Re-design castle nut to make it more machine friendly?

i_r_machinist · Mar 5, 2019

I have a very tall castle nut that I'm designing to send out for a quote. It will require a bore that will be threaded 1"-8unc with a depth of at least 3.125" in a blind hole. I thought I would put about 1.5" of thread then relieve behind it. I have found one bar so far that would work for the relief, but it would be extended almost 7d out of the holder. I can change the design so that it is just a deep threaded hole. I'm looking at extended spiral taps now, but don't have much personal experience with such. I can spec the material to a free cutting stainless. If you were going to cut this, what would you rather deal with?

Thanks in advance
i_r_

alphonso · Mar 5, 2019

303 without a doubt.

i_r_machinist · Mar 5, 2019

Thanks!
i_r_

mjk · Mar 5, 2019

agree on 303 if it meets strength and corrosion criteria
that bore as shown would be pain as it would trap chips and sure to be an inspection argument waiting to happen

I'm not sure if they make form taps that big but if I was tapping to a blind hole I'd look into it.

FredC · Mar 5, 2019

If just stainless is specified on a print I will always go 416HT. Machines faster, easier, is stronger, and looks better just not as "stainless".

If it is exposed to H2S tempering at 1050 to 1100F will make it a bit more resistant, but in that case I would go with 303 or look at 17-4s specs to see if it is suitable.

Oldwrench · Mar 5, 2019

How is this a castle nut? The slots are missing.

i_r_machinist · Mar 5, 2019

Oldwrench, You're right. Bastard acorn. The nut will have a finish on the bottom for a rubber gasket. Engineers believe we are getting leaks along the mounting studs of a blind flange at the bottom of a flooded cavity. No chlorine/H2S concerns. Got called off to do something else in a.m., but back on taps now.
Thanks
i_r_

mjk · Mar 5, 2019

if you need a seal and your designing the "castle bastard acorn" why not a groove for an o-ring as a seal.

gaskets probably going to get twisted and torn unless encapsulated with a washer

Oldwrench · Mar 5, 2019

IR, if the quantity isn't prohibitive, it might be easier to design a standard-height nut with a relatively short sleeve extending from it so you could insert a closed tube and weld or silver-solder it in.

Edit: I second the motion on the O-ring

rcoope · Mar 5, 2019

+1 on the O-Ring although if these nuts and studs are holding a plate onto a flange of some sort, you also need to have the flange-plate interface properly sealed with an O-Ring. I would think if you went with the first idea and relieved it you wouldn't have to be super concerned about finish quality inside the bore, so sticking the boring bar 7xD out wouldn't be the end of the world.

By the way, McMaster has 303 Hex bar in sizes up to 3" so doing the things from hex stock would save machining the hex, or you could get fancy and turn the hex off in the upper part so it looked like a conventional acorn with hex at the bottom. I wouldn't call this a bastard acorn nut, I'd call it a giant-child-of-another-mother acorn nut.

mjk · Mar 5, 2019

If they're getting leaks around the studs the leakage is coming thru the vessel bottom/stud/nut interface. Regardless of the type gasket used for the flange the leakage will probably still occur.
Straight threads just ain't made to seal

mhajicek · Mar 5, 2019

mjk said:
If they're getting leaks around the studs the leakage is coming thru the vessel bottom/stud/nut interface. Regardless of the type gasket used for the flange the leakage will probably still occur.
Straight threads just ain't made to seal

I believe that's why it's an enclosed acorn nut design. No thru hole.

david n · Mar 5, 2019

I'd make it as-is............outa 303 o'course..............Drill, bore the relief(may have to flush the chips a few times), ID thread....................303 will break a nice tight chip.......I make a part that I have to reach close to 3" deep with a 1/2 boring bar in 1045........stingy chips. OP's part would be much easier...............

Comatose · Mar 5, 2019

I'd just single point it as a deep threaded hole. Then you shouldn't need to bore much (if at all.) Why remove all the material with a single point operation (boring) when you can remove some of the material with a single point operation (threading)?

i_r_machinist · Mar 5, 2019

O-ring was the first design I came up with, but was shot down because mating surface will have a uneven coated surface. They use a full face gum rubber gasket that extends past the 24" BHC to the edge of the flange. customer

mjk · Mar 5, 2019

Just to clarify

Sealing the top of the nut with a cavity eliminates one area, but the bottom contact area will also need a seal

What material and surface prep will the bottom of the nut be seated against?

Triaged · Mar 5, 2019

Crush washer then. Dead soft aluminum or copper.

SeymourDumore · Mar 6, 2019

Am I the only one who is completely lost on what allaya're talkin' about?

The OP is asking about a castle nut, then shows a picture of a "blind" nut.
And then it has to seal something, which enters into the blind area somewhere....
Why does it need to be a "deep threaded hole"?

One would like to hide the actual part is fine, but why not come up with a somewhat germane image instead???

neilho · Mar 6, 2019

As mjk says, the soft gum rubber gasket is gonna get twisted and torn unless there's something stationary between the nut and the gasket.

An O-ring at the end of the nut, sealing against a washer that contacts the gum rubber could work. Just so the washer doesn't spin and crush or shear the gasket, it could be made large, with a relieved edge, or perhaps even convex. If the underlying surface is so uneven that that won't work, spherical washer sets, with sealing material between.

Seems to me a dismissal of the Oring idea on the basis of unevenness, is offhand, at best. How much uneven? is one of the questions apparently not being considered by engineering.

The other is, how much force will/should/can/must be applied to the gasket? The answer to that drives the washer contact area.

i_r_machinist · Mar 6, 2019

Seymour, you are exactly right and I apoligize. When this was brought to me "castle nut" was implanted in my head and I never questioned it.
For some reason, these studs are extended 3.5 +/- 1.4" from the floor of the cavity, which makes no sense as they hold down a 1/2" plate. The area in pink is the normal sealing surface, which while the unit is running only serves as a air dam. While refueling, a blind flange is placed in this position and the cavity is flooded up to about 30' of water. People are going to be working directly below these cavity penetrations and this water dripping on them would be a bad thing, thus seal off the area around the studs. Welcome to my world.
have fun
i_r_

Re-design castle nut to make it more machine friendly?

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