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Improving the CG-5 Equatorial Mount

Declination Axis, Part 3

Getting Started
Declination Axis
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Dec shaft and gear
With the worm assembly removed, the worm gear can slide off the Dec shaft. Note that there is a large plastic washer at both ends of the silver worm gear. When I disassembled my CG-5, one washer remained inside the Dec housing, stuck in the thick grease.

Here are the main parts comprising the declination axis. From left to right, we have the Dec housing, Dec shaft, worm and worm gear, setting circle, ring nut, and retaining ring.
  Dec parts

Now that the declination axis is fully disassembled, it's time to clean. Using your solvent of choice, clean the old grease from the components. Pay special attention to the worm, which will require the toothbrush to scrub out the old, glue-like grease. Also use the brush to clean out the teeth in the worm gear.

Dip each plastic washer into the solvent and carefully wipe it clean using a cotton cloth. Do the same with the ring nut and retaining ring. The idea is to remove as much of the old grease as possible, along with dirt and metal shavings left over from the manufacturing process.

Deburring and Polishing
This part of the job is messy and time-consuming, but it's the main reason for the whole project. Take your time, and do it right.

There are numerous metal-to-metal and metal-to-washer bearing surfaces inside the Dec assembly. Run your finger across all of these surfaces, feeling for rough spots. To make the mount operate smoothly, you'll want to smooth out any rough spots you find.

It's sometimes difficult to determine whether or not a particular surface actually comes in contact with another surface. If in doubt, put a small dab of grease on a surface, slip the component into place, and rotate it. Then remove the component and see if the grease has been distributed across the surface. If it has, you can be fairly certain that the surface should be smoothed and polished. You don't want to waste time and effort on surfaces that don't make contact with anything.

Inside my CG-5, there were some very rough, almost jagged burrs. I used a sharpening stone to file them as smooth as possible.

Next, I used the Crocus cloth to polish all of the bearing surfaces. I spent a lot of time at this, perhaps more than necessary. I used small pieces of Crocus cloth so that I could reach into various nooks and crannies.

In my mount, some of the plastic washers had been roughened by contact with jagged metal. I used a hobby knife to gently scrap and shave off all rough spots I found on the washers. I was able to get them quite smooth. If you do this, use care not to cause further damage.

When everything is smoothed and polished to your satisfaction, clean the parts with your solvent and a clean cotton cloth.

Applying Grease
You want a thin, even coat of grease on all bearing surfaces. As a non-expert in such matters, I can only guess whether I applied the correct amount of grease to my mount.

Apply grease to both sides of the two large washers that go on opposite ends of the worm gear. Put a few evenly-spaced dabs of grease on both sides of each washer. Grease the portions of the Dec shaft that bear on the inside of the worm gear. Slide the first large washer, then the gear into place on the Dec shaft. Place the second large washer into its location in the Dec housing.

Next, grease the bearing surfaces on the bottom end of the Dec shaft. Grease the entire outside of the worm gear (or the entire inside of the Dec housing). Make certain the gear teeth of the worm gear are completely greased. Put the Dec lock button into place, then slide the Dec shaft into the housing until the parts are fully seated.

To check whether the internal surfaces are sufficiently smooth, assemble the components, rotate the Dec shaft a few times, them pull the shaft from the housing. Examine the bearing surfaces and washers to see if the grease is evenly distributed, with no gaps. Pay special attention to the large washers on the worm gear. If the dabs of grease have been spread around the whole washer after a few turns of the Dec shaft, the bearing surfaces are probably smooth enough.

There are three main areas of adjustment on each axis. They should be performed in the order listed below on both the Dec and RA axes:

  • Ring Nut
  • Worm Shaft End Play
  • Worm Gear Mesh
Ring Nut
Screw the ring nut onto the bottom of the shaft. Don't forget the plastic washer that goes between the ring nut and the shaft. Tighten the ring nut, using your spanner tool if necessary, until the shaft turns freely, but does not wobble or feel loose. Then, tighten the small set screws in the ring nut to hold it in place.

When adjusting the ring nut, bear in mind that the assembly will loosen up a bit after a period of use. If it is too loose now, it will only become looser later. There is a very fine line between too loose and too tight. You will find yourself making very small changes in the tightness of the ring nut to achieve the best compromise.

Worm Shaft End Play
If either of the worm shafts is able to slide back and forth in its bracket (end play), you'll have excessive slop in the mount. To adjust the end play of either worm, start by removing the large silver nut on one end of the worm bracket. This reveals a threaded insert (or sleeve) through which the worm shaft passes. This insert can be turned to adjust the amount of end play in the worm. Too loose, and the axis will be very sloppy. Too tight, and the axis will be stiff. It may take several tries to find the happy medium.

