Lathe workholding: face drivers
If you work in turning, you’ve seen parts which use center drilled
holes. These holes are drilled on the part centerline and are used
in turning and grinding processes.
For shaft-type pieces, the part centerline
is often a datum and other features are toleranced from the
centerline axis. These center drilled holes also provide a physical
feature from which the part can be measured, especially for
straightness and concentricity.
Centers often have no function in the final
use of the part; they are only used to help manufacture the piece.
When machining a shaft part, the first step
is often turning; grinding and gear cutting sometimes follow. The
center drilled holes are machined during the turning operation to
assist with the subsequent grinding and gear cutting operations.
Some people produce the turned shaft by
putting the part blank into a lathe and turning one side at a time.
The first half is faced, drilled and turned. It is then taken out of
the chuck, turned around and the second half is also faced, drilled
and turned. This process can result in many problems.
This two-step turning process allows
inaccuracies to enter the process. Moving the part from the chuck
eliminates the continuity of the machining process. Parts can be
eccentric and holding lengths can be a problem.
Face drivers help to offset these
challenges. Face drivers are used to help hold a shaft part in such
a way that the entire turning process can be accomplished in one
operation.
A face driver is a device that grabs onto a
part face to provide a superior workholding condition. It uses a
spring-loaded center as well as carbide insert “teeth” to
penetrate into the part face.
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The part is set between the face driver’s spring-loaded center and
a tailstock center in the machine tool. The part is programmed to
machine from the tailstock end toward the end held by the face
driver. By turning is this direction, the force of the first cut
pushes the part into the face driver.
This force ensures the inserts grip the
part securely. Once the first cut is made, the shaft is well seated
on the driver.
With the shaft blank held securely, the
part can be machined in a single turning operation. All machining is
completed with a single set-up, providing more accurate lengths and
diameters.
Process Considerations
There are a few points to remember when
using a face driver.
- The feed rates must be reduced when “back” turning, away
from the spindle.
- The diameter of the face driver should be a least 1/2 the
diameter of the blank shaft diameter. This 2-1 ratio is a useful
rule of thumb.
- The face driver can be bolted directly to the machine spindle
or held in bored soft jaws.
Use of face drivers is common in
production of motor shaft parts, and other shafts up to 40"
long.
I know what you’re thinking – “The
shaft blank still needs center drilled holes for use of a face
driver. How do you easily face and drill each end of the blank?”
There are special purpose machines that
perform this operation very efficiently. These machines easily face
each end of the blank as well as putting in center drilled holes.
The blank is held in twin vises and twin in-feed machining heads
face and drill centers in each end.
This may seem like a lot of work and there
are still two machining operations (facing and drilling on one
machine, then turning on another), what’s the benefit?
Companies that manufacture shaft type parts
in large quantities find this two-step process more efficient and
more accurate then the old method of turning each side separately. A
facing and centering machine is quick and easy and the increased
accuracy of a single turning operation makes using a face driver an
efficient method for shaft production.
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