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  #11  
Old 03-08-2011, 01:57 AM
David Gardiner David Gardiner is offline
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Hi Ian, You can't blame the sandblaster, he did his job. I almost never have anything sandblasted. I the heat generated by the sandblasting is what has caused your problem and of course the pressure. I always strip all paint and underseal off panels by hand. (You may notice a theme runing here I beleive in doing things by hand, (machines are fine but often they cause more work than they save.) When the paint etc. has been removed the rotten area will need to be replaced leaving only good metal. If any areas need cleaning up I use a strip and clean disc I any areas will not clean up in this way I use a little low pressure sandblasting kit. As a last resort I may occasionally have the edges of a panle sandblasted after all coatings and filler have been removed by hand, this way the blaster only needs to use low pressure.

You will have a hell of a job sorting this out. My advise is make the flat section of wing and replace it. The wings are simple anyway.

David
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Last edited by David Gardiner; 03-08-2011 at 02:02 AM.
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  #12  
Old 03-08-2011, 03:26 AM
robert robert is offline
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strange problems these oilcans
the basic what i learnt is ,when you heat metal it expands, when it cools down it shrinks more then it expanded.

this said i would expect damage by sandblasting is to be repaired
by stretching , but every time i have to fix this i am shrinking with a torch

i shrink it with a torch more then it need , then flatten it out in the e wheel
i do this in the center of the oilcan.

remeber every dent is a week place that makes oil can stronger, so dont shrink a second time before you flattend it out completly

this is what works for me ,but stays a strange problem to me
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  #13  
Old 03-08-2011, 06:07 AM
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Richard K Richard K is offline
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Quote:
Originally Posted by robert View Post
strange problems these oilcans
the basic what i learnt is ,when you heat metal it expands, when it cools down it shrinks more then it expanded.
Robert, Your statement above is exactly correct. When a portion of a piece of metal is heated; it's ability to expand is restricted by the cooler metal around the heated area. Thus the heated area thickens and/or bulges toward the heat source. As the heated portion cools, it cools unevenly, because the surrounding cooler metal dissipates (or soaks away) the heat faster at the edges of the heated spot. This effect is called an upset area.

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Originally Posted by robert View Post
this said i would expect damage by sandblasting is to be repaired
by stretching , but every time i have to fix this i am shrinking with a torch
Robert, Again, your observation is correct.

Robert L. Sargent, author of "Automobile Sheet Metal Repair" says: "A very common mistake is to confuse the effect of upsetting with stretched metal. An upset area will have a gathering, or drawstring, effect on adjoining metal, particularly in a low-crown section. Such areas will often pop in and out; this is often called oil-canning. Oil-canning can also be caused by stretched metal, but, unless there is evidence that it has been subjected to tension or has been worked to much with a hammer and dolly block, it is very probable that it is the result of an upset in an adjoining area."

"Oil-canning can also be caused by stretched metal, but, unless there is evidence that it has been subjected to tension". In this case, sandblasting is evidence of overall stretching.

Again quoting Robert L. Sargent "When the exact difference between stretched and upset metal is recognized, the problems of stretched metal become relatively simple. Stretched areas of metal rarely rise above the surface level to become unacceptable bulges. When they do, the metal man who has learned the proper technique of shrinking finds them relatively easy to repair".

Quote:
Originally Posted by David Gardenir
Hi Ian, You can't blame the sandblaster, he did his job. I almost never have anything sandblasted. I the heat generated by the sandblasting is what has caused your problem and of course the pressure.
In the case of this wing it appears stretching caused by sandblasting caused the oil-canning. The posts above focusing on locating and correcting an oil-canned section are appropriate "fixes".

As is often the case, my posts are lengthy and detailed. My interest lies in understanding why and how things happen. For some, understanding is the key to knowledge. Others may learn in different ways and that is OK. Our common interest is to develop and grow in the skills we need to enjoy this, often confusing and complex, craft of metalshaping.

Reference note:
Book Title: Automobile Sheet Metal Repair
Author: Robert L Sargent, Mechanical Engineering Laboratories, General Motors Institute
Third Printing 1965
Published by the Chilton Company
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  #14  
Old 03-08-2011, 07:36 AM
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Marty Comstock Marty Comstock is offline
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This one time, at Gator meet....

I was working on Gators rear door on his truck, and boyyyyyy did I have issues. The weld seam needed to be repaired and re flowed to get a fair curve. (Stole that from you Richard). I worked for quite some time chasing it and chasing it. I stretched and shrunk like a mad hatter using a shrinking disc and an air planishing hammer.

Pop told me something that really spoke to me, "make it work as one big unit, then you can address the panel as a whole". I was having 2 oilcans, fixing one, making the other worse, so on and so forth. (really big low crown door here) So I continued with the thought of not trying to fix the oil can, but whatever I do, make it work over the entire panel as a whole, in effect, making the oilcan LARGER. GOOD NEWS! once I had it to that point, I was able to work the panel as a whole and wash it with the air hammer, gently stretching it and Viola, no more oilcan. YAY!

