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Holding Workpieces Between Centers.

 
  
 


MILLING MACHINE OPERATIONS - OD1644 - LESSON 1/TASK 1
(4)
Holding Workpieces Between Centers.  The indexing fixture is used to
support workpieces which are centered on both ends.  When the piece has been
previously reamed or bored, it may be pressed upon a mandrel and then
mounted between the centers, as with a lathe.
(a)
There are two types of mandrels that may be used for mounting
workpieces between centers.
The solid mandrel is satisfactory for many
operations, while the mandrel having a tapered shank is preferred when
fitting the workpiece into the indexing head of the spindle.
(b)
A jack screw is used to prevent springing of long slender workpieces
held between centers, or workpieces that extend some distance from the
chuck.
(c)
Workpieces mounted between centers are  fixed to the  index head
spindle by means of a lathe dog.
The bent tail of the dog should be
fastened between the setscrews provided in the driving center clamp in such
a manner as to avoid backlash and prevent springing the mandrel.
When
milling certain types of workpieces a milling machine dog may be used to
advantage.
The tail of the dog is held in a flexible ball joint which
eliminates springing or shaking of the  workpiece and/or the dog.
The
flexible ball joint allows the tail of the dog to move in a radius along the
axis of the workpiece, making it particularly useful in the rapid milling of
tapers.
(5)
Holding Workpieces in a Chuck.
Before screwing the chuck to the
index head spindle, it should be cleaned and all burrs removed from the
spindle or the chuck.
Burrs may be removed with a smooth cut, three-
cornered file or scraper.  Cleaning should be accomplished with a piece of
spring-steel wire bent and formed to fit the angle of the threads, or by the
use of compressed air.  The chuck should not be tightened on the spindle so
tightly  that  a  wrench  or  bar  is  required  to  remove  it.
Cylindrical
workpieces, held in the universal chuck, may be checked for trueness by
using a test indicator mounted on a base which rests on the milling machine.
The  indicator point  should  contact  the  circumference of  small  diameter
workpieces, or the circumference and exposed face of large diameter pieces.
While checking for trueness, the workpiece should be revolved by rotating
the index head spindle.
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