long range ammo MiniBoy Mark I

[MrCrowley]

what dia is your piston

Roughly 50mm, it'd take quite a bit of machining but 2.5" aluminium could work I guess.

[dewey-1]

I still want that CAD model! (this means you, Dewey! )

Some of you youngsters need to learn a little tact in asking for something from somebody elses efforts.
There is a big difference in a request and a demand.

Here is my response to you in the same sentence style that you used.
I still want that CAD model requested in a proper manner! (this means you, ramses!)

I have always offered most of my CAD files to others by this statement; "Please request any other formats through PM."

You are not in a demanding position if you want any results, where as I can be, for you to get the end result.

[ramses]

I still want that CAD model! (this means you, Dewey! )

Some of you youngsters need to learn a little tact in asking for something from somebody elses efforts.
There is a big difference in a request and a demand.

Here is my response to you in the same sentence style that you used.
I still want that CAD model requested in a proper manner! (this means you, ramses!)

I have always offered most of my CAD files to others by this statement; "Please request any other formats through PM."

You are not in a demanding position if you want any results, where as I can be, for you to get the end result.

First of all, I would like to apologize. I was not aware of your statement regarding other formats. I worded my request in a joking manor, similar to what other members here do. Based on some of your posts, it seemed like you enjoy joking around. I'm sorry that it offended you.

You have attained the reputation as the forum's CAD modeler, and I was knew that you had a model made. My intent was similar to jokes involving JSR and epoxy, Poland Spud and air cylinders and directional control valves, BtB and sawn off barrels, etc.

In my defense, I did request a model on pg 10 of this thread, but that was for a different purpose(deciding if the holes in the pedals would jeopardize the structural integrity of the sabot).

So, allow me to re-phrase:

Hey dewey-1, would you mind PMing me a model of the projectile in its sabot so I can try to come up with a drag coefficient? My software imports inventor assemblies (*.iam files).

Thanks

[dewey-1]

ramses;
No offense taken, just the wording.

Here is a method to import dwg file.

http://autodeskmfg.blogspot.com/2009/11 ... odesk.html

PM me your e-mail address and I can send a dwg file.

[Fnord]

but you forgot the force acting on the back of the sealing face. That is 806 lbs pushing to the left.

Yes, I know there is force holding the sealing face shut in the diagram, but the problem I was trying to point out was it doesn't work out that way when the face is only partially attached; the pressure essentially sticks it to the seat like a suction cup, and the piston is more or less "free floating".

Crowley:
For a piston material, I'd go with a composite design.

You need high compression strength along the piston's axis, which honestly I think a high-strength steel or T6 tempered aluminum should work well for. While fiber-reinforced composites may have a very high strength to weight ratio, it only applies in tension loads, and they don't 'yield' like metals do.

For the washers, however, a fiber-reinforced material is ideal. Kevlar has the highest strength-to weight ratio, followed by graphite and fiberglass.

You can even make washers yourself from cloth sheets of these materials; fiberglass is cheap and readily avalible, graphite(carbon fiber) you can buy online(or ask auto-body shops if they have any scraps leftover). Kevlar you'll most likely have to order, but make sure it isn't 'waxed'.

You want a solid neoprene or silicone bumper, not foam. The bumper shouldn't be part of the piston since it'll only be extra mass to slow down.

[MrCrowley]

I've bought a keel roller to use as my new sealing face, I'll just use the end bit. If you're not familiar with keel rollers, they're made of very hard rubber that doesn't compress the slightest under human strength. Hopefully, if I file down the barrel seat, this sealing face wont be cut to shreds.

[ramses]

but you forgot the force acting on the back of the sealing face. That is 806 lbs pushing to the left.

Yes, I know there is force holding the sealing face shut in the diagram, but the problem I was trying to point out was it doesn't work out that way when the face is only partially attached; the pressure essentially sticks it to the seat like a suction cup, and the piston is more or less "free floating".

Oh, wait... This is allowing the pilot valve to open DUH!!!!! Now we just need a BIG fender washer to hold the sealing face open, and another piston is probably not necessary). More piston travel before the pilot openes would help. Great catch on the mechanics of this!

As far as the aluminum goes, I'll talk to my foreman on Monday (assuming they don't stick me on the Haas mills again).

[MrCrowley]

Now we just need a BIG fender washer to hold the sealing face open

Hold it open? Even though the sealing face is epoxied down, it could morph enough under pressure?

When I remove the sealing face today and replace it with the keel roller, I'll try add the QDV style pilot as well. I have a wooden dowel o-ring piston that would work perfectly, I just have to remove some stuff and re-epoxy it


Edit: Attached photo shows new sealing face. I cut off about 25mm from the end and epoxied it on the piston. I couldn't believe how difficult it was to remove the old sealing face, I had to heat the crap out of it before the washer *popped* off.

[ramses]

I think adjusting the QEV will help a lot. The new sealing face may not have too much of an effect.

[MrCrowley]

So is the sealing face flexing forward and pushing the piston back unseating the pilot? That's the only way that I can think of which would open the valve though I'm not able to calculate the physics which is what you did on the previous page I assume.

[Fnord]

(^Not sure who exactly your post is addressing but-)

Based on what I know about your valve it's impossible to tell.

Any squishy or stretchy parts you can eliminate from the design will give you a better shot at success, and using a harder grade of rubber will help reduce any unwanted movement when pressure distortion comes into play. You may have to keep the sealing face greased since it'll take more force to deform and thus seal properly, but overall I think a tougher face will help.
If it doesn't, then the only thing I can tell you is to de-sensitize your pilot by using a QDV design. If you can give it about a centimeter of travel before the pilot actually "opens", I think that would be more than sufficient.

[ramses]

Bad news: my CFD software cannot handle compressible flows, and therefore is useless for this (mach number >0.3).

[MrCrowley]

Tested the new pilot valve today at low pressure. Seals well at 90PSI and I got the barrel to seal with 60PSI in the chamber but started leaking above that so hopefully with more pressure in the pilot (200PSI +) the leaking will stop.

Tomorrow: High pressure testing and hopefully I'll fire the cannon and make sure the valve still opens without bouncing.





Edit:

High pressure testing completed.

Pilot filled with 260PSI leaks to 250PSI over two minutes (time it takes to fuel + add air for 10x mix).
Chamber filled to 140PSI (with 250PSI in pilot), no leaks.

I'm satisfied with that, combustion testing on Wednesday

[MrCrowley]

[youtube][/youtube]

Things are working well. The piston doesn't open prematurely, it also opens fully without bouncing, there doesn't appear to be any leaking and it is louder than ever before.

Three things about the second shot in the video:
a) I did wear ear protection for the first shot but as you can see for the second shot I forgot to wear them and left them laying behind me.
b) I believe the cannon wouldn't fire initially because of dust in the stun gun circuit. Dust has been a minor problem before when changing the battery as it sometimes caused the stun gun to short circuit. Immediately after this shot the ignition didn't work again but after a shake or two it worked (hence me believing it was dust in the circuit and not the spark gap being too large)
c) Yes, that is a girl who screamed and not me or my other friend

[jackssmirkingrevenge]

Nice... so we have a date set for the perforation of the stratosphere?