Tuesday, March 10, 2015

So, You Want To Make A Minigun

Hey everyone, This is Killbucket, formerly of Air-Sharp.

I now have my site at mrminigun. Sorry, but I no longer offer the CAD packages. I'm sorry to say, they resulted in few completed costume M134's, because they took a depth of skill outside of what I'd originally planned. I also emphasize this point: it was my OLD design work, belt drive with heavy truck bearings as part of the layout. Not easy to completely see through.

You can really, really get mired in the details, and un-neccessarily so. From insanely detailed to cartoonishly pastiche, a costume minigun WILL get Love. They are just inherent crowd pleasers. Everybody wants a picture taken with one. Walking into a room with a minigun really steals a show in a BIG way.
About the WORST mistake you can make in building YOURSELF a mini is: throwing money at it. Go cheap, because you WILL make the parts more than once, and changing your mind mid-stream is a LOT easier if you aren't "married to" what you have together already. And making something on a budget just naturally enhances creativity.
Make your objective a project that can be completed. Focus on the biggest parts, and the most robust construction to ensure an item with longevity and structural integrity. Skip extraneous detail in your first few builds (NObody ever makes just ONE mini, it is its own "sickness", making these. So make plans to get the main parts clear, then start elaborating on them.) , and make a really simple setup you can throw around and let people handle, minus explanation or warnings.
I'm sure you've seen countless home made costume miniguns. Most are throw-together amalgamations, components held in situ by whatever means were available. There's nothing wrong with this, because miniguns are confusing jumbles of parts for most observers. As long as the general out line suffices, it's all very well and good for most of our needs. Getting down to the nitty-gritty in detail may be possible, but it will go un-noticed by most observers.

Keep the overall weight of your chosen components in mind when designing parts and especially how they attach to each other, and the main assembly. Hardware seems lightweight, but individual bits really add up in number, FAST. Try to have single fasteners do multiple tasks, and eliminate that which does nothing. For an elaborate build, materials choices can really help. Balsa and PVC foam are great replacements for smaller, un-stressed details, and are easy to whip up with normal hobby tools.
For a given actor or user, the finished size of the unit must be realistic. We aren't all built like Heavy Weapons Guy. Make it wieldable. Overall lengths over 3 feet are NOT crowd-friendly, weights over about 12Lbs are tedious to hold at events, or for retakes in filming. Long ammo belts look great but limit mobility. Incidentally, ammo belts of any length that look good are NEVER cheap, period.

Ideally, the minigun's CG should be located such that holding only the front carry handle allows the gun to hang level or almost level. This allows an actor or wielder to pivot the weapon naturally, and makes long-term carry much more comfortable. If you add a Body Harness Ring like I do, be sure it also balances the unit.
I made my first fake minigun builds (over 500 units ago) out of foamed PVC board and plumbing parts, and all the PVC components were cut out using a coping saw and a bar clamp, out on the kitchen table. I glued everything together using plumber's cements, and even put airsoft M14's into a few of them. This was all chronicled in the old Killbuckets website, which no longer exists. Today, I build exclusively with 6mm thick ABS sheet from Interstate Plastics, and the minigun parts are machined by my ShopBot CNC.

