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Old 03-31-2021, 08:01 PM   #1
ItWillBeSlow
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Roof patch causing warp / oil canning

I am fixing an old CB antenna hole in my roof. The hole was in the center of the cab and I cut a 2" x 2" square to patch in with new metal.

About half way through tacking, I realized I had some major oil canning going on, and a hump on the sides and a valley on the top and bottom. The hump was about 1/8" up and the "valleys" also about 1/8" deep. I tacked fairly slow, 4 or 5 at a time and I don't think heat was the issue. These issues extended almost a foot around my patch in all directions.

I decided to cut out what I had put in and everything settled back to where it needed to be and there is no warp.

What did I do wrong with this patch, and how to I weld in this piece without the warp and oil canning?
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Old 03-31-2021, 10:13 PM   #2
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Re: Roof patch causing warp / oil canning

Heat will do it...you need planishing
And I wouldnt do a square patch...round patch or radius the corners...
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Old 03-31-2021, 10:19 PM   #3
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Re: Roof patch causing warp / oil canning

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Originally Posted by mongocanfly View Post
Heat will do it...you need planishing and shrinking to keep it tight..
And I wouldnt do a square patch...round patch or radius the corners...
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Thanks for the feedback.

Can you elaborate why a square patch would be problematic for this, and how a circle would help?'

I don't have any other tools for planishing, etc. I put a small radius in my previous square patch by rolling it over my rubber hammer to get it close to the crown of the roof. I'm not looking for perfection and expected a minor skim coat of filler, but not the warping I'm seeing..
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Old 03-31-2021, 10:27 PM   #4
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Re: Roof patch causing warp / oil canning

Near impossible to weld without warpage unless you can planish (stretch) the weld afterward in a low crown area like that. The weld tacks will shrink when they cool down, and that shrink pulls from all directions. So as the weld shrinks, about 6" or so out from the weld area, the panel remains cool and unaffected by the heat. The area between these two areas, caught between two differing forces (one shrinking, one staying the same) results in a buckling, a sinusoidal wave of distortion. To remove the wave adjacent to the weld, you stretch the weld to eliminate the pull. If ANYONE suggests using a shrinking disc, it is the incorrect process. You don't fix a shrink with more shrink. You may remove some of the wave but you will only be shrinking the center even more, resulting in a low area for more oil canning. Fix the cause, not the result.

When you cut around the perimeter of the welded area, the roof returned to normal as you "disconnected" the pull of the welds...


More helpful hints:

A 90* tight corner concentrates the shrinking effects on the inside of that corner for a noticeable pucker. A radius in corners helps to balance the shrinking effects on either side of the weld for an easier task of planishing out the shrinking deformity. For that small circle I would have used a round patch to fill the hole, not a square one. (food for thought)

The tighter the patch (as close to zero gap as one can get) the better. A gap offers no resistance to the panel pulling closer together when the weld cools and shrinks, so any gaps will result in the outlying area pulling in more than with tight joints.. Where the crown of any panel is there to provide support and stability for the panel across its entire length, any disruption of that support risks oil canning. So for a low crown area like a roof skin, this means any additional shrinking/pulling in the center from gaps results in an even larger low area in the middle of the roof as that arc (cross section depiction) turns into more of a straight line. Which results in loss of that structural support that the crown gives, and an even greater likelihood of oil cans..
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Old 03-31-2021, 10:31 PM   #5
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Re: Roof patch causing warp / oil canning

Square corners cause a stress point...round or radius corner patches eliminate that

I was thinking backwards above about shrinking..I was thinking you had a high spot out away from your patch...that needed shrunk
.Robert set me right again..so I edited the above..
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Old 03-31-2021, 10:54 PM   #6
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Re: Roof patch causing warp / oil canning

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Originally Posted by MP&C View Post
Near impossible to weld without warpage unless you can planish (stretch) the weld afterward in a low crown area like that. The weld tacks will shrink when they cool down, and that shrink pulls from all directions. So as the weld shrinks, about 6" or so out from the weld area, the panel remains cool and unaffected by the heat. The area between these two areas, caught between two differing forces (one shrinking, one staying the same) results in a buckling, a sinusoidal wave of distortion. To remove the wave adjacent to the weld, you stretch the weld to eliminate the pull. If ANYONE suggests using a shrinking disc, it is the incorrect process. You don't fix a shrink with more shrink. You may remove some of the wave but you will only be shrinking the center even more, resulting in a low area for more oil canning. Fix the cause, not the result.

