Paint or Bare?

[Fasttrack]

I've got another job coming up (thank goodness! It's been 4+ months since I ran a machine or picked up a stinger): Weld a stiffening plate onto the stick of a trackhoe. It's a 12" wide 1/2" thick piece of A36 about 11' long.

Now, my question is about the paint. Should I remove all of the paint on the stick between the plate or should I leave the majority of the area painted? My vote is to have bare metal across the entire surface. I will, of course, remove the paint anywhere I will be welding, but for the area that just gets sandwhiched between the stick and the plate ... bare or painted?

[sasquatch]

I've seen this done both ways by different people, also seen the clean ground pieces greased a bit then clamped up and welded.

[shipto]

I dont see any problem with sandwiching some old paint as long as you clean a reasonable way back from the welding area. Especially for something like that.

[Black_Moons]

I would'nt leave any paint where it might get hot enough to vaporize and contaminate the weld arc. I sure would'nt add grease anywhere except maybe as a splatter guard if the area nearby was decorational.

[rollin45]

Enlighten me, what possible difference could it make? As long as the weld area is clean, and you leave a hole near the top during the welding for the hot gas and air to escape, then once almost done on the final pass, fill in the hole and finish the cap.

rollin'

[Fasttrack]

I seem to recall an engineering handbook noting that bolted joints should be bare for maximum strength. Paint or other material between the interface of the parts could weaken the joint. I wasn't sure it would matter for a welded joint.

[Boostinjdm]

Clean the area to be welded and go to town. I don't think that paint in between will make a difference. It might even slow down any rust jacking.

[wierdscience]

I would buff the paint off about an inch either side of the weld and let it go at that,it won't matter either way,but why go to all the extra work/cost.

[danlb]

I have no value to add to the thread. Sorry.

I am curious about the way a stiffening plate works. When you weld that 12 x11 plate, are you plug welding it or only around the edges? It seems to me that cleaning the paint off will result in a plate that is parallel in all planes with the member that is being stiffened. I think that means that it will not flex independently of the part that it is welded to.

On the other hand, I think the paint will provide a sizable gap due to the thickness of paint on heavy equipment. I seem to recall 1/16 inch thick as not uncommon. I think that makes the stiffening plate part of a shallow box structure. Is that stronger or weaker?

Dan

[becksmachine]

I seem to recall an engineering handbook noting that bolted joints should be bare for maximum strength. Paint or other material between the interface of the parts could weaken the joint. I wasn't sure it would matter for a welded joint.

As for bolted joints, according to AISC it depends on the paint. Google "paint faying surfaces". Some paints are ok.

Dave

[Fasttrack]

As for bolted joints, according to AISC it depends on the paint. Google "paint faying surfaces". Some paints are ok.

Dave

Interesting. Thanks for the Google tip - turned up lots of good hits. I wouldn't have known the term "faying" otherwise.

Dan - I was planning on plug welding in a few locations, although most plates I've seen are not plug welded. I am welding another plate to the stiffening plate to support a hydraulic cylinder for a thumb/ demo grapple so I will plug weld the stiffener there and then cap it with the other plate. Sort of hard to describe without a picture.

Regarding your question, I assumed I'd want the surfaces bare instead of in the "box" configuration you describe. The main idea of this stiffening plate is to prevent the box of the trackhoe stick from buckling when being used in conjunction with demo grapples or a thumb. In addition, it will provide some added rigidity to prevent the stick from twisting, which can happen with inexperienced operators doing demo work, etc. This is my understanding of purpose, but I'm just the welder. The booklet for this thumb says to weld on plate before installing a thumb, so that's what I'mm going to get paid to do!