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I have a hollow inox tube 40mmx20mm with 2mm thick walls that I would like to drill through-holes into and then tap them M5 suitable for M5 inox bolts. This tube will be used on a piece of furniture as a type of pivot arm etc...

So, I have been looking at the available M5 manual tap tools on the market and they all seem to have a maximum thread length of only 16mm. This means that once I finish tapping the first top hole, the thread won't be enough to continue tapping the second hall, since the gap inside the tube amounts to 16mm (20mm - 2x2mm).

I would like to have a continuous tapped thread in these through-holes to be able to screw M5 bolts and them cut them off flush at the end, then polish the surface of the tube etc. for a clean finish.

I am not sure how I can do this if most of the taps have the 16mm thread limit. Is there a way to overcome this, say by inserting an M5 nut underneath the first hole (maybe with superglue), which acts as the "extra" material for the tap to be able to continue tapping the second hole with continuity to the first tapped thread ? I thought about stacking up a few M5 nuts inside the hollow part of the tube (4 x 4mmM5 nuts), but not sure if this would work.

Any thoughts or suggestions would be appreciated.

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  • You say 40x20mm. Is your tube a rectangular cross section?
    – Huesmann
    Commented Jan 12, 2023 at 14:34
  • Question is confusing. Is it box-section tubing, and the bolt will go through the 40mm side to the other 40mm side, or the other way. All of my taps are longer than 16mm (1.6cm, way less than 3/4"), and all would travel through even the longer part of the tube. Why does it have to be a bolt/set screw?
    – Tim
    Commented Jan 12, 2023 at 15:24
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    Do you need threads in both walls? Could you tap one wall and drill the other slightly larger to make a clearance hole?
    – spuck
    Commented Jan 13, 2023 at 0:05

6 Answers 6

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I would not tap the tube, I would redesign the joint. You're using a bolt as something it isn't and that's the problem. Your design intends to grind the bolt down anyway, to make it flush with the tubing, so avoid using a bolt (which avoids the tapping of the hole).

Instead drill a hole and weld a rod into place, then cut the rod to desired length, grind / finishing non-exposed end flush with the tube and thread the exposed end.

Any drilling of the tube and then tapping the tube will create problems. Unless the tube is very thick, the threads will easily strip. That's why in (cheap) furniture they often tack-weld bolts instead of thread the tubing. Doing it twice (once for each side of the tubing) will create this problem twice, and add in an additional problem, that of making sure the threading is aligned between the two walls (so the bolt bits / doesn't seize on the other side of the tubing).

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I would find a bar that slides inside the tube you have and clamp it so it does not move.

Then I would mount the tube in a vise and drill through the tube and bar completely.

Once the hole is cleaned then the tap can be run through from one side only with the bar making sure that the thread is "consistent" for both sides of the tube.

Do NOT tap each side separately, tap from one side all the way through the far side in one operation.

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  • Hmmm, yes this could work. I will need to clamp it in place. Could be wood or plastic as you all point out. As long as the thread carries through enough to the second hole for the tap to continue the thread. I may just look into getting/cutting a piece of 36mm x 16mm bar of something to slide inside and may not even need to clamp anything down as this would be kept fixed by the steel tube itself. Thanks that'll get me going ;-)
    – citizen
    Commented Jan 12, 2023 at 14:05
  • Better to just weld in a rod, through the desired hole in the tubing, clean up the welds, and then add threads to the rod. This approach will work, but leaving the bar that guided the tapping in place puts a lot of friction on the bolt / tap. Removing it allows thermal expansion / stresses on the tube to make the threads lose their alignment, even after taking such care to get them in nicely.
    – Edwin Buck
    Commented Jan 12, 2023 at 18:48
  • I've read through this twice and still don't understand the idea. I also don't understand why "quotation marks" were used on a seemingly arbitrary word. Maybe add a bit more detail for the slow folks.
    – isherwood
    Commented Jan 12, 2023 at 19:56
  • Solar Mike use the word "consistent" in quotes because there is a problem with tapping both sides of the tube. Two nuts welded to a plate of steel, such that one could pass a threaded rod through both of them at the same time doesn't always work because sometimes the second bolt is rotated such that the threads don't align. So, they need to be consistent to each other, as well as being threaded correctly to match the rod. You can't just tap each side, you have to tap them both at the same time. There's not a great term for this, hence the quotes to bring attention to it.
    – Edwin Buck
    Commented Jan 12, 2023 at 20:05
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I'd pose a frame challenge. 2mm tubing walls have enough flex to accommodate half a thread thickness (the most the two taps could be off). I'd drill through and tap from both sides and move on with my day. The bolt will likely thread in just fine.

I also wonder how a tapped bore can be considered a pivot. They're generally unthreaded.

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Similar to the above answers but maybe easier to find a tube, broomstick, etc to fit in the existing tube and drill through it. Then run the tap through it. You're just looking to carry the threads through, once you hit your tube on the second side, you're home free, the tap will do the rest.

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After drilling holes suitable for tapping, you can skip the tapping step and simply use self-tapping screws. These can be either thread-cutting or thread-rolling type; both would work.

If you can't find a bolt with the head style and finish you want in self-tapping style, install any self-tapping screw of sufficient length to go completely through the tube, then remove it. Presto, two tapped holes properly aligned for the bolt you want to use.

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  • The bolt head I would like to use is flat pan head with a hex key so I'd rather keep this as is and is readily available etc... Any other time I'd probably consider a self-tapping type.
    – citizen
    Commented Jan 12, 2023 at 14:14
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    @citizen Sure, understood, so as suggested in the second paragraph, use a long self-tapping screw as a substitute for a long tap. Insert, remove, and your holes are now tapped and ready for your preferred non-tapping bolts.
    – MTA
    Commented Jan 12, 2023 at 14:44
  • I had a quick look at the availability (in my region) of the self-tapping M5 with 0.8mm pitch that I need to use... Not much arround really, or at least the data that is available is not complete enough for me to know if they are the right tap for my M5s. So at least with the taps, I can get them easily on RS for a few € each, the M5 and an M10 (same story with the M10).
    – citizen
    Commented Jan 12, 2023 at 15:51
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I thought about suggesting self tapping screws, but someone else did that, and I can't find any I'd consider self tapping for metal. Self tapping for wood, yes, but not metal.

So I started looking for longer working length manual taps. I found a couple that would span the full 20mm tube you have. These are the few I could find that had the actual thread length mentioned in their listings. I couldn't find a M5x0.5 tap.

M5 x 0.8, 20mm working length: https://www.amazon.com/dp/B09766V534/
M5 x 0.8, 22mm working length: https://www.amazon.com/dp/B081SV9L5Q/?th=1
M5 x 1.0, 22mm working length: https://drillsandcutters.com/m5-x-1-hss-metric-hand-tap/

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