Short Projection Pipe tap? (2024)

Hello all,

I have a relatively thin(1")6061 aluminum part with two 3/8 NPT female threads, through the same through hole, from opposing sides. Without running the risk of damaging one side while tapping the other, would a short projection tap be what i'm looking for? As is, with a standard 3/8"-18 NPT tap, we're pretty good, i'd just like to see a little more engagement of the fittings into the part as it's for a hydraulic application with some pressure behind it (~1000psi)

Corey

Could you use a bulkhead fitting?
McMaster-Carr

Ray Behner said:

Could you use a bulkhead fitting?
McMaster-Carr

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Unfortunately, I don't think so. I have a compression fitting to 3/8" tubing on one side, 90* MxM npt elbow going into a flow control on the other.

rbent said:

See Also

National Pipe Thread (NPT): Dimensions& FormulasTapping NPT pipe threads

I've cut a pipe tap shorter before, started with a normal one then take my shortened unit with the tip cut off to finish the job.

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That was my initial idea, but the guys in the machine shop weren't too keen on the idea.

HuFlungDung said:

Thread mill it. I've never had occasion to try it, but it might even be possible to threadmill the bottom half of the hole with the required reverse taper if you use a single point thread mill.

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Yeah that's a no go as well lol. Our machinists are older gents who learned CNC from running manual machines in the 70s-80s. Learned enough to do the basics and that's it. No CAM, no profiling,etc. Everything done by hand, at the machine, 1 operation at a time basically. Very frustrating sometimes lol, but i'm a junior engineer 2 generations in front so it's not my place to say anything. Helical interpolation is as fancy as they get. So I have to give them what they want unless I want to end up machining 1000 different parts manually. Although it sounds pretty bad, it's not a bad gig here. Just got to play the game.

HuFlungDung said:

I'm an old f*ck, and I'd ask your old f*cks how do they intend to do it then? Shortening a standard tap should work just as good as one that is 'made' shorter. WTF.

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I think i'll just have them tap it with the standard, then go through them with an old ground one during assembly. I only need another rev or so. They were saying something about loosing the OG thread/cross threading it/idk something along those lines. I try and choose my battles wisely. For reference in the future tho, would the "short projection" be what i'm looking for? Say for going into a blind hole or some other application where I wouldn't have room for the standard tap?

capocoreyollo said:

That was my initial idea, but the guys in the machine shop weren't too keen on the idea.

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I can't see you've much option, the taper is the taper, for a given angle / diameter ratio the length has to be the length,...that said, you might be able to trim off a thread or two (that's 0.055 /thread) or reduce the tip dia' of the tap so if it does go a tad deep it won't damage the opposite thread.

OTOH, use a parallel female thread and cram the male taper into it, ............the purists with whinge but in the real world it's done every day of the week.

Limy Sami said:

OTOH, use a parallel female thread and cram the male taper into it, ............the purists with whinge but in the real world it's done every day of the week.

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How true. This is probably a stainless fitting as well which someone will over tighten and damage the aluminum threads. You had might as well get the benefit of the metal-on-metal seal. For stainless-on-stainless pipe fittings I've converted to this method with a specified TIG weld to ensure the seal. The threads provide the mechanical strength and the weld provides the sealing. Unlike the OP my uses are typically for gas applications and I've yet to have any issues. The machine shops like it too since a damaged fitting results in additional parts.

here's the assembly

Comfortably Numb said:

If those were stainless fittings and tubing that begs for a shop with an orbital welder. Those would be done before you could finish pouring the morning coffee.

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If you took a look up top, I talk about our machine shop a little bit. You should see the welding department....lol. If your asking for anything but MIG welding mild steel in a straight line, good luck! That would definitely "seal the deal" Although getting those cylinders out when they need repair/replacement may be tricky.

To get threads to register from both sides tap first for 3/8" NPS (admittedly, an extra step) and use the first pass to guide/register follow-ups with unmodified NPT tap. (Easy peasy, even for OFs like me/us. ) How many holes?

If you can start a NPT fitting into a NPS-tapped hole, typically steel hydro fittings and 'soft' alum is where it'd seem to work best/easiest. I'd compare how your tap/fittings would match up for that, and want 4-5 threads of engagement min (WP <3k PSI, of course, and if you can turm 'em in that far without snapping them off.)

It looks like there is plenty of room in the rear where the tubing comes in. Why not use thicker stock and slot the middle if you need to keep the other fittings in the same location?

Comfortably Numb said:

It looks like there is plenty of room in the rear where the tubing comes in. Why not use thicker stock and slot the middle if you need to keep the other fittings in the same location?

View attachment 156160

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Totally could/should have just made them thicker. Dropped the ball tho, already had material ordered and cut for 24 pieces.

Lessons learned:
1) Don't trust the "hole wizard" for necessary pipe tap depths
2) Ask more questions/get more approvals before pushing a part

24 pieces are for 6 machines (2 assembly's per unit) and I'm building 1 as a trial run now, so it would be months before I even try these, let alone use the other 20, so when it comes time and if it leaks, then ill beef it up and get new ones made.