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Tag: 3D Printing

3D Printing Custom Propellers

For fun, I wanted to custom design a propeller so that I could effectively match it to the hydrodynamic characteristics of the AUV and a drive train to try and improve efficiency. Luckily, I found a design tool which makes the propeller design easy — MH Aerotool’s JavaProp. Although the JavaProp is meant for airplane propeller design, it’s easy to change the medium parameters by inputting the density, kinematic viscosity, and speed of sound for water in the options tab to produce designs suitable for usage in water.

Recall previous posts where I used OpenFOAM to analyze the flow around the AUV hull to determine the drag and velocity profile in the propeller disc. I can input these parameters into JavaProp to tailor the design to my AUV! Granted the flow profile is a fairly simplistic linear input with 2 parameters, it’s probably good enough for my purposes. Inputting the drag and dimensions into JavaProp is straightforward. For a preliminary cut, I chose the following parameters:

Propeller Design Inputs

Clicking “Design It” gives me a quick design. After some modifications, the rough design gives me about 76% efficiency, and that’s without really thinking too much into it. Lots of information is generated as well to better understand the propeller’s performance.

Test Propeller Performance

 

The best part, however, is that it outputs the optimum propeller geometry given a selection of profiles for different points along the propeller:

Test Propeller Geometry

The awesome part is that you can export the profiles as a surface to manipulate in external programs – Through some CAD elbow grease with your program of choice, it’s possible to convert the surface into a complete propeller. One caveat, the trailing edge needs to be thickened to be manufacturable later, but I accomplished this easily with the custom settings available in JavaProp.

The 3D prop was created in OnShape by intersecting the exported surface along a series of planes, which creates a series of profiles used for a loft feature for a single blade. From there it’s just a matter of rotation pattern feature to make three blades, adding a hub, and adding some fillets at the root.

Propeller CAD Design

 

Shapeways has recently introduced a trial of the HP Jet Fusion 3D printer, so I figured that would be a good material to do a trial print in. The results were surprisingly good! I had originally planned to smoothen the propeller out and use it to make a plug to create a mold, allowing me to cast propellers out of a much stronger urethane material. Although not smooth enough to be a perfect propeller, this new material may be tough enough to use the water itself!

3D Printed Propeller

The keen eye will notice that I didn’t print the specific design shown above — I actually 3D printed an earlier revision, which didn’t have an adjusted flow profile. Once adjusting the flow profile for reduced flow near the root of the blade, the chord of the profile at that point increased to compensate.

Next steps will be to experiment with different ways to smoothen and stiffen the propeller. Either filling and sanding or adding a very thin layer of fiberglass tissue and epoxy could work. Either way, 3D printing propellers seems very feasible, and a very reasonable cost. The best part is, I can experiment with different ways to optimize and easily print multiple variations to actually test.

One unknown, however, is how the print will deal with pressure if used directly at the AUV’s test depth — Any porosity may cause problems so some testing will be required before putting it into use.

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QTTX 131600 Heavy Duty Flat Car

I’ll admit I’ve always been a train geek, and have recently started collecting some N-Scale trains but unfortunately living in Vancouver I don’t have room for a permanent layout.

When I first discovered Onshape, the project I set out on to learn and test the tool was creating a model of one of my favorite rail cars, a QTTX 131600 series 12-axle heavy duty flat car.  (I didn’t feel like going through the hassle of getting permission to post a real photo here, but there are plenty of great photos at that link). The goal was to attempt to 3D print the design to see how well it would turn out. This is just a quick summary, with a more detailed build thread up at n-scale forums.

First up, I modeled the buckeye trucks. These specific roller bearing buckeye trucks are not readily available in N-Scale, so I had to design my own. Furthermore, the mounting height had to be non-standard so that I could stuff them under the bolster mechanism. I ended up using several overlay photos, combined with the QTTX131600 rough dimensions from TTX’ materials that I dug up after some research. This took a lot of messing around with, but the end result was pretty good. Below is an early version of the truck.

Roller Bearing Buckeye Truck
Roller Bearing Buckeye Truck

The rest of the flat progressed fairly well. The aim was to make it accurate, rigid, yet with as little material as possible, as 3D printing can be expensive. Again using plenty of reference photos and dimensions that I found while scouring the internet, I came up with something pretty close. I had to make a couple of alterations underneath the body to enable smooth flow over un-prototypical 11″ radius curves and omitted any of the hidden air brake equipment. I also whipped up a quick pressure vessel load. I used tiny magnets embedded in the deck and the load supports to keep tings in place.

QTTX131600 heavy duty flat car, with load.
QTTX131600 heavy duty flat car, with a load.

After multiple iterations with revisions here and there trying to stiffen up thin features so they’d survive the 3D printing and cleaning process, I sent it off to Shapeways to be printed. After a tense couple of weeks, I finally got the parts back in the mail!

QTTX 131600 3D Printed Parts
QTTX 131600 3D Printed Parts

The parts came out well, and even the thin features like hand rails, hand brake and the ladder survived the process although not my handling — I had to glue some back on). The features are incredibly delicate, at sub-millimeter dimensions. Some of them I beefed up to non-prototypical dimensions, but they look well regardless.

First, I assembled the trucks and bolsters to test how they rolled. The first version trucks were a bit tight, so did not roll as smoothly as I would have wanted. I eventually printed another set of trucks with looser tolerances which performed much better.

Buckeye Trucks
Buckeye Trucks

After playing around with the fit of all the parts, I cleaned heavily (I used a combination of hot water, simple green, and an ultrasonic cleaner to try and de-grease the residue left over from printing. I hear Bestine is the best way to go, but didn’t have any readily available and am generally not keen on overly harsh substances if I don’t absolutely need them (not that simple green is harmless, but I had it on-hand for servicing scuba regulators).

Painting was a learning experience, as I had to get an airbrush and learn how to paint! I used general art grade acrylics as opposed to any model specific paint and attempted to mix my own colours. The big mistake I made was not using a suitable primer, as I found certain colours (notably the yellow blend I used) would run, likely because of something in the white paint. Any mixtures without white adhered much better. The second big mistake I made was being bit heavy handed as opposed to using thin coats. I ended up going several itterations of painting, and at one point completely  stripped the paint with alcohol and started from scratch. A warning here is that alcohol can warp the parts, but luckily I designed the deck and frame with registration pins to maintain the correct curvature when properly assembled.

Finished 3D Printed QTTX131600 Flat car
Finished 3D Printed QTTX131600 Flat car

The end result wasn’t too bad! I definitely learned a lot along the way, the paint job isn’t perfect, but I’m happy with how it turned out. There are some artifacts showing from the 3D printing process, but overall it’s pretty good and smooth looking. It looks great being pulled behind an ES44AC. I’m not sure if I’ll venture down the road of making any other train cars (I have enough on my plate with work and the AUV project as it is), but it was an interesting project. I may, however, attempt to see how much of this I can reproduce on my mill, once I get it converted to CNC.

If anyone’s interested, I’ve made the models available for purchase on Shapeways:

N-Scale QTTX 131600 Series Heavy Duty Flatcar

N-Scale Roller-Bearing Buckeye Truck (Modified for a generic ride height for use on other cars)

N-Scale Pressure Vessel Load

 

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