When you cut around the perimeter of the welded area, it returned to normal as you "disconnected" the pull from the welds...


More helpful hints:

A 90* tight corner concentrates the shrinking effects on the inside of that corner for a noticeable pucker. A radius in corners helps to balance the shrinking effects on either side of the weld for an easier task of planishing out the shrinking deformity. For that small circle I would have used a round patch to fill the hole, not a square one. (food for thought)

The tighter the patch (as close to zero gap as one can get) the better. A gap offers no resistance to the panel pulling closer together when the weld cools and shrinks, so any gaps will result in the outlying area pulling in more than with tight joints.. Where the crown of any panel is there to provide support and stability for the panel across its entire length, any disruption of that support risks oil canning. So for a low crown area like a roof skin, this means any additional shrinking/pulling in the center from gaps results in an even larger low area in the middle of the roof as that arc (cross section depiction) turns into more of a straight line. Which results in loss of that structural support that the crown gives, and an even greater likelihood of oil cans..
Thanks for the feedback. So it sounds like my next task is to cut a circle out around my current square, and get as tight of a patch fit as possible. If I weld this in slowly, is this achievable without any additional tools (or skills)?

I canít tell for sure if you are saying this is an inevitable issue, or if the tips you proposed should avoid the issue.
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Old 03-31-2021, 11:00 PM   #7
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Re: Roof patch causing warp / oil canning

In the center of a low crown panel you are hard pressed to NOT have any distortion without planishing the welds.

It looks like your truck has the inner steel roof skin/headliner. This may work in your favor in providing some support if the shrinking occurs, but I think you will still have some distortion.

Questions:

When the antenna was installed, where did the cable come out? Did the antenna go through both layers?

Does the cab have a cargo light that may provide access for a dolly in the form of a leaf spring?
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Old 03-31-2021, 11:32 PM   #8
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Re: Roof patch causing warp / oil canning

Not that it helps in the center of a roof, but the following should help in understanding warpage.


I hear people all the time suggesting to only use the bare minimum amount of a patch panel needed to repair the rust. I prefer to look at it another way, For example, most rocker panels have an outward crown that, if we cut out a patch for the front 1/3 only, we are left with a vertical seam. The shrinking that will indeed take place is going to give us a nice valley right at the vertical weld seam. So IMHO, it makes more sense to install a FULL rocker panel, where most are installed with spot (plug) welds at top and bottom flanges. Without using a vertical butt weld seam that you'd need for a partial patch, there is virtually no warpage at all in installing the full panel.

For any other patches, (quarter panel, door corners, etc.), I worry less about using the least amount of patch possible, and more about putting a weld seam where you do have access to planishing. Additionally, a half height quarter patch puts your weld seam in the middle of a low crown area for a guaranteed warpfest. Using a taller quarter (even if not the OEM full height) puts the weld seam up higher where the profile usually has a higher crown, which helps to control the warping a bit better. So if given the choice here, I am using the tallest quarter available and putting the seam:

1) where I have access for planishing

2) in a higher crown area to help control warping

3) near body crease to help control warping (keeping enough distance for dolly placement).

Other body panels would follow same 1-3 considerations in that same order.

