$4 Ball Link Z Axis Backlash Adjuster
for Sherline Mills
By Robert Anderson
Though
Sherline equips their milling machines with a backlash adjustment
mechanism for the X and Y axis there is no such adjustment for the Z axis. For
the manual mills this has not been a significant problem because in most
operations you would set the Z axis to the cutting height that you desire and
then use the locking lever on the Z axis lead screw to lock the headstock which
effectively eliminates backlash on the Z axis. With the advent of affordable CNC
milling and many users adding CNC to their systems where the Z axis is no longer
set to a specified depth and then locked, backlash on the Z axis became an
important consideration. In fact, starting in 2004, Sherline actually removed
the Z axis lock on their CNC equipped machines to avoid situations where the
user would lock the Z axis and then forget to unlock it during CNC operations
(However, I have heard that Sherline has added the Z axis lock back to their CNC
machines but have provided it with their Z axis backlash upgrade).
Users quickly figured out that the Z lock mechanism could be modified to serve
to adjust backlash if it could be held securely just short of the locked
position. Sherline has since added their version of a modified Z lock mechanism
which does just that to CNC and CNC-ready systems available direct from Sherline
which utilizes the Z lock lever as a backlash adjuster...this modified Z lock
mechanism is available as an upgrade from Sherline for $35. You can view the
modification at the Sherline web site...http://www.sherline.com/4017Zinst.htm.
After converting my Sherline mill to CNC operations years ago, I simply
ignored the issue of backlash on the Z axis because I seldom needed the Z axis
precision and merely used the CNC to cut 2D shapes in flat parts that only required that I
merely cut deep enough to cut through the part. As I moved on to more
sophisticated operations which involved milling at various depths I worked
around the problem by setting the gibs on the headstock saddle loose enough so
that gravity always allowed the weight of the headstock to pull any Z axis slack
from the system. This worked fine so long as I was milling soft materials which
did not "push back" during milling operations (until recently, I worked almost
exclusively in non-metallic materials such as plastic, wax, and resins) this
worked just fine. However, one day I had the need to cut a moderately contoured
metal part and just a test run proved that running a "loose" Z axis and relying
on gravity to control Z axis slack and backlash just would not work. I knew
about the Sherline upgrade but didn't want to wait while the upgrade was ordered
and shipped. So, to finish the part, I resorted to holding the Z axis lock lever
just short of its locked position by hand while the part was milled. Not a
desirable way to do things but it worked for the one part and started me
thinking of a quick way to modify the Z axis lock into a backlash adjuster.
Being a model builder and having a stock of components for setting up
linkages in radio-controlled models, I found a couple of "ball links" in my
supplies bins. Ball links are basically small devices which allow the connection
of the control pushrods to the various movable surfaces of model airplanes
without slack or looseness in the linkage while allowing a very wide range of
motion. The ball link looks and works just like the ball-hitch you find on the
rear bumper of cars or trucks for towing trailers. The particular ball links
that I selected are from Du-Bro (
www.dubro.com/hobby ), part number 181. They are available from a variety of
suppliers of R/C model supplies and just about every hobby shop that caters to
the radio control enthusiast will have them in stock for about $1.50 each. These
are not the only ball links that are potential candidates for this modification.
The number 181 ball links are very small but Du-Bro has a variety of ball links
that can work just as well. Since the number 181 ball links are what I had and
used (and have performed very well), they will be the basis of the instructions
for this modification.
The only modification that you will need to perform physically to the mill
will be the drilling of two small holes...one on the Z axis saddle and one on
the Z axis lock lever. The mill does not need to be disassembled for this
operation (particularly if you use a small rotary tool such as a Dremel Moto
Tool to drill
the holes) and the modification does not affect the manual operation of the
machine or the ability to quickly lock the Z axis if you so desire.
