DHEM: another DIY Harmonic Equatorial Mount

What the hell is this DHEM thing?

DHEM (DIY Harmonic Equatorial Mount) is an equatorial mount for observation or astrophotography. It is based on the use of a harmonic reducer. The mount is designed to be inexpensive in relation to its performance, transportable, with a relatively large load capacity. Most importantly, this mount is easy to build: no need for machine tools or other laser-cutting-water-jet-plasma-turbo-piston-5-axis machines. A shopping list, a screwdriver, a saw and some resourcefulness: you've got everything you need to make a DHEM. The ultimate aim is to distribute its design as an open-source product for personal use.

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The technology used in this mount is similar to that used in robotics or industrial production lines. The main mechanical gearbox is a harmonic gearbox (wikipedia).

It can produce surprising results for an astronomical mount at first glance; the mount appears grossly undersized and unbalanced, whereas the whole assembly is extremely robust and stable, with enormous torque. Conventional equatorial mounts require the use of counterweights. The aim of this development is to make available the design of a harmonic mount that can be built in your garage, with few tools or mechanical knowledge, for a price of less than 1000€. Of course, the proud aim is to achieve performance equivalent to that of commercial harmonic reduction mounts.

Harmonic gear motor
BOM less than 1000€
No need for machine tools: all you need is a saw and a screwdriver!
Lightweight: max 5 kg
Minimum load weight without counterweight: > 12kg (to be tested)

The first results of the performance tests are up to my expectations for the moment. I hope this github repository can help you design or reproduce this equatorial mount. Please be patient, I'm only working on this project in my spare time. I'll update the testing progress on this readme. ✍️

Support

Hey! Help me out for a couple of :beers: !

Design

3D design

The 3D design was carried out on SolidWorks 2022 and OnShape. Here's a sharing of the design on OnShape. The project is a bit of a mess, but it includes all the components needed to assemble the mount. You will find all stl files for printing in this repo.

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Bill of materials

Here's the shopping list for two axes. Prices are approximate: I already had quite a few parts in stock when designing the DHEM. The total investment is around 900€ for the mechanics, and I estimate the cost of making an OnStep MiniPCB2 controller at 100€.

The estimated total cost of the equatorial mount is less than 1000€.

I think you can save a lot of money by going to other supply sites. For example, I got about €70 worth of Bosch M4 T-nuts from RS Components, when I could have got them for 5 or 10 times less with no-name (for example, another example). I had Bosch in my stuff, so I didn't deprive myself.

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| Name | Quantity | Reference | Price € | Link | | -------- | -------- | -------- | -------- | -------- | M4x8 | 32 | - | a few | M4x10 | 16 | - | a few | M4x16 | 20 | - | a few | M4x20 | 20 | - | a few | M5x10 | 8 | - | a few | M5x20 | 6 | - | a few | M6x16 | 12 | - | a few | 20mm Bosch Rexroth profile | 1 | 212-3292 | 25 | RS components T-nuts | 64 | 466-7281 | 70 | RS components M4 Angle Bracket | 24 | 180-9136 | 30 | RS components M4 inside M6 insert | 4 | B096VSWCBQ | 8 | Amazon FR Dovetail Clamp | 1 | EUF9144C | 30 | Amazon FR Dovetail Mounting Plate | 1 | EUF9175B | 20 | Amazon FR Arca clamp | 1 | B0BW3SSHY1 | 20 | Amazon FR Arca mounting | 1 | B0771KW7TK | 15 | Amazon FR | Nema 17 with 10:1 Planetary Gearbox | 2 | 17HS15-1584S-MG10 | 60 | StepperOnline Harmonic Drive | 2 | CSF-17-100-2UH | 550 | ebay (datasheet) RJ45 connector | 2 | MRJ5380M1 | 35 | Farnell OnStep controller | 1 | MiniPCB2 | ~100 | oshwlab

Assembly

Harmonic drive

Harmonic gearboxes (wikipedia), or more precisely strain wave gears, are a type of gear mechanism that transmits much more torque than other common gears. This is mainly due to the fact that there are many more teeth engaged at once. One of the major advantages of these gears is that they have virtually no backlash, partly because they use deformation. It's a bit like belts, which are supposed to be more or less backlash-free, but harmonic gearboxes use deformable metal spring steels, which are much better than belts.

Put another way, in the case of an astronomical mount: they can drive much heavier loads without the need for counterweights, all with a very low self-weight!

The harmonic reducer model chosen CSF-17-100-2UH is the same as Alan DIY mount. You can find lots of them on ebay. This one has a reduction ratio of 1/100. It has the good fortune to meet my requirements, and to have been tested for another equatorial mount. Why deprive yourself? According to datasheet, it can handle more than 50Nm of torque and it also has a very comfortable permissible radial load of over 70Kg.

Using OnStep's configurator spreadsheet, I chose to drive the harmonic gearbox with a planetary gearbox 17HS15-1584S-MG10, in order to achieve good tracking resolution, a good slew rate and a good number of steps/deg. Its reduction ratio is 1/10 and it is given for a low backlash (around 30 arcmin). Seen through the telescope, this low backlash will be divided by 100 by the main gearbox. You can probably use another motor with a planetary gearbox.

Motor holding

We start assembling the mount, attaching the motor and its planet gear to the 3D-printed motor spacer with M4 screws. I printed most of my parts in PETG or ABS, with an Ultimaker S7, in 0.15mm layers, slow speed, high infill, and 101% flow. Two 20mm Bosch Rexroth profile bars are cut at 120mm, and tapped on both sides with M6. An M6 (external) to M4 (internal) insert is housed on one side of the two Rexroths. These two bars are then positioned in their housings, and secured with 4 T-nuts. The M6/M4 insert is located on the motor shaft side. It will be used to attach the assembly to the harmonic gearbox. Note the orientation of the Nema 17 motor in relation to the two 120mm bars. The motor wires must be oriented in the space available on the bar side.

Once the CSF spacer has been centered on the harmonic gearbox, the assembly is secured to the gearbox using two M4 screws. Make sure you fit your 2 M4 screws into two plain holes in the gearbox flange. Do not use the tapped holes (every other hole is plain). Don't forget to insert the key on the motor shaft, and secure the shaft in the harmonic gearbox with the two M3 set screws. Make sure that the bars are properly aligned, perpendicular to the bearing plane on the harmonic gearbox.

As can be seen from the images, the 3D printed motor spacer has

DHEM

Install / Use

README