Comet Tooling

Comets are in a class by themselves. With a bright, fiery head and a long, full tail, they are one of my favorite display effects. Add to this the surprise of a crossette finish and you've got a real crowd pleaser every time. The tooling described on this page will enable you to make most varieties of comets for a 3 inch mortar. The tooling techniques should be scalable to any size comets you want to make.


First, we need to gather materials to use for the base and sleeves in the tooling set. I used a piece of oak with dimensions of 3.5" X 6" X 5/8". The sleeves are cut from a piece of 2.5 inch schedule 40 PVC conduit pipe. One is a 1/2 inch slice and the other is 2.5 inches long. Each piece of conduit has been cut along its length by using a coping saw with a very narrow kerf.
A 1/2 inch hole is drilled at the center of a 2.5 inch circle on the base block. A short length of dowel is placed in the hole so that it protrudes to a height of 1/2 inch. The other end of the dowel is flush with the bottom of the base block.
The inside of the 1/2 inch slice of conduit must be coated with vaseline or bearing grease to act as a mold release. It is then tacked in place by using hot glue. The exposed surface of the small dowel should also be coated with mold release. This assembly can now be used to mold a raised cylinder onto the base block.
I chose to use auto body putty because it's cheap and readily available. A $5.00 can of Bondo will be sufficient for this entire set of tools. Another option would be epoxy or some other resin system. Of course, if you have a wood lathe, you could make all the necessary shapes from wood and skip all the fuss of casting them. When you are done, the base should look like this picture. You should retain the small length of dowel as part of this tool set. It will be used to fill the hole in the base when you are pressing a comet which has no central core.
The 2.5 inch length of PVC sleeve is now used in a similar manner to cast a rammer piston with a hole in it. It is placed over the base cylinder to hold it in place, so it shouldn't be necessary to tack it down with hot glue, as before. Again, the inside of the sleeve must be treated with mold release grease. The dowel is cut to be even with the height of the sleeve and is wrapped with a few layers of tape so the hole in the piston will be a little larger than the dowel. I also placed a circle of wax paper in the bottom to protect the base cylinder. The wax paper and the dowel are also treated with mold release. Unless you are extremely skillful at using Bondo, I suggest you cast the rammer piston in several steps instead of with one batch of putty. Bondo will gel within 4 minutes, which gives you very little time to mix it, get it into the mold and get out the air bubbles before it's too late. I used three small batches of 80 grams each. Again, if anyone could suggest a better resin system, I would love to hear about it.
With a little luck, your rammer piston should look like this.
These rammers have been formed using the same techniques described above. The solid one on the left is used to press comets that have no central core. The one on the right is used to create the interface between priming compositions and hard to ignite comet compositions, such as flitter and other high metal formulas. The shape of the rammer face produces ridges of comet composition that are covered with prime composition. These ridges are much easier to raise to ignition temperature than a flat surface would be. This rammer was made by cutting pieces of hardwood to form the ridges and gluing them on the face of the rammer with epoxy.
The last part of the tool set is the crossette insert. I started by marking the face of a 1 inch hardwood dowel as shown. The arcs are drawn with the aid of another piece of 1 inch diameter dowel. A hole is drilled in the center to accommodate the insertion of a tapered pin later. A length of 11/16 inch of the drilled dowel is cut from the end.
The opposite face of the dowel piece is now marked with the same pattern inside a 3/4 inch circle. This time, the arcs are drawn using a piece of 3/4 inch dowel. The pattern must be precisely aligned with the pattern on the opposite face.
Straight cuts are made which create a square on each face of the dowel, one being smaller than the other. This creates the desired taper in the insert.
The curved surfaces are carved out by any of several possible techniques. A Dremmel tool would work nicely. I used a straight edge bit in a router mounted in a router table. Even hand carving with a pocket knife would work if you have a little patience. To finish the crossette insert, the completed dowel piece is glued to a short length of a 1/2 inch dowel and the tapered pin is set in place with epoxy. The length of this pin will determine when the crossette will burst while in flight. I make the pin a little long and shorten it to taste after experimenting with a few test comets. Finally, the shaped part of the insert should be sealed with a good finish of some kind. I used a thin coating of epoxy followed by light sanding.
In use, the insert is placed in the base, as shown, to form the burst charge cavity in the end of the comet body. The shape of the cavity is specifically designed to ensure that the crossette will burst into 4 equal pieces, creating a beautiful glittering cross in the sky.

