05/11/08

$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.

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.

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).

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.

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).

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