Thread
Cutting
Unit 55
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Objectives
Recognize and state the purposes of six
common thread forms
Set up a lathe to cut inch external Unifiedthreads
Set up an inch lathe to cut metric threads
Set up a lathe and cut internal threads Set up a lathe and cut external Acme
threads
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Threads
Used for hundreds of years for holding parts
together, making adjustments, and
transmitting power and motion
Art of producing threads continually
improved
Massed-produced by taps, dies, threadrolling, thread milling, and grinding
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Threads
Thread
Helical ridge of uniform section formed on
inside or outside of cylinder or cone Used for several purposes:
Fasten devices such as screws, bolts, studs, andnuts
Provide accurate measurement, as in micrometer
Transmit motion
Increase force
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Thread
TerminologyCopyright The McGraw-Hill Companies, Inc.
Permission required for reproduction or display.
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Thread Terminology
Screw thread
Helical ridge of uniform section formed on
inside or outside of cylinder or cone
External thread
Cut on external surface or cone
Internal thread Produced on inside of cylinder or cone
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Major diameter
Largest diameter of external or internal thread
Minor diameter
Smallest diameter of external or internal thread
Pitch diameter Diameter of imaginary cylinder that passes
through thread at point where groove and threadwidths are equal
Equal to major diameter minus single depth ofthread
Tolerance and allowances given at pitchdiameter line
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Number of threads per inch
Number of crests or roots per inch of threadedsection (Does not apply to metric threads)
Pitch
Distance from point on one thread tocorresponding point on next thread, measured
parallel to axis
Expressed in millimeters for metric threads
Lead
Distance screw thread advances axially in one
revolution (single-start thread, lead = pitch)
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Root
Bottom surface joining sides of two adjacentthreads
External thread on minor diameter
Internal thread on major diameter
Crest Top surface joining two sides of thread
External thread on major diameter
Internal thread on minor diameter
Flank
Thread surface that connects crest with root
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Depth of thread
Distance between crest and root measured
perpendicular to axis
Angle of thread
Included angle between sides of thread
measured in axial plane
Helix angle Angle that thread makes with plane
perpendicular to thread axis
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Right-hand thread
Helical ridge of uniform cross section ontowhich nut is threaded in clockwise direction
When cut on lathe, toolbit
advanced from right to left
Left-hand thread
Helical ridge of uniform cross section onto
which nut is threaded in counterclockwise
direction
When cut on lathe, toolbit
advanced from left to right
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Thread Forms
April, 1975 ISO came to an agreement
covering standard metric thread profile
Specifies sizes and pitches for various threads
in new ISO Metric Thread Standard
Has 25 thread sizes, range in diameter from 1.6
to 100 mm Identified by letter M, nominal diameter, and
pitch M 5 X 0.8
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Table 55.1
ISO metric pitch and diameter
combinationsNominal Thread Nominal Thread
Diameter Pitch Diameter Pitch
(mm) (mm) (mm) (mm)
1.6 0.35 20 2.5
2 0.4 24 3
2.5 0.45 30 3.5
3 0.5 36 43.5 0.6 42 4.5
4 0.7 48 5
5 0.8 56 5.5
Portion of table taken
from textbook
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ISO Metric Thread
60 included angle, crest = 0.125 times pitch
D (depth of thread) is 0.6143 times pitch
Root is 1/4thof pitch,
Flat on root (FR) wider than crest (FC)
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ISO Metric Thread
x.250
x.125
x54127.0(external)
PFR
PFC
PD
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American National Standard Thread
Divided into four main series, all havingsame shape and proportions
National Coarse (NC)
National Fine (NF)
National Special (NS)
National Pipe (NPT)
Has 60 angle with root and crest truncatedto 1/8th the pitch
Used in fabrication, machine construction
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American National Standard Thread
N
PF
NPD
.125orx.125
.6134orx6134.
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British Standard Whitwork
(BSW) Thread Application is same
as for American
National form threadN
PR
NPD
.1373orx.1373
.6403orx6403.
