Why 6/8mm?

inthetallgrass said:

bobgengeskahn said:

That is the idea... however, I have a few modifications in mind for our applications. I'd like to try a boat tailed version and a concave version with a flared end similar to a pellet... this is all going to depend on when I can get a test platform set up.

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So like a blade BB. Also don't start with the dimpled BB's again.

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Yes, no more dimpled BBs. I believe Beck was trying to pull our chains

And yes again, similar to the blade. My whole design process has been to start with something along those lines and work to try to tackle some of the other design problems that I see with the blade to come up with prototype ideas and hopefully boil it down to 3-5 models and test them on a set barrel platform.
There area a few design parameters that I am trying to keep in mind with them, ease of machining is a big one, as well as injection molding. Right now I am debating the size issue (obviously : ). However, finding precision tube down to +/- 0.001mm or even +/- 0.01mm is near impossible, which means I many be stuck with a standard airsoft barrel :-/. Sad because I have a feeling that the flared end idea would work best... but at only 6mm it will be hard to get the fins to have enough surface area to be effective, and making them bigger would break the "easy to machine" contingent.

Gotta spend money to make money ... however this doesn't count when its money spent on smokes... I still cant figure that one out...

ME as in Mechanical Engineering.

I think boat tail, mostly because it work very well with match grade RS bullets.

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And this is my thinking for trying it, however the seal that is created in RS vs. the 'seal' (or lack of) that is created in airsoft may cause an unacceptable bleed of pressure. The other concern I have with a boat tail would be exposing the fins to the air flow from the cylinder (planning for a HPA set up) would cause the BB to rotate counter to the intended spin once it leaves the barrel. I may just be over emphasizing the effect in my head... all the more reason to test

On a slightly more serious note, I have been looking all day and have yet to find a way to build an acceptable mold. I have mainly been looking into injection molding and some rapid prototyping techniques. Anyone have any ideas for an accurate cheap way to cast things?

They wouldn't melt or deform? The silicone molds I have typically seen are very soft and would (I imagine) be prone to bloating when filled. Not particularly desirable in something where the tolerances are 0.001"

sniperninja said:

They have plastics that don't need heat, you just mix and pour it into the mold.

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I can imagine it wouldn't be the greatest quality... but... if it is some form of simple epoxy resin it could be pretty amazing stuff... as far as precision/weight go I'm not sure, but it'd be worth a shot over trying to make a precision injection mold, especially since I cant think of any way to break it down to less than 3 parts, and 3 part molds get very very... very complex to deal with :-/

Edit: Advantage to an epoxy based over an injection mold (if it is epoxy) would be that if it is done cold, epoxy generally expands in the curing process, where as with an injection mold there is always the possibility of air pockets getting stuck in the mold, causing the injection material to combust/the material contracting when cooling; all resulting in imperfections in size and tolerances. Professionals have very complex machines to get the consistency they do, i.e for mainstream BB production, however even for a store bought personal home press you run into a lot of these issues if you are trying to make precision parts.

There are a few ways to make molds depending on the manufacture process you're going to use. Most likely since this would be a "garage style" build I'll have to get one pellet built out of 7075 or some sort of steel and use that to cast off of... and it honestly wouldn't be that difficult as long as I can draw it in CAD first (I know some people that would be able to cut it, might take a while, but free is worth waiting ).

Another possibility would be to go straight to the mold and design that on CAD and have it cut, but like I said, designing a 3 part mold is very difficult to get right, especially on a 6mm by 12mm round, or even if I go to a 8mm. (I am still looking for a 10mm/0.40" tube since that is where I'd like to end up, just cant find anything precise enough to be the barrel, so if anyone knows anywhere...)

Unfortunately with both of these ideas is that if I were to bore a stock barrel the wall would be too thin (stock barrel OD is usually only about 10.5mm) and I couldn't assure 10.0mm +/- 0.001 or even +/- 0.01 with a drill that long.

I have found some 10mm ID tube (tube is measured by OD, pipe is measured by ID for those that may not know the difference), however they only guarente accuracy down to +/- 0.05 usually or worse, which in airsoft barrel terms is HUGE as most of us know

If I can find a precision 7 or 8mm barrel I'll try to go with that, but the barrel hunt will have to wait till I can decide on an actual size for the BB.

I have spent most of the day looking at different casting techniques and started reesarching different .410 sabot/slug designs. There are both rifled and unrifled slugs for the .410 and it seems from the forums that I have read that the rifling on the slug does not actually spin the slug at all and is more for looks than anything else. What do you guys think? I have not heard anything else either way, however after thinking about it I could see it going either way. On the one had it would make sense that curved v. straight cuts would make a difference, on the other hand the control surfaces would have to be rather large to make a difference.

My original plan was to make rifled rounds and find a way to hook up a high speed video along the flight path to see if they actually did spin... but for ease of manufacture it would be nice if I could clear this up before custom building a rifled mold (because that would only be about 10x the cost and about 100x the headache of not rifling it )

Well, I seem to have reached a mental block recently so perhaps someone here has experience with some of these things I'm looking at.

For the material I am looking at a number of options. I will probably play with some of the 2 part epoxy resins that they have at Tap Plastics since there are a few locations around where I live, but. They have some good properties for casting, but I am 90% sure they will end up too heavy. The 'mainstream' plastics I am looking at are LDPE and LLDPE.
http://en.wikipedia.org/wiki/Low-density_polyethylene
http://en.wikipedia.org/wiki/Linear_low-density_polyethylene

Both are commonly used for injection molding and have good handling characteristics and elasticity and are used in thin applications i.e. plastic wrap. This was an ideal characteristic to look for since I will be wanting to keep the skirt as thin as possible (somewhere in the realm of 0.2-0.25mm).

For the manufacturing process I am debating between building a cast for injection molding or swedging... or a blend between the two. The problem is that injection molding is very common with plastics, and swedging is very common with metals, but I can't find any information on swedging plastics. The reason I am so interested in swedging is because it is a much more precise and repeatable process as opposed to casting. There is no expansion to worry about since it is done cold and there is no combustion in the mold due to lack of vents because the material is pressed between two casts.

The problem is that plastics (especially LDPE) is elastic and does not retain a form when pressed well like metal does. I am debating creating a method of 'hot swedging' so to say, where the material is cut to a rough shape and then compressed between two hot molds... but I have no idea if something like this would work.