Web site by: Phil Broad

The body and pods at the tips of the fins seen here on the rocket "Luna" from
Destination Moon are examples of axially symmetric shapes
 

What is an "Axially Symmetric" shape?

Simply put, an "axially symmetric" shape is anything which could be produced on a lathe.  Or put another way, it is a shape with a profile that is rotated about a central axis.  Axially symmetric shapes are often found in model building projects but usually the scratch builder will shy away from such subjects because they lack the very expensive "lathe" required to do the work.  Subjects such as "flying saucers" from science fiction films, the capsules used in manned space exploration, rocket bodies, rocket engine expansion bells, submarine hulls, nose cones and spherical tank domes of any size or cross section are often a great source of concern. Well they need no longer be!

A lathe simply turns a hard pattern material so a cutter can be applied to shape it into the desired profile.  What if the material is semi-liquid and therefore soft?  In such a case, a much slower turning speed could be used to achieve the same results.  There is a technique which can be applied to such shapes which can be used for any size pattern, up to and including full scale.  And it doesn't require any expensive equipment.  It is as follows.

Basic requirements for making an axially symmetric pattern:

Instructions for making a "saucer" shape
(the sample subject here is the upper half of the
Jupiter II saucer from Lost in Space)

Making the tool
(see illustration above)

1. Fix a pivot post in the center of your working area.  It should be high enough to protrude above the pattern you are trying to create.  Make it as square to the working surface as possible but in the end, this is not critical.  Don't worry if it is wobbly, once you begin making the actual "part" the pivot post will become quite rigid.  The working surface will have to be larger than the completed pattern (is that too obvious?) and it should be expendable or protected with an expendable rigid layer because "mold making" will follow "pattern making".  A good method is to use rubber cement to temporarily attach a .60 inch thick sheet of styrene plastic to a piece of heavy plywood or other working surface.

2. Attach a piece of sheet material, with the desired profile cut into it, to your pivot post.  The fit of the template pivot to the pivot post itself should be “snug” with no slop.  The template will represent one half of the desired final shape or the profile as it extends from the axis of rotation to the outer edge.  It should pivot freely around the post and extend all the way down to the working surface.  This profile template should be as rigid as possible yet the edge, which is cut to the profile shape, should be as sharp as possible (it doesn't have to be a "knife edge", 1/16 of an inch thick along the cutting edge will be fine).  Sometimes a stiffening rib will need to be added to the profile template (in the case of very large templates or templates made of thin material). You do not want the template to bend or flex.  A coat of  “Pam” cooking spray will help keep plaster from sticking to the template as you work with it. The tool is now ready for use.  Easy huh?

Making the pattern
(see illustration above)

3. Start construction of the actual pattern by building up the area which will form the interior of the pattern with balled-up newspaper.  Then cover that with layers of newspaper strips which have been dipped into a mixture of plaster (or Hydrocal), which has been mixed to the consistency of whipped cream and let harden (the same way model railroad mountains are made).  This should form a rough shape which is about ½ inch (or more) below the desired final outer shape.  Swing the template around the pivot to check the clearance.  This is only for the purpose of saving materials, if the final pattern is only going to be a few inches across then you may not need to do this step.  Instead, simply build up the small interior space with a lump of plaster mixed to a thick, stiff consistency.  If it is going to be 24 or more inches across then you will probably want to save the materials required to fill the void by following the above procedure.  This is a judgment call on the part of the builder.  Really large patterns may require an internal stiffening structure to help take the weight.

4. After the filler hardens you can begin building up the pattern to its final shape.  A rough coat should be built up to about 1/8th inch from the final surface no matter what size the pattern is.  In the case of a small pattern this build-up may only be one coat.  In a larger pattern you may wish to build it up in successive coats to keep from handling batches of material which are too large.  Your final goal here is a semi smooth surface, 1/8 inch below the desired final shape.  Once again, used the template by swinging it around the pattern to check for clearance.  Have a chisel or other flat tool handy to scrape off excess material and to push it into areas which need filling.  Fingers also make excellent shaping tools too so don't be afraid to get in there and get your hands dirty.  These “build up” coats are mixed thick (but not so dry that they are sticky and hard to work with) and applied quickly.  Wetting your hands periodically will help prevent too much material from building up on them.  Keep a bucket of water next to you to clean your hands and tools often while you work.  You don't want excess material building up on your tools during construction either.  By dipping your hand into the water and then letting the water drain off onto the wet pattern you can smooth it before it hardens, much like a potter at the spinning table.  Remember: it is always better to shape something while its wet rather than after it hardens.  Use a chisel to knock down high spots while the plaster is steaming and still semi soft.  At this point it will have a "wet chalk" like consistency.  Before it hardens completely, get in there and chisel off the parts that are too high.

The final coat

5.  Now your pattern is complete up to 1/8 inch below (approximately, this isn't rocket science so keep the calipers in the toolbox) the final desired surface.  The last coat to be applied to the pattern will be mixed to the consistency of whipped cream.  It will be quite runny and thin yet have just enough body to keep it from running off the pattern while working with it.  Be sure to mix your material well, kneading out all lumps until uniformly smooth.  Make sure your template is mostly clean of any dried plaster too.  It has probably picked up some during the previous steps and you need it to be as clean as possible now.  Spread a generous coat of this thin material over the pattern (just smear it all over like icing on a cake) and then swing the template around in a 360 degree rotation, making sure to hold in contact with the base (don't let it "skip").  Chances are that this first pass will produce a shape remarkably accurate in appearance yet now you have a problem, all the excess has bunched up in front of the template like a bow wave and is running down the side of the part.  Grab your trusty chisel and scrape it off.  But now there is a gouge in the part!  Don't panic.  Grab a little bit of the plaster from the mixing bowl and spread it over the gouge (don't use the material that you've just scrapped off. It may be too dry now or too wet,  get new material from the mixing bucket) and swing the template over it again.  Drip water over the part to prevent the surface from becoming too dry.  You will find that as you run the template over the part it may have a tendency to squeeze the water out of the plaster causing it to crack if the template passes over it more than once.  Simply drip water on it and gently run the template over the affected area again.  Keep working it until you have gotten the shape as good as you can get it.  But wait! Now it has defects such as ridges running radially from the pivot to the outer rim where the template must have skipped up over something during the turning process!  These are easy to remove with a sharp chisel (can you tell that a chisel is going to be important?) while the plaster is still steaming.  Simply run the chisel down the ridges and cut them off.  The wet forming is now complete.

Finishing

Chances are that the pattern still has various minor defects, rough areas, slight ridges or low spots.  Wait until the pattern is completely dry (it may take a day or two or three, depending on its size) then use all the normal techniques for finishing any pattern.  Dry plaster or Hydrocal will take putties and fillers and can be readily sanded.  Once final shaping is complete, coat with automotive primer (I use gray "Rustoleum"), sand smooth with fine grain paper and finish paint to a high gloss.  Apply several coats of gloss paint (I get best results with "Krylon").  Use black, it shows surface defects well, then rub it smooth with rubbing compound (available at most automotive supply stores).  This surfacing process includes the base too, as it will form the separation point of the mold. Make the mold out of fiberglass or RTV synthetic rubber.  In the case of a deep cone, such as a Gemini capsule (see illustration below), the mold may need to be split in two pieces if it is made of rigid materials like fiberglass.  Mold design is another issue.

Method of making long
"spindle shapes" such as rocket bodies
(see illustration below)

Now you can make patterns for any subject that can be made on a lathe.  The Enterprise from the original Star Trek series is an excellent subject for this process since it is all axially symmetric shapes (except for the pylons).  Have fun.