For trimming, I try to keep absolute tight joints and if using MIG, set the welder hotter, adjust wire feed faster if it tends to blow holes, and control heat with shorter length of time on the trigger pull. first and foremost, we need to insure full weld penetration, so some practice on scraps of same material thickness and clamped in mid air to simulate what is on the car. (metal welding table is a heat sink) To further explain gap vs no gap, anything welded is going to shrink when the weld cools. If you have a gap, the weld will pull the panels together as it cools, and each subsequent weld will pull it that much closer together. So if your panel had been trimmed correctly, now you have low areas being put in as the panel is moving. If the new patch is trimmed for a tight joint, then it will still shrink, but you will only need to planish to overcome weld shrinkage, and should not have panels moving together creating low areas. In essence, it will still need planishing as would a gapped joint, but with no gap it should remain more consistent where any planishing required is also consistent. With panels pulling together in a gapped situation, you will have differing planishing requirements based on how much the panel pulled in that particular area. Hope my rambling makes sense.


I had done some test welds a few years back and I think the pictures taken will help out in understanding the weld location and shrinking. The tacks were done using the TIG and NO filler for minimal warpage. This also means we need absolutely tight joints... Here's the tacking process, and as said in video, amperage is set at 70. Based on 18 gauge thickness this should have been about 45, but as we also do with MIG "dot" welding, higher amperage and less elapsed time on trigger pull = flatter welds, less HAZ.


https://www.youtube.com/watch?v=aTqQJoecqCw



Note minimal weld size, minimal HAZ with the higher amperage, shorter burst...





Patches started out flat and for the most part remained so..








Adding a weld pass we are quick to see some distortion...








Examining this further, even though we have absolutely tight gaps for less instance of the panels pulling together, we still see distortion.. This is your typical weld shrinkage as the weld cools. Note in the next picture the panel is still fairly flat along the edges (red line), some shrinking at the weld (yellow arrows) and show a dramatic pucker between the two. Note that the weld has yet to be planished, so the weld shrinkage is pulling the metal alongside it together, the areas unaffected by heat remain largely unchanged (red line) and the area between the two are forming a bulge due to these differing forces. Here we address the problem, not the result. Planish out the weld to stretch it in length and the bulge will disappear. Don't make a habit of chasing the result, a shrinking disc on the bulge is not the correct resolution; if this were a crowned panel that action would be causing a severe low area.





Referring back to an earlier statement I made on weld location:


......let's try this same scenario using a crowned sample near a body crease so we can take advantage of all 3 choices...





Weld pass....











Here we can see how the weld location and panel features (crown, body crease) helped to control and limit any warping effects. The weld will still need planishing to restore the crown of the center bead, as no doubt it has pulled in slightly, but this is hands down a dramatic improvement over the flat "patches" we did the first time. This shows how these features in your body panels can help out in controlling weld distortion, so take advantage of these in weld location and leave the limiting of panel size as your absolute last consideration.
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Old 04-01-2021, 06:36 AM   #9
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Re: Roof patch causing warp / oil canning

Thanks for the detailed response and the additional info Robert.

The antenna cable was fished through to drivers pillar and the hole did not go through the inner panel. My truck does not have a cargo light or any other way to get behind this patch.

In my first attempt, I definitely had some gaps in my patch, and was blowing holes and turned my welder heat and wire speed down low, so it seems I had a major offense in at least two categories.

I am not familiar with planishing techniques, is this something I will need to do if I canít get behind the patch?

Once I get my circle patch cut, is there a recommendation for what order to tack in?
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Old 04-01-2021, 03:15 PM   #10
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Re: Roof patch causing warp / oil canning

As it relates to metalshaping, planishing is a smoothing action. If you were to rough out a shape in a panel using a blocking hammer into a shot bag, it would add shape but the resulting finish would look about like a bag of walnuts. Planishing would be the process to smooth out those "walnuts" to a consistent finish. It would be accomplished using a planishing hammer, then English wheel, etc.

