Writing tech articles can be tough sometimes, I need to keep it simple and concise to keep readers from falling asleep throughout the article as well as still being informative. I try to avoid writing super long back stories or intros to keep tech articles to the point and get right into the meat and potatoes of the project. Recently I wrote a quick DIY article on How to Make your own Tuck Shrinking Fork from scratch using junk you have laying around the shop. I had assumed (you know what they say about assumptions!) that most of our readers of our “Tech Tuesdays” emails would at least be slightly exposed to the process of “tuck shrinking” a panel. Turns out I was wrong! We probably had as many comments of “What the heck does that tool do? Show us how!” than anything! We even had readers thinking I was trying to scam or market our metal fab classes to them! Not so at all! So recently I decided to show everyone the process of tuck shrinking sheet metal both with my DIY tool and a tree stump.
Now I’m not a metal fabrication/shaping historian, but the process of tuck shrinking a panel goes way back and isn’t by any means a new process and there’s many different ways to do it, I’m just showing one way. The simplest way to describe how metal moves or reacts when you shrink or stretch it is to imagine pizza dough. When you stretch the dough out to make a larger pie you’ll see it gets larger AND thinner as you stretch it out. If you watch the process they start with a small, thick, round piece of dough that they kneed out until the dough is the desired thickness and put the excess material on the edges for the “crust” The same if they wanted to make the pie smaller, you’d need to gather the dough together creating bunches and smooth it all together until it was the desired shape. Metal reacts almost EXACTLY the same, just not as tasty when you heat it in the oven for a while (but it does look a lot cooler when finished!).
So what are you doing when you shrink metal? You’re gathering the metal particles or molecules together pulling the surface tight and making it thicker in that area. This puts shape into the panel. The number of shrinks and where you shrink helps determine the final shape. As you shrink the metal it needs to go somewhere or it will just sit in a bunch or ripple. The key is how you make that area smooth and still retain its shape. The only tools needed to make a tuck shrink is a teardrop mallet and a stump or tuck shrinking fork.
First I’ll show you with a stump. This tree stump has had a shallow bowl cut out and sanded smooth. A good stump for shaping and hammering metal is another nice FREE addition (no subliminal sales message to sell tree stumps here!) to your shop that you can use often. Start by placing the sheet metal over the bowl so it covers the bowl and lays as flat as you can hold it. Then take your teardrop mallet and make one hard blow on the metal driving down and towards the outside of the bowl.
If you’ve done it right this action will cause the metal to “pucker” on the edge and give you a little ripple or ridge. This metal has been “gathered” together to create that pucker and can be easily reversed by hammering on the top of it to let it fall back in place. To truly shrink the panel into that area you must smash the metal together into itself. That part of the process is the trickiest to get the hang of, but it’s a simple concept.
This is where the tuck shrinking fork from a few weeks back comes into play. Start by inserting the fork into the edge of the sheet metal at the desired depth of the shrink you want to make.
Then twist the handle of the tool to twist the metal around one side of the fork.
Then remove the tool and move it over so the fork will roll the metal over the other side of the fork by twisting the metal the other direction. By twisting the metal the other direction you’ve pulled the metal together into the same shape tuck as with the hammer. The only big difference is that you can produce tighter, more precise tucks rather than depending on the accuracy of your hammer swing. Also this is possible to do ON the vehicle or on a part where a stump can’t be used.
Start by using your teardrop mallet to either round over the open end of the pucker or hit alternatively on the left and right side of the pucker to lock the shrink in. If you don’t lock the shrink in it can escape as you hammer on it, reversing the shrinking you’ve just done.
Then you want to alternate your hits left and right starting from the back of the pucker all while working your way forward. Keep your hits accurate and tight together to minimize lumpy areas.
Once you have the pucker flattened out you have effectively shrunk the metal and it is forever in that position until you manipulate it again by stretching or shrinking further. You can also put multiple tuck shrinks in a panel to quickly rough in the shape of a panel and then hammer each pucker out. This is a similar process to blocking (hammering) out a rough shape in a sandbag; you’re just shrinking the edges instead of stretching the center of the panel with a sandbag. You can also shrink multiple times over the same edge, but the metal gets thicker and harder to shrink each time you repeat the process.
Finally you can use a planishing hammer or a hammer and dolly and hammer lightly to smooth out imperfections until the metal is smooth enough to move on with. Above is after a quick planish. You can see some minor ripples still in the metal that we can planish out with a few more passes. Just be careful you don’t hammer too hard on-dolly as you can actually stretch the area back out; you’re planishing or smoothing metal, not driving a nail in a 2×4!
You may be asking “Can’t I do this with a shrinker?” The short answer is “yes” you can shrink metal with a kick or hand operated shrinker. The difference is that the shrinker is basically making tiny little “puckers” in the metal and only gathering it together. It isn’t actually forcing the metal particles into that shape they are just bunched up and under tension. If the panel or that shrunken area is going to be welded on this will become an issue as the bunched up metal will relax as soon as the heat from a welder is put on it. Shrinkers are great for areas like window channels, door frame surrounds, etc., but not for a seam where you will be butt welding a patch panel together. It will fall out of shape and become a warped mess.
I’m sure others are asking “well when would I ever need this, I’m not building entire panels from scratch?”. This method is also adjusting the edge of a panel that maybe is overstretched or there’s “ripples” in the metal ON THE VEHICLE. It’s very hard to use a hand/kick shrink on the vehicle as the tool really needs to be fixed to the ground/table. Maybe you were fitting a patch panel and the edge is popping in and out like there’s too much metal, but you have to weld right on that edge? Tuck shrinking will pull the extra material together and you can planish it smooth so you will still have a nice clean weld joint.
This method may not be for everyone, but I think it is a good thing to learn and have in your metal shaping skill set in case you ever need it! Much like teaching your wife to drive your stick shift car in case of emergency, she may hate it, but could be a life saver in an emergency! Thanks for reading along, feel free to drop us a line if you’d like to see any other tutorials on similar techniques!