Screw Cutting
Standard and
non-standard threads can be cut on capstan and turret lathe. All types of
standard threads can be cut on this machine with the help of collapsible taps
and self-opening die heads. These machines are developed mainly for production
work. So most of the jobs produced contain standard threads. Single point
cutting tools are also used for thread cutting on capstan and turret lathes.
These are used in conjunction with leaders and followers. These are used to cut
a special or large diameter screws (non standard) thread only.
(a) Collapsible Tap - Threads of large
diameter are not generally cut with solid tap but with collapsible tap. This
consists of a body with slots for carrying the blades. Inside the body there is
a plunger with a taper on the end of which supports the blades in their
operating position. A trip ring, which may be set at a suitable longitudinal
position, is connected to inner plunger and the action of this ring arrests the
inner plunger whilst the blade and body moves forward. This removes the taper
support from the blade bases allowing them to collapse so that the tap may be
withdrawn without stopping or reversing the machine. It is used when the
diameter exceeds 1”. The range is 1 ¼ to 3 ¾” inches.
(i) Alt Collapsible Taps
Five
standard tap bodies and 33 standard tap heads enable ALT taps to produce
1-1/4" to 13-1/4" I.D. and 1-1/4" to 12" nominal NPT and
BSTP pipe threads. Recommended where wide range of work requires the
versatility and wide range coverage offered by detachable tap heads.
(ii) LL Receding Chaser Collapsible Taps
A receding
chaser type tap primarily intended to produce longer length American Petroleum
Institute (API) threads. Can also be used to cut NPT and BSTP taper pipe as
well as straight threads. Four bodies with 26 standard tap heads allow 1"
to 12" nominal pipe sizes to be tapped.
(iii) 2 LLSR Collapsible Taps
A receding
chaser type tap for use where overall length restrictions prevent the use of
the longer LL Tap. The 2 LLSR's overall length is 10-1/16" regardless of
the tap head size used. The 2 LLSR tap body can be supplied with 1-1/4",
1-1/2", and 2" size heads to produce threads on 1-1/4", 1-1/2",
and 2" NPT, BSTP, and API line pipe, without chaser leave-off marks.
(b) Self-Opening Die Head and Dies - This
attachment is capable of holding the standard dies in position and is used to
machine external standard threads. Figure below shows Self-Opening Die Head.
Dies may be fitted in self opening type of holders.
The self-opening
type die holder is used for cutting an exact length of thread, and where quick
removal of die head is necessary to reduce the production time. When the
desired length of thread has been cut, the die head opens, thus clearing the
die pieces from the thread and permitting the die head to be withdrawn without
stopping or reversing the spindle rotation.
The die pieces are
four in number for a particular dimension, which are fixed in die head slots
provided for them consecutively in clockwise direction. Particular care should
be taken in selecting the die pieces size, serial number and the type of
thread. In operation the head is partially rotated to close the dies and then
fed on to the work end with a light pressure. The range of dies pieces
available is ¼” to 4 ½” diameters. The following table shows
the recommended cutting speeds in feet per minute for die heads.
Recommended
Speed for Various Materials
Material
|
Speed
Ft/min
|
Tough
steel
|
5-8
|
General mild steel
|
10-20
|
Free
cutting steel
|
25-50
|
Cast
iron
|
10-15
|
Brass
and copper
|
as
per turning
|
Leaders and Followers - Most the screw cutting on these
machines is accomplished by means of taps and dies. But where large or
non-standard screws are required to be made, it is not possible to cut with the
tap or die. In such cases leaders and followers are used. The leader is a short
lead screw and the follower consists of two bronze half nuts which are held
firmly in place by cam. Cam is operated by a light pressure on a conveniently
located hand lever.
Operations Carriedout on Turret Lthe Machine by qhuntblog
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