Tools for Pressing Rockets

WARNING AND DISCLAIMER:  If you are underage, then consult with your parents or guardians before attempting any of this.  You are on your own - I'm not responsible for your actions or harm you may bring to others because of your actions.  Making the items described below  can result in injury or death to you or people in your vicinity. Some things mentioned here may be illegal to make in your city, county, state, or country so check the laws that apply to you before you attempt anything described here. These notes are not complete on purpose. If you are reading them and new to pyrotechnics, then you are making a mistake. Stop now - this page is not for you. Get a beginning book on fireworks (see Skylighter or American Fireworks News (very quick shipping)  for a start) and read up. You can't make any of this work without more information so read up or join a club or ask someone to help you.

For an advanced tool, see powerpack.html.

You will need to press rockets if you use compositions other than BP or if you want to insure your BP grains are consistent. Hydraulic presses are commonly used for 3/4" and bigger rockets. Arbor presses can be used for 5/8" and smaller rockets.

10 Ton Grizzly Press with gauge (grizzly.com)

A 10 ton press is a good size to start with. Smaller and larger will work, but the 10 ton seems to be suitable for most hobby pyro uses. Some presses come with a pressure gauge on the hydraulic bottle (get one of those if you can, it is much more convenient). At the time of this writing, the Grizzly press is a good example - see the above picture. This gauge will read the pressure in the bottle so you have to convert that to piston pressure and that to pressure on the composition. Once you figure it all out (don't worry, you can do it), you can make a chart for the common rocket sizes and won't have to calculate it again. See the next section  for a complete discussion on calculating pressures on the compositions.

Strap reinforced H frame

Many H frame presses (like the Grizzly in the first picture above) will flex a bit when the pressures get high. This can cause the ram going into your rocket motor to lean over and damage itself or your tooling. You might want to reinforce your press with cross straps. They are easy to install - requiring only a drill and a hacksaw for tools and some 3/16 x 1 inch black iron (available from hardware stores). The tighter you can make the press frame, the better it will be. Some advocate squaring the press frame and then welding it together to eliminate flexing as much as possible.

 

Shielding

I recently saw a video of the results of a six pound motor exploding on a press. The incident made up my mind to over-shield my presses. So here is the result of me getting a bit paranoid.

As you can see - the shield covers the press! It is a 5/16" steel steam tunnel cover. The cover itself weighs 180 pounds.

Here is the side view. It is easy to put a rocket motor on and step back and run the hydraulic controls. The whole thing is on a dolly so it can be moved with just a simple push.

Making A Rocket Press From Scratch

(sort of)

While the seed of the following project was a Harbor Freight log splitter, the resulting modifications were significant enough that making one from three pieces of I beam would have been just as easy. 

You can build your own press from channel or box iron or I beams. Look at the Grizzly press in the very top picture to get an idea of how to construct one. Most rocket press makers, when making their own, make them narrower than the general shop press.  The one below is an attempt to make a lighter weight press that can be transported easily.  The log  splitter is rated at 10 tons but six tons or so is more like it - during tests the back beam starts to slightly flex at 10,000 lbs on the jack. However, that is good enough to press 4 lb rockets. Indeed, up to about 10,000 lbs, it is just as stiff as my larger reinforced H frame press.

This kind of press either needs to be strapped to the back of a bench top or it needs a base plate made (from angle iron or similar) so it is stable with in use.

The log splitter as it comes from Harbor Freight

Add ears to the top of the beam to hold a blast shield. Add a plate with a 1.5" pipe cap on it to the 'V' part.

Remove the jack that comes with it and replace with a vertical jack. Weld a U bolt base to the main beam (made from flat iron). The U bolt base is shaped to match the contour of the jack body. It is important with this kind of press that the jack be very firmly secured. The base of the jack is drilled for 1/4" bolts to secure it to the base of the press.

Finished beam assembly

Reassembled press with 7 ply blast shield. Extensions can be added to the pipe cap to accommodate different rocket lengths. The spring return on the jack is not strictly necessary. The base plate on the jack was modified from the one on the log splitter; however, you could just as easily make one from scratch. I've since fallen in love with this press, replaced the plywood with 1/2" Lexan and now use it as my main press. It works great.


