M16A Adding Pressure Gauge

[elaerico]

Hi. It's me again. I just bought a new spa M16A, but it has only one pressure gauge, for the plenum, but not in the deposit, so I want to add one.
The drop block who holds the bottle is aluminum, I don't know what alloy is it, in its thinnest part, 9.5mm wide. The foster fitting is a M10x1mm, and inside has a tiny 3mm hole for the air passage.
So, I want to add the pressure gauge to the side of that block. I have a 1/8 bsp gauge and a 1/8 bsp tap. I was thinking drill the first 6.5mm with a drill, and make the thread with the tap, and finish the hole with a 4mm drill, for having a delrin disk for sealing. How do you recommend me to proceed? Do you think it will be enough the 1/8bsp to hold on the forces?
As always, thanks a lot!

[subscriber]

Please post a sketch of where the new hole would go.  While you show the part, it is hard to know if the way the passages would intersect with your proposed gauge port would be correct.

Is the regulator adjustable on that block?  If not, why would the regulator need a gauge?  It seems odd for a PCP air rifle not to have a fill pressure gauge.  That is how you know not to overfill it...

[elaerico]

Here I attach the drawing. It will intersect perfectly with the air passage.  Yes, the regulator is externally adjustable, hence the plenum manometer. The force in the manometer will be, for the maximum rating of 250 bars, will be 190 kilograms, or 400 pounds.
I found this online calculator, and make some calculations, assuming its a M8x1mm thread, becouse is the most simillar to the 1/8bsp, and get those results. I hope you help me check it.
Thanks a lot!

[subscriber]

Roberto,

If I read your computation screen capture correctly, it predicts a material stress of 546 MPa to be carried by a material with a shear strength of 207 MPa?  That looks like a no-go.  Or did I read that wrong?

If the gauge is rated to measure 3000 PSI I would expect the threads on the gauge to carry whatever force is developed at the effective area that the air acts against.

As a lot gauges bodies are machined from brass, I would take this a step further:  Easily machined 360 brass has a yield strength of 15000 PSI.  That is less than half that of 6061 T6 Aluminum.
https://www.mcmaster.com/brass/ultra-machinable-360-brass-rods-and-discs-8/

Now, we don't actually know the grade of brass, nor if it has been work hardened above the annealed listing given by McMaster.  It may not be unreasonable to assume that your aluminum block has similar strength to the gauge body threads.  So, it the gauge is rated at 3000 PSI (or more to use up to 3000 PSI), then the thread in your aluminum block should also be strong enough.

I would bracket that statement by saying the threaded hole needs to be just slightly deeper than the threaded stud on the gauge, to ensure full thread length engagement.

Drilling the hole undersized, then enlarging it should provide a more accurate and smoother hole.  Normally, the hole drilled for a thread such as M8 is larger than the root diameter of the thread.  This is to speed up the threading process, as a smaller volume of cuttings have to be removed.  The result in a slightly weaker thread.

If you make a hole that is tighter than the conventional drill size for M8 x 1mm, you can get the full thread strength.  If you do this you will need to reversing the tap to break the chip and clean out the chips much more often.  If you don't you will strip the thread or break the tap.  So, I would not go straight to a 7 mm drill for an M8 x 1 pitch.  Start with 6 mm or 6.35, then ream the hole with a drill between 6.35 and 7 mm diameter.
https://www.natool.com/wp-content/uploads/2019/06/tapdrillsizes-inmet_web_catp113-116.pdf
https://www.wlfuller.com/html/tap_drill_chart.html
 

Can you show an image of the face of the gauge that you want to use?  And threaded end.

[elaerico]

Thanks for the answer! I show you the gauge I will be using. It has a 1/8 bsp thread. So I was thinking in make the hole with a 8.75mm, but I think I should start then with a 8.5mm.
Here is the picture. I was thinking making a through hole, and sealing the air with an oring in the base of the gauge. What do you think about that?

[subscriber]

The smaller the area exposed by the O-ring, the smaller the force trying to pull off the gauge.  So, the green D in the sketch below should be as small as practical.

For a face seal on the O-ring you will need to make a "nest" to keep it in place; and to prevent it from blowing radially outwards.  This nest could possibly be made from a tube insert, as what I show in the very fast hand sketch (using mouse curser as pencil) will be difficult to machine into the block.   Unless you have a cutter like this, to shape the O-ring seat: https://www.mcmaster.com/6850A21/  And use a mill, or very steady drill press.