As it relates to welding in a patch.... As you weld, the heat will cause some expansion of the metal as it gets hot. This takes place at the weld and heat affected zone (HAZ-the metal surrounding the weld that has changed color) When it cools off, the molecular structure of the steel realigns such that it shrinks back down more so than what it was previously. Side note- whether you are welding sheet metal or 1" thick plate, any welding process will result in shrinking of the weld and HAZ.

Planishing in this case would be the process to add a bit of stretch to the area that has shrunk in order to restore the original shape of the panel. It is done directly on the weld itself, using a hammer and dolly. When you strike a hammer face into a dolly you will hear a PING. When you are planishing, it is hammer on one side, dolly on the other, and you want to hear that PING. You hear the ping, you are stretching. So when using the hammer and dolly to planish welds, it is advantageous to have a weld proud on both front and back side of the sheet metal, as these give you a "target" that is off the surface of the sheet metal for a more effective stretch of that one weld dot by its lonesome self. Dolly on one side, hammer on the other to strike the weld dot. I will typically grind down the individual weld tacks after planishing, as this get the weld proud out of the way for when you start overlapping, and it returns the metal thickness to what we set the welder up for.

Looking at your weld "dots", they have a divot on many of them. This is an outgassing that occurs as the puddle cools. I think you can remove some of that by using a hotter weld. Turn your heat up, add more wire feed speed if it tends to blow out, and shorten the trigger pull elapsed time.. So in essence, despite the heat being hotter from the machine's perspective, because the trigger is not pulled as long, the heat the panel sees is the same or less. (if we can get the operator adjusted as well ) I think you'll see that a hotter, shorter time-weld has the puddle sitting there boiling less, so the pin holes and divots SHOULD be less an issue. It should also give a flatter weld for less cleanup.

For your welds on a round patch, I would follow a similar pattern as you'd do with doing lug nuts on a wheel.. Where you said you turned down the heat due to blown holes, that may be due to having a gap in the panel. I would strongly suggest getting some scrap metal the same thickness as your roof skin, and practice on that, and not the truck, until the process and settings are dialed in.. and don't weld on a steel work bench. Your practice welds should be in free air, just as the panels on your truck are.




I've got a weld test coming up probably next week, a gap VS no-gap on two virtually identical panels so we can do a quantitative monitor of any difference in the results. Started with two panels stacked and added some low crown on the power hammer, then smoothed them out on the English wheel.







Then some profile templates were made so we can monitor any changes...







Ö.one for each direction....



















We'll cut equal sized "patches" out of each, leave a gap with one and the other will be tightly fitted. Both will be welded using MIG to keep that a constant. You may or may not be done when I get to that, but wanted to let you know we will be doing that. We should be doing some videos to better show the process...
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Old 04-01-2021, 07:32 PM   #11
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Re: Roof patch causing warp / oil canning

Quote:
Originally Posted by MP&C View Post
As it relates to metalshaping, planishing is a smoothing action. If you were to rough out a shape in a panel using a blocking hammer into a shot bag, it would add shape but the resulting finish would look about like a bag of walnuts. Planishing would be the process to smooth out those "walnuts" to a consistent finish. It would be accomplished using a planishing hammer, then English wheel, etc.

As it relates to welding in a patch.... As you weld, the heat will cause some expansion of the metal as it gets hot. This takes place at the weld and heat affected zone (HAZ-the metal surrounding the weld that has changed color) When it cools off, the molecular structure of the steel realigns such that it shrinks back down more so than what it was previously. Side note- whether you are welding sheet metal or 1" thick plate, any welding process will result in shrinking of the weld and HAZ.

Planishing in this case would be the process to add a bit of stretch to the area that has shrunk in order to restore the original shape of the panel. It is done directly on the weld itself, using a hammer and dolly. When you strike a hammer face into a dolly you will hear a PING. When you are planishing, it is hammer on one side, dolly on the other, and you want to hear that PING. You hear the ping, you are stretching. So when using the hammer and dolly to planish welds, it is advantageous to have a weld proud on both front and back side of the sheet metal, as these give you a "target" that is off the surface of the sheet metal for a more effective stretch of that one weld dot by its lonesome self. Dolly on one side, hammer on the other to strike the weld dot. I will typically grind down the individual weld tacks after planishing, as this get the weld proud out of the way for when you start overlapping, and it returns the metal thickness to what we set the welder up for.