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Unified Thread
Developed by U.S., Britain, and Canada forstandardized thread system
Combination of British Standard Whitworth and
American National Standard Thread
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NPF
NPF
NPD
NPD
.250
orx.250thread)(internal
.125orx.125thread)(external
.5413orx5413.thread)(internal
.6134orx6134.thread)(external
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American National Acme Thread
Replacing square thread in many cases
Used for feed screws, jacks, and visesD=minimum .500P
=maximum .500P+0.010
F=.3707P
C=.3707P-.0052
(for maximum depth)
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Brown & Sharpe Worm Thread
Used to mesh worm gears and transmitmotion between two shafts at right angles to
each other but not in same plane
D=.6866P
F=.335P
C=.310P
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Square Thread
Being replaced by Acme thread because ofdifficulty in cutting it
Often found on visesand jack screws
D= .500P
F = .500P
C= .500P + .002
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International Metric thread
Standardized thread used in Europe
F = 0.125P
R= 0.0633P (maximum)
= 0.054P(minimum)
D= 0.7035P (maximum)
= 0.6855P (minimum)
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Thread Fits and Classifications
Fit
Relationship between two mating parts
Determined by amount of clearance orinterference when they are assembled
Nominal size
Designation used to identify size of part
Actual size
Measured size of thread or part
Basic size: size from which tolerances are set
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Allowance
Permissible difference between largestexternal thread and smallest internal thread
Difference produces tightest fit acceptablefor any given classification
The allowance for a 1 in.8 UNC Class 2A and 2B fit is:
Minimum pitch diameter of the
internal thread (2B) = .9188 in.Maximum pitch diameter of the
external thread (2A) = .9168 in.
Allowance = .002 in.
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Tolerance
Variation permitted in part size
May be expressed as plus, minus, or both
Total tolerance is sum of plus and minus tolerances
In Unified and National systems, tolerance is pluson external threads and minus on internal threads
Maximum pitch diameter of the
external thread (2A) = .9168 in.
Minimum pitch diameter of the
external thread (2A) = .9100 in.
Tolerance = .0068 in.
The tolerance for a 1 in.8 UNC Class 2A thread is:
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Limits
Maximum and minimum dimensions of part
Maximum pitch diameter of the
external thread (2A) = .9168 in..
Minimum pitch diameter of theexternal thread (2A) = .9100 in.
The limits for a 1 in.
8 UNC Class 2A thread are:
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ISO Metric Tolerances and Allowances
ISO metric screw thread tolerance system provides forallowances and tolerances defined by tolerance grades,tolerance positions, and tolerance classes
Tolerance grades specified numerically
Medium tolerance indicated by number 6, number below 6indicates finer tolerance and number above 6 indicates greatertolerance
Tolerance for thread at pitch line and major diameter may beshown on drawing using the following symbols:
External threads:eindicates large allowance
gindicates small allowance
hindicates no allowance
Internal threads:Gindicates small allowance
Hindicates no allowance
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Tolerance Example:
An external metric thread may be designated as follows:
Pitch Outside
Nominal Dia. Dia.
Metric Size Pitch Tolerance ToleranceM 6 X 0.75 - 5g 6g
The thread fit between mating parts is indicated by
internal thread designation followed by the external
thread tolerance:
M 20 X2 -6H/5g6g
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Symbols Used to Indicate
Allowance For external threads:
eindicates a large allowance
g indicates a small allowance
hindicates no allowance
For internal threads:
G indicates a small allowance
Hindicates no allowance
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Three Categories of Unified
Thread Fits External threads classified as 1A, 2A, and 3A and
internal threads as 1B, 2B, 3B
Classes 1A and 1B Threads for work that must be assembled
Loosest fit
Classes 2A and 2B Used for most commercial fasteners
Medium or free fit
Classes 3A and 3B Used where more accurate fit and lead required
No allowance provided
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Thread Calculations: Example 1
To cut a correct thread on a lathe, it is necessary first to make
calculations so thread will have the proper dimensions.
D=single depth of thread
P=pitch
in..061.100x.61343
x61343.
in.100.10
11
PD
tpiP
Calculate pitch, depth, minor diameter, and width of flat
for a
10 UNC thread.
in.0125.
10
1x
8
1
8flatofWidth
in.628.
.061)(.061-.75
)(-diaMajordiaMinor
P
DD
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Thread Calculations: Example 2
P=pitch= 1 mmD=0.54127 x 1
= 0.54 mm
What are the pitch, depth, minor diameter, width of crest and
width of root for an M 6.3 X 1 thread?
