Thread Cutting Operation on Lathe
Threads can be cut on a cylindrical surface by forming or
machining process Thread cutting by the machining process is done using a
lathe. Threads of any pitch, shape and size can he cut on a lathe. The
following figure shows the set-up of a lathe for thread cutting.
Thread cutting operation is done on a lathe using a
single-point tool called thread cutting tool. The workpiece is held between centres or in a
chuck and the tool is held in tool post. For producing threads of pitch p mm, the tool must
travel a distance equal to p [mm] as the workpiece makes one complete rotation.
The definite relative rotary and linear motion between the workpiece and tool
is achieved by locking or engaging the carriage with the lead screw through a screw
and nut mechanism and fixing a gear ratio between the headstock spindle and
lead screw. This is done by using change gear mechanism or gear box between the
spindle and lead screw. To cut the threads, the tool is brought to the start of
the workpiece and a small depth of cut is given to the tool using cross-slide. The
carriage is engaged with the lead screw, the cut is made on the entire surface
and at the end of the workpiece, carriage is disengaged. The tool is pulled out
of the job and brought back to the starting position. The process is repeated
until the full depth threads are obtained. The following relationship is used
to determine the gears/wheels required to generate threads of definite pitch.
All the lathes are provided with a set of change gears,
usually having 20 to 120 teeth, with a variation of 5 teeth. In addition, a
gear of 127 teeth is also provided which is known as translating gear. It is
used in cutting metric threads. After the determination of above gearing ratio,
the next step to follow is to multiply the numerator and denominator of the
fraction by the same number in order to find out the number of teeth of the
change gears.
Gear trains:
The following two types of gear trains are employed in
cutting threads,
• Simple gear train
• Compound gear train
Simple gear train:
A simple gear train consists of a driving gear [mounted on
the stud], a driven gear [mounted on the lead screw] and one or two
intermediate gears. The intermediate gears are known as idle gears, have no
effect on the speed ratio but are used only to fill up the gap between the
driver and driven gears, and to obtain a desired direction of rotation of the
lead screw.
Compound gear train:
A compound gear train consists of two studs instead of one.
The second stud is suitably mounted on the bracket or quadrant carrying the
change gears. Evidently there are two drivers and two driven gears. The first
driver '1' is mounted on the first stud, which meshes with the first driver '2'
on the second stud. The second driver '3' is also mounted on the second stud and
it meshes with the second driven '4' mounted on the lead screw. Such a gear
train is employed when the desired gear ratio is such that it is not possible
to arrange a simple gear train out of the given set of change gears.
Gear Train for Screw Cutting:
Types of Threads:
Gear train for cutting metric threads on a lathe with
English lead screw:
Pitch of lead screw is given in inches. An additional gear
with 127 teeth is incorporated in gear train.
For Multi Start Threads,
n=number of starts
pL = Pitch of Lead screw
pC = Pitch to be cut
Note :
Screw cutting calculations:
Modern lathes are fitted with change gear boxes. Using these
gear boxes we can select the required rpm for the lead screw for thread
cutting. But in ordinary centre lathes these gear boxes are not available. So
it is necessary to calculate and arrange the change gears to get the required
rpm for the lead screw.
Example 1
The pitch of lead screw is 6 mm. The pitch of the thread to
be cut on the work piece is 3 mm. Find the change gears.
Solution:
A simple gear train is used:
Lead screw gear has 60 teeth
Spindle gear has 30 teeth.
Example 2
Find the change gears for cutting a thread of 1.6 mm pitch
on a lathe having a lead screw of 6 mm pitch.
Solution:
Compound gear train is used:
80 teeth gear [spindle gear] drives 100 teeth gear. 100
teeth gear and 20 teeth gear are on the same shaft. They are called
intermediate gears. 20 teeth gears drives 60 teeth lead screw gear.
Example 3
Find the change gears to cut 16 T.P.1 thread on a lathe with
a 6 T.P.l lead screw.
Solution:
END
1 Comments
2tpi lead to cut 120mm pitch
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