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Piston shape

A place to ask general spud cannon related questions.
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Unread postAuthor: Technician1002 » Sat Jun 13, 2009 3:17 pm

psycix wrote:Sweet piston!

Though I am concerned about the o-ring grooves, they are really shallow. I'd make the diameter of the material next to the groove larger, so that you've got a deeper groove.
This way, the o-ring will handle pressure the best way, with no chance of getting pushed out of its groove OR being extruded/ripper apart under high pressure.
For moving parts, floating o-rings would be best.

Oh, and you could try to drill a hole through the piston, so it becomes lighter.

EDIT: As tech said, optimally the clearance between the material and the wall has to be like a few thousandth of a mm.
Tolerance depends on your working pressure though.


A few thousandths of a mm is probably too tight. You do want the piston large enough so the sides of the o ring seal against the walls of the groove and have no chance to squeeze through the space. Attached is a close up shot into the outlet of my 2 inch cannon. You can't even see the o ring. Without the o rings on the piston it is loose enough to fall through without pushing. I hope this helps. The piston rings are not squeezed down to the bottom of the grooves. There is space under them for air. The o rings are just large enough to hold their place in the cylinder without the piston so the ring itself provides the pressure to seal against the cylinder wall.
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PistonClearance.jpg
Piston clearance in the 2 inch QDV cannon is small.
O-rings.JPG
O ring test fit. Just large enough to hold itself in place. Air pressure later provides a tight seal.
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Unread postAuthor: daccel » Sat Jun 13, 2009 9:37 pm

Thanks guys, very helpful! The back rings weren't floating, but I've seen the light, and will change accordingly.

I was using 4 layers of tube that just happened to all sleeve nicely together, but it restricted me with the size of piston/port etc.

So I picked up a 5/8 aluminum rod which is the ID of the valve casing, and I'm just going to drill it to work for the piston rather than trying to make the piston work for what I have. Also it means I can save the piston which took a lot of effort with my limited tools. And will be stronger and better in the end.

The area differential between front and back will also drop, hopefully giving the piston more 'pop' and a faster opening time.

Picked up some new o-rings too, so given I can't add material back to the piston, and I don't want to restrict the port too far below barrel diameter, clearance on both ends of the piston will be .015" total to cylinder. Hopefully that's sufficient.

Now my new question is given the width of the grooves I cut on the back is twice what I now need, could that be a problem with a floating o-ring? I know it will seal fine, but I'm thinking of the transition when high pressure switches from the spring to the chamber, the o-ring will slide back, and presumably lose the seal, which would change my spring pressure shot to shot. What if I doubled the o-rings up, that shouldn't create too much additional friction in a floating set up, and would minimize blow by?

I had thought about drilling the center, but will hold off to see what it looks like with deeper grooves cut, rather have it heavy and not break on the first shot.
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Unread postAuthor: Technician1002 » Sun Jun 14, 2009 12:03 am

daccel wrote:Thanks guys, very helpful! The back rings weren't floating, but I've seen the light, and will change accordingly.

I was using 4 layers of tube that just happened to all sleeve nicely together, but it restricted me with the size of piston/port etc.

So I picked up a 5/8 aluminum rod which is the ID of the valve casing, and I'm just going to drill it to work for the piston rather than trying to make the piston work for what I have. Also it means I can save the piston which took a lot of effort with my limited tools. And will be stronger and better in the end.

The area differential between front and back will also drop, hopefully giving the piston more 'pop' and a faster opening time.

Picked up some new o-rings too, so given I can't add material back to the piston, and I don't want to restrict the port too far below barrel diameter, clearance on both ends of the piston will be .015" total to cylinder. Hopefully that's sufficient.

Now my new question is given the width of the grooves I cut on the back is twice what I now need, could that be a problem with a floating o-ring? I know it will seal fine, but I'm thinking of the transition when high pressure switches from the spring to the chamber, the o-ring will slide back, and presumably lose the seal, which would change my spring pressure shot to shot. What if I doubled the o-rings up, that shouldn't create too much additional friction in a floating set up, and would minimize blow by?

