SHCS size for pinning a valve

[BigBird]

I am trying to determine the screw size needed for pinning a PCP tube (valve and fill end) that is 1.25" x .095", 1020, a513 using a working pressure of 3000 psi air.

I had: Area = 0.8825 x 3000 = 2647.5 lbs force on internals (?).  Is that the correct calculation?

If I counterbore the screw heads of 4 -  1/4" x 28 Grade 8 SHCS screws (these are grade "9" but I have to go back to get documentation) so the screws are flush with the apex of the outside tube would that give at least a SF of 4?  Do I need bigger screws, or grade "9"?

These screws will be in a 303 SS valve bored out (concentric) to 0.5" with screw locations similar to a Bontrager (one head width away from other holes).

Also I have not seen pinning screws added to the breech end of  the valve (when the valve is long enough and diameter allows).  If all the tube holes are correctly placed so as minimal, equal settling do those screws require a different calculation?

I will pin the fill end as well.  I believe there is a calculation on GTA somewhere for that.  I was planning on using 4 screws.

Here are the screws with a look at head, and pattern.

[rsterne]

A Marauder uses the same size tube and a 3000 psi fill pressure, and the valve is made of aluminum.... It uses three high-tensile 10-32 SHCSs at 3, 6 and 9 o'clock, with the valve body counterbored the diameter of the screw head (5/16") so that the top of the head is flush with the apex of the outer edge of the tube, or a few thou proud.... That makes the shear plane through the head of the screw, rather than the threaded portion.... There is certainly no problem with that installation at 3000 psi.... so I would think your installation is more than enough....

Bob

[BigBird]

Great! That is good news!  I had not studied the Marauder enough.  I always assumed somehow the breech screws went all the way through.
Thank you!

[rsterne]

There are two 4-40 screws that hold the receiver on at the front which also thread into the valve.... but the holes in the tube are oversize, and they cannot really take any load unless the three main 10-32 screws shear off.... so I never include them in my consideration of the valve anchoring on the MRod….

Bob

[BigBird]

Closing in on the end of this project (or rather the start of chasing a bell curve or a sensible Korean cliff).

I am really taking my time on some procedures (less on others which probably shows).  However this is more critical.  I have to pin the fill end an I'd like to get some more info to make sure I am doing it right and for background info.  I have printed my 3 and 4 hole template (white ring on tube) so I can get it parametric (hope thats the right word).  I've definitely had some that aren't quite parametric that work fine and tested fine.

I was confused by this post which says the head diameter doesn't matter it is the calculation of the distance from the end.  Kinda makes sense to me except if the diameter was too small wouldnt you create like a knife blade shearing through the end?

https://www.gatewaytoairguns.org/GTA/index.php?topic=61069.msg585339#msg585339

I use 1.5 x head size but i was wondering based on the link, is countersinking the SHCS head counterproductive?  I was going to use 4 high tinsle 10-32 SHCS in this end.  I don't really care if they stick out or not.  Its actually easier not to countersink of course.  I also have some 1/4-28 but those would be rather big sticking out and I don't have enough distance to countersink those UNLESS I don't have to (countersink) and want them sticking out.

Anyway I thought I was ok till I read that and didn't totally understand it.  Then I freaked out and wondered if the fill end would come flying out of my 30 caliber 

Thanks in advance.

[Rob M]

i always align my side pin screws with the transfer port.this way , even off they punch into the valve it has no effect on sealing , since the screws are entering behind the poppet..I take the stem out to drill and tap them with the valve body in place to assure they share loads .., then drive the  valve body back out for deburring, adding orings  and so on.. I also love to use  1/4 inch screws, as it eliminates the sanity issue in most cases.

[BigBird]

This is for the fill end.  Sorry I posted under the previous valve topic.  However, still the same pressure vessel configuration.  Yes.  I used three 1/4-28 in the valve but in the opposite configuration that you describe.  One goes down toward the grip.

I hesitate to post hideous pictures but this is the fill end.  It started round until my makshift mill bit it.  It will do for testing. The holes in the tube however are more permanent and whatever replaces this will just have to conform.  This one is for testing.  I've already had it unpined in a 12" tube with ends blocked to test the plug in the .192" hole pictured.  It allowed filling then held 800 psi and I stopped.  That has nothing to do with pinning though.

More information about this plug which affects this operatiion.  It has a "hole" down the center and an actual hole .192" on the side connecting to the fill port.  I have a 303 SS plug (pictured) that is very nice looking but decided to use 6061. It is way out there and the gun is so off balance anyway.  I will make another 6061 plug.

[Rob M]

makes sense.. for the flats are you using a milling attachment on the lathe or an end mill in the drill press?

[BigBird]

makes sense.. for the flats are you using a milling attachment on the lathe or an end mill in the drill press?

I have my $39.99 Horbor Freight drill press with a cross slide vise. It wanders but was less secure before using the Vise jaw pads.  Not ideal for steel but better than using a dremel.  Using a 1/2 end mill on a piece of steel caught it on the side and twisted the whole drill press neck.  I had to resecure the single set screw that holds the drill on the pipe (neck).  I need a mini mill.

[Rob M]

one idea i always had for the mini drill presses.. take a large tupperware , lay the drill press into the tupperware with the top belt cover well secured and taped shut.. THEN , pour concrete into the tupperware essentially making the entire back end one piece (-= the concrete would need to be deep enough to lock the base to the head unit , but shallow enough to still allow milling in the Y axis
never tried it , i have a decent size mill but it would be fun.. Epoxy granite would be better.

[BigBird]

That would probably be something I'd do!  If we are hypothesizing we might as well use 15 minute super (reinforced) epoxy.  Use a lot of modelling clay to create Corvette curves.

My problem is I have no "Z" so the cross slide is at a fixed position.  My larger drill bits dont fit.  Endmills do but they dont plunge as good as drill bits with a tapered point.

[Rob M]

once it was a huge casting , it would likey handle most ops like a mini mill.problem would still be the XY table , youd need something lower profile , more like a real mill table.. and at that point , seeing as those are 200 bux , you may as wel just get a mini mill.. But for a lot of basic milling , the harbor freight drill press in a stone / expoxy casting would do the trick..then forthe quill , add a real z lock .. i did one on mine cant remeber the details it was 5 yrs ago .. the only end mill that does ok plunging is a center driill endmill. Those arent common aside from  2 flutes. and even then , i take it super slow plunging those.. as for the drill , you need a stub set..even for the mini mill clearance is tight