Massive Hit

[audietron]

During our last match on Friday at championship we had a slight problem with our bumper brackets.

We also bumped in to pink team (233) during the same match.
Notice the corner of the frame.

[JohnSchneider]

At Hub City we had a pretty fun collision with 16 :rolleyes: . You can see the numerous places our frame split. We Covered the whole thing in tin foil and had it rewelded (This time without the faces being ground off). No issues. http://i.imgur.com/h1bglpH.jpg

[mdituri]

In MAR, we seem to take a lot of hard hits, much harder than we experienced at Worlds, for sure. When I removed our bumpers after Bridgewater, we had dented diamond plate on the front of our robot. CRAZY. MAR is so aggressive and defensive, where Worlds was about scoring, scoring, scoring...In MAR sometimes I think people enjoy just smashing into each other at full speed, where at Worlds, they were avoiding each other so they could rush to the end of the field and score.

[Kevin Sevcik]

At Hub City we had a pretty fun collision with 16 :rolleyes: . You can see the numerous places our frame split. We Covered the whole thing in tin foil and had it rewelded (This time without the faces being ground off). No issues.I have to say, I have little sympathy for people who go to all the trouble of welding up a frame, only to grind all the reinforcement off. As you discovered there, unless you've done a proper bevel prep on the edge, a butt joint is quite likely to have low penetration and little fusion of the base metal. And so you can end up with fractures that follow the original seam quite impressively. Any time one of my mentors suggests we weld something up and grind it flush, I give them the evil eye and they usually decide that perhaps we don't actually need to grind it, actually...

[Mike Marandola]

During our last match on Friday at championship we had a slight problem with our bumper brackets.

We also bumped in to pink team (233) during the same match.
Notice the corner of the frame.

On an unrelated note, how do you guys like those Vex pro wheels?

[jbsmithtx]

We too had a big collision with 16 at Hub City... They must have earned something with that world championship of theirs from last year... Their climber stuck out a little bit, and when we hit them, we got a bit under them, and they bent our frame, and we suspected to bend their climber... Successive times after that they couldn't climb... Was a big hit and bent our entire chassis out of whack. The C-channel in front was bent in and then pushed out, and the rest of the frame had shifted in different directions.

[JTN]

We too had a big collision with 16 at Hub City... They must have earned something with that world championship of theirs from last year... Their climber stuck out a little bit, and when we hit them, we got a bit under them, and they bent our frame, and we suspected to bend their climber... Successive times after that they couldn't climb... Was a big hit and bent our entire chassis out of whack. The C-channel in front was bent in and then pushed out, and the rest of the frame had shifted in different directions.

As a member of 16's drive team, I am not entirely aware of what incident you are referring to, but I can give you a real explanation as to why we "couldn't climb." The answer is simple, one of the two CIM motor wires got pinched in the robot during rotation of the climber somehow, nothing to do with the piece sticking out. Also to keep you from worrying, you did not do any damage to our climber. The part you hit was made of two 1" diameter steel threaded rods that can withstand a 2000 lb load.

Thank you for your concerns, and sorry about the broken frame. :o
-JTN

[JohnSchneider]

I have to say, I have little sympathy for people who go to all the trouble of welding up a frame, only to grind all the reinforcement off. As you discovered there, unless you've done a proper bevel prep on the edge, a butt joint is quite likely to have low penetration and little fusion of the base metal. And so you can end up with fractures that follow the original seam quite impressively. Any time one of my mentors suggests we weld something up and grind it flush, I give them the evil eye and they usually decide that perhaps we don't actually need to grind it, actually...

When you drop the frame off to be welded and it comes back like that, you dont really have much of a choice :(

[audietron]

On an unrelated note, how do you guys like those Vex pro wheels?

We have had no problem with them at all. We did buy spares because we heard they rub down quickly but after 2 regionals of defense and offense the front wheels were only somewhat worn with a lot of the weight at the front. The five dollar price just can't go wrong and during champs they did not rub down very much. We plan on using them for a west coast style drive as well.

[James Tonthat]

When you drop the frame off to be welded and it comes back like that, you dont really have much of a choice :(
I've just always stayed with the frame while it gets welded to make sure it was done right then put it into powder.

[Kevin Sevcik]

When you drop the frame off to be welded and it comes back like that, you dont really have much of a choice :(Erm. Well that certainly is a problem. Maybe proper weldment drawings in inventor with callouts for the welds? Then you'll atleast have papers to shake at someone when it comes back that way.

Who'd you have weld it up that didn't know that basic rule of welding, anyways?

[fb39ca4]

Huh. My browser loads the first picture sideways. I've never seen that happen before.

[JohnSchneider]

Erm. Well that certainly is a problem. Maybe proper weldment drawings in inventor with callouts for the welds? Then you'll atleast have papers to shake at someone when it comes back that way.