mm125.0
1x125.0
x.1250crestofWidth
mm22.5
.54)(.54-6.3)(-diaMajordiaMinor
P
DD
mm25.0
1x0.25
x25.0rootofWidth
P
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Procedure to Set the Quick-
Change Gearbox for Threading1. Check drawing for thread pitch required
2. From chart on quick-change gearbox, find
whole number that represents pitch in
threads per inch or in millimeters
3. With lathe stopped, engage tumbler lever
in hole, which is in line with the pitch
4. Set top lever in proper position as
indicated on chart
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5. Engage sliding gear in or out as required
6. Turn lathe spindle by hand to ensure that
lead screw revolves
7. Recheck lever settings to avoid errors
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Thread-Chasing dial
Lathe spindle andlead screw must bein same relative
position for each cut Thread-chasing dial
attached to carriage forthis purpose
Dial has eight divisions
Even threads use any division
Odd threads either numberedor unnumbered: not both
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Thread Cutting
Produces a helical ridge of uniform section
on workpiece
Performed by taking successive cuts with
threading toolbit of same shape as thread
form required
Work may be held between centers or inchuck
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Procedure to Set Up a Lathe for
Threading (60 Thread)1. Set lathe speed to speed used for turning
2. Set quick-change gearbox for required pitchin threads per inch or in millimeters
3. Engage lead screw
4. Secure 60 threading toolbit and check
angle using thread center gage5. Set compound rest at 29 to right; set to left
for left-hand thread
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6. Set cutting tool to height of lathe center
point7. Mount work between centers
Make sure lathe dog is tight on work
If work mounted in chuck, it must be heldtightly
8. Set toolbit at right angles to work, using
thread center gage9. Arrange apron controls to allow split-nut
lever to be engaged
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Thread-Cutting Operation
Procedure to cut a 60 thread
1. Check major diameter of work for size
2. Start lathe and chamfer end of workpiece
with side of threading tool to just below
minor diameter of thread
3. Mark length to be threaded by cutting
light groove at this point with threading
tool while lathe revolving
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4. Move carriage until point of threading tool
near right-hand end of work5. Turn crossfeed handle until threading tool
close to diameter, but stop when handle is at
3 o'clock position
6. Hold crossfeed handle in this position and
set graduated collar to zero
7. Turn compound rest handle until threading
tool lightly marks work
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8. Move carriage to right until toolbit clears
end of work
9. Feed compound rest clockwise about .003 in.
10. Engage split-nut lever on correct line of
thread-chasing dial and take trial cut along
length to be threaded
11. At end of cut, turn crossfeed handle
counterclockwise to move toolbit away from
work and disengage split-nut lever
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12. Stop lathe and check number of tpi with
thread pitch gage, rule, or center gage
13. After each cut, turn carriage handwheel to
bring toolbit to start of thread and return
crossfeed handle to zero
14. Set depth of all threading cuts with compound
rest handle
See Table 55.2 and Table 55.3
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When tool is fed in at 29, most of the cutting
is done by the leading edge of toolbit.
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Table 55.3: Depth setting for cutting 60
ISO
metric threadsCompound Rest Setting
Pitch (mm)
(mm) 0 30 29
0.35 0.19 0.21 0.21
0.4 0.21 0.25 0.24
0.45 0.24 0.28 0.27
0.5 0.27 0.31 0.310.6 0.32 0.37 0.37
0.7 0.37 0.43 0.43
0.8 0.43 0.5 0.49
Portion of table taken
from textbook
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Table 55.2 Depth settings for cutting 60
national form threads*
Compound Rest Setting
tpi 0 30 29
24 .027 .031 .0308
20 .0325 .0375 .037
18 .036 .0417 .041
16 .0405 .0468 .046
14 .0465 .0537 .0525
13 .050 .0577 .057
11 .059 .068 .0674
Portion of table taken
from textbook
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15. Apply cutting fluid and take successive
cuts until top (crest) and bottom (root) ofthread are same width
16. Remove burrs from top of thread with file
17. Check thread with master nut and take
further cuts
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Six Ways to Check Threads
Depends on accuracy required:
1. Master nut or screw
2. Thread micrometer
3. Three wires
4. Thread roll or snap gage
5. Thread ring or plug gage
6. Optical comparator
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To Reset a Threading Tool
Must reset when need to remove partlythreaded work from lathe, tool removed forregrinding, or work slips under lathe dog
Procedure
1. Set up lathe and work for thread cutting
2. Start lathe, toolbit clear of work, engage
split-nut lever on correct line3. Allow carriage to travel until toolbit
opposite any portion of unfinished thread
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4. Stop lathe, leaving split-nut lever engaged
5. Feed toolbit into thread groove using onlycompound rest and crossfeed handles until
right-hand edge of toolbit touches rear side
of thread6. Set crossfeed graduated collar to zero
7. Back out threading tool using crossfeed
handle, disengage split-nut lever, andmove carriage until toolbit clears start of
thread
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8. Set crossfeed handle back to zero and take
trial cut without setting compound rest
9. Set depth of cut using compound rest
handle and finish thread to required depth
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To Convert an Inch-DesignedLathe to Metric Threading
Metric threads may be cut on standard quick-
change gear lathe by using a pair of change gears
having 50 and 127 teeth respectively. Lead screwhas inch dimensions so necessary to convert pitch
in millimeters to centimeters and then into threads
per inch
1 in. = 2.54 cm which gives ration 1:2.54
teeth127
teeth50
50
50x
54.2
1
(spindle or drive gear)
(lead screw)
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Procedure to Cut a 2-mm Metric Thread on
Standard Quick-Change Gear Lathe
1. Mount the 127-tooth gear on lead screw
2. Mount the 50-tooth gear on spindle
3. Convert the 2-mm pitch to threads per
centimeter:
cmP
cmmm
thread/52
10
110
4. Set quick-change gearbox to 5 tpi
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5. Set up lathe for thread cutting
6. Take light trial cut7. Reverse spindle rotation until cutting tool
has just cleared start of threaded section
8. Check thread with metric screw pitch gage
9. Cut thread to required depth
Note: Never disengage the split nut untilthe thread has been cut to depth.
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Procedure to Cuta Left-Hand Thread (60)
Basically same as for right-hand threads,
with few exceptions:
1. Set lathe speed and quick-change gearboxfor pitch of thread to be cut
2. Engage feed-direction lever so that lead
screw will revolve in opposite direction tothat for right-hand thread
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3. Set compound rest to 29 to the LEFT
4. Set up left-hand threading tool and squareit with work
5. Cut groove at left end of section to be
threaded
Gives cutting tool starting point
6. Proceed to cut thread to same dimensions
as for right-hand thread
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Cutting a Thread on
a Tapered Section
Use same setup as for regular thread cutting
Most important that threading tool be set at
90 to axis of work
Do NOT set it square with tapered surface
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Common Methods ofMeasuring Threads
1. Thread ring gage
2. Thread plug gage
3. Thread snap gage
4. Screw thread micrometer
5. Optical comparator6. Three-wire method
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Three-Wire Method ofMeasuring Threads
Recommended by Bureau of Standards andNational Screw thread Commission
Three wires of equal diameter placed in thread,two on one side and one on other side
Wire used should be hardened and lapped to three timesaccuracy of thread to be inspected
Standard micrometer used to measure distanceover wires (M)
Different sizes and pitches of threads requiredifferent sizes of wires
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The Three-Wire Method ofMeasuring 60 Threads
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Permission required for reproduction or display.
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To Calculate theMeasurement over the Wires
NGDM
5155.13
where M=measurement over the wires
D=major diameter of the thread
G=diameter of the wire size used
N=number of tpi
Any of the following formulas can be used to calculate G:
PN
PN
PN
.505or.505ireSmallest w
.57735or.57735
wiresize-Best
1.010or
010.1
reLargest wi
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Multiple Threads
Used when necessary to obtain increase in leadand deep, coarse thread cannot be cut
May be double, triple or quadruple, depending on
number of starts around periphery of work Pitch: distance from point on one thread to
corresponding point on next thread
Lead: distance nut advances lengthwise in one
complete revolution Single-start: pitch and lead equal
Double-start: lead twice the pitch
Triple-start: lead three times pitch
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Single ThreadDouble Thread
Triple ThreadCopyright The McGraw-Hill Companies, Inc.Permission required for reproduction or display.
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Multiple Threads Maybe Cut on a Lathe
1. Using accurately slotted driveplate or
faceplate2. Disengaging intermediate gear of end gear
train and rotating spindle desired amount
3. Using thread-chasing dial Only for double-start threads with odd-number
lead
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Procedure to Cut an 8-tpiDouble Thread
1. Set up lathe and cutting tool as for cuttingsingle-start thread
2. Set quick-change gearbox to 4 tpi3. Cut first thread to half depth required for 4 tpi
4. Leave crossfeed handle set to depth of
thread and not reading on compound restgraduated collar
5. Withdraw threading tool from work usingcompound rest handle
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6. Revolve work exactly turn
Remove work from lathe with lathe dogattached
Replace work in lathe with tail of dog in slot
exactly opposite one used for first thread
7. Reset crossfeed handle to same position as
when cutting first thread
8. Cut second thread, feeding compound rest
handle until graduated collar at same
setting as for first thread
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Thread-Chasing Dial Method of
Cutting Multiple Threads Double-start threads with odd-numbered
lead may be cut using thread-chasing dial
1. Take one cut on thread by engaging splitnut at numbered line on chasing dial
2. Without changing depth of cut, takeanother cut at unnumbered line on chasing
dial (second thread will be exactly inmiddle of first thread)
3. Continue cutting thread to depth, takingtwo passes for every depth-of-cut setting
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Square Threads
Found in vise screws,jacks, and other deviceswhere maximum powertransmission required
Square threading toollooks like short cutting-off tool
Both sides of squarethreading tool must be
ground at angle toconform to helix angleof thread
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Square Threading Tool
Helix angle of thread (angle of square threadingtool) depends on two factors:
Changes for each different lead on given diameter
(greater lead, greater angle) Changes for each different diameter of thread for given
lead (larger diameter, smaller angle)
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T C l l t th H li A l f th
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To Calculate the Helix Angles of the
Leading and Following Sides of a
Square Thread
diametermajorofncecircumfere
threadofleadanglefollowingTan
diameterminorofncecircumfere
threadofleadangleleadingTan
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Clearance
Square toolbit must beprovided with approximately1 clearance on each side
Makes it thinner at bottom
For leading side of tool
Add 1 to calculated helixangle
On following side
Subtract 1 from calculatedangle
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Procedure to Cut Square thread
1. Grind threading tool to proper leading andfollowing angles Width of tool should be ~.002 in. wider than
thread groove
2. Align lathe centers and mount work
3. Set quick-change gearbox for required
number of tpi4. Set compound rest at 30 to right, whichwill provide side movement if necessary toreset cutting tool
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5. Set threading tool square with work and
center6. Cut right-hand end of work to minordiameter for approximately .060 in. long
7. If work permits, cut recess at end of threadto minor diameter
8. Calculate single depth of threadN
500.
