Stepper motor assembly V22.1

V22.1 (.1 version has more margin between parts)

Oil system

The stepper house is filled with sunflower oil. Sunflower oil has lower viscosity and longer expiration date than other super marked oils. Food oil has been used in many previous projects. Other oils was considered but it was either very expensive or an environmental hazard. The main downside with food oil is the expiration date. The oil will turn rancid, smell bad and be sticky and hard to remove. 
Water will get inside the housing if no oil is used, even in shallow water. 
A system for filling oil was made. The design has a syringe house and a vent. Oil can easily be filled inside the syringe when the vent is open. The hole can be covered with the second part of the syringe and a screw cover. The idea behind this design is to make filling easier. Pressing the syringe will also make overpressure inside the stepper house. The overpressure will help keeping the water out. 
Cable is slowly leaking oil into the electronics tube. The cables penetrator is now filled with epoxy. 

Printing material

The stepper motor house is printed with PETG. PETG is stronger and has better water resistance than PLA. (layer adhesion is better). Part cooling fan speed was set to 0 to reduce warping of the part. Infill was set to 100% to increase water tightness and compression strength.
PETG has some problems. The main problem is excess material sticking to outside of the printing nozzle. The material will slowly build a clump and then suddenly drop it on the print. The clump can make a gap in the surface. Oil was leaking from the parts with a clump in the surface. Low viscosity super glue was used to fill the gaps. This stopped the oil leak.

Magnets and sensor

Magnets and a magnet sensor is used for calibration. The sensor is fixed to the stepper housing. The magnet holder is screwed to the shaft with the same screws for the stepper motor. It houses two magnets with a gap between them. TODO: insert angle The wing is level when the sensor is in the middle between the magnets. Calibration sequence is further explained in the StepperWing library ReadME. TODO: link
Note: Magnet holder must be in correct position (calibrated) when assembling and disassembling the stepper assembly. The magnets is conflicting with the syringe. The port side syringe is broken because of this. 

Shaft spacer and bearings

The spacer is there to keep the bearings apart. It has holes for oil to flow. It also has a tab on the side to make sure holes are not blocked if inserted the wrong way.  One of the holes should be pointing towards the syringe. This is to make sure no air bubbles get trapped when filling oil. 

The two bearings are holding the shaft tight.

The shaft seal only worked on one of the assemblies. Oil was leaking probably between the 3d-printed surface and the seal. A new seal was glued to the outside of the stepper house. This did not leak.

Problems with this design

Gears are inaccurate. The wing is not rigid because of small gaps between the gears. It is therefor possible to rotate the wing a few degrees. This could make controlling it harder.

One of the shaft seal is leaking oil. The 3D printed surface is uneven. Contact between surface and seal is not watertight. Sanding the surface will help, but it is hard and time consuming. A new design should have the seal on the outside of the house. Sanding would then be easier. 

The rubber end cap is hard to find/buy. It limits the size of the stepper assembly. The assembly could be thinner and lighter.

Motor shaft 

The shaft connects stepper and wing. It is made from a 1m anodized aluminum shaft.

There was an attempt to make the shaft with hardened steel. The metal was very hard to cut and drill with the available tools.
The result was poor. Therefor the shaft was made with aluminum instead. Aluminum is weaker and has less accurate dimensions. 

Method

1. Shaft was cut to size with a Femi metal saw and lubricant oil. 
2. Center hole was drilled with a mini lathe. 
   The hole was first drilled with a center drill. Then it was drilled with a 4 and 7,5mm twist drill bit.
   Make sure the shaft is not wobbling before drilling. The shaft needed a lot of adjustment before it was stable.
   It is also important to use lubricant oil when drilling. 
3. Chamfer on the sides of the shaft was also made on the mini lathe.
   A Form tool was carefully used at an angle. Moving the tool too fast resulted in the shaft rotating off axis.
4. Holes on side was drilled with a drill bench.
   A straight line was first drawn to use as a reference. Hole on top and bottom must be aligned.
   Hole for the wing was marked by placing the wing on the shaft, aligned with the line.
   The first hole for the magnet holder, was measured from the edge.
   The second hole was marked by aligning the magnet holder on the first mark. 
   All holes was first pre-drilled with a smaller twist drill.
   Using a center punch is recommended to make sure drill bit is not drifting.
   Drilling speed was selected from the table at the wall. (fast with small bits, slow with thick bits)
5. The two holes for the stepper and magnet holder was tapped to fit a machine screw.
6. All hole edges was smoothed by hand with a larger drill.

Reference

CAD files is found on github TODO: link

Electrical schematic is found under the Electrical Schematics page.

Parts are found under the Materials page. TODO link