Looking at your weld "dots", they have a divot on many of them. This is an outgassing that occurs as the puddle cools. I think you can remove some of that by using a hotter weld. Turn your heat up, add more wire feed speed if it tends to blow out, and shorten the trigger pull elapsed time.. So in essence, despite the heat being hotter from the machine's perspective, because the trigger is not pulled as long, the heat the panel sees is the same or less. (if we can get the operator adjusted as well ) I think you'll see that a hotter, shorter time-weld has the puddle sitting there boiling less, so the pin holes and divots SHOULD be less an issue. It should also give a flatter weld for less cleanup.

For your welds on a round patch, I would follow a similar pattern as you'd do with doing lug nuts on a wheel.. Where you said you turned down the heat due to blown holes, that may be due to having a gap in the panel. I would strongly suggest getting some scrap metal the same thickness as your roof skin, and practice on that, and not the truck, until the process and settings are dialed in.. and don't weld on a steel work bench. Your practice welds should be in free air, just as the panels on your truck are.




I've got a weld test coming up probably next week, a gap VS no-gap on two virtually identical panels so we can do a quantitative monitor of any difference in the results. Started with two panels stacked and added some low crown on the power hammer, then smoothed them out on the English wheel.



Then some profile templates were made so we can monitor any changes...



Ö.one for each direction....



We'll cut equal sized "patches" out of each, leave a gap with one and the other will be tightly fitted. Both will be welded using MIG to keep that a constant. You may or may not be done when I get to that, but wanted to let you know we will be doing that. We should be doing some videos to better show the process...
Today I cut a circle patch and scribed the area I need to remove on the roof. I haven't cut the roof yet. If I can get the roof as "perfect" as the patch I should be in good shape on fitment. Unfortunately, I now have a 3.25" circle patch to fix a .5" hole due to my ignorance.

I am planning on tacking this in tomorrow, pending some more thought and review of the info above. If I understand correctly, I don't really have any real planish or "correction" options with this fix since I cant get behind it. My best bet is a good patch fit, slow welding, hotter welding to reduce the overall heat input?

Should I be looking for / hammering down high spots as I tack, if they come up??

I am using a patch that is cut from leftover replacement floorpan material. The material is tri plus or key parts and just thick (if not thicker) than original. Ill probably do a small test as you mentioned with the heat higher than I am used to just to get comfortable.
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Old 04-01-2021, 08:27 PM   #12
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Re: Roof patch causing warp / oil canning

For the situation you are in, I would use some metal adhesive (Fusor 208 would be my choice) to put the piece(s) in & limit your "effected" area. Over the years I've done several antenna/light holes in ex-police vehicles, ect. with no issues.

A round "backer" slipped in, half under the original skin & half to sit the patch on will allow for a better/flatter finish. You'll want to do it in two stages, put in the backer...let it dry, then install the filler piece. This way you can work through the hole to hold the backer in place until it dries

I'm sure you have realized now that it would have been best a week ago to ask: "How would you guys go about filling this 1" round hole in this roof?", but hey...thats what makes it all a challenge. Lorne
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Old 04-02-2021, 09:20 AM   #13
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Re: Roof patch causing warp / oil canning

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For the situation you are in, I would use some metal adhesive (Fusor 208 would be my choice) to put the piece(s) in & limit your "effected" area. Over the years I've done several antenna/light holes in ex-police vehicles, ect. with no issues.

