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Bridgeport Boss head teardown
micrio (the at sign) mv (a dot)
I picked up an old Bridgeport Boss head. I believe that it is a Boss 3 because the
stepper motor cooling fins are horizontal rather than vertical as all the newer
versions had. This is the first
generation of Bridgeport CNC machines. I believe that the basic Boss Z axis drive remained the same up through
the Boss 10. I believe that this drive
is the same as is used in the rigid ram mills even though the housing is different. The Boss Z axis drives have a reputation of
being bullet proof. The Boss head Z
drive was a mystery to be before I took this one apart. If it is a mystery to anyone else then this
article may help.
I intend to replace the head on my
Bridgeport conversion mill with this Boss
head. The mounting flange and bolt
arrangement is identical between the two. The current head on my mill is a manual vari-drive head with a Wesel Z
axis drive unit. I am unhappy with the
backlash of the
unit and hope that the Boss hear will do better. This is my
Bridgeport in my way too cluttered basement.
This is the Wesel Z axis drive.
This is the Boss head that I am going to use. I have removed the motors and most of the
vari-drive to make it easier to handle.
The spindle drive section of Boss mills is very similar
to the vari-drive manual mills. The
principle difference is that the motor is mounted with its drive shaft upward
in the Boss mill. The manual mill had
the motor shaft facing downward. Thus,
the aluminum housings that contain the vari-drive are different even though
they look similar. The forward
vari-drive assembly is the same between the Boss and the manual mills.
I believe that the motor and the vari-drive shives are
the same between the manual mill and the Boss mill. They are different is the way that the sheves
are assembled. In the manual mill the
spring is outermost on the shaft and the fixed sheve is next to the motor. The Boss has the spring next to the motor
and the fixed sheve outermost. I have
assembled both and either one could be assembled for the manual mill or the
Astute observers will notice a bolt on the end of the
shaft. This is a modification that
holds the fixed sheve in place because the shaft and the snap ring grove get
messed up from long use.
I am not going to talk much about the vari-drive since it
as about the same as for the manual mills and is not much of a mystery.
This picture shows separation of the Z axis drive section
from the quill section. As you separate
them be sure that the ball nut does not unscrew from the ball screw. If this happens then all of the small ball
bearings will go flying.
At this point you can remove the quill by pulling it out
with the Z axis drive section.
This is a view of the top of the ball screw. The ball nut is visible around the ball
screw. As you can see there is nothing
to stop the ball nut from coming off the end of the ball screw. If this happens then all the ball bearings
will fall out.
If you choose the pull the quill out with the Z axis
drive then you must remove the quill guide device as shown below. This prevents the quill from rotating and
serves to actuate the limit switches.
If you want to separate the quill from the ball screw,
you must remove the 6 Allen screws that attach the ball screw to the top of the
quill. Be very careful that you don't
let the ball nut unscrew from the ball screw.
Once all the screws are out you can separate the two
This is the bottom of the Z axis drive assembly. The cog pulley that is driven by the stepper
motor is visible.
. At this point I
am pressing the ball screw assembly out of the housing. The large ball bearings the mount the ball
nut can be seen. The ring that holds
the bearings into the housing can be seen below the pulley.
There is the ball screw assembly separated from the
housing. Notice the piece of wood
holding the ball nut up. Without the
wool the ball but would unscrew from the ball screw due to gravity.
This is the inside of the housing. The 4 Allen bolts hold the ball nut into the
I am beginning to slide the cog pulley off the ball
nut. You can see the ball bearing
channels under the pulley. Above the
pulley are two large ball bearings that hold the ball nut in the housing. The ball nut is spun by the Z axis motor and
the ball screw does not spin.
Here I have the cog belt pulley removed from the ball nut. You can see the ball bearing recirculating
channels. These are hidden by the
pulley. The pulley is held in place by
the large snap ring.
I am removing the quill from the lower housing. The lower housing used on the Boss is the
same as is used on the manual mill except that unused features are not
machines. Thus, the Z axis feed areas
exist but are not machined. I believe
that you could put a Boss Z axis drive on top of a manual mill lower housing.
The quills between the Boss and the manual mull are not
exactly the same. On the left is the
quill from a manual mill and on the right is a Boss quill. The manual mill has a rack cut in the quill.
The manual mill does not have the holes for the ball
screw drilled into the top of the quill.
The spindle is longer in the Boss mill. The spindle on the left is a Kwik switch
200. You can see that it is not drilled
for a draw bar. The Boss spindle on the
right is a NMTB 30 that is drilled for a draw bar.
I hope that this helps people understand the Boss Z axis
drive. If you have
any questions please email me.
Jan. 21, 2007
Reprinted with permission. This content is hosted free of charge for the benefit of the machinists community.