Grain Casting Fixture

 

 

  

 
 

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Casting Types

Casting methods first depends on the type of grain.  You can cast the propellant directly into the motor case which means you are using a case bonded method.  You can cast a single long grain or multiple short grains as with BATES grains.  You can also cast a number of types of cores besides just a round core.  See Propellant Configuration on the Overview page.  What I am going to discuss here is a casting fixture for BATES grains for a 1" diameter PVC motor.

For a number of reasons I decided to start small.  My first thought was to be able to make motors for low power model rockets.  I could also make a lot more motors and test a lot more a lot faster and for less money.  Another reason is that I hadn't seen much in the way of small sugar motors on the web and so wanted to fill a semi-void.  I also was interested in making a larger number of grains and motors at one time -- batch mode.  I started with 1/2" PVC motors and found that the larger the size of grain, the easier to cast.

Casting Fixture Goals

I went through a number of iterations of making molds trying to find faster and easier methods of both making the molds and for casting the grains.  I ran into a number of difficulties and found ways to overcome them.  That is a big part of the fun of experimenting. 

The biggest problem is that I used steel core rods and first, the propellant wants to stick to the rods.  Second, when you push the rod through the propellant and through the bottom of the mold, a plug of propellant always is ejected through the bottom.  This plug then can stick in the hole below the bottom of the main holding fixture and then prevents the rod from going through the bottom on subsequent castings.

What I found worked best was to use a piece of sheet metal, I used galvanized steel roof flashing, with a hole the size of the coring rod with a larger diameter hole below it.  The thin metal acted to wipe the rode and the larger hole below it allowed the ejected propellant core to be ejected out of the mold.  To do this, you also need legs on the mold so there is a clear space below it for the propellant slug to fall into.  This way ejected propellant doesn't accumulate in the bottom hole.  You still need to either freeze the rods or use silicone grease on them.

Fixture Design

 

 

Fixture Fabrication

 

  
I used a piece of 2X4 for the fixture body, a piece of 1X4 for the base, and pieces of 2X4 for the legs. I just took some pieces out of my scrap box.  The piece for the body was already painted.  Also in the picture is the piece of flashing I cut from a roll and the adjustable wood bit.  The adjustable wood bit can only be used on a drill press and can rip large chunks of flesh off your body if you try to use it with a hand drill.  If you don't have an adjustable wood bit, you can use a flat speedbore wood bit that is larger than the OD of your body tube.  You will have to wrap tape around your mold tubes so they fit good in the fixture base so the core rod hole is aligned in the center of the grain.
   
The first thing is to make sure that you can align all the pieces, take them apart, and put them together again in the same place because you are going to drill the holes in the pieces separately.  To do this, I drilled holes in the four corners of all three pieces, the body, base, and wiper plate.  I then put nails that just fit snuggly into the drilled holes.  I also made a paper template of what I wanted the hole configuration to be.  In this case, two rows of four holes.  This was laid out to get the most room between the holes.  It is then just used to center punch the fixture pieces.
   

Here the holes are being center punched.  Notice that the base is the side being punched.  My paper template was a little short on one end but I made sure the set of holes were centered on my fixture and that the holes would not interfere with where I was going to mount the legs when I am done.

Next pilot holes are drilled where the board was center punched.  I used a 1/8" bit for these and used a drill press to make sure the holes were straight and perpendicular to the base.
   

The body is removed and holes are drilled through the wiper plate and the base.  This fixture will be used to make BATES grains for 1" PVC motor cases which will have 3/8" diameter cores.  This grain will work for motors with one to four grains.  When these holes are drilled, the wiper plate is removed and the holes in the base enlarged to 1/2" or 5/8" diameter.  You have to use a twist drill to enlarge the holes.  Alternatively, you could drill the wiper plate and base separately and use a flat speed bore bit on the base and a twist drill on the wiper plate but the alignment may not be quite as good.

Here the body holes are being drilled.  To use an adjustable bit, you have to drill several test holes in scrape wood and check the diameter with a piece of PVC pipe -- 1" in this case.  Be sure to tighten the screw on the bit very tight and check the diameter often because it will tend to get bigger.  The pilot holes drilled in the first step guide the adjustable bit.  When the adjustable bit breaks through the bottom, it doesn't do that with finesse.  It kind of makes a mess.  It isn't being done here but it would be much better to clamp the body piece to another piece of wood while drilling so that the splintering isn't so severe when it breaks through.

Here is the top side after the holes are bored.

This is the bottom side of the fixture body.  You can see that there is a lot of chunks of wood ripped off the surface where the bit broke through.

I decided to compensate for the splintering by sawing a slice off the bottom.  My 10" table saw would not saw all the way through the 3-1/2" width of the board so I had to saw part way through and then turn it over and finish it.  The surface was then sanded to even up the surface.  This step isn't really necessary because you will use pieces of PVC pipe in these holes and the pipe will fit against the wiper plate so the splintered areas don't effect the casting of the grain at all.  The ID of the PVC pipe pieces and the wiper plate are the surfaces that form the grain.

Here is the finished product with all the pieces screwed together with screws through the original alignment holes.  Notice that the legs are on the outside of the outermost body holes.  Now you should paint the fixture except for the holes so that spilled propellant won't stick to the fixture and so you can show off your fixture and show everyone what a quality carpenter you are. (Unlike me).
   
This is a top view of the fixture body with the PVC grain molds in place and showing how the core rod clearance holes are centered in the grain molds. This shows the propellant plug clearance holes in the base.  It would have been better to have 5/8" holes here but since I had drilled the 3/8" holes through the base and wiper plate at the same time, I couldn't use a 5/8" speed bore bit and I didn't have a 5/8" twist drill -- only a 1/2" twist drill.
The last parts to make are the cap which is optional and the coring tool, not optional.  The purpose of the cap is to center the coring tool as you push it through the grain. It also helps form the surface of that end of the grain.  Like the other side, you need aluminum foil as a liner inside the cap.  The first caps I made were what is shown in the middle for a 3/4" grain.  I didn't like how these worked because when I pushed the coring tool down through the propellant, it pushed the cap up off the top of the grain mold unless I didn't fill the mold all the way full.  It is difficult to gage how much extra room to leave.  so when I made my caps for the 1" mold, I put a bar across for the coring rod hole so the excess propellant could extrude out along the sides.  Then I just took a sharp knife and cut off the excess.  That worked a lot better.  I drilled the large hole in a larger piece of wood first, then drilled the coring rod hole, then cut out the cap from the larger wood, and finally sanded it round (sort of). The coring rod I use is steel. I had some scrap rebar that I used for the T-handles but the body here is 3/8" steel rod.  I welded my handles on but I have seen where other people have made wooden handles.  You need to be able to have some good leverage to twist and pull them out of the propellant grain and that is why I chose the T-handle configuration and welded them.  You could also use a straight rod and a pair of vice grips on it if you wanted to.  Notice the long taper to a sharp point.  I experimented with different configurations--round tip, short taper and finally the long taper.  The long taper to a sharp point was definitely the best.  You don't need a whole bunch of coring tools and caps for the smaller grains, at least 1" or smaller and maybe bigger.  They cool fast and you want to pull them out before they set up very much so generally, two or three are enough regardless of the number of molds in your fixture.

 

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