An Easy-to-Make, Non-Binding, High-strength Tangent Arm Nut for Telescope Slow Motion Controls

by Jim Sapp

The Background:

The typical mechanical solutions to the problem of telescope slow motion fall into two major categories:

The former method can offer a full 360 degrees of travel and is the preferred way to provide siderial tracking, but for fine manual control it requires a large radius worm wheel which can be expensive, heavy, difficult to implement with simple tools due to the required degree of precision, and an inefficient use of material.

When only a small degree of movement is required, the tangent arm is much easier to implement with simple tools and provides fine slow motion at greatly reduced weight and cost. The typical layout consists of an arm temporarily clamped to the axis housing of the mount. The end of the arm farthest from the mount's axis carries a threaded nut through which passes a threaded rod (the lead screw). The lead screw (threaded rod) is captured in a bracket which is attached to the tube of the telescope (or other part to which the slow motion is to be imparted). Turning the lead screw pulls the nut (and hence the arm) toward one end of the screw, providing the desired slow motion.

The Problem:

The primary difficulty encountered in using this method is the tendency of the nut to bind against the lead screw when it approaches either end of the screw, since the movement of the end of the arm naturally describes an arc in relation to the straight line described by the lead screw. This limits the length of travel and results in rapid wear of the nut and/or screw, which eventually leads to backlash and a sloppy fit.

This binding is caused by two main factors: As the arm/nut approaches the end of the lead screw,

This problem is usually overcome by mounting the nut with a pivoting pin (which allows the angle of the nut's axis to remain aligned with the lead screw) to a slug which can travel a short distance in a slot milled into the end of the tangent arm (which in effect allows the arm's length to adjust, maintaining the position of the nut in relation to the lead screw's axis).

However, this presents a number of difficulties to the telescope maker with limited tooling:

Two more, but less severe sources of binding and wear that are not addressed by this solution are:

These two items are inobvious at first but can be seen to add additional stress to the nut's pivot pin as well as the lead screw and it's bearings.

A Solution:

Described below is a strong but simple solution that provides accurate captured movement for the nut on all axes and can be implemented with only a drill press using metal or even plastic or wood materials. It eliminates the need to mill a slot in the end ot the arm and eliminates the weakness, rapid wear, and subsequent backlash of the typical nut/pin/sliding slug assembly. I have built several of these units using only a drill press, files, and reamers and have enjoyed excellent success each time.

I'll dub this the "Cylinder Block" method. The idea is to provide universal motion for the nut, similar to the way a ball joint works, but limited in travel and well caged for strength. Refer to the photos below.The nut consists of a short length of brass rod that is drilled through the side and tapped to accept the lead screw's thread. This brass cylinder then fits into a hole through the side of a larger steel cylinder, which has a second perpendicular hole drilled through it to clear the lead screw. This steel cylinder then fits into an aluminum block which is drilled and tapped to be fixed to the end of the tangent arm.

This aluminum cage block is drilled through three axes:

The brass nut cylinder is free to rotate in the steel cylinder which corrects for the angular displacement at the ends of the lead screw's travel, and is also free to slide up and down in the steel cylinder to correct for a bit of displacement of the "tangent arm to saddle plate" dimension. This allows the telescope mount maker a bit of lattitude in the mount's bearing placement without interfering with the function of the tangent arm's mechanicals.

The next set of photos show the wide lattitude of angular misalignment and lead screw displacement that this nut mounting scheme can accomodate. On the left you can see the arc through which the arm can swing without binding the nut, and on the right you can see the amount of lead screw bearing misalignment that can be accomodated. The result is a firmly fixed, solid, and very strong tangent arm nut that is free to float enough to negate a wide variety of improper or missed tolerances in the construction of the mount.

The photos below show a simple implementation of this idea on the declination axis of a German equatorial mount. As you can see, as with most tangent arms, the arm is clamped to the declination axis housing, while the slow motion's lead screw is attached to the mount's saddle plate.

Construction Notes (in no particular order):

I hope this little idea helps to give the reader great success in constructing a fine telescope mount. It has worked very well for me and even after many years of use these tangent nuts remain solid and backlash free.

- Jim Sapp
Summer, 2003

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