A round "backer" slipped in, half under the original skin & half to sit the patch on will allow for a better/flatter finish. You'll want to do it in two stages, put in the backer...let it dry, then install the filler piece. This way you can work through the hole to hold the backer in place until it dries

I'm sure you have realized now that it would have been best a week ago to ask: "How would you guys go about filling this 1" round hole in this roof?", but hey...thats what makes it all a challenge. Lorne
Can you elaborate more on the purpose of the backer? Is this to help absorb heat, or support the patch before it goes in? I dont understand how it would help the shrinkage, if that is the root cause of the issue here.


Quote:
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I'm sure you have realized now that it would have been best a week ago to ask: "How would you guys go about filling this 1" round hole in this roof?", but hey...thats what makes it all a challenge. Lorne
Definitely agree with this statement. However I have already done cab corners, rockers, floor patches, pillars, cab supports on this truck. This is my first rodeo obviously but I was looking at this hole as one of my last "easy" metal work tasks.... (wrong!).
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Old 04-02-2021, 11:16 AM   #14
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Re: Roof patch causing warp / oil canning

If you have any spare sheet steel around, I would practice the same patch on about a 12" x 12" piece to see how things react. make changes to see if one method works better than the other.
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Old 04-03-2021, 03:17 PM   #15
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Re: Roof patch causing warp / oil canning

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Can you elaborate more on the purpose of the backer? Is this to help absorb heat, or support the patch before it goes in? I dont understand how it would help the shrinkage, if that is the root cause of the issue here.


Definitely agree with this statement. However I have already done cab corners, rockers, floor patches, pillars, cab supports on this truck. This is my first rodeo obviously but I was looking at this hole as one of my last "easy" metal work tasks.... (wrong!).
Im thinking you misunderstood what I have suggested. What I was referring to is to use a metal bonding adhesive & glue the piece in...not doing any welding at all. That way you can't/won't have any warping issues. The backer is to sit the patch on so it fits flush with the original Roof
I look at it this way, years ago someone made a mess by drilling the hole & then tightening down the antenna, raising up the area around it with the little bar/foot (Im guessing was the reason for you to cut the bigger hole?)...you didnt help matters by welding in the square cornered patch. Without cutting some inner cab away to properly work the weld area, the whole thing is going to be a compromise at best. The middle of a Roof (or Hood) is NOT the place to learn/practice...get something stuck in there to keep the water out & move on.
You may see a "ghost line" around you patch under certain light being the sun heat will be beating on it some, but the outcome for the situation (no access, somewhat limited tools & knowledge) I feel its the best answer. Like I said earlier, Ive done several antenna holes (Roof & Trunks) just like what you had starting out without a problem. A couple I just tapped down a bit & glued a piece on top. NOT Robert-like, but for a 6 or 8 year old ex-police car...it works real well
If its something you have interest in, I can go into detail of how to prep the parts, ect for the best results. My intentions are not to be-little your efforts or ability, just being a realist with the situation. Lorne
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Old 04-04-2021, 08:50 AM   #16
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Re: Roof patch causing warp / oil canning

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Ö...NOT Robert-like.....
Point taken Lorne, most just go ahead and say anal retentive.

Sometimes we do what we have to do. In this case as long as the truck is stock height nobody should see any ghost lines..
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Old 04-04-2021, 09:21 AM   #17
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Re: Roof patch causing warp / oil canning

THAT is my point! I love to follow along to your posts & have some pretty good skills...I think that is why I appreciate what guys like you can do. Sometimes the vehicle design, time, tools & skills should just lead in another direction for some folks.
Get it done & enjoy the truck is my thoughts. If the worst/most noticeable spot on the whole truck is that ghost line on the roof...the man did a mighty fine job for his first rodeo! Just my Thoughts, Lorne
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Old 04-04-2021, 11:08 AM   #18
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Re: Roof patch causing warp / oil canning

This guy has some great videos and explains everything in great detail without getting to technical

https://youtu.be/_u31t13QO6A
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Old 04-04-2021, 09:28 PM   #19
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Re: Roof patch causing warp / oil canning

