Machining welding beads from I-beam unions

[drodrig]

Hi there,

I am designing a "square" made with wide flange beams. It is made with HEB160 beams and the outer dimensions are roughly 700x800 mm2.The plan is welding it.

I am concerned about the accuracy of the whole system, what tolerances we have in the overall dimensions after welding. What is the typical precision one has in this kind of structures?

We plan to make a double V butt welding

One problem we face is the bead interferes with the parts which go bolted up and down this structure. So I was thinking about putting the welded assembly in a milling machine and getting rid off the upper and lower beads. Is this recommended? Just from one side of the V-butt-welding, maybe doing this is crazy...

thanks

 

[KENAT]

There are standard weld symbols for grinding weld flush or under flush as required. How it is achieved is a secondary consideration normally. Exceptions would obviously be if it materially impact the end performance in which case you may need to give more information on acceptable dimensions & tolerances.

As to typical process capability (achievable tolerance), if you machine the holes before welding then generally wouldn't be great but I'm not close enough to that kind of industry to give a good idea. With accurately cut beams initially, the right fixturing and good technique etc. you may be able to do surprisingly well.

However, if putting the finished piece on a machine is a realistic option you could always consider drilling the holes after welding.

Important thing is functionally what tolerances do you need on the holes?

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[gtaw]

It is not generally a good idea to have welds intersecting at 90 degrees from each other. The resulting residual stresses can exceed the tensile strength of the steel and produce cracks where the welds intersect. You can treat the residual stresses in the longitudinal direction as vectors to appreciate how the resultant can easily exceed the minimum specified tensile strength of the steel member.

I suggest you obtain a copy of AWS D1.1 or another structural welding standard that depicts typical joint details and "good industry practices."

If the assembly is to be machined, you should consider a thermal stress relief operation so you can more easily hold the necessary tolerances.

Best regards - Al

 

[racookpe1978]

Absolutely do NOT drill holes before welding the assembly.
Grinding such welds flush is a standard process in many industries - but do it ONLY for the surfaces that must sit flat.

If the bottom of the assembly does not actually have to "flat" then you will probably find that your machinists can support a simple assembly on the milling machine ABOVE the milling machine plate for basic operations like drilling bolt holes with little problems. An automated milling machine can drill the holes, but you MUST decide where you need to index the clamped assemblies FIRST (before making jigs and fabrication assemblies!) so that all of your stacked tolerances end up with acceptable hole locations.

Alternative: Drill all of the holes, put the drilled parts into a jog with pegs or bolt studs to locate the already-drilled bolt holes, THEN weld the plates and beams together. But you will still likely get some weld shrinkage and frame warping as the welds cool.

 

[thaidavid40]

Completely weld this according to proper procedures, then completely stress relieve the whole welded assembly in a kiln, then mill and drill as required.
Dave

Thaidavid

 

[drodrig]

Thanks for all the answers,

We need a precision better than 1 mm. This structure will sit on two parallel rails (two rollers each).

I am now concerned about the perpendicularity of the welds. Isn't perpendicular welding pretty standard? If one looks industrial buildings. What if we cut in the flange like this?:

We get two additional bead lines perpendicular to the original one.

About the machining the guys in the workshop will know; but do we really need to put it in the kiln? I guess not everybody anneals all the welded structures... but I am just guessing

thanks

 

[gtaw]

While it is common to have welds perpendicular to each other, it is common practice to stop the welds short of intersecting each other. The exception of course would where fluid tight joints are a necessity, in which case I would stop the welds 25 mm short of the intersection. Once all the welding was completed, complete the welds in the corners to form the fluid tight joint.

With regards to the thermal stress relief, the maximum temperature is between 1150 and 1250 degrees F with a hold time of about 15 minutes for each 8 mm of thickness. This is to reduce the residual stresses to a tolerable level that will allow one to hold the necessary machining tolerances.

Best regards - Al

 

[MintJulep]

Does the loading really require CJP welds of cross braces so massive?

What is the load path between whatever you are putting on this and the rails?

 

[tbuelna]

As others noted, normally you would weld, stress relieve, and then finish machine. With a large amount of welding, over time the structure will move quite a bit if it is not stress relieved and this can affect the accuracy of finish machined features. It should be no problem finding a mill capable of handling a 700mm x 800mm weldment.

To get a better fit with the mating part contact surfaces, it is common practice to weld local raised pads at the bolt hole locations, and then finish machine the pad faces after welding/stress relief. This approach does not require any material to be removed from the I-beam flanges.

 

[drodrig]

gtaw, thanks for the hints, we will take this into account, but do you recommend the line welding or cutting the flanges to have the two additional perpendicular welding lines?

MintJulep, I must admit the design is an overkill. Here are the forces, about 3t up and 2t down. They go directly to the beams perpendicular to the rails, the other two beams (parallel with the rails) are just connecting beams. I was thinking decreasing the size of these connecting beams, but I wanted continuity and everything connected on the flanges:

tbuelna, you mean an additional small plate welded and then machined?

thank you all

 

[gtaw]

The last details looks fine. No interconnecting continuous welds - Good. Use run-off tabs so there are no unfilled craters at the ends of the joints. The tabs can be trimmed and ground smooth after all welding is completed. Notches and partial joint penetration groove welds are an invitation to fatigue cracks if cyclic loading is involved. Fatigue generally isn't a serious concern if the stresses are in the elastic range and there are less than 10 000 load cycles. AWS D1.1 can provide some good direction regarding fatigue.

You could probably reduce the size of the connecting beams, but the detail shown provides rigidity. No dimensions are provided, but structural steel is typically provided in 20 or 40 foot lengths. So, if all four components can be cut from the same length, there is no cost savings if two 20 foot beams are cut to t he proper lengths and half of each twenty foot length is scrapped.

Best regards - Al

 

[racookpe1978]

I would use the 4x small plates, machining only those flat for the 4x mounting pads to rest on.
The plate assembly is certainly very rigid.

But! The twisting forces will be transmitted to the left-right rollers to the tracks - which are NOT rigidly attached to anything. Rather, the track rollers have a gaps between roller and roller body, roller and tracks, and the tracks themselves are only attached at intervals to the "floor" - whatever that might be.

Thus, your very rigid, very strong frame will stay rigid, but will easily rotate as a whole under that twisting motion.

 

[gtaw]

I would consider a diagonal bracing from one corner to the opposite if rigidity is a concern . It all boils down to how this is to be used.

Best regards - Al

 

[MintJulep]

Machine joints to fit. Machine joint prep.
Weld weld weld weld weld.
Grind a bunch of that weld off.
Stress relieve.
Machine holes.

That's a bunch of work, leading to a part that is far more expensive than it could be if is was designed to do what it needs to do.