3000:1 harmonic telescope drives for RA and DEC

3000:1 harmonic telescope drives for RA and DEC

These PETG drive modules are for the right ascension (RA) and declination (DEC) axes of an equatorial telescope mount. Each uses a NEMA 17 stepper motor, an EG17-G50 planetary gearbox with a 50:1 reduction, and a 60:1 printed strain-wave stage. The modules have been printed, assembled and operated on the author’s build.

The two stages work in series:

Motor → EG17 gearbox (50:1) → strain-wave stage (60:1) → axis output.

The nominal total reduction is 50 × 60 = 3000:1: 3000 motor-shaft revolutions produce one axis-output revolution with the clutch engaged. The 60:1 figure refers only to the strain-wave stage, not to the complete drive.

A hand-operated dog clutch disengages the output for manual positioning and balancing without forcing the reduction stages. Both modules use the same drive mechanism. RA has an hour setting circle; DEC has a degree setting circle and a counterweight bell for an M12 rod.

The download includes printable STL files, separate RA and DEC STEP assemblies with English part names, and illustrated assembly manuals in English and Italian. The manuals show the assembly sequence, insert locations and required hardware, including how to load the harmonic drive’s 44 balls. Use the STEP assemblies to inspect dimensions or adapt the parts to your own build.

How the mechanism is arranged

• The wave generator runs on 44 loose steel balls, Ø5 mm, between its race and the flexspline.

• A separate ball retaining ring closes the assembly with four M3 screws.

• Two 6812-2RS bearings support the output shaft in a removable cartridge.

• An open 6817 bearing carries the clutch actuation load. Three identical printed collars connect it to the operating ring.

• Two printed retainer halves secure the 6817 bearing.

• A purchased 6200-ZZ bearing, 10 × 30 × 9 mm, sits in the dog clutch plate and guides the flexspline mount. Its solid Ø9.9 mm pin enters the Ø10 mm inner race; the Ø14 mm shoulder contacts only that race. The mount has an R3 root fillet and no retaining disc or pin-end fastener.

• The housing has separate upper and lower sections joined by six M5 screws.

• The drive pin can be removed from the Wave and fitted to the gearbox shaft.

• QR011 and Vixen interfaces connect the modules to the telescope mount.

The clutch is a positive engagement mechanism, not a torque limiter. Support the telescope before releasing an axis, and engage the clutch fully before powered movement.

Files and variants

Use HARMONIC_MOTOR_RA.step for the RA assembly and HARMONIC_MOTOR_DEC.step for DEC. The illustrated guides are MOTOR_MANUAL_EN.pdf and MANUALE_MOTORE_IT.pdf. They show assembly order, screw locations and insert positions with arrows.

The updated 6200-ZZ seat and flexspline guide pin are included in the STL files and assembly manuals. The existing RA and DEC STEP files do not include this update.

The current printable set is listed below. Quantities refer to one motor unless marked otherwise. P10, P11 and P12 in the manuals and STEP files identify the three collar positions; print the same collar STL three times.

For both axes, print two copies of each common part, six collars, one RA setting circle, one DEC setting circle and one counterweight bell. One ball-loading tool can serve both assemblies. Bearings, balls, motor, gearbox, inserts and other commercial hardware are purchased parts, not printable substitutes.

Printing in PETG

Orient each part to keep its mating surfaces clean and reduce supports. Export orientation is not a guarantee of the best printing orientation. Inspect the sliced preview before printing. Print the flexspline, circular spline and wave generator without supports; support scars on their teeth and ball tracks interfere with operation. Keep seams away from working teeth, ball tracks and bearing contacts.

Use wall counts, solid surface layers and infill appropriate to the load path. Infill alone does not establish strength. Dry the filament according to its manufacturer’s instructions and use temperatures and cooling suited to that material and your printer.

Calibrate your own printer and material

Before printing the full set, tune extrusion flow and pressure advance, then check dimensional samples made with the same material, layer height and print speeds as the intended parts. Keep pressure advance under one consistent configuration: slicer or start-code commands must not override the calibrated firmware value with a different setting.

Measure both small screw holes and larger circular openings. Set hole expansion separately from horizontal (XY) and vertical (Z) shrinkage compensation: one correction should not hide an error in another. Check seam quality and support release as well as dimensions. Use the validated results in the production profile; do not copy another printer’s flow, pressure advance or compensation values.

Complete hardware list

The tables give quantities for one RA module, one DEC module and the pair. Screw lengths are measured below the head; all listed screws are ISO 4762 / DIN 912 socket-head screws. The motor-to-gearbox kit needs its own manufacturer-specified fasteners in addition to this list.

Screws

The four gearbox-to-housing M4 × 14 screws engage the purchased gearbox’s threads. Match their usable engagement to the actual gearbox. Check that every screw clamps its joint before reaching the bottom of a pocket. The DEC rod clamp uses one M6 × 25 screw without an added washer in that joint.

Brass inserts

Use the specified insert family and length, not just a matching thread designation. The M3 inserts are the short, 4 mm-long version. The manual identifies the receiving holes on each part.

The seven M3 inserts per motor serve the bearing retainer (2), clutch indicator (1) and Wave retaining ring (4). DEC has one additional M6 insert in its counterweight bell. Check insert retention in your printed parts, especially the four Ø4.4 mm Wave seats.

Bearings, drive and other purchased parts

Buy one 6200-ZZ bearing for each motor, or two for an RA and DEC pair. It is a commercial bearing, not a printed part. Match the purchased QR011 clamp and Vixen bar hole patterns to the STEP interfaces. Select counterweight mass and position for your telescope and accessories; the listed rod length does not prescribe a payload.

Assembly outline

The illustrated manuals contain the full sequence and the part-by-part insert maps. In brief:

1. Prepare the parts and the inserts shown in the maps. Assemble the two output bearings, spacers and shaft in the cartridge.

2. Fit the cartridge in the lower housing and join the housings with six M5 × 14 screws.

3. Fit the 6200-ZZ in the dog plate, applying insertion force to the outer race. Attach the plate to the output shaft with six M5 × 14 screws before fitting the flexspline mount, which would cover the screws. Assemble the sliding ring, 6817 bearing, two-piece retainer and three collars. Connect the operating ring.

4. Insert the flexspline mount’s Ø9.9 mm pin into the 6200-ZZ and seat its shoulder against the inner race only. Do not fit a retaining disc or pin-end screw. Secure the circular spline and flexspline, then complete the Wave and drive-pin assembly in the following steps before handling the module by the flexspline mount.

5. Insert the drive pin into the Wave, then seat the loading tool against its stop. Rest the tool on the secured circular spline so the Wave enters the secured flexspline.

6. Feed all 44 Ø5 mm balls individually through the Wave’s loading slot into the race. The balls then hold the Wave in position.

7. Remove both the loading tool and the drive pin. Secure the ball retaining ring with four M3 × 6 screws.

8. Fit the pin to the keyed gearbox shaft, then engage it with the Wave. Install the gearbox housing and motor.

9. Fit the setting circle for the selected axis, the clutch indicator, QR011 clamp and Vixen carrier/bar.

10. Close RA with six M5 × 35 screws. For DEC, add the counterweight bell with six M5 × 50 screws, then fit and clamp the M12 rod.

Start powered checks slowly without a telescope. Confirm clutch operation and inspect for binding before applying load. Do not force the output by hand through the reduction stages. Balance the mount and inspect load-bearing prints, inserts and bearing seats during use. The author’s successful assembly and operation do not establish a rated payload, tracking accuracy or service life for every printer and material.