Who'd you have weld it up that didn't know that basic rule of welding, anyways?

Our sponsor who is world (And technically out of this world) class. I dont think the welder knew the kind of loads we abuse our machine with and thought it would look nicer (that part is true). We sent them some pictures after we had a good laugh about it, and we'll be collectively sure to make sure to be extra concerned with exposed frame.

The kids managed to drive an entire match without noticing, and we're pretty sure 16 hit us on our exposed face with those huge claw things they had at the time. A full speed collision with something like that would probably cause a lot of frames some grief.

[three_d_dave]

The problem with welding thin items like this is that full penetration welds can easily become full-blow through holes. It does look like no weld-prep was done, but if you don't call for any you won't get any. The other factor is to look at the teensy weld section you'd end up with.

A better approach is to bevel, weld, grind flush, and then put a doubler plate over the joint and a fillet weld around the doubler, which will end up with about 10 times the shear area you have there.

It's hard to see how the inverted-V is to fit in, but it also has very little to work with.

[Brian Selle]

A flush weld with the proper relief bevel and penetration would have been fine (it was 1/8" wall 1x2 6061 tubing). Had the welds not been ground flush it also would have been fine (as was the case for the NASA repair welds on one side only that lasted another regional and the championships). We told our sponsor NOT to grind these welds but after they were smooth and sexy the welder assured us that there was plenty of penetration. Lesson learned... we should have sent it back.

[three_d_dave]

No matter how skilled a welder, there are material properties to consider.

In extruded form it would have been 6061-T651:
tensile strength of about 45ksi and yield of 40ksi.

After welding is in the -T0 condition:
tensile strength of about 18ksi and yield of 8ksi.

Eventually 6061 will age back to T4 or so, which is pretty close (70%), but it requires heat treat to get to T6/T651 strength levels.

Compared to the base (original) material, a welded area in aluminum can be a significant weakness that needs reinforcement.

[JamesCH95]

At Hub City we had a pretty fun collision with 16 :rolleyes: . You can see the numerous places our frame split. We Covered the whole thing in tin foil and had it rewelded (This time without the faces being ground off). No issues. http://i.imgur.com/h1bglpH.jpg

I have to say, I have little sympathy for people who go to all the trouble of welding up a frame, only to grind all the reinforcement off. As you discovered there, unless you've done a proper bevel prep on the edge, a butt joint is quite likely to have low penetration and little fusion of the base metal. And so you can end up with fractures that follow the original seam quite impressively. Any time one of my mentors suggests we weld something up and grind it flush, I give them the evil eye and they usually decide that perhaps we don't actually need to grind it, actually...

This x 2.

I would prefer an open root weld in this situation over a bevel, less prep effort and virtually no chance of a LOF defect. I say having written WPSs and earned an AWS certification in TIG welding aluminum.

A flush weld with the proper relief bevel and penetration would have been fine (it was 1/8" wall 1x2 6061 tubing). Had the welds not been ground flush it also would have been fine (as was the case for the NASA repair welds on one side only that lasted another regional and the championships). We told our sponsor NOT to grind these welds but after they were smooth and sexy the welder assured us that there was plenty of penetration. Lesson learned... we should have sent it back.

Unless the welder has (or had) a certification in the weld they did, they have no qualification to make such a statement. Welders are not generally structural engineers, "plenty of penetration" might not be strong enough even if the weld did have full penetration.

Also, as mentioned by three_d_dave, welded material is generally weaker and needs the extra material provided by the weld bead even with full penetration.

This is what a single-sided, single-pass aluminum weld should look like:

http://www.fronius.com/internet/img_usa/ST/Grundlagen_ST_CMT_abb06_rdax_100.jpg

[Brian Selle]

No matter how skilled a welder, there are material properties to consider.
...
Compared to the base (original) material, a welded area in aluminum can be a significant weakness that needs reinforcement.


Agreed. I actually ran some FEA on the front portion of the frame and felt it was adequate (had it been properly welded) even with the reduced strength in this area. The front was actually lower on my list of concerns as there are fairly large cutouts and holes in a WCD frame that reduce the strength more than 70% in certain areas.


Unless the welder has (or had) a certification in the weld they did, they have no qualification to make such a statement. Welders are not generally structural engineers, "plenty of penetration" might not be strong enough even if the weld did have full penetration.


I should have inserted this in quotes or added an icon... I said this rather tongue in cheek. There was virtually no penetration.

[Kevin Sevcik]

For extra overkill points, the next time a welder says that, you can send the part off for ultrasonic inspection to see if it really does have plenty of penetration.

[JamesCH95]

For extra overkill points, the next time a welder says that, you can send the part off for ultrasonic inspection to see if it really does have plenty of penetration.

Ha!

Heck, even a mirror to get a look at the back of the weld...