Featured Technical Fender bumping advice

@tb33anda3rd -I thought tire chains also...
I got a shrinking disc about 10-12 years ago when I fixed the length of the roof above the driver's door on my Packard where the shed it was in fell over on it. Unfortunately no before pictures but it turned out prettydecent. I've used it a lot since then. Wolfe metal fab I think, nice and well built.

 

Regarding shrinking discs, I’ve an older metal 8? Inch grinder. Gotta weigh 15lbs. I****ume just let the weight of the grinder do the work? Or is it better to get a lighter weight one and apply some pressure?

 

I use an electric grinder. Weight? Maybe fatigue while using it.

 

Kyle is great! His HAMB handle is “66gmc”

 

I was referring to the weight of the grinder. The big one is a heavy thing, nothing like a 4.5 inch angle grinder.
I was asking if it’s better to use a heavier grinder vs a small light grinder and apply pressure

 

My understanding is that a small diameter grinder isn't the way to go, but I have no experience of this! Too much speed (10k rpm) and the small disc size.

I have had a Sunchaser disc for many years. 9" diameter. Requires 6k rpm, and my Dewalt machine is quite weighty. No real pressure is required, and most crucially, the larger disc finds the highs and floats over the lows, something the smaller disc is less able to do, which kinda defeats the objective of just heating the high spots. It works great but the serrated, almost knife like disc is something to be most wary of. I usually have a convenient table in the work area to safely set the slowing down machine down on and be able to quickly grab the wet cloth and spray bottle.

Chris

 

Also, keep the heat lower than you'd think, if you get too hot (like red hot) the metal is stretching and when you hit it with the damp rag the net shrink isn't as great as you'd expect. I hit with heat (shrinking disc or torch) and cool it quickly, you'll see a little steam, if you hear a hiss then its getting more to the too hot side of things. With torch and a small bump you'd hit it for less than 2 seconds, it's hard for 1 person to do it.
A disclaimer: I don't hear as well as I used to, perhaps even with a slight steam there is a hiss that I don't hear, I forsure hear it if its too hot.

 

I’ve done it with a torch before, but I used compressed air to cool it down. Maybe I was getting it too hot, was leaving like slag where I heated it.

 

I only use a torch when you really really have to shrink a whole bunch. Think about it. Heat makes metal shrink, and as soon as you get the torch down there even for a few seconds, it sends heat all over the panel.
Shrinking disc only needs you to count to maybe a thousand and three, otherwise the heat on the high spots will start creeping into the low spots and the next thing you know, is you're using a hammer and dolly to bring back the overshrunk panel. I used to do that a lot.
I have both of Wray's large and small shrinking discs and I use both of them a lot. The small one was designed to get into tighter areas that the big one just won't fit.
You can see how crooked this box side is. It was far worse than that, but with the help of some dollies and a********, and mucho time with shrinking disc, I have got it pretty much perfect now, and these box sides are thick metal. I should probably update my thread.

 

Peter Tomasini made a series of MetalShaping DVD's and one of the techiques he illustrates is using a torch to take out the bumps, he puts some walnuts in a piece of metal and uses a torch, hammer & dolly to take them out and then hits the piece of metal with black paint, he does it in real time and the finished piece is perfect.

 

I really like the shrinking disc and my other 'go to' is a slapping spoon...
I made my first shrinking disc out of the base plate for a (big truck) diesel oil filter....it already had the perfect center hole and the underside was disc shaped...it worked great on my small Makita grinder with speed control
The bigger discs also work well on a Milwaukee battery powered angle grinder....not so heavy as the one with a cord