The Double-Petal Ball Shell, and A Jig For Making Them

by Victor Chaney

I have made a jig for easy creation of double-petal ball shells: an inner burst of stars, surrounded by an outer burst of different stars.

I had so much fun making these shells that I didn’t want to make any more single-petal shells. Twice as many stars, and more is better! Two kinds of stars mean prettier fireworks.

I got inspired to do this by Cliff Pearson, a master firework maker and teacher of fireworking techniques.

Making double-petal shells

This technique of double-petal shell making involves creating an inner half-shell to hold the inner stars.

One technique involves placing a 3-inch ball shell hemi to hold tissue paper in place while burst powder is packed around it.

It is possible for that hemi to slip around in position or depth, and attention is required to hold it still while packing the burst.

The jig I designed holds a half-ball suspended and fixed in place, so that both hands are free to pack in the burst powder. 

The burst powder needs to be packed in firmly to hold the outer stars in place, and to give a rigid tissue-lined hemisphere to hold the inner petal of stars.

First, we'll cover the technique, and later, how to build the jig out of wood or 3D print it out of plastic.

It starts like making a single-petal shell, placing the stars around the inside of a hemi. I find that spraying a layer of Loctite spray adhesive inside the hemi helps hold the stars in place.

Building the First Layer of Stars in a Shell

A larger ball half is wrapped with tissue, and placed into the stars, to get the tissue down into the hemi without disturbing the stars.

Inserting Tissue Paper Barrier Into Shell

Now the ball half is removed, and some burst powder is put into the bottom of the tissue.

This gets burst at the bottom of the shell, where it is hard to pack it in after the jig is placed. Here, I also caught on to using a cardboard take-out soft drink holder as a stand for the hemi, with the aluminum pan to catch the spills.

Burst Powder Added to Shell

Tissue is wrapped around the half ball of the jig, and the jig is fastened into position. Burst powder is packed into the space with a wood dowel. If it is desired to put a rim of different stars just below the rim, leave them out until the burst is packed up to 1/2 inch short of the top, and then those stars can be placed and held in place by the burst powder as it is added.

The Double Petal Jig

I found out about making double petal shells from Cliff Pearson about 2 weeks before the big Western Winter Blast firework convention put on by the Western Pyrotechnic Association in Arizona, 2020.

I pumped Cliff for information on how to do it on the phone, then looked at fireworking.com for more tips and came up with a wood jig to make it all easier.

The jig holds a half ball of wood suspended into the Hemi shell, so that burst powder can be placed inside of the outer layer of stars, saving space for the inner layer of stars. Below you'll find information on both 3D printed and wooden versions of this jig.

A 3D Printed Jig 

One way to make this jig is to print it with a 3D printer. Once it is designed, one merely prints the pieces, glues them together, and there you have it.

This is designed for a 6-inch ball shell. By using the scaling functions of 3D printing software, jigs can be made for other-sized ball shells.

I think 6-inch shells are a great size for this. I made wood jigs for 5" and for 6" shells, and the 6" shells were so much fun that the 5" jig did not get used much at all.

3D Printed Jig

The 2-7/8-inch diameter half ball with its attaching connection is screwed to the upper crosspiece. This way, different sized balls can be used, even giving the possibility of making a triple petal shell. If you don’t have a 3D printer, it is possible to have a 3D printing service print them for you (or consider making the wood jig, which does the same thing).

The design of this 6-inch jig was done with Autodesk Fusion 360. This program is free for hobby use.

My Fusion 360 design file may be accessed by downloading the ball shell jig files.

This includes the design file that can be opened in Fusion 360, and the .stl files that are ready to print. The ones with disks have “mouse ears” on the corners to aid in sticking to the print bed, especially useful if you print in ABS. The disks are easy to trim off after printing.

The .stl files may be used with a slicer program to load into your printer, or uploaded to have the parts made for you.

3D Printed Jig Pieces

The bottom pieces are printed with supports everywhere for the overhanging lip that rests on the edge of the hemi, but the horizontal holes for the eye screws are blocked from supports with support blocker. 

The other parts are printed with no supports, and are almost all printed right-side up, except the ball is printed with the round part facing up, and the ball connector is printed with the gluing pegs up. I used 3 mm wall thickness and 35% infill. For ABS, I used a glass bed heated to 100°C, covered with a layer of Kapton tape, and MagiGoo Original as an adhesive. 

The printer is enclosed. Nozzle temperature is 243°C. I printed two bottom pieces together, and the crosspiece, ball, and ball connector together in a separate print.

Four pieces are ready to fasten together. Two circles of wood are made to hold the bottom supporting pieces in the correct position, and the crosspiece has matching holes for the pegs sticking up from the lower pieces.

Gluing the Pieces

Wood circles are 5-5/8" and 6-3/8" in diameter. Pieces are put onto the wood circles, and test lined up before gluing.

You could also draw the circles on a piece of paper and clamp the lower pieces onto the circles. There are pegs in the bottom pieces that mate with holes in the crosspieces to help line it up.

For ABS, they are glued with clear PVC glue, sold for gluing PVC pipe. For PLA, roughen the glue surfaces with sandpaper and glue with epoxy. Wet both surfaces in each glue joint before assembling.

Gluing Jig Pieces

I used lead weights to hold it in place while the glue dried. Woodworking clamps also work well here.

There are holes printed in the sides to place the eye screws.

The size of the holes is important. If they are too small, then the plastic will crack when screwing in the eye screws. They are drilled out with a #2 drill (0.221”).

The eye screws are brass, as steel screws would present a danger from possible sparks, which could be a big problem while working with pyrotechnics!

My jig is printed with ABS plastic. It has good strength, and also slight flexibility to help prevent cracking when putting in the eye screws.

Using PLA plastic will probably work fine, especially Tough PLA, but you need to be careful with the plastic softening and distorting if left in a hot car.

The holes need to be just the right size for PLA to avoid cracking the plastic when placing the screws.

The eye screws didn’t seem to be available in brass machine screw threads. There are some nice knob screws available for a big jump in price. The brass eye screws work well and require no tool to tighten. Again, don’t use steel! 

Brass eye screws can be found at mcmaster.com part number 9497T36. 

The brass wood screws for attaching the ball are brass #8 x 1.5 inches flat head wood screws.

The holes in the crosspiece are drilled out with a #19 drill (0.166”) for a loose fit of the screw shank, and the holes in the ball connector (glued to the ball) are drilled out to a #26 drill (0.147”) to engage the threads of the wood screws.

The Wood Jig