Or an adjustable boring head in a full size milling machine: https://www.mcmaster.com/boring-tools/boring-tool-holders-for-milling-machines-4/

[elaerico]

I was thinking in this kind of seals, so I'll drill to 4mm before the chamber with the 8.5mm drill and thread to that, and then a 2 or 3mm hole to the chamber. I'll have to figure it out how to make the bottom flat surface, I think I can use an 8mm flat-end mill cutter, and then insert a 2.5mm x 8mm in diameter seal.
I tried to draw it. In orange is the seal.

[Rob M]

I was thinking in this kind of seals, so I'll drill to 4mm before the chamber with the 8.5mm drill and thread to that, and then a 2 or 3mm hole to the chamber. I'll have to figure it out how to make the bottom flat surface, I think I can use an 8mm flat-end mill cutter, and then insert a 2.5mm x 8mm in diameter seal.
I tried to draw it. In orange is the seal.

those work very well , i use them aall the time for gaauges and foster nipples.. be sure to only drill the hole aabout .400 deep , any deeper the gauge flat will never crush the seal
use a 4 flute end mill to make the bottom flat, only after you haave already drilled the center hole down to the opening( ideally a 9mm

[subscriber]

Roberto,

Those seals should work fine, if you make the pocket perhaps 0.5 mm shallower than the seal.  The gauge foot needs to compress the seal just a little.  If the seal measures 8 mm in diameter an 8 mm endmill will work, because the hole will end up about 0.05 to 0.1 mm larger than the cutter.

I would use a two flute endmill because those are center cutting.  Plunging with a four flute endmill takes more force, even if it is center cutting.

The through hole can be 1 mm in diameter for the gauge.  Small drills are just so fragile and like to break off in the hole...  I do not like to drill any hole smaller than 3 mm diameter to any depth.  2 mm if not drilling deeper than perhaps 6 mm - "peck drilling" to clear the chips very frequently; using a milling machine's quill, or a drill press. 

I would try to keep the total drilling depth on the small center hole as short as possible.  Thus, I would use the 8 mm flat endmill first. before drilling the center through hole.

Obviously you need to do all drilling and tapping operations in one clamping setup so as not to lose the centering.    Obviously the size of the endmill for the flat should not remove the threads above it....

[Rob M]

subscriber is right about the 2 flute, but since most people dont have 2 flute end mills in small diameters  , i suggested 4 which are common and dirt cheap..

[elaerico]

Thank you both. I do have an 8mm 2 flute mill luckilly. Something who concerns me now is how far should I make the hole for not make too weak the wall who goes between the chamber and the seal? How much do you think will be reasonable? The gauge thread lenght is 7mm, and the wall thickness was 9.5mm, so will it be reasonable to leave 2mm without the big drill, and make a 1mm seal?
I've seen that material is PTFE. I cant get that here. Can I use teflon, or grilon, or delrin?
Thanks a lot!

[subscriber]

Roberto,

Please make a sketch of your proposal with dimensions indicated. 

If I understand correctly, there is a 1 mm tall seal option?  I would absolutely use that so that you can make the floor thickness 3.5 mm + whatever the "crush" depth is of the seal.  So, perhaps 3.75 mm deep floor under the seal; to compress the seal 0.25 mm.  If you go thinner than this on the floor, the aluminum may bulge when you tighten down the gauge.

If you point us to your image source for the seal, perhaps we can find out the material.  It may be Teflon.  It has to be something with a little bit of "give".  I must be squashed so that it makes good contact with the gauge and the metal floor.

[elaerico]

Oh, I eddited the previous post at the same time you were writing the new post! It's a PTFE seal, but I dont have any, so I have to make one in my lathe, and I can make in any dimensions I want, so I can use 1mm x 8mm diameter, with a 3mm hole. I infeer from your answer I can use Teflon,right?

[subscriber]

Same material:  Teflon is the commercial name for PTFE: Polytetrafluoroethylene

[elaerico]

Well, thanks a lot to all who have helped me, especially to subscriber, who has a lot of patience with me.
I did it. Just not in the way we concluded, becouse my tap is tapered in the tip, so that won't allow to make the threads all the way to the seal. So I make a through hole, and make a 1/2" 1.5mm deep oring groove to seal with the face of the pressure gauge, and so far so good. I pump it to 250 bar, and let it in another room for a couple of days, just in case 

[Rob M]

nice!