Another welding question

[Marc]

Hi there! ......... I have yet another welding question. Have to make two new anchoring points for my pontoon since I don't trust the one I have in aluminium. I have to weld two flat bar 100x15, 150 long to a plate that is 150x300x10. This is a rather critical weld since it will hold the whole structure. Conceded there will be 4 of this flat bar welded to two plates..... How much chamfer should I give the flat bar and what sort of angle, in order to get a complete weld to the plate? ... your replies as usual are very appreciated.

[DoctorWu]

Hi there! ......... I have yet another welding question. Have to make two new anchoring points for my pontoon since I don't trust the one I have in aluminium. I have to weld two flat bar 100x15, 150 long to a plate that is 150x300x10. This is a rather critical weld since it will hold the whole structure. Conceded there will be 4 of this flat bar welded to two plates..... How much chamfer should I give the flat bar and what sort of angle, in order to get a complete weld to the plate? ... your replies as usual are very appreciated.

Are the 2 150x100x15 bars welded standing at right angles to the 150x300 plates? With a fillet weld all round then just make sure the 15mm edge is FLAT and clean. This should require minimum 5 passes and the root pass would be about 150+ amps.

[Marc]

Yes at right angle.
I am thinking to grind the edges of the flat bar so that the weld penetrates all the way inside the joint.
Just not sure at what angle I should grind the bevel and how much to leave without grinding.
Also preheat or not?

[Michael G]

Rule of thumb is that anything over 3mm should have weld prep if you want full penetration welds, so if you are chasing full pen on a 15mm thick bar I would say prep minimum 6mm each side. The angle is not necessarily anything special. The narrower it is the less weld is needed to fill the "hole" (and so less heat). It just needs to be wide enough that you can get in to weld.
As for preheat, you are going to be putting a lot of heat into one side of a plate so there will be a tendency to distort. I think it would be one of those things that you either preheat and weld at a reasonable pace or don't preheat and wait between runs for things to cool.

Michael
(far from a welding expert)

[DoctorWu]

Yes at right angle.
I am thinking to grind the edges of the flat bar so that the weld penetrates all the way inside the joint.
Just not sure at what angle I should grind the bevel and how much to leave without grinding.
Also preheat or not?

If 2 plates are welded together at 90deg to each other the weld required is a Fillet weld, the plates themselves form the bevel. A manufactured bevel is required when 2 plates thicker than say 4mm are welded together in the same plane and this is called a Butt weld.

[Marc]

I understand that the plate and the bar make a 90 degree angle and that I can weld a fillet around it. I was wondering if grinding the bar I would have a better weld because it will penetrate between the end of the bar and the plate.

[DoctorWu]

I understand that the plate and the bar make a 90 degree angle and that I can weld a fillet around it. I was wondering if grinding the bar I would have a better weld because it will penetrate between the end of the bar and the plate.

To get good penetration in a Fillet weld you need a flat clean fit, eg. NO gap and high amperage on the root run. For 15mm plate I would use at least 150amps and good quality hydrogen controlled rods, 3.2 for the root run and 4mm for the others. Make sure you have solid tacks on both sides before doing the root pass and then alternate each weld from one side to the other.

[Marc]

Forgot to say I will be using a mig. 250 amp

[DoctorWu]

Forgot to say I will be using a mig. 250 amp

Same plate setup applies but I couldn't help you with the welder settings as I haven't done mig to certification level. Thing to remember when doing a proper Fillet weld is to really burn in on that first pass and this is more important than the look of that run.

[Marc]

No problem with the set up, thank you for that.
the other two anchoring point I made where similar, with 12 mm plate and a 25mm shackle welded to it, to take a stay cable. The way I did that one was cutting the shackle above the eye where the pin goes in and leaving just a U. Then I drilled 2x 26mm holes and put the shackle through it, so I could weld from the back and a fillet around at the front. Turned out real good.
Applying the same logic with the flat bar I thought in making a tenon in the flat bar and a slot in the plate and weld front and back. May be I am overthinking this.
i have to make two of this. If I tack weld the two plate back to back before welding the flat bar at 90 to it, would that stop it from buckling or do I need something more substantial?

[Gavin Newman]

Marc

From my reading of your set-up you will have a foot of weld on each joint. If that doesn't hold the two pieces together then a tenon, back welded, isn't going to make it any better. It just complicates the whole set up.

[Marc]

Yes, well not a foot but 230mm however I get your point. I know no one does that sort of plug welding, if it is even the right term. Like I said, I probably am overthinking it.
What about backing one plate against the other to avoid bending?

[DoctorWu]

Marc

From my reading of your set-up you will have a foot of weld on each joint. If that doesn't hold the two pieces together then a tenon, back welded, isn't going to make it any better. It just complicates the whole set up.

I agree. Fair bit of extra work for no realistic added advantage.

[DoctorWu]

Yes, well not a foot but 230mm however I get your point. I know no one does that sort of plug welding, if it is even the right term. Like I said, I probably am overthinking it.
What about backing one plate against the other to avoid bending?

If you want to avoid bending just alternate your beads from side to side. Or clamp a piece of sturdy angle(10-12mm) to the offside before you do the first root run, remove it and do that side and so on.