Quote:
Originally Posted by HAULIN' IT View Post
Im thinking you misunderstood what I have suggested. What I was referring to is to use a metal bonding adhesive & glue the piece in...not doing any welding at all. That way you can't/won't have any warping issues. The backer is to sit the patch on so it fits flush with the original Roof
I look at it this way, years ago someone made a mess by drilling the hole & then tightening down the antenna, raising up the area around it with the little bar/foot (Im guessing was the reason for you to cut the bigger hole?)...you didnt help matters by welding in the square cornered patch. Without cutting some inner cab away to properly work the weld area, the whole thing is going to be a compromise at best. The middle of a Roof (or Hood) is NOT the place to learn/practice...get something stuck in there to keep the water out & move on.
You may see a "ghost line" around you patch under certain light being the sun heat will be beating on it some, but the outcome for the situation (no access, somewhat limited tools & knowledge) I feel its the best answer. Like I said earlier, Ive done several antenna holes (Roof & Trunks) just like what you had starting out without a problem. A couple I just tapped down a bit & glued a piece on top. NOT Robert-like, but for a 6 or 8 year old ex-police car...it works real well
If its something you have interest in, I can go into detail of how to prep the parts, ect for the best results. My intentions are not to be-little your efforts or ability, just being a realist with the situation. Lorne
Thanks, I did misunderstand your initial post about using the metal bond, I was thinking this was for backer-only and not the actual install of the patch. you are also correct that I cut a larger square (d'oh!) due to the dimple around the original antenna hole. I could have gotten away with a 1.5"circle if I hadn't stacked mistakes to this point.

I have not looked into the metal adhesive up to this point, and havent heard of it to be honest (did I say I was ignorant yet?). I must say I am a bit skeptical of a product like this but do like the idea of avoiding this issue altogether. Is there a drawback to the metal adhesive? is there a risk this bond will break in 10 years and I will have a major issue? I do need to look into this in more detail.

One additional option I thought about was to take hybrid approach to the recommendations so far: I could cut this circular patch as precise as possible and weld in as clean and slow as possible and cross fingers/hope/pray there is no warping. If there is warping, I could cut a square out of the inside roof liner to get a dolly behind it if needed and properly correct the warping. I could then metal adhesive the roof patch back in place, and put a headliner in the truck to cover the patch since there are some grooves in this metal here I could never clean up to look 100%.

I'm still mulling this over and thinking through the advice so far before taking any action - please keep comments and advice coming, thanks everyone for the help.
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Old 04-04-2021, 09:37 PM   #20
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Re: Roof patch causing warp / oil canning

They install body panels from with factory with adhesive..
Martinsr did a bunch of tests with panel bond with great results..
Is it right for what your doing? I dont know that..but somebody will..
Sounds like Lorne has done it any times..
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Old 04-04-2021, 09:41 PM   #21
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Re: Roof patch causing warp / oil canning

http://67-72chevytrucks.com/vboard/s...d.php?t=241030
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Old 04-04-2021, 09:54 PM   #22
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Re: Roof patch causing warp / oil canning

Quote:
Originally Posted by 72c20customcamper View Post
This guy has some great videos and explains everything in great detail without getting to technical

https://youtu.be/_u31t13QO6A
This is a cool video and interesting technique. It seems theoretically I could follow this same procedure and reduce the warp risk. The only notable drawback or risk may be that I am currently planning a smaller patch (3.25" circle) and may not have the easy manipulation/flexibility of patch to push the patch back into the hole after back-cutting at the 45 degree, compared to the larger square he is using in this video. I would have to tack 4 times on the circle, and then cut one quadrant at a time to push this section back flush prior to tacking it in flush. I hope that makes sense - basically I am saying my patch is less flexible. I am curious what others on this thread may think of this technique.
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Old 04-05-2021, 02:03 AM   #23
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Re: Roof patch causing warp / oil canning

Some casual observations... I would not recommend any novice use a 24 or 36 grit sanding disc on a 7" disc grinder for removing welds, as was shown in the video. Too much heat generated, can't see where you're sanding, a recipe for disaster when someone sands half the panel thickness away, or worse yet, sands through. For the most part his welder is not set up hot enough, or there would not be so much rework after grinding. If you need to reweld after grinding, turn up the heat, you are not getting a full penetration weld. And now you need to grind again.