Basically, do something. if it gets better, good. if it doesnt, reverse the process back to zero and do something different. Continue this process untill you are satisfied with the outcome. If you do this enough, you will (hopefully) intituativley know which process to use the next time. Or have a better feel for the next time.

Richard makes very good points here. 2 people can look at the same panel and come to differing conclusions as to what is going on, and its such a fine point, it is really difficult to say what exactly happened, stretch or shrink? Either way, it needs repair, or replacment as David suggested.

As for sandblasting, this is almost as bad as the 1018/1008/AK/DQ/DDQ debate I sandblast alot if not all my bodypanels, and I have little or no issues with it. Generally the panels I work on are higher crown, and i do shy away from sandblasting thinks like underside of hoods (thats the rare place I have seen issues in sandblasting). maybe thats for another thread

Marty
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  #15  
Old 03-18-2011, 01:15 AM
Overkill Overkill is offline
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Default Still struggling with an oil can

Richard,

I think your explanation of "upsetting" is what I'm struggling with.

In my case, I'm working with .063 5052. I annealed about 4" of the edge and rolled it over a 1" tube that has a slight crown in it. Between rolling it over, and the annealing, the oil can formed.

I've blended the area with a shrinking disk, tried heat and wet rag, etc. All I can get it to do is to grow with the heat, then return to the same low crown contour - not the flat panel I desire.

I've re-read the entire thread, I think the fact that it returns to a low crown time after time, leads me to think that it's the previously annealed and rolled over edge that has locked the oil can crown into place.

I've cut off most of the area I annealed, and that did help with the problem quite a bit, but I can't cut it all off. So now I need to address that last inch along the edge, that I now believe is "up set". Following the logic posted here, I'm thinking it likely shrunk after annealing, so I'll have to stretch it.

Plan on giving that a try tomorrow. If anyone thinks I should do otherwise - speak now --- so I don't have to clean up another mess.

Thanks, John
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  #16  
Old 03-18-2011, 01:36 AM
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Richard K Richard K is offline
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John,

The "upsetting" that I describe is more likely to happen in steel. Aluminum transfers heat so fast that it may negate most of the upsetting effects.

The inability to get any cooperation from the metal "may" be due to the alloy 5052 and the fact that you attempted to anneal it as one would for 1100 or 3003.

See http://allmetalshaping.com/showthread.php?t=1898 Posts #9 and #11.
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Last edited by Richard K; 03-18-2011 at 01:40 AM.
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  #17  
Old 03-18-2011, 01:45 AM
robert robert is offline
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in my understanding auminium does not shrink like steel
if you heat up and cool down alu goes back to original size and steel will shrink a littlle

so i dont think your panel shrunk by anealing i suspect it stretcht at the moment you were forming
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  #18  
Old 03-18-2011, 05:00 AM
Dyce Dyce is offline
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I'm going against the shrinking by heat. I beleive there is heat there, but there is also pressure from the sand hitting the panel. Steel will streatch easier with heat involved. I've done repairs from careless sandblasters that throw there rocks at panels and inside body supports and I always have to shrink.
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  #19  
Old 03-18-2011, 07:46 AM
TheRodDoc TheRodDoc is offline
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I agree with Jeff.

I also have repaired a bunch of blasted panels in my years.
Sand blasting doesn't shrink metal. Only stretches it. There is no heat to speak of.
It is the peening of the metal surface that stretches it.
The more air pressure you use the harder the sand peens the surface side of the sheet.

What makes it very hard to fix is the fact that the stretching is very uneven across the panel. Depending on how close, how long and what angle the blast nozzle was from the sheet at any given point on the panel.

So to even have a chance to repair the damage you need to know that only stretching has caused the warping of the panel from blasting.
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  #20  
Old 03-18-2011, 08:08 AM
TheRodDoc TheRodDoc is offline
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Proper blasting is much like painting. You need to make even passes completely over the whole part. If there was a 6" round area of rust in the center of a hood but the rest only paint with shinny rust free metal under it, and say it takes 4 passes with blaster to remove that circle of rust but only 1 pass to remove the paint from the rest, Then you would have to go over the complete hood with those 4 passes keeping the nozzle the same distance from metal, same angle from surface and same speed to stretch the whole hood evenly.
Just as you would when painting it. Stay in one spot a little to long you get a run. with blasting, Stay in one spot a little to long and you get a spot stretched more then the rest.

To show why some people might think blasting shrunk the metal, reverse the rust area to all of the hood with only a 6" circle of paint in the center of the hood. say the blaster made 4 passes on all of the rust but only one over that circle. Now it would look like to some that the circle area was shrunk when it just wasnt stretched as much as the rest of the hood in reality. This might be fixed by stretching only the 6" circle area to match the rest of the hood.

Where in the first example you could shrink the same 6" circle area to fix the hood. Anyway, this is what makes it hard to read and fix the damage. It all depends on what the blaster did. But for sure no metal will be shrunk from blasting.
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Last edited by TheRodDoc; 03-18-2011 at 08:32 AM.
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