I need to affix a fastener deep inside a tube

682bear said:
I like your idea about a thicker piece of flat stock placed inside the tube, but instead of 'pinning' it, I would drill holes in the tube, locate the flat stock, and then 'plug weld' the stock to the tube... then locate the 1/2-13 holes and drill/tap with the flat stock in place.

FWIW, I have welded flange nuts in steel tube also...

-Bear
Click to expand...

Hi Bear

I like this idea.

I could pre-drill and tap the 1/2-13 holes in the flat stock, bolt it in place inside the tube for registration, and plug weld the two together. Remove the temporary bolt, and voila!

If the plug welds were far enough away from the threads there would be no HAZ issues.

The plug welds would basically be a "heavy tack" correct? How big should the holes for the plug weld be, practically speaking?
 
My rectangular tube is 2in x 3in x 1/4in wall. It will be oriented flat for less tendency to roll when raising the mill (lowering the feet) to get the pallet jack under it.

Already had the materials when I learned of the flat stock approach.
 
jmarkwolf said:
Hi Bear

I like this idea.

I could pre-drill and tap the 1/2-13 holes in the flat stock, bolt it in place inside the tube for registration, and plug weld the two together. Remove the temporary bolt, and voila!

If the plug welds were far enough away from the threads there would be no HAZ issues.

The plug welds would basically be a "heavy tack" correct? How big should the holes for the plug weld be, practically speaking?
Click to expand...

The size of the holes really wouldn't be important... as long as they are large enough to get a sufficent weld... probably 5/16 or 3/8 would be plenty.

The welds are only there to hold the bar in place until it is bolted to the bottom of the machine... after it is mounted, the welds serve no purpose at all.

-Bear
 
Take a chunk of 1 inch solid bar. Tap it for you screws. I assume you are using 1/2 inch.
Drill a 1 inch hole all the way thru the tubing.
Push the chunk of bar into the the tube and weld a circle on each side of the tube.
Done.
That's how I put bomb proof mounts in tubing on dune buggy frames.
 
Yep, don't worry about the nuts doing funny things when you weld on them. We build heavy duty commercial trucks and use weld nuts inside the frame to hold everything on - been doing it for years. Three heavy tacks will do fine. However, welding a nut inside a tube is going to be tricky without opening up a big clearance hole on the bottom side, which also kills your strength unless you cap over it when you get done. I like the big bar inside plug welded to the side of the tube idea. Plus that gives your feet something more substantial to rest on (if the bar sticks out that far). I agree that the feet thing looks wiggly - any vibrations are carried by 4 points, maybe four 1/2-13 bolts. The bolts in the tube through the base are not really doing much of anything structurally, more positionholder. Rule of thumb for plug welds: min diameter is 1.5 times material thickness for structural applications, let's you get some heat in the hole before you close it up.
 
OP here.

I might've gotten ahead of myself.

I am an amateur welder, with a Millermatic 211 mig welder (capacity 3/8in). Probably shouldn't expect a decent plug weld with a 1/2in thick bar stock and 1/4in tube wall.

I can take it to a real welder.
 
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