Variable Hypocycloid

Variable_Hypocycloid

A way to convert rotation to an adjustable length linear motion on the fly; i.e. the stroke length can be changed while the machine is in motion. Giant steel and iron versions of this mechanism have been around for over a hundred years but my mostly-plastic interpretation is small enough to be useful on a hobbyist's workbench. Alternately it's fun to watch. :-)

This was published (without fanfare) on Dec 31, 2025 and I'm still writing the assembly manual!! If you find your way here please let me know what's missing so I can improve it. :-)

This is an easier to assemble version of one I made some time ago. Here’s a crummy video of that one in action: https://www.youtube.com/watch?v=dtayrCuVh3o&list=PLci-Ej4vBzCHKBcdSM4zriQGkbCiM2Efj&index=2

…And here’s a better explanation of it by my guru for all things mechanical, the late great Robert Murray-Smith: https://www.youtube.com/watch?v=zzdOSxIejfo&t=12s

Printing the parts:

-Here are quantities and fill percentages:

In addition to the printed parts you will need the following:

Tools:

Heat-set insert tool

Metric hex keys

Arbor press or vice

Hacksaw or abrasive cutoff blade

Sandpaper or a file

Small Phillips head screwdriver

2.3 mm drill bit

4.0 mm drill bit

Hand drill

Arbor press or bench vise

Materials: (all from Amazon)

Many: M3 x 5 x 4mm Heat-set inserts

18: M3 x 10mm socket head cap screws

1: M3 Shoulder Bolt: 4mm dia x 5mm long shoulder

1-2: M3 nut spanners or pliers

1: 34 cm Gt2 timing belt, 6mm wide

4: 5mm id Shaft Collars

1: WINSINN GT2 Timing Belt Pulley 20 Teeth, 5mm Bore, for 6mm Width Belt

1: 5mm x 5cm D-shaft with flats for pulley setscrew and crank handle

1: 16mm od x 8mm id x 5mm wide bearing

1: M3 x 10 socket head setscrew

30mm: M3 Allthread

2: M3 nuts

Some blue Loctite

Electronics:

1: NEMA 17 stepper motor

1: 12v 2A power supply

1: stepper motor controller: 

https://www.amazon.com/Stepper-Controller-Integrated-Forward-Communication/dp/B0B1HNS9L2?ref_=ast_sto_dp

Assembly:

Insert 2: M3 Heat-sets into the top of the Big_Gear_Cage_Lower

Snap Crank Handle_01 into Hand_Crank_04

Insert 4: M3 Heat-Sets into the Motor_Base_Tall_02

Insert : 4 M3 Heat-sets into the top of the Plinth, 2 into the side and 4 more into the front

Insert 1: M3 Heat-set into the Red Crank

Insert 1: M3 Heat-set into the Stepper Crank

Insert 1: M3 Heat-set into the Tensioner

Insert 1: M3 Heat-set into Stepper_Crank_17 

Fasten the Big_Gear_Cage_Lower to the front of the Plinth with 4: M3 x 20mm BHMS; aim to get it centered. If you don't have BHMS you can use SHCS but you'll need to add washers.

Fasten the Pillow_Block_02 to the side of the Plinth with 2: M3 x 10 SHCS; leave it loose for now.

Secure the Plinth to the Base with 4: M3 x 10mm SHCS

Mount the stepper motor to NEMA_17_Mount_09 with 4: M3 x 10 SHCS, making sure that the cable connector faces down.

 Attach  Stepper_Crank_17 to the stepper motor shaft and secure it with the M3 x 5mm setscrew.

Place the Big_Gear into the cradle on the top of the Big_Gear_Cage_Lower and secure it with the Big Gear Cage Upper with two: M3 x 10mm SHCS

Press the bearing into the recess in the back side of the Small_Gear.

Slide the Small_Gear and bearing onto the end of the Stepper_Crank, being careful to mesh the teeth with those on the Big_Gear. 

Slide the Red_Crank onto the hexagonal boss on the Small_Gear. Secure it with the M3_Washer and an M3x10mm SHCS

Attach the Slider to the outer end of the Red Crank using the M3 shoulder bolt. Use a dab of Loctite on the threads to secure it, making sure that the Slider still spins freely. The slider will interface with accessories that can be driven by the Hypocycloid. The first one will be a die filer, followed by a razor saw and a hacksaw. Ideas welcome for more accessories!

Attach the Bracket_07 to the boss on the lower side of the Big_Gear_Cage_Lower by sliding a 5mm rod through the matching holes. Continue sliding the 5mm rod until it passes through two Shaft_Collars and the Pillow_Block. Tightening the Shaft Collars at the two extremities of their travel will lock the 5mm in place.

Now tighten the bolts holding the Pillow_Block to the Plinth.

Snap the Tensioner into Bracket_07 using the two slots as a guide. 

Wrap the Timing Belt around the Timing Belt Pulley, then insert it into the top of Bracket_07; secure all with the 5mm dia. D-shaft. Tighten the Pulley setscrew on the flat of the D-shaft.

Wrap the free end of the Timing Belt around the Big_Gear.

Insert a length of M3 allthread into the Tensioner and screw it in until it hits the 'bumper' on the side of the Big_Gear_Cage_Lower. Keep turning the screw until the Timing_Belt is just secure, no more.

Slide the Hand_Crank onto the end of the D-shaft sticking out of the Timing Belt Pulley. Give it a turn or two and make sure it turns the Big Gear without any slop that would cause the Timing Belt to lose steps.

Now all that's left is to power up the stepper motor and give it a go. Although the motor can turn either way and the output will be the same it's best to make it run in a direction that will tend to tighten, rather than loosen the Shoulder_Bolt holding the Slider.  The thing will work by itself as a demo or you can carry on and build the various output devices. Only the Die Filer is ready but more will come along. Here's a link to the new, improved version, Die Filer #6: