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The Offcial Machinist Thread: Revised 01/04/2014

A place to ask general spud cannon related questions.
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Unread postAuthor: jrrdw » Thu Jun 07, 2012 8:21 am

MrCrowley wrote:Unfortunately the lovely finish is no longer as I had to shave 0.1mm off it to get it to fit better in the piston housing. I'm planning on restoring it back to its shiny finish if anyone has some tips (sandpaper finer than a couple hundred grit seems to be difficult to find at the nearest hardware stores).


Wet sand it with 1500/2000 grit Wet/Dry sand paper. Use 1 drop of dish washing liquid to 1 quart of water for the wet sand. Keep your piston wet with the mixture while sanding. Looks like you can screw a piece of allthread into it and chuck it in a drill and make quick work of it.

The trick is to keep the piece wet. This will make a mirror finish...
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Unread postAuthor: FighterAce » Thu Jun 07, 2012 8:24 am

Perhaps I didn't explain the situation good enough... imagine a standard piston valve... once you vent the pilot and chamber, the high pressure air remains in the area between the piston body and piston housing between the 2 O rings.

The air then pushes on the O rings to make a seal... O rings are pressed against the piston housing and thats whats causing the friction.
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Unread postAuthor: velocity3x » Thu Jun 07, 2012 9:43 am

MrCrowley wrote:Has anyone done flow simulations to see if there is a turbulence caused by a piston design like that?

I did a sim (1-1/2 valve) in Floworks last month. A "Tee" type valve with your piston design will have only a modicum ....if any turbulence.
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Unread postAuthor: Goats spudz » Thu Jun 07, 2012 11:08 am

Gippeto wrote:Goats....are you planning to remove the pin valve from the regulator?
i have done that im gonna put pics on now i forgot all about this :roll: Any tips on painting aluminium tanks? i was thinking of etch-primer-spray-bake?
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Unread postAuthor: MrCrowley » Thu Jun 07, 2012 4:12 pm

FighterAce wrote:Perhaps I didn't explain the situation good enough... imagine a standard piston valve... once you vent the pilot and chamber, the high pressure air remains in the area between the piston body and piston housing between the 2 O rings.

The air then pushes on the O rings to make a seal... O rings are pressed against the piston housing and thats whats causing the friction.

Well if both o-rings seal 100% individually, you wont end up with any air between them. I think the forces in a piston hybrid are large enough to overcome any extra friction; I had no problem with my last dual o-ring design.
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Unread postAuthor: MrCrowley » Thu Jun 07, 2012 10:51 pm

Double post:

Okay so the o-ring grooves are a touch too shallow; probably similar to the o-ring dimensions on the original piston. I tried using a variety of fine files (the only ones that could fit in the groove) with the piston attached to a drill but that has no noticeable affect.

Perhaps I'll get the grooves touched up by a machinist and reduce a 2.5-3cm portion at the front down to 30mm diameter. That should be enough for structural purposes, right?

edit: back-of-the-envelope calculations for the piston suggest removing ~130g of weight will decrease its kinetic energy from ~5300J to ~3700J. I don't expect these numbers to be correct, just a rough estimate. Sound about right? Theoretical opening time was less than 2ms but that assumed the pilot pressure had been completely evacuated at time of opening, no friction, and the combustion pressure was constant.

velocity3x wrote:
MrCrowley wrote:Has anyone done flow simulations to see if there is a turbulence caused by a piston design like that?

I did a sim (1-1/2 valve) in Floworks last month. A "Tee" type valve with your piston design will have only a modicum ....if any turbulence.

Is that with a solid piston design?

jrrdw wrote:The trick is to keep the piece wet. This will make a mirror finish...
I got some 800grit stuff for now and that works pretty well. Perhaps when everything is done I'll get some 1500 grit to make it look nicer.
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Unread postAuthor: jackssmirkingrevenge » Fri Jun 08, 2012 6:44 am

MrCrowley wrote:Perhaps I'll get the grooves touched up by a machinist and reduce a 2.5-3cm portion at the front down to 30mm diameter. That should be enough for structural purposes, right?


Yup...
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Unread postAuthor: velocity3x » Fri Jun 08, 2012 10:25 am

MrCrowley wrote:
velocity3x wrote:
MrCrowley wrote:Has anyone done flow simulations to see if there is a turbulence caused by a piston design like that?

I did a sim (1-1/2 valve) in Floworks last month. A "Tee" type valve with your piston design will have only a modicum ....if any turbulence.

Is that with a solid piston design?


Yes...solid piston. I checked at D/2, D/4 and full retract (1 1-2") for the piston positions. ALL positions had extremely little or no turbulence, but the full retract position had tremendous flow volume! Velocities using 1kpsi hpa went as high as 1633/fps. I checked the TEE configuration against other valve styles (sprinkler and coax). The TEE definitely wins out with almost zero turbulence and greater flow.
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Unread postAuthor: MrCrowley » Sun Jun 10, 2012 4:55 pm

Received a quote from a local company. The guy didn't think it would take long to modify the piston, probably only an hour or so. $125. Yeah, f**k that :D

I'll try Brugar (the place I got my S/S barrel welded) but if that doesn't work out there's not much else I can do in terms of reducing weight.

Maybe I'll drop in on my old High School shop teacher but he seems like the type who wouldn't be willing to do it because the lathe is school property.
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Unread postAuthor: Fnord » Sun Jun 10, 2012 6:16 pm

Okay so the o-ring grooves are a touch too shallow; probably similar to the o-ring dimensions on the original piston. I tried using a variety of fine files (the only ones that could fit in the groove) with the piston attached to a drill but that has no noticeable affect.



Using an un-modified file won't do much. You'll need to grind the end off so you have nice flat cutting surfaces, then grind your cutting edge on the tip to around a *60 degree angle or so. Keep the file cool when grinding; if it turns yellow. you're loosing hardness.
Sharpen the edge lightly with a belt sander, or just stick some sandpaper to a bench and do it manually.

That's how I made this piston:
http://www.spudfiles.com/forums/files/4piston_205.jpg

Be aware this will take a while to turn down. Mine was aluminum and it took a while. Improvise a tooling rest and don't push too hard, as you'll wear out the bearings in your drill.

Also, try to practice on some scrap washers first. It takes some trial and error to get a smooth clean cut with a ghetto setup like this.

*edit; 60 degree angle- had it backwards.
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Last edited by Fnord on Sun Jun 10, 2012 8:50 pm, edited 1 time in total.
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Unread postAuthor: MrCrowley » Sun Jun 10, 2012 8:06 pm

Cheers Fnord.

I just got a quote from Brugar Engineering, they want $85 for an hours worth of machining. Guess I'll be using the file method to get the o-ring grooves right and I'll have to put up with the weight for now :wink:
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Unread postAuthor: Crna Legija » Sun Jun 10, 2012 9:45 pm

LOL you got it really cheap, it took me maybe 1/2 and hour to machine that piston, would take less then 10 min to cut a chunk out.
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Unread postAuthor: MrCrowley » Sun Jun 10, 2012 11:54 pm

Yeah I felt like saying to them "I'll give you $30 if I can use your lathe for 15min" :D

Fnord's file technique worked pretty well and now the o-rings are a good fit. Got the sealing face epoxying over night but it'll be a few weeks before I can get any testing done.

Thanks again Crna, if you didn't machine it for me any local businesses probably would have charged me $200 + materials :wink:
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Unread postAuthor: jackssmirkingrevenge » Mon Jun 11, 2012 12:58 am

I think I heard it from Jay Leno, something along the lines of "In the past, technology was expensive and labour was cheap, now the opposite is true."
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Unread postAuthor: sagthegreat » Sat Jul 07, 2012 1:31 pm

I was wondering if someone could machine me a inline vortex block, it has to fit in a 2 inch tube, and be for 6mm bb's. Any suitable material will work. Im in the usa, so someone located there would keep prices down
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