Welding tolerance for DOM tubing through plate?

[red92s]

I've begun a new role designing some weldments (new to me) and hoping to get a bit of input regarding how best to design them for fit-up and welding.

We frequently have DOM tubing "piercing" through a flat steel plate. The tubing is typically 1.25"-2.5" OD, and the plate is typically .375"-.5" thick. The holes in the plate are most frequently laser cut, although occasionally they are machined. What would be a good starting point for the nominal size on the holes in the plate, to aid in easy assembly while keeping things from getting too sloppy? I've had some parts made where the holes are +.045" from the nominal tube OD, and it feels very loose. Any input appreciated.

 

[SceneryDriver]

I think your first question should be, "how consistent is the OD of your tubing?" Also, "how consistent is your laser cutter / machinist?" My laser cutting guy can hold -0, +0.005" tolerances, but others may be more or less precise.

Personally, I've always used the actual dimension + 0.005" and gone from there. Is a slight bevel on the end of the tubing acceptable to aid insertion? +0.005" is not much wiggle room, but the slight press fit it can afford (without damaging anything) is nice for fit-up. How large are these assemblies? Are these the kind of weldments that can be moved by a person and "helped" together with a 5# deadblow hammer, or do you need a crane to move the finished pieces around?



SceneryDriver

 

[red92s]

Typical OD tolerance on DOM is ~+.005" for 1.25" OD, and ~+.007" for 2.5" OD. Not sure about the tolerance on the laser cut diameters, but I will inquire with the supplier.

Chamfers on the tube ends can be done on parts that are made on the lathe, but some are "dumb" tubes saw-cut to length, so adding that feature would be cost prohibitive. These assemblies are usually smaller than a breadbox. Using a small deadblow to coax things together is certainly an option if necessary, and much preferable to stopping to open up the holes wit a die grinder.

 

[SceneryDriver]

Can the saw-cut tube pieces be deburred with a flap wheel disk on an angle grinder? That should be all the bevel you need.


SceneryDriver

 

[red92s]

They can be, but doing both ends manually on the 100 or so pieces of tube needed for a production run isn't real appetizing.

 

[Mr168]

If you aren't willing to debur the saw cut tubes, your needed hole dimensions will likely be all over the place. Given that a tube that diameter can be deburred on one end in 2-3 seconds using a belt sander, and you're looking at relatively small runs of tube, I'd strongly recommended going that route to take burs/trash out of the equation.

 

[SceneryDriver]

I'm with Mr168 - a quick sanding should be all you need. Do you de-burr or otherwise remove the burr from the inside of the tube, or does your product not require it?

I work in the entertainment industry and also special effects, so much of what we build in the scene shop is just that - scenery. Our carpenter/welders saw cut all the parts for a piece of scenery, and then do a quick de-burr on everything with a flapper wheel grinder or a stationary belt sander, depending on how large the pieces are. A piece of scenery can easily have over 100 parts of box tube that need to be de-burred, and it doesn't take more than about 20 minutes or so. If these are round tubes and you get a stationary belt sander, it will take much less time.

What type of saw are you using to cut your tube? Cold saws are a good choice, but I've actually found that the dry-cutting carbide blade saws (Milwaukee, Dewalt, and others too) make the cleanest cuts. I use a 14" dry-cut carbide saw to cut DIN rail and I don't have to do any de-burring at all.


SceneryDriver

 

[MadMango]

At .375-.50" plate, your ~3deg cut bevel with a laser will be more of a factor depending on which side the tubes get inserted from. Hopefully your welding jigs hold the parts where they need to be.

"Art without engineering is dreaming; Engineering without art is calculating."

Have you read faq731-376 to make the best use of these Forums?

 

[mfgenggear]

http://www.stockcarsteel.com/dom-steel-tube-drawn-over-mandrel/

I agree with all of the above comments.

what is the wall thickness of the tubing?

The degree of precise alignment will be held by the welding fixture.
I am talking out loud here cause my welding experience needs refreshing.
it seems that a snug fit would be prefered but not necessary. it would seem to me it would be easier to tack weld, then weld it.
The hole tolerance +/-.010 should be adequate.
let's say 1.250-1.255 dia of hole should be 1.265+.020/-.000 dia should be adequate.
the hole's should be verified with 1.265 dia pin to make sure it will assemble.
these are suggestions.

depending on the quality of welds required, is it x-rayed? mag particle inspected?
a critical weld must have a prepped & cleaned edge.
plus a welding schedule to make sure the correct temperature & filler metal is applied.
the application is not specified.

HTH






Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.

 

[mfgenggear]

also take into account that the welding will consume material the length of the tubing will shorten, this is a fact.
must allow for weld shrink.

HTH

Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.