Part of the reason we use butt welds is to duplicate the original metal "flavor and feel" so that when someone looks in the trunk or under the floor pans, they don't see a flange repair, or have to contend with ghost lines on exterior painted panels. If we can effectively cut out damaged (rusty) metal, butt weld in a new section, planish/metal finish/filler as needed, we can have an invisible repair that for all practical purposes should be a permanent repair. The patch is normally already shaped to match (in the case of repops) so that the crown of the patch matches the existing panel. (NORMALLY) Part of the reason you see recommendation to use TIGHT butt welds, is that it eliminates open area that allows any panel movement, ie: as the weld shrinks, the panel pulls together. If the crown of our patch used to match prior to welding, weld shrinking along with the panel's pulling closer together (due to the gap) is going to pull in some of the crown. If it is a flatter crown, such as the middle of a quarter panel or top of a roof/hood, it is more noticeable as the crown in these areas is normally minimal/just enough to maintain the shape, and may result in oil canning. If it is a concave crown, such as a reverse where the wheel opening flare comes outward, then the weld seam has a tendency to pull outward as it shrinks. The following pictorial shows an exaggerated crown and gap primarily so you can see it in such a small area. But it shows what happens as weld shrinkage and panel movement pull at the surrounding area. In the bottom view, the red line depicts where the crown was originally.





With tight butt welds we do need to planish in order to remove any deformation caused by shrinking, and add some stretch back into the area. This should be the extent of our planishing effort, as the tightly fitting panels prevent the panel pull from adjacent areas (that a gap allows, resulting in loss of crown). In the case of any gappage around a patch, we would need to planish even more to add enough stretch to overcome this loss of crown, or add filler as needed.


Looking at the quarter panel in the video, and welding that took place, one would expect in the case of a normal butt weld that the edges of the panels against each other will be a positive stop against any pull from adjacent areas. The only planishing we should need to do is to overcome any shrinking issues due to weld heat. While the theory looks good on paper, the angle cut did indeed close up the gap when compared to a straight cut. But it did not totally eliminate it. My concern with this type cut is that two angled surfaces do not provide the positive "stop" that a truly butted panel does in preventing panel pull from adjacent area. After the grinding process is done, checking the weld seam with a 6" ruler may show how flat that single weld is, but our main concern should be the crown across the entire patch to see if it has pulled inward causing a loss of crown. A defect such as that will be a pain to correct, filler or otherwise. I'm not intimately familiar enough with the method to say this will indeed be a concern. But these thoughts are more cautionary, to give you things to be aware of, to look for.. if you do give this method a try.. For the quarter panel and the upper and lower weld line he used, I would expect any shrinking/panel pull to cause the following issues to the crown:





Ö..and to a lesser extent, the slight crown from front to back, as the weld shrinks, will pull slightly inward. Armed with this theory on shrinking and panel pull, these are merely things to look out for. More planishing perhaps may be needed to remove any defects that might show up.


If you plan on using this method for your roof skin, I would find something to practice on first for some real world experience..
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Old 04-05-2021, 04:02 AM   #24
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Re: Roof patch causing warp / oil canning

MP&C is amazing and the detail he provides is nuts. He has a thread on a 55 Chevy that he and his minions show great work that everyone should look at from beginning to end.
Welding on a large low crown bit of metal w/ no ability to planish the welds is going to be a warping issue. Sorry.
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Old 04-05-2021, 09:44 AM   #25
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Re: Roof patch causing warp / oil canning

Robert...great explanation of gap vs no gap....
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