Strong Helically Geared Telescoping Rod with Attachments (Print in Place)

Overview

I designed a telescoping rod that receives various attachments. The attachments included today are:

• A back scratcher
• A pivoting axis that takes a 32mm neodymium magnet.

I am also planning a few more:

• A pivoting axis that accepts a 50mm mirror.
• A hook
• A croupiers rake. 

The Design

My design criteria were:

1. Must print in place as oriented on the bed.
2. Must be strong enough for intended use.
3. Must fit on the smallest Prusa printer.

The telescoping rod, and the pivoting axis attachment are both print in place with no support material.

Due to the inner most shaft of the rod only being 10mm in diameter, I have added a cavity along the inside of this that accepts a cuboid shaft, which is printed separately flat on the bed. This adds a high amount of strength to the inner shaft. I guess technically because of this it's not strictly print in place, but I gave myself a pass. ;)

I have designed this model with quite tight tolerances (will upload a version with looser tolerance later).

It was an incredible challenge to get the tolerances right, have strength in the part, and make the handle small enough in diameter to be comfortable to hold. I went through several complete remodels before landing on something that worked for me.

Printing the Models

I printed all parts on my Prusa XL-5T using the following:

Prusament Azure Blue PLA
Prusament Lime Green PLA
Prusament Galaxy Black PLA
Prusament Pearl Mouse PLA
Fulament Matte Purple PLA

Print Settings

.2 mm layer
3 perimeters
15% infill

The scratcher needs a brim, but other parts do not; I printed this on a core X/Y machine, so there isn't a lot of wobble on the z axis. If you are printing on a bedslinger , you probably want to split the telescoping rod in to objects and enable an outer brim on the outer shaft of the model. 

Post Processing

A little post processing is needed.

Cut the mouse ears off of the cuboid shaft. (added these into the model so it wouldn't curl off the bed).

Tap the cuboid shaft into the telescoping rod with a small mallet (or your table top. You can add superglue if you want, but I have found it is not necessary. 

Depending on the tolerances of your printer, you may need to tap the bottom of the threaded rod gently a few times to be able to actuate the helical gears. If you want, you can add lubricant to the teeth, but I like mine to have a bit more friction. The teeth will wear in with use, so I'd advise against using a lubricant until you're comfortable with the resistance of the rod.

Designing Your Own Attachments

The rod has standard M14 thread, with 0.4mm fillets on both the root and crest. I then offset the root by 0.1mm and crest by -0.1mm, to smooth out the thread angle. 

I've also included step files for both the telescoping rod and the pivoting axis attachment.

A Note on Strength

The internal shaft being 10mm in diameter would normally be a weak point where the rod could fail. The cuboid shaft printed horizontally seems to add a significant amount of strength. When I was going between printing iterations of this model to find the right tolerances, I had one that was a bit too snug on the top, but I thought I might be able to work the gear into the tread with a pair of pliers. With I tight grip, I wrenched on it, putting quite a bit of torque on it, to the point the shaft was starting to twist. Eventually, it did fail, but not along the layer lines like I would have expected, but rather in a spiral pattern, starting at a seem and winding down the shaft. I thought that was pretty cool.