9. Start lathe and just touch tool to workdiameter
10. Set crossfeed graduated collar to zero
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11. Set .003 in. depth of cut with crossfeed
screw and take trial cut12. Check thread with thread pitch gage
13. Apply cutting fluid and cut thread to depth,moving crossfeed in from .002 to .010 in.for each cut Depth of cut will depend on thread size and
nature of workpiece
Note: Since thread sides are square, allcuts must be set using crossfeed screw
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Acme Thread
Gradually replacing square thread because
stronger and easier to cut with taps and dies
Used extensively for lead screws because29 angle formed by its sides allows split nut
to be engaged during thread cutting
Provided with .010-in. clearance for bothcrest and root on all sizes of threads
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Procedure to Cut an Acme Thread
1. Grind toolbit to fit
end of Acme thread gage
2. Grind pointof tool flat until
it fits into slots
of gage
Indicates number
of threads per
inch to be cut
Copyright The McGraw-Hill Companies, Inc.Permission required for reproduction or display.
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3. Set quick-change gearbox to required
number of threads per inch4. Set compound rest 14 to right
5. Set Acme threading tool on center and
square it with work using gage6. At right-hand end of work, cut section
.060 in. long to minor diameter
7. Cut thread to proper depth by feedingcutting tool, using compound rest
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Measuring Acme Threads
One Wire Method Single wire or pin of correct diameter placed inthread groove and measured with micrometer
Thread correct size when micrometer reading over
wire is same as major diameterof thread and wire is tightin the thread
Diameter of wire to be
used can be calculated
as follows:
Wire diameter = .4872 x pitch
Copyright The McGraw-Hill Companies, Inc.Permission required for reproduction or display.
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Internal Threads
Cutting threads in a hole
Operation performed on work held in chuckor collet or mounted on faceplate
Threading tool similar to boring toolbit exceptshape ground to form of thread to be cut
Procedure
1. Calculate tap drill size of thread
N
1-diametermajorsizedrillTap
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2. Mount work to be threaded in chuck orcollet or faceplate
3. Drill hole approximately 116 in. smallerthan tap drill size in workpiece
4. Mount boring tool in lathe and bore hole totap drill size
5. Recess start of hole to major diameter ofthread for .060-in. length
6. If thread does not go through workpiece, a
recess should be cut at end of thread tomajor diameter; recess should be wideenough to allow threading tool to run outand permit time to disengage split-nut lever
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The compound rest is set at 29 to the left
for cutting right-hand internal threads.
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7. Set compound rest at 29to left, to right forleft-hand threads
8. Mount threading toolbit into boring bar andset it to center
9. Square threading tool with thread center
gage10. Place mark on boring bar, measuring from
threading tool, to indicate length of hole tobe threaded
11. Start lathe and turn crossfeed handle outuntil threading tool just scratches internaldiameter
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7. Set crossfeed graduated collar to zero
8. Set .003-in. depth of cut by feeding
compound rest outand take trial cut9. At end of eachcut on internal thread,
disengage split-nut lever and feed crossfeedhandle into clear thread
10. Clear threading tool from hole and checkpitch of thread
11. Return crossfeed handle back to zero and set
depth of cut by turning compound rest outdesired amount
12. Cut thread to depth; check fit with a screwor threaded plug gage