Use
of Diaphragm Chucks
Most
of us are familiar with the standard 3-jaw chuck used for either
first or second turning operations.
This type of chuck has a sliding master jaw that moves to
grip the part.
This
chucking method is adequate for first operation work from a saw cut
blank or a forging/casting material.
However, using this type of sliding jaw chuck for second
operation turning may not be as easy.
Producing
a true and concentric second operation can be difficult when holding
the part with a standard sliding jaw chuck.
What options are there?
A diaphragm chuck.
Diaphragm
chucks are similar to old-fashioned diaphragm oil cans. The machine
draw bar pushes the face of the diaphragm and bends the plate
forward. This results in
an outward movement of the jaws, sometimes as much as 1/8,
allowing the jaws to open 0.080 in clearance.
Note
on this sketch that the face of the chuck is pushed (bent) open,
allowing the part to be loaded.
When the stroke of the draw bar is reversed the chuck returns
to its natural state and grips the part.
Diaphragm
chucks are a good choice for more difficult applications where
repeatability and minimum concentricity characteristics are a must.
Standard sliding jaw chucks often wear, resulting in loss of
performance especially in regard to concentricity.
I
have always been a fan of the diaphragm chucks produced by Woodworth
Chucks, although this type of chuck is available from several
sources.
Installing
the Chuck
When
initially mounted, the master ring is ground true with a grinding
attachment. This ensures the best conditions for achieving
consistent, near perfect concentricity.
Hardened
pads on the jaws grip the individual part to the required size.
These hardened insert pads are located on the chuck by means of the
ground master location ring. The
pads are manufactured to high quality standards, further ensuring
the inherent quality of the work holding method.
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This
type of chuck provides a pull-back effect; as the part is gripped, the
chuck pulls the part back against the location face.
The
sketch illustrates a 7 long part approximately 4.00 in diameter.
The chuck achieves a maximum speed of 2500 RPM and the grip
length is only 1.25 long. This
part is hanging out a long way and spinning pretty fast -- but the
chuck provides a secure gripping performance considering the forces
that affect the part.
By
its very nature this type of chuck is designed for second operation
type work. The machining process must be controlled to limit the
amount of facing stock to be removed.
A
rule of thumb of 0.080 maximum facing stock works well.
There should be no situation where a greater amount of facing
stock is needed. Remember,
the most dangerous point in a facing operation is when the maximum RPM
is reached with the tool at centerline.
Disadvantage
of Diaphragm chuck
There
is always a down side, so lets consider a disadvantage of the
diaphragm chuck. As the
jaws open and close, the face of the diaphragm chuck is consistently
flexing. The net result is
potential metal fatigue as the steel eventually starts to show
hairline cracks.
Some
work holding companies make the flexible device from a forged blank
instead of a saw-cut billet. The
idea is that a forging has a longer life due to the internal grain
flow of the material.
The
flexible component of the chuck eventually starts to fail. The
manufacturer should be able to provide guidance regarding the number
of cycles that can be expected prior to failure.
An
inspection procedure should be developed and used to regularly look
for cracks in the face of the diaphragm.
In my experience, it is only a matter of time.
The life of the diaphragm is a serious factor to consider when
selecting the type of chuck for your application.
The
use of this type of work holding provides consistent repeatability and
great concentricity. The application shown in the sketch has been in
service consistently for over 15 years.
This might be a good method if you have a difficult second
chucking operation.
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