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Explosive Drop Testing

Performing a drop test on an explosive is a great method to judge its sensitivity. However, most hobbyists rely on a hammer to test it. This is inaccurate for multiple reasons - you could put in amounts of power without even noticing, and there is no accurate way of measuring it. Even if you let it free fall and use a yard stick, there are still too many variables. This is where a drop testing rig comes in. A drop testing rig is a device where you can place a small amount of explosive in, and drop a weight at incremently increasing heights until it sets off the explosive. It is a very controlled system, using something to direct the weight, and it only uses gravity to decide the amount of force present. I have seen many representations of a device similar to this, all of which are rather shotty in both design and craftsmanship. Because of this, I had to design my own.

I wanted a rig that is easily adjustable, very versatile (for primaries all the way to insensitive blasting compounds), was very acurate, easily repaired (if neccesary), simple in construction, easy to transport, and above all performed to 100% of it's expectations 100% of the time. Since this is a very simple device, those were all easily obtained.

It consists of three parts - the chamber, the barrel, and the plunger. If you choose to build your own, remember - it would be hard to find the exact dimensions of every part that I have found. It would be more wise to build one to the same tolerances, instead of dimensions. For example, the inside pipe OD is ~1" (25mm) and the outside pipe ID is ~1" (27mm), just find two pipes that are 2 milimeters apart, roughly. This is so they slide in nicely. Above is a perfect example of why the emperical measurement system is crap.

This is the chamber.

It is a peice of steel pipe [OD 1 1/4" (32mm) / ID 1" (27mm) / 12" (305mm) long]. The hole was cut into it using a mill, however a dremel or even a simple file could be used to make it. I used a mill for three reasons: It was available, I knew how to use it, it would make the best looking hole of all tools). The hole is is 2 7/8" (73mm) long and 1" (2.5cm) wide. I simply eye-balled it. You can do the same, or round it to the closest value (3" or 70mm) if you choose to measure it. This was welded to a steel plate, 4 1/2" x 4 1/2" (115mm x 115mm) wide, 1/2" (13mm) thick. Again, dimensions don't matter, I just used what I found. Weld process was GMAW (MIG) because GTAW (TIG) wouldn't be strong enough for this material, and SMAW (Arc welding) would possibly weld the inside of the tube, preventing the plunger from making full contact with the base plate.

This is the barrel - possibly the simplest part to make.

It is only a steel pipe [OD 1" (25mm) / ID 13/16" (20mm) / 39 3/8" (1m) long] I had to sand about 1ft of the bottom so it would fit in the chamber easily.

This is the plunger.

It's basically the engine to this freight train. The plunger body is made of a 5" (127mm) long peice of 1/2" (13mm) diameter copper tubing. It has two end caps on it, one of which was sweat to the pipe, the other is not. The plunger handle/height gauge is a 3ft long (91.5cm) long peice of 1/8" (3mm) welding filler rod. This is attached to the plunger by bending a hook shape into it, and feeding it through a hole of similar diameter in the attached end cap, then covered in molten solder to prevent it from moving. The weight is checked and lead shot is added to bring it to 150grams total weight. The opposite end cap is simply pushed on, so if any repairs are necessary, it can be disassembled again. More plungers can be made in this same fashion, for example a 1kg, 2kg, even 10kg if an explosive cant be set off with the 150gram plunger. These can obviously be easily swapped and dropped in place of eachother.

All burs and flashings are ground off the pipes to promote a smooth operation, then the barrel is slid into the chamber. To load, lift the barrel up and place the desired quantity of explosive in the center of the baseplate in the chamber. Slowly bring the barrel back down until it makes contact with baseplate. Insert the plunger in the barrel, and set at desired height. This can be milimeters up to decimeters, you decide. It can either be held with your hand, or you can use a spring loaded clamp. I would recommend the spring loaded clamp in case the plunger ever decides to shoot back out. It shouldn't shoot out from pressure, as there is 1/8" of play all the way around the plunger for gas to escape. It's just for safety purposes and peace of mind that you use a clamp.

Above is a picture of the clamp with 6" (152mm) of drop armed.

Weighing in at only 8.5lbs (~3.5kg), including clamp and plunger, it's very easy to transport and takes up very little room. I have met all my goals I described in the beginning, so I declare this a 100% yeild - since it is a chemistry website after all. Changes to the design and other updates will be posted as made.