You will need two ball link kits and a short length (about 2") of either
1/16" diameter steel rod or 2-56 threaded rod (the threaded rod will be
available at the hobby shop or from Du-Bro for less than $1 for a one-foot
length and is preferred over using steel rod as you will see). If you really
want to be "mechanically correct" you will need a 2-52 tap and drill set (also
available from Du-Bro or your hobby shop as Dubro part # 360 for about $6).
However, if you don't have (or can't quickly acquire) the tiny 2-56 tap you can
make do with just a 5/64" or #47 drill bit. I'll confess that when I did my
modification that I did not have the tap and just used a #47 drill bit.
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Contents of the Ball Link kit: Wire coupler (top), washer and nut
(middle), ball coupler and nylon link (bottom). For an installation using a
short length of 2-56 threaded rod you will only need the ball coupler and nylon
link.
The ball link kits come with one steel ball coupler (which has an integral
2-56 mounting stud), a nylon link which snaps over the ball, a brass wire
coupler that has a 2-56 thread on one end, a washer, and a 2-56 nut. If you have
decided to use 2-56 threaded rod you will not need the wire coupler or the
washer and nut. If you have decided to use unthreaded 1/16" rod you will need to
retain the kit's wire coupler.
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Locking Lever drilled and with the ball coupler installed.
Begin your modification by drilling through the end of the Z axis locking
lever parallel with the Z axis lead screw using the #47 drill. If you want to be
"mechanically correct" drill through and tap with the 2-56 tap. Screw the ball
coupler into this hole (if you just used a #47 drill, the hole will be slightly
undersized and the ball coupler will "self tap" into the hole...just to be sure,
I wicked in a bit of thin CA glue [cringe!] to secure my ball coupler).
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Z axis saddle drilled and with ball coupler installed
Drill a second hole (either with the #47 drill or with the 2-56 tap and drill
set depending on your preference) in the Z axis column saddle about 1/4" from
both the back right corner and the back left corner of the saddle. Drilling a
hole near each corner gives you the option of using either corner as the anchor
location for you adjuster though only one can be used at a time. Thread the
second ball coupler into one of the corner holes (select the corner hole which
gives the shortest distance to the locking lever ball coupler when the locking
lever is in the locked position).
Snap a nylon link over each of the ball couplers.
Cut a length of the 2-56 threaded rod that is adequate to reach from nylon
link to nylon link plus between 3/16" and 1/4" extra to thread into the nylon
link. (If you decide to use the brass wire couplers, you will need to thread the
couplers about half way into each nylon link and then cut a length of 1/16 rod
to span between the two wire couplers. You will need to solder the wire couplers
to each end of the 1/16 rod. The use of a threaded rod, in my opinion, is
preferable to using the wire couplers that come with the ball link kit.) Thread
a nylon link onto each end of the threaded rod.
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Adjustment is quick and easy... Just unsnap one end of the link and thread
the nylon coupler in or out to adjust backlash. It's easy to get a consistent
and maintainable adjustment of .002" backlash.
It's obvious now how this works. You simply snap a nylon link over the ball
on the locking lever and the other over the ball on the saddle. To adjust the
locking lever to being just short of "locked", the position which will remove
almost all backlash (you can adjust backlash to as little as .001"), you simply
unsnap one of the ball links from its ball coupler and screw it in or out on the
threaded rod to fine tune the length.
I was able to perform my modification in about 15 minutes (I took my
time...with these instructions you can do it in five or ten minutes) and it
required no disassembly of the mill. You will get more room to work and drill
the required holes if you run the headstock all the way to its lowest position
before beginning. I performed this modification over a year ago and have done
many milling operations with no issues. As the Z axis led screw, lead screw nut,
and axis lock wear over time, you can swing the end of the link you have just
made over to the opposite side of the saddle (the reason for drill two holes in
the saddle). You may think that the tiny nylon links are either too weak or
themselves have too much "slack" but this is not the case. If you are a bit
worried about this kind of thing, your hobby shop (or Du-Bro) has heavier duty
links that are larger (if you so desire) and some that are all-metal (overkill,
in my opinion).
E-mail with suggestions or questions