This is the complete set of comet tooling for 3 inch comets. There is a short dowel to fill the hole in the base cylinder when pressing solid comets. Another longer length of dowel is used to press comet "cookies" with a hole in them for cored comets. These cookies dry out much sooner than entire comets would. They are then stacked and glued together with nitrocellulose lacquer. Next there is the crossette insert followed by three specialized rammers. One for pressing cored cookies, one for pressing solid cookies and one for priming flitter comets. There are two PVC sleeve lengths. One is used to press the cookies, which are 1/2 to 5/8 inches thick. The longer sleeve is used to press crossette sections and complete comets when the cookie method is not used. Both sleeves use hose clamps to prevent expansion during pressing. Last in line is the base, which has a 1/2 inch hole in the center of the raised cylinder. When you are ready to used these tools to create breath-taking comets, check the forthcoming projects pages to see them in actual use.

Your feedback is welcome. Please offer suggestions, refinements, critiques or cudos in the submission form below. You must be a registered pyro enthusiast (or at least a wannabe) to contribute.

fogoforumadmin	Feedback from Richard J. Kinch - Jan 24, 1999	October 26, 2007
Resin casting suggestions: Silicone grease works splendidly as a mold-release agent for polyester resin. Spread it with a fingertip as thinly as possible. The pistons slip effortlessly from the PVC tubes. This grease is sold in small plastic containers for lubricating O-rings in plumbing and swimming pool apparatus. It won't melt from heat like Lloyd's petrolatum (although with my other ideas below the mold will never heat up anyway). To make a mold bottom, cut and glue on a disc from heavy overhead transparencies instead of Lloyd's aluminum disk. Use cyanoacrylate glue and make an airtight joint (you can see the seam through the plastic sheet, and this will require a clean, square tubing cut) for purposes explained later. The cheapest transparencies at the office supply are the suitable PET plastic, like soda bottles (which would be another source if you flatten the bottle curvature). No mold release is needed for PET, you can see if any bubbles are stuck on the bottom, and you get a perfectly smooth casting. I believe this is in fact what is sold as costly "release film" for use with Bondo. The expensive transparencies for copiers or laser printers are some other plastic (acetate?) which may or may not work. Put a pinhole in this clear bottom, and use compressed air (even a bicycle pump or a good puff on a rubber tube would do) to gently push the piston out with an even, pneumatic force (1st reason for an airtight glue seam) while it is solidified but still soft. Immerse it in water to keep it cool and prevent distortions and cracking. As soon as it is firm, but not rock-hard, use a file to dress the piston corners. One might also try sanding off the concavity at this time, if you're a perfectionist. A mold top works almost the same way as the bottom, but stays loose instead of being glued on. Pour resin to overfill the tube slightly, then push on the plastic piece, and move it around to work the bubbles out past the tube edge. Peel it off as soon as the resin just starts to solidify, releasing the suction caused by the material shrinkage. This way all the shrinkage concavity will migrate to the top of the piston (composition end) where it doesn't matter, instead of on the bottom (press plate end) where it should be flat. Suction at the bottom will prevent shrinkage from forming a concave bottom, since the mold is airtight there but open at the top. To mark a fine calibration groove in the piston, re-insert the hot casting into the mold so the shoulder of the tube is at the location to be marked, and scribe the cheesy resin with a pin or sharply cut piano wire, using the mold tube's shoulder as a guide. For added flair, put your initials or other trademark on the top PET mold sheet using a bead of epoxy, or attach any solid convex shape. Then your piston will have a mold for your mark, and the composition pucks will have a positive image. This serves no purpose other than the vanity of the toolmaker. The PET plastic could also be used to avoid having to make the wooden base you describe for casting the comet tools. The clear plastic can be marked to facilitate alignment.

jeffrocket55 nice tooling January 19, 2009
when i make celluclay / burnhams watter putty clay rouns hemis I coat the plastic shell with wesson oil makes a great mold release even if you bake something in the oven it wont evaporate.You should do a pictorial tutorial on them.Make hemis for pennies!Love your site.Homemade is always better.Your tooling ideas are right on.