BELT DRIVES: are simple are relatively easy to make up yourself. You can use an ordinary RC car motor and a Kenmore vacuum cleaner belt to spin a barrel pack. An aluminum or steel tube can be used to extend the motor shaft. It's a bit fiddly to get the belt tension just right, but the trade-off is something that almost any home cobbler or tinker can construct. The downside of belt drive is that it side-loads the barrel pack, and bearings or lubrication become critical. Employing a conservative Duty Cycle is a must. Running the minigun too long can overheat parts.
GEAR DRIVES: have other considerations: reliability is paramount. In a costume minigun, the barrel pack normally sees very little torque inputs. It spins freely outside of the drive mechanism. However, add one ham-fisted "friend". Stopping the barrels suddenly or blocking them while driving the motor puts tremendous loads on gear teeth.
Availability is elusive, even the largest RC Truck gears come up short: cost-prohibitive and intolerant to errors in installation. I was unable to purchase suitably large-toothed gears, my calculations specified 2- module gear teeth, larger than any fullsize truck uses on the starter! In the end, I designed and now make all of my own CNC cut gears. The teeth are over a third of an inch high, and are estimated to take several times the loading that they will ever see.
Some tool catalogs list plastic gears with larger teeth, for some builder's use, these may be suitable. Grainger's and McMaster Carr are two sources, but prices are stiff.
The final speed of the barrel pack is also an important design issue. The current M134D fires at max 4,000 rounds per minute. Divided by six barrels, that's 666.6 RPM at the most. For safety, 500RPM is the maximum that I would recommend for a costume or display minigun. Also, the barrels must easily disconnect from the drive train without damage if obstructed or accidentally caught in something. Most belt drives would pass if tested for this feature, but there can be an expected trade-off in slippage vs safe disengagement.
I used to employ ordinary RC Car NiCad battery packs in my creations, with separate packs and isolated wiring harnesses, if the item also contained an airsoft setup. This is because barrel drives draw considerable amperage, and the airsoft gun's firing speed would suffer otherwise. A cordless drill power pack is ideal. I use the 18V packs meant for Harbor Freight Drills, because they are universally available. Spare charged packs are a must for filming unless you have some type of DC power source. To connect the pack to the minigun, you can run a cable to the gun inside your ammo belt, or stash it under the gun on a quick-release dock, like I do.
On the gun, 16ga wire suffices, but for umbilical or power cables, use at least 14ga or larger. Trigger switches become a problem.
The tendency and usual design process is to incorporate a video game joystick handle for it's looks, ergonomics, button layout, and ease of purchase. However, most are lightly constructed, containing digital switches that are limited in current capacity.
So a Relay-equipped wiring layout was a must, to allow both circuits to operate together (or independently if you want to spin the barrels without "firing"). Mandated use of relays or switch substitution couteract the ease of adaption to DC motor switching: It's important to note that the suitable heavy-duty microswitch will not fit inside any current vidgame controller, and I have tried almost all of them. I use quality 5-amp rated microswitches mounted in custom-designed handles. Any airsoft guns get automotive, 30-amp relays. I rarely if ever make one of these nowadays.
Making a multi-barrel Gatling Gun pack can drive you CRAZY. First, you need all six barrel tubes to be of equal length. Some tolerance in length is allowable, but extreme differences will stand out like a sore thumb. Also, the tubes need to be of a reasonably rigid material, I've found plastics to be great for static models that do not spin, but on costume model miniguns with a powered barrel pack, the curvature of plastic pipes will be apparent to observers. Metal conduit makes for a much more solid assembly, and straightness is not an issue.
The separator or spacer discs for a minigun barrel pack will also take care and planning to execute successfully. My early efforts at cutting round discs were troublesome, and getting the holes equidistant from the axis took great care. But it can be done, and enough times to make a barrel pack (mine contain seven discs, two are just spacer rings) that can be presentable. Mine are all CNC cut, identical. My best advice is to make one and then use it as a pattern for the rest.
Assembling the barrel pack straight and true can also be accomplished with care and planning. I start with just two of the barrel tubes, the drive gear, and one barrel disc. Clamped up or in my production jig (sorry no-can-see!), getting the initial two tubes located is key. Once those are bonded or other wise fixed into place, the rest will align properly, given matching hole locations in the barrel discs. Liberal amounts of Loctite Professional Super Glue in slotted assemblies hold mine together.
The barrel discs themselves need to be axially and radially aligned to their rotational axis. In other words, when they spin, you don't want them walking end-to-end, or wobbling side-to-side. This spoils the visual effect if it's excessive.
A slight amount of misalignment is OK, but the best test is spinning the pack to check its behavior. Again, alignment jigs or blocks go a long way in getting parts tightly aligned, as well as taking time in the process. The barrel pack is THE most "make it or break it" item in the assembly, from an aesthetic standpoint. Be prepared to make a second one, to preserve your sanity.
The spacing of the barrel discs themselves is mostly a cosmetic or user preference. Pictures of real M134 miniguns show one layout, most costume, movie, or prop miniguns vary. The thick discs in the middle of the pack double as the bearing point for the front of the pack, so the center axle must reach at least to that point in the assembled gun.
This can be silly-expensive, or laughably cheap. I stand behind my exhortation to GO CHEAP unless you plan to make a row of them: repeatability changes almost everything. Making ONE mini, you can just wing it, fudge things and figure it out as you go along. Found objects being used, literally ensures a UNIQUE and recognizable prop gun.
Planning to make even two, let alone two hundred, minigun movie props dictates careful choice of all components, and also underlines the need to repeat operations and finishing in similar fashion. Availability and cost of individual components can easily derail the process. Ensuring a supply chain, monitoring quality, and incorporating customer feedback into product improvements go a long way towards a repeatable success.
You can do this.

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