 

 

When you start pressing with a hydraulic press, you will need to put a sleeve on your tubes. See: http://www.wichitabuggywhip.com/fireworks/rockets/extras.html for more information about sleeves.

Wolter PtoF Gauge (www.wolterpyrotools.com) 

If your press doesn't have a guage, you will need a PtoF  gauge and learn how to convert the gauge reading to actual pressure on the composition. Pressure gauges are available from a couple of places, including wolterpyro. There are lots of threads on that on rec.pyro, UKRocketry, Passfire, etc. See just below this section (here) for a summary of how to calculate actual pressures on the composition.

 

Harbor Freight 1 Ton Arbor Press (www.harborfreight.com)
 (these have to be modified to get a longer throat for rocket pressing as in the one below)

1/2" threaded rod and some black iron make a simple extension for the arbor press

If you want to get by cheaply and use smaller rockets, you can use an arbor press without a gauge. Just put about 80 lbs on the handle when pressing each increment. Why? With 80 lbs on the arm of a 20:1 1 ton press, there is about 1600 lbs on the arbor piston and, for a 5/8" motor, about 4800 on the comp. With smaller rockets, you can put proportionately less on the handle. You can also put a torque wrench on the arbor press to get consistency. The picture below shows an arbor press with a 1/2" grade 8 bolt installed on one end and a torque wrench attached. That arrangement lets you get very good consistency.

A micrometer torque wrench (3/8") attached to an arbor press


With rockets larger than about 5/8", go to 6500 to 8000  lbs on the comp and use a hydraulic press.


Calculating Pressure on the Composition

Pascal defined pressure calculations in a closed system - once you go through the wuz-wuz of understanding his law, you can KNOW why the following works. However, until then, you can figure the pressure on your composition by doing the following.... BTW - only do this once - make a chart and tape it to your press.

Find out what the ram that goes inside your rocket tube is receiving in pounds. If we are using a Harbor Freight one ton arbor press (see pictures above) and exert 70 pounds on the handle, then about 1400 lbs will be felt on the ram that goes inside the rocket motor because there is a 20:1 ratio on the Harbor Freight one ton arbor press. However, check to be sure with your press - it may be more or less than 20:1.  Let's say the ram is 5/8" in diameter. You can calculate the pressure on the composition by using the following formula:

Force = Pressure x Area
or
Pressure = Force/Area

We are solving for Pressure, so we need to find out what the Area of a 5/8" ram is and divide Force (1400) by that. The area of the end of a round ram, in this case the one going inside your rocket motor, is calculated using the following formula:

Area of a circle = Pi x Radius2

1. Find the Radius of the tool end:

If you have a 5/8" tooling, then (5/8)/2 is the radius. That is  5/16ths or .3125.

2. Find Radius squared:

.3125 * .3125 = .0977 (which is Radius2)

3. Use Area of a circle formula to find square inches:

.0977 x Pi  or  .0977 x 3.1416 = .3068 square inches on the end of a 5/8" round ram

4. Use (Pressure = Force/Area) formula to find pressure on composition:

1400/.3068 = 4563 lbs

The pressure on the composition of a 5/8" motor when applying 70 pounds to the handle of a Harbor Freight 1 ton arbor press will be 1400/.3068 or about  4563 lbs (very roughly)

Of course, these calculations are rough guides - the first ram on a cored BP tool set will not be round and blunt at the tip , and its area will be less because of the spindle hole. You will be pressing harder than you calculated for the composition over the spindle. That won't be a problem with an arbor press since you are working at the lower ends of the required compression for whistle and BP anyway. In addition, perfect transfer of 20:1 is seldom realized. 

To be very accurate, you could use one of the PtoF gauges sold by Wolter (see picture above) and know exactly what was applied to the top of the ram. Use that number, divide by .3068 for a 5/8" tool and you are done. However, it is more important to be consistent that extremely accurate.

