Making an HPA probe

[WobblyHand]

Key items here:
Make sure that your groove volume is bigger than your o-ring cross sectional area.
The extrusion gap (the space between the two parts that the O-ring is being pushed into) needs to be pretty small at 5ksi. Just a few thousandths per side.
You need to have a lead-in chamfer to compress the o-ring. 20 degrees is a good gentle angle.
Process compatible lube is your friend

Your full radius groove is unusual. I assume you are worried about tensile stresses in the part... leave room for the rubber: it does NOT change volume. Just shape!

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Worried about stress risers at 5150 PSI.

 

[Weldingrod1]

24% squish is a LOT... gonna be a pain to get into the hole. Check the existing lead in chamfer!

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[WobblyHand]

24% squish is a LOT... gonna be a pain to get into the hole. Check the existing lead in chamfer!

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I know. It's actually 23.6%, but yeah 24%. I need to measure the stock part some more. It's making me a little worried. I'll check how far the stock part's o-rings protrude, or at least get an estimate. I have no reason to believe that the original design is particularly sound, lets just say some rules could have been bent. I think they use NBR80 o-rings. I'm using NBR90.

 

[WobblyHand]

What holds this in the gun?

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3 o-rings and someone's good looks?

 

[Weldingrod1]

The right hand (QD end) o-ring seems a lot bigger, or in a shallower groove.

Ah! I get it! This is pressure balanced! The pressure is between two O-rings. I really wonder about the third one...

Photos of where it fits into???

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[WobblyHand]

The right hand (QD end) o-ring seems a lot bigger, or in a shallower groove.

Ah! I get it! This is pressure balanced! The pressure is between two O-rings. I really wonder about the third one...

Photos of where it fits into???

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I think the RH o-ring is in a shallower groove. It's diameter is 8mm and is proud of the QD end. Seems to act as a stop for the probe. The pressure is between the bottom two o-rings. Those o-rings seem to be about 7.1mm OD, but that's real hard for me to estimate measure. Putting a mic on an o-ring isn't easy to do accurately. I think the middle o-ring is a hair bigger diameter than the bottom one, based on drag, but that's tough to measure.

The manual states that all three o-rings are the same and are 1.5(CS) x 4 (ID). I'm going to pull the o-rings and attempt to measure the groove depth. I'll also place some drill bits in the groove to get a bound on the size. Don't have any gauge pins.

Figured someone would ask. BRB for a picture. I don't have one with the hole exposed... Usually have a plug in there.

Hole is just above the trigger guard. Best that I can tell it is 7mm ID. (Measured with calipers. Yeah, I was lazy. Think I will get my half ball inside measurement tools to get a better ID.)

 

[WobblyHand]

Half ball gauges aren't so easy to get right. I measure somewhere around 7.01- 7.015, I think. Maybe a little less. You want some drag, but not much. Set the gauge to 7.000mm on my mic, was able to get it in hole with a little drag. Given the application, think it is 7.000 mm. An I drill goes in the hole (0.2720") and a J drill (0.2770") does not. 7mm is 0.2756".

 

[WobblyHand]

This didn't occur to me to even measure, but the inlet hole is 2.5mm only for 13mm. The rest of the hole continues, but at 1.5mm diameter. This gives more meat in the HPA area, and adds 0.5mm more to the distance between the oring groove and the hole. Hmm. Well this part, err prototype, is no good. Things make a little more sense. Design needs more updating. Feel a lot more comfortable with nearly a 40% increase in wall thickness.

 

[WobblyHand]

The o-ring grooves are different than first thought. Need to redraw stuff due to all these changes. Good thing this is a hobby, not a business, I'd go broke.

Probably ought to put on the drawing the small radii on the o-ring grooves. Not the best drawing, but it helps me think through stuff.

Slowly getting there.

 

[WobblyHand]

Getting a little spooked by that small hole. Got a small chuck and am making a sensitive drill sleeve for it. Don't know that it will help here, but it's something to add to my capabilities.