I had thought about drilling the center, but will hold off to see what it looks like with deeper grooves cut, rather have it heavy and not break on the first shot.


Wide grooves is fine. Mine lose seal when moving just like you anticipate. Mine seals up (I can hear it) on the first application of air. Even when using a hand pump, it seals with a good first stroke on all 3 of my cannons.

You shouldn't have too much problem unless the space under the rings is excessive. I cut the grooves so the ring will just touch bottom when the top is pressed in flush with the surface of the piston. My space underneath is about the same as the piston clearance.

Take a look at this photo;
Image
The side of the o ring is about even with the top of the groove. The ring needs to seal against the wall of the groove, not ride over the top. This is OK if the ring is squeezed smaller to put it into the cylinder as long as it isn't squished against the bottom. The less you squeeze it the easier it moves. Too small and it may have trouble making a seal.

Now look at the photo below. The o rings barely extend above the surface of the piston.

The 4 inch piston on the previous page has an excellent o ring and groove. Awsome job on that 4 inch 3 inch porting monster. :salute:
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CoreAssy.JPG
QDV Core assy showing O rings in relation to the OD of the piston.
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Unread postAuthor: USGF » Sun Jun 14, 2009 6:13 am

Technician1002 wrote:
The 4 inch piston on the previous page has an excellent o ring and groove. Awsome job on that 4 inch 3 inch porting monster. :salute:


Tech, Thanks. We did relax the ID of the groove so the piston moves very freely. We use gland specs such as these.

http://www.engineersedge.com/general_en ... linder.htm


I know it is hard for some members to measure to this level. Gives you a guidline though. The piston should be close enough that the o-ring does not extrude into the clearance.

BTW, cheap calipers are a must anymore. They are about $20 from Harbor Freight. I can't say about outside of the USA. They work really well considering.

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Unread postAuthor: Technician1002 » Sun Jun 14, 2009 6:25 am

USGF wrote:
Technician1002 wrote:
The 4 inch piston on the previous page has an excellent o ring and groove. Awsome job on that 4 inch 3 inch porting monster. :salute:


Tech, Thanks. We did relax the ID of the groove so the piston moves very freely. We use gland specs such as these.

http://www.engineersedge.com/general_en ... linder.htm


I know it is hard for some members to measure to this level. Gives you a guidline though. The piston should be close enough that the o-ring does not extrude into the clearance.

BTW, cheap calipers are a must anymore. They are about $20 from Harbor Freight. I can't say about outside of the USA. They work really well considering.

USGF


Thanks for the link. Good info. For most stuff a fit by feel works fine. Don't ask the depth or width of my grooves. I never measured them. I cut them to fit the ring I got to fit the pipe. The o ring was the measuring tool.

Groove deep enough? check! Wide enough for a tiny bit of wiggle? Check! Ring large enough to stay put in a pipe without much squeezing? Check. Done.

Many calipers won't measure large diameters because the opening throat isn't deep enough. Digital with a pair of machinists calipers are a winning combination.
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Unread postAuthor: john bunsenburner » Sun Jun 14, 2009 8:04 am

Hey, id like to ask how oyu cut the O-ring grooves, Gippeto said he uses his parting tool which is ground square at the end, i still don't have a parting tool(just a cut of with a sharp end atm) so i would like to ask if i could use that, or if there is any other recomendations.
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Unread postAuthor: Technician1002 » Sun Jun 14, 2009 8:36 am

john bunsenburner wrote:Hey, id like to ask how oyu cut the O-ring grooves, Gippeto said he uses his parting tool which is ground square at the end, i still don't have a parting tool(just a cut of with a sharp end atm) so i would like to ask if i could use that, or if there is any other recomendations.


If you have a broken drill bit and a grinding wheel, grind a tool out of the stump. Works for me on the drill press as a lathe. I clamped it in the vise and slid the vise over to the chucked piston and cut the groove.

Sorry, no pictures at this time.

The other tool I have used as it is easier to clamp is a wood chisel ground down to a narrow tool.
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Unread postAuthor: Gippeto » Sun Jun 14, 2009 9:50 am

More o-ring info. This is where I turn when I just "gotta" know. :)

http://www.hitechseals.com/pdf/oringbrochure.pdf

John, what kind of cutters are you using??