If you use a hydraulic press that has a bottle gauge, you will have to make interim calculations to find out what the pressure is on the ram going into the motor.  Here is an example:

The gauge on the Grizzly reads hydraulic pressure inside the jack - and that pressure will be applied per square inch across the entire area of the jack piston. The total force of the jack piston will be higher if it is more than 1 square inch and less if it is less than 1 square inch. 

To find out, you have to convert the pounds per square inch on the gauge to pounds (force) at the piston. To do that  calculate the area of the jack piston and multiply it times the pressure reading (this is the first formula at the start of this section).  Force = Pressure x Area

For instance, if you are reading 2000 lbs on the gauge on the Grizzly and the piston is 1.5 inches wide (1.77 square inches), then the force delivered to the top of the rocket tube ram will be 2000 * 1.77 = 3540.  Note the internal diameter of the piston in a jack is usually about 1/2" bigger than the external diameter.

Now that you have the force on the ram going into the motor, you can calculate pressure on the composition (as we did at the beginning of this section). If you are pressing a 5/8" motor, then 2000 on the Grizzly gauge would equal 3540/.3068 or about 11,500 lbs! At these pressures, the composition tends to flow around the tool and cause it to stick. If you have this problem and still need these pressures, then use smaller increments of composition when pressing.

 


Sample solutions for several motor sizes:

1/4 (0.250) area = 0.0491

force applied to ram in motor:  250 lbs = 5092 on comp

3/8 (0.375) area = 0.1104

force applied to ram in motor 500 = 4527 on comp
force applied to ram in motor 750 = 6790 on comp
force applied to ram in motor 1000 = 9054 on comp

1/2 (0.500) area = 0.1964

force applied to ram in motor 750 = 3819 on comp
force applied to ram in motor 1000 = 5092 on comp
force applied to ram in motor 1250 = 6366 on comp
force applied to ram in motor 1500 = 7639 on comp
force applied to ram in motor 1750 = 8912 on comp

5/8 (0.625) area = 0.3068

force applied to ram in motor 1000 = 3259 on comp
force applied to ram in motor 1250 = 4074 on comp
force applied to ram in motor 1500 = 4889 on comp
force applied to ram in motor 1750 = 5704 on comp
force applied to ram in motor 2000 = 6518 on comp
force applied to ram in motor 3000 = 9778 on comp

3/4 (0.750) area = 0.4418

force applied to ram in motor 1500 = 3395 on comp
force applied to ram in motor 1750 = 3961 on comp
force applied to ram in motor 2000 = 4527 on comp
force applied to ram in motor 3000 = 6790 on comp
force applied to ram in motor 4000 = 9054 on comp

7/8 (0.875) area = 0.6013

force applied to ram in motor 2000 = 3326 on comp
force applied to ram in motor 3000 = 4989 on comp
force applied to ram in motor 4000 = 6652 on comp
force applied to ram in motor 5000 = 8315 on comp
force applied to ram in motor 6000 = 9978 on comp

1 (1.000) area = 0.7854

force applied to ram in motor 3000 = 3819 on comp
force applied to ram in motor 4000 = 5092 on comp
force applied to ram in motor 5000 = 6366 on comp
force applied to ram in motor 6000 = 7639 on comp
force applied to ram in motor 7000 = 8912 on comp

1 1/8 (1.125) area = 0.9940

force applied to ram in motor 3000 = 3018 on comp
force applied to ram in motor 4000 = 4024 on comp
force applied to ram in motor 5000 = 5030 on comp
force applied to ram in motor 6000 = 6036 on comp
force applied to ram in motor 7000 = 7042 on comp
force applied to ram in motor 8000 = 8048 on comp
force applied to ram in motor 9000 = 9054 on comp

1 1/4 (1.250) area = 1.2272

force applied to ram in motor 4000 = 3259 on comp
force applied to ram in motor 5000 = 4074 on comp
force applied to ram in motor 6000 = 4889 on comp
force applied to ram in motor 7000 = 5704 on comp
force applied to ram in motor 8000 = 6518 on comp
force applied to ram in motor 9000 = 7333 on comp
force applied to ram in motor 10000 = 8148 on comp
force applied to ram in motor 11000 = 8963 on comp