weld size (stupid question) 1

[duk748]

hello - hope everyone had a nice holiday - i have a problem w/ a supervisor over a weld size - i have a plate 20mm thick welded to 2 80x80x6 square tubing - a 100mm dia. shaft is then welded thru the 20mm plate w/ a re-enforcing plate welded on the back side of the shaft also welded to the 2 tubes - i believe that i know what the size of the weld should be to weld the 2 20mm plates to the 2 80mm sq. tubes -
following lincoln welding specs the weld should be .75* the thinnest material - i would like to use a 10mm weld all around the 20mm plate to the tubes (not following lincoln) - supervisor says that the weld should be 15-20mm - i believe that this is over welding - he claims that our overseas manufacturing is not welding our parts well enough so he wants to increase the weld sizes on everything we design - this seems like such a waste to me - any help or thoughts would be greatly apprecaited

 

[dhengr]

A sketch with several views and dimensions and loads would go a long way toward getting you a meaningful answer. Imagine the number of configurations we could draw from your description, and still not know the loads or weld stresses. Furthermore, I doubt that Lincoln Electric says the weld should be sized at 75% of the thinnest material, the way you are interpreting it. And finally, “not welding our parts well enough” can cover a multitude of sins other than weld size.

 

[duk748]

hello again - here is the sketch -

 

[Ron]

Based on a quick view of your sketch, the weld size would probably be no greater than the thinner part; however, the direction of stress plays a part in that decision.

 

[MintJulep]

If your fabricator is not welding good enough now what makes you think that they will do MORE welding better?

 

[duk748]

hello again & thank you for all the advice - yes it is true - we have some very bad internal problems in our manufacturing facility overseas - our production managers are not paying attention to the details & the hiring of qualified people in the country we manufacture in is not the greatest - i am not high enough in the company to even voice my opinions about quality control & the like for fear of losing my job - i am in engineering & even then i sometimes have to use forums such as these to re-enforce my own knowledge & education even though my opinion is over ruled by my supervisors & our director of engineering 1/2 of the time - i am sure all of you have at one time heard the expression "it's my way or the highway?" - once again i thank all of the very knowledgeable people here to help me

 

[dhengr]

No one would ever have conjured your sketch from your first description, and your sketch only slightly clarifies your question. It is not a stupid question, it is just being asked that way. And, you have to develop some imagination and some real engineering intuition and a meaningful ability to really explain your problem. Weld design and sizing is not just a matter of some rules of thumb or max./min. vs. plate thickness. You have to explain how this little structure works, its orientation w.r.t. (with respect to) the floor, etc. We can’t see what you are looking at, and what you have been puzzling over for days, from here, you must explain how this works and is loaded so that we can understand how the welds are loaded. And, you have to show the welds so we can comment on their location and size as they counteract the loads and moments, etc. Per your description: is see the 100mm dia. shaft above piercing the two 80mm pls., but not likely leaving enough edge distance around the shaft; I see the two 20mm pls., but they are very different in size and welding lengths; I see the two 80mm tubes, and they appear to be bolted to some sort of support; but you tell us nothing about anything at or below this point, except that it appears that 500# induces some moment on the support at an unknown distance below the support, if that’s the only loading on the system. You need to be smart enough to know what info. and data is required to solve your own problem, maybe with some help, but otherwise you shouldn’t call what you are doing design or engineering. I understand that the boss gets the final say in what happens, but as MiltJulep and I said earlier, if you have the right facts in hand, a very strong argument can be made that more of a poor weld is not better, it is just more expensive to make and fix later.

 

[desertfox]

Hi duk748

If the joints are to be fillet welded then the weld cannot exceed the the thickness of the thinnest component.
The size of weld upto that limit should be determined by the loads the component will be subjected to and not what somebody thinks might cover poor welding, how does a bad 10mm fillet weld perform any better than a poor 6mm fillet weld?
Have a look at this site for welding calculations:-

http://www.roymech.co.uk/Useful_Tables/Form/Weld_strength.html

If you provide more info on weld types loads etc we might be of further help.

desertfox

 

[tr1ntx]

"he claims that our overseas manufacturing is not welding our parts well enough so he wants to increase the weld sizes on everything we design"

I went through this very same thing years ago with Chinese suppliers. Their weld beads were slightly larger than what we had done on our in-house welding, but the welds broke at the interface, i.e., they "pulled out" of the base metal. Obviously their weld procedure was flawed, but our management wouldn't attempt to force correction. They were too in love with the low prices and the butt-kissing, pandering salesmanship they got from the Chinese reps. The company president's solution was to have them put larger weld beads. Of course, they held more force before they broke, but they still broke the same way. Neither I nor any of the welders or supervisors would come forth and spell it out for them because we suspected that we would all lose our jobs to "outsourced manufacturing". Such poor manufacturing practice should have immediately disqualified them as unfit, unknowledgeable amateurs, but it didn't.

So duk748, the supervisor's solution may or may not work. It depends on where the overseas manufacturer is not welding "well enough" and why that is not "well enough". On the leg of the fillet to the 80x80x6 tube, anything over 6mm is theoretically wasted welding. You are limited by the 6mm wall thickness' strength, so welding a bead further out along that wall doesn't gain anything. It just shifts the leading edge of the HAZ, and gives a larger HAZ, which is generally not what you want. I could see going halfway up the 20mm edge of the plate, i.e., 10 mm, with that leg of the fillet to "make it look right", but the governing factor is the weld throat depth. Using 75% of 6mm might be the proper theoretical design, and might be all you really need, but a 4.5mm fillet sure wouldn't look right with a 20mm plate edge next to it, plus to make sure you get all the way around the tube's corner the bead will likely need to be larger than 4.5mm.

A 6x10 fillet might be hard to do and would actually end up ~7.5x10, but that's the limit of what I would do (assuming no out of the ordinary loads). That tubing is going to melt much quicker by the weld arc than the 20mm edge of the plate so the arc needs to start against the plate edge and spend more time over it. The supervisor's solution of a 15-20mm weld is definitely overkill against that 6mm tube wall. A lot of people with a little exposure to general welding have seen those big, thicker-than-the-base-metal fillets done in multi-pass welding on big structural pieces but don't understand the reason behind it. "Bigger is better", right?

So what is going to happen here is that the supervisor will get his way with the 15-20mm welds, which will likely be strong enough to hold, so he'll say "see, I told you so" thinking he solved the problem. Of course, this merrily neglects the doubled or tripled welding time. Then eventually the overseas manufacturer will try to raise the price, or skimp and cut back somewhere else, like on material quality.

 

[duk748]

hello again & thank you to everyone for all your help - tr1ntx thank you for your well versed understanding of my problem - you have it correct as i thought in the begining - thank you for the links to the weld calculations also - i will try & hang in there considering how the economy is right now but it is really getting hard to accept fiction for fact here - my best to all -