If you have HSS tool blanks, you can simply grind a tool blank to whatever profile you require.

http://busybeetools.ca/cgi-bin/product1 ... 4092477300
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Unread postAuthor: Technician1002 » Sun Jun 14, 2009 11:23 am

Gippeto wrote:More o-ring info. This is where I turn when I just "gotta" know. :)

http://www.hitechseals.com/pdf/oringbrochure.pdf




Gippeto, you found a real jewel on that PDF. Great stuff. For "floating rings" the groove is cut slightly deeper which permits some minor leakage as it moves with a trade off of lower moving friction. They seal on the side wall and cylinder bore instead of the top and bottom of the o ring at the sealing surfaces shown in the PDF. I saved the file. Thanks much. Other than that, the stuff is gold.
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Unread postAuthor: john bunsenburner » Sun Jun 14, 2009 12:03 pm

Well i think my calipers are off by a mm, and so i am planning to get my self new ones, for makign thinks like pisotns, and O-ring groves ect. should i get a digital caliper, or is there something else you recomend?
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Unread postAuthor: USGF » Sun Jun 14, 2009 3:34 pm

Technician1002 wrote:Groove deep enough? check!

Many calipers won't measure large diameters because the opening throat isn't deep enough. Digital with a pair of machinists calipers are a winning combination.


Also, remember to use the depth measuring feature of a digital caliper to simply measure the depth. Once you have a formula it becomes very easy. Somthing like this: Machine the piston with a .005 to .010 clearance. If you have .010 clearance and you are running 1/8 inch cross section orings, then the depth would be about .130". Nominal .138 -.005 for clearance - .003 for squish. Don't get confused between radius and diameter readings.

Width is not critical but you want to have a groove at least .020 -.030 wider than the thickness of the o-ring. Good surface finish is the next important thing to consider. The grooves are humbug to sand. A good sharp cutter as Gippeto mentions is the key. Do all you can to eliminate chatter. A large machine makes it easy. More skill is needed on smaller lightly built machines.

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Unread postAuthor: daccel » Sun Jun 14, 2009 5:10 pm

I guess it should be ask twice, cut once. Oh well, I had 34" of stock left :P so I decided to start fresh taking into account all your input.

I didn't work exactly to the specs in those links, but they definitely helped to visualize and know what to shoot for.

The back rings have as small a bottom and side clearance as possible while still floating to minimize seal loss on opening. Unfortunately the hole I drilled was off center a bit so I had to shave down the back below ideal clearance to bring it back into center with the front. Hopefully they don't extrude.

Weight is now 4.0 grams, compared to 5.8 before. The hole in the center stops 1/16" short of the front ring.

I tapped the hole and then fixed the tap in the drill press to spin the rod, as the chuck is 3/8" and the rod was 1/2".

I just used a hacksaw and a few different files, because I don't have a grinder to make cutting tools, and then sand paper to finish it. I do have digital calipers, which is more convenient than checking with the oring.
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piston 1 vs 2.jpg
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piston2 bare.jpg
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Unread postAuthor: Technician1002 » Sun Jun 14, 2009 8:04 pm

:blob7: :blob5: :wav: :walk: Great.. Now damage pics.. I think you have a winner.
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Unread postAuthor: daccel » Sun Jun 14, 2009 11:53 pm

:shock: You want me to use this as ammo after all that? :wink:

Damage pics are a ways out. Taking my time to make this a quality build and it keeps evolving. But rest assured I shan't deprive the forum of video and pics when it makes its debut.

Thanks for the help guys, saved me from learning the hard way, do enough of that as is.
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Unread postAuthor: Technician1002 » Sun Jun 14, 2009 11:57 pm

daccel wrote::shock: You want me to use this as ammo after all that? :wink:

Damage pics are a ways out. Taking my time to make this a quality build and it keeps evolving. But rest assured I shan't deprive the forum of video and pics when it makes its debut.

Thanks for the help guys, saved me from learning the hard way, do enough of that as is.



Oh yea! :D Can't wait for damage pictures. :bounce:
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