On both of my CG-5s, there were several very small, very thin plastic washers in the worm assemblies. These washers are nearly invisible when buried in the thick factory grease, so it's easy to miss them. Look carefully!

Another description of this adjustment comes from fellow CG-5 owner Jim McKay. Note that the worm assemblies on Jim's CG-5 contained no plastic washers! This is indicative of the variation you can expect between different examples of the CG-5.

I went ahead and removed the nut [on one end of the worm housing]. The nut holds a threaded sleeve in place. One end of the sleeve bears directly on the worm while the other end of the worm bears directly on the worm housing. There are no plastic washers on either end of the worm as there are inside the mount. The threaded sleeve can be threaded in and out of the housing thereby increasing and decreasing the amount of endplay in the worm. You then lock it in place with the hex nut. It would appear that you need a small amount of play otherwise the worm becomes very difficult to turn. I was able to take most of the play out and retighten the nut again. The nut is 16mm and is on fairly tight (I used a 1/4" drive metric socket set). I first took the worm housing off the mount and put it in a vise (with something to cushion the metal jaws of the vise), but I later found out that it can be loosened while the housing is still attached to the mount. Adjusting the sleeve made a BIG difference in the slop that I was experiencing in the RA axis. The DEC axis was fine and I didn't touch it. My guess is that without any washers between the worm and the housing, wear at the ends of the worm will necessitate adjustment at a later date.
Although I did not have any endplay in the worms of the first CG-5 I refurbished, my second CG-5 required this adjustment. Unlike Jim's mount, mine had plastic washers in both worm assemblies. I was able to remove the endplay almost entirely while maintaining very free movement of the worm.

Worm Gear Mesh
This is the most tedious, but arguably the most important adjustment of all. Here, we are adjusting how deeply the worm meshes with the teeth of the large silver worm gear.

The small set screw between the two larger silver hex head screws on the worm assembly controls how deeply the worm meshes with the teeth of the worm gear inside the mount. If the gears mesh too tightly, the worm shaft will be stiff and difficult to turn. If the two gears do not mesh enough, the axis will have backlash and "slop."

Turning the set screw clockwise pushes the worm farther from the worm gear, loosening the axis. Turning the set screw counter-clockwise lets the gears mesh more deeply, tightening the axis and reducing backlash. Proper adjustment consists of making small changes in the depth of the set screw until you achieve a good compromise between ease of motion and lack of backlash. Note that this sounds counter-intuitive (turning the screw clockwise to loosen up the axis), but it is correct.

Start by applying some grease to the worm and re-attach the worm assembly to the housing. Replace and tighten the four silver hex head screws, starting with the two on either side of the set screw. Try to tighten these two screws equally. Then tighten the other two hex head screws holding the worm assembly. Now grasp the upper end of the housing and turn the shaft.

If it is too tight and does not move freely, loosen the four silver hex head screws and turn the set screw a fraction of a turn clockwise to loosen up the axis. Re-tighten the four hex head screws and check the axis again.

If it is too loose and wobbly, loosen the four silver hex head screws and turn the set screw a fraction of a turn counter-clockwise to tighten up the axis. Re-tighten the four hex head screws and check the axis again.

As little as a tenth of a turn of the set screw can make a noticeable difference in the tightness of the gears. In addition, tightening the four hex head screws deforms the castings slightly, pressing the gears together and causing them to mesh a little tighter. The soft metal used for the castings is easily deformed by tightening the screws.

So you must use some trial and error to get it right. You want the worm tight enough against the worm gear so that the axis doesn't wobble, yet not so tight that the worm shaft is hard to turn.

Fellow CG-5 owner Bob Supler suggests lubricating some of the screws in the worm assembly:

As for the worm bracket screws, I lubricated all three on each axis – the center hex, as well as the two on either side. I also learned to put a dot of grease on the tip of the center hex because that grinds directly against the mount.

I didn't do this, but based on discussions with Bob, it seems like a good idea. When I need to adjust the worms again, I'll follow Bob's advice.

Between adjusting the tightness of the ring nut and adjusting the worm assembly, it's easy to spend an hour or more on each axis. That's what it takes to find the best compromise between stability and ease of motion. Don't skimp on this part of the project. Failing to properly adjust the mount can negate all the hard work you've done to clean, smooth and re-grease it. It's well worth your time.

After adjusting the ring nut and worm, replace the setting circle and its retaining ring. Don't forget the washers here: four of them under the setting circle and one under the retaining ring. You may have more or fewer washers in your CG-5.

You've completed work on the declination axis. Congratulations!

RA Axis »»

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