I just drill and tapped four 5/16 x 18 into some 0.200 wall AL tubing. I can
see this won't hold long term, the threads will strip. What can I do for
strong tap theads in this situation? Some sort of insert???
The best option depends on the details of the connection. If the tubing
is large dia so the curvature isn't too bad, "riv-nuts" should work. If
it's smaller dia tubing with too much curvature for a riv-nut, a larger
insert inside the tubing with the threads would work if you can get it
into place. If you can drill oversized and do a through bolt with a
sleeve that would work as well.
Insert a block in the bore and tap into it, threaded inserts, sleeve
the outside, or a combination of all of them. Just depends on what
you intend the load to be and what direction it occurs. Also what
alloy you're using, extruded hardware store gum isn't the same as 7075-
If it's threading the inside of a small tube...
Maybe use a pipe-thread (tapered) tap? It would leave near full
thickness at the narrow end, but engage fully at the mouth
of the tube,
Or, is this drilled/threaded perpendicular to the axis of the tube?
For that, an aluminum-soldered (or welded) boss might be
stronger; it's how backpacks are assembled with thinner
Thanks for helping think outside the box. If I can find a way to reach down
inside the tube and start a bolt from the inside, then twist it tight and
form a stud. reaching two feet inside a 4x4 tube and starting a bolt should
I've never heard of anyone trying to do this with a hand drill... Hmm...
I'm guessing you might not be able to make enough thrust, etc...
Maybe give them or "form Drill" a call and see what their thoughts are.
I've had discussions with both and found one to be much more helpful than
the other... But I don't remember which. Figures, eh?
The poor man's version of that is to drill a hole much smaller than the
tap drill, use a long and very tapered punch to extrude metal into the
hole. Drill and tap normally. For the 4"x4" tube you might have to have
a heavy backer block with a suitable hole and work upside down. :)
Joe AutoDrill wrote:
It will be easier if you cut a screwdriver slot across the end, and
use a split screw-holding screwdriver to pull it into the threads.
Even better is turning 1 - 2 threads off so it will drop into the
hole. You could position it with smaller tubing with a vee in the end,
and a loop of string through the tube to hold the bolt in place but
let it turn.
Is this a square or rectangular tubing, or round/curved? If the
former, I would consider a Rivnut for the task. It slides in, and is
drawn up to grip on the inside of the mounting surface. The threads
already exist, and are used for the drawing. There are speed tools (for
smaller sizes), wrench tools (at least to 1/4" and maybe larger), and
pneumatic or hydraulic tools.
They were invented by B.F. Goodrich, and now seem to be made by
"Böllhoff-Rivnut® Inc" according to a Google search.
They have downloadable PDF documents describing the products, and a
certain amount of tooling as well. Or, you might want to make your own
tool. The wrench style tools have a thrust ball bearing with the outer
nut being turned around a bolt which is held still by a large Allen key
which produces the pull necessary to set the Rivnut. This is probably
what I would try to make to set 5/16" Rivnuts if I did not already have
the Hydraulic tool.
Note that Rivnuts can be obtained with through holes or blind
holes, as well as with flat head or countersink design, and those
without the knurled sleeve have available a key which will fit in a
notch cut by a tool made for the purpose so the Rivnut will not spin in
the hole even if you do not set it fully tight.
Here are a couple in the 5/16-18 size, but not quite right for
320393707983 Steel, but grip range from 0.027" to 0.150",
not your 0.200".
370230283081 Aluminum, with a grip range of 0.040" to 0.200"
The more serious ones (larger, wide flange) take a more serious
tool than the vendor is offering. I've got a hand-pumped hydraulic tool
of the sort, as well as the speed tools for lighter weight things.
370185982532 This one looks nice too, but too short a grip
I don't see any of the hydraulic tools in my old saved searches
right now. But I do like Rivnuts for the type of job you are