Making propellers from scratch
by Kurt Greiner, from Warship Models Underway! |
Please note - this is a more advanced operation, and involves using
chemicals and high heat for soldering. Please read and observe all safety rules for
your materials and equipment! |
If you have
access to a lathe and small milling machine, you can make props that are at least as good
as those available commercially. In fact, you could probably make them without those
tools, but of course it would be a lot more work. The nice thing about making your
own props is that you can customize them to perform exactly the way you wish. Also,
it is a way to get an unusual prop that not available elsewhere. |
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A home built prop
from Kurt's DDG-53 model
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After you have determined the size of the finished propeller, the number of
blades and the pitch angle, you are ready to cut metal. The first thing I do
is take some brass round stock, and cut it to about .060 ( ~ 1.5mm or a sixteenth of an
inch) over the final size of the hub. I then face the end, making sure that it
is 90 degrees to the hub; this will be the bearing surface where the prop meets the shaft
or strut and a care to get a good finish will be rewarded with a prop that does not bind
or wobble. I then drill a hole slightly under the size of the prop shaft, and ream
to the final size. Reaming assures good fit, but careful drilling might do an
adequate job as well. Once the propeller hub blank has been reamed to final shaft
size, I part it off from the remainder of the bar stock, using a cut off tool. A
stub of prop shaft is temporarily superglued to the hub blank, and then this is chucked
into the lathe. |
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The
hull is machined
down with very light cups, creating the profile in steps. I have found
that by mounting the hub on a section of shafting, the prop hubs revolves extremely true,
without noticeable wobble, a problem that had plagued some of my earlier efforts at
propeller making.
Once the blank is roughed out by turning, a
smooth cut file smoothes out the transitions between the steps. Sanding with 120 to
400 grit sandpaper puts a nice final finish on the blank.
I
used a Sherline lathe and Milling machine (although
the label says Craftsman, it was made by Sherline). Other brands, such as Taig could also be used of course.
I am using Sherline's WW collet set to hold
the shaft, as it is a bit more accurate than the three jaw chuck. Every bit of
accuracy improves the final product. |
The
hull is machined down with very light cups, creating the profile in steps. I have
found that by mounting the hub on a section of shafting, the prop hubs revolves extremely
true, without noticeable wobble, a problem that had plagued some of my earlier efforts at
propeller making.
Once the blank is roughed out by turning, a
smooth cut file smoothes out the transitions between the steps. Sanding with 120 to
400 grit sandpaper puts a nice final finish on the blank. |
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Once
the hub blank is finished, it is time to mill the slots for the propeller blades.
This is an important step, as a tight fit between blade and hub
eliminates the need for a jig to hold the blade in place while soldering. I use a
jeweler's slotting blade - these are available in many sizes from .010 to .125 and
thicker. I find that .030 works for props in this scale, as it is slightly less than 1/32
of an inch. If you do mill the slot a bit too wide, don't worry, there are some
adjustments that can be made later. Do try for a tight fit though, with the blade
having to be lightly tapped into place. The angle of the hub to the slotting cutter
is the propeller root pitch angle, generally between 30 and 45 degrees. |
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With your
hub turned and milled, it is time to make the blades of your propeller. These are
brass sheet, cut roughly to shape with tin snips. |
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They
are then stacked together to assure a uniform shape. I was using Shurtape double sided tape (a great tape for most
machining operations), but have since used a
small pair of needle nose Vise
Grips to hold the stack together. I put a small sacrificial strip of brass on
the top and bottom of the stack to avoid marring the blade surface. ( I am
considering purchasing a pair and grinding the jaws smooth for these types of operations).
However you hold them together, file and sand them until you get the blade shape you
desire. |
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When
the blades are shaped, you are ready to solder them to the hub. You will need a full
sized propane torch, as a micro torch does not supply enough BTU's of heat to overcome the
mass of a solid brass hub.
First, you will have to remove the stub shaft
that was used to turn hold the hub for the machining operations. Using a small vise
and a pair of pliers, gently heat the hub until you can pull the shaft free. I use a
miniature vise loosely gripping the shaft, and pull against the jaws of the vise.
Have a heat resistant pad under the hub to catch it when it falls free, or you risk
marring the surface. Caution - heated CA glue fumes are toxic, you
should do this operation in a well ventilated area!
Once your hub has cooled, clean the mounting
hole and the slots for the blades thoroughly. The reamer or drill you used to bore
the shaft hole will work well for this area; a file of the correct size or folded
sandpaper will clean the slots. You should try and get the area bright and free from
contamination prior to soldering.
To solder the blades in place, apply flux to
the blades and hubs, and then heat the hub and not the blades, as your heat will spread
from the hub when it is
sufficiently hot. You risk overheating the blades if you apply heat directly to
them. When the flux begins to bubble, apply the solder sparingly to the joint - you
want to see a thin perimeter of solder around each blade, not a huge glob. When the
hub cools, you will see a black residue that is easily cleaned off with water.
To inspect your solder joint, first look at
it - a good joint will be shinny and have no dull spots are areas of balled up solder.
If it looks good, hold the propeller by the shaft hole with an undersize bar, and
then take a brass bar and lightly tap each blade. You should hear a clear,
bright tone. If you hear a buzz or dull noise that is a sign that the joint
will fail.
Once your blade has been soldered, you can
sand the blades lightly with 600 grit, and then polish to the degree you wish.
Remember that most ships propellers have a milled finish, but of course it is very
tempting to shine the heck out of you nice brass prop! If you do decide to polish
it, a soft buffing wheel mounted in a Dremel Tool and automotive rubbing compound will do
the job in short order. |
Article © Copyright 2000 Kurt Greiner, Warship Models Underway!
Article used with premission.
You can send a tip here, or send
Email to: roger@rktman.com
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