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kenb 04-03-2011 12:08 AM

Tuck Shrinking details for rookie Metalshapers
Hello Metalshapers,

I decided to write up this thread to discuss the basic process of shrinking with tucks, for the benefit of the newcomers to our craft. Getting my head wrapped around this technique took me a while when I first started shaping metal, as I was lacking a sufficent understanding about what was actually happening to the metal itself during the process. I found it easy to find information about the specific shrinking techniques, but it took me an extra little while to get the actual mechanics of the shrinking process down pat. Hopefully, this thread will be helpful for the guys who are looking at trying some tuck shrinking for the first time.

There are quite a few ways to shrink sheetmetal (shrinkers, shrinking discs, various hammer methods, heat, etc.), but I'd like to concentrate this discussion on the specific process of shrinking metal with tucks, which is the most likely method to be used by a rookie to this craft using only readily available hand tools.

I'm aware of two methods for shrinking with tucks, which is using either tucking forks, or shrinking metal in the hollow of a wooden stump. I originally learned how to shrink metal using tucking forks, but I've been using a stump more and more often these days for the same operations because the process is much kinder to the metal, and it's also a whole lot faster in most cases. I'll never toss my tucking forks though, as they are terrific for localizing a bit of shrink exactly where I want it.

Since much of this discussion will be concentrated on the behavior of the metal itself during the tuck shrinking process, I used a set of tucking forks to form the tucks in the pictures below for clarity purposes. I'll move on to discussing the same process with a stump hollow a little later on.

Here's the 10" diameter piece of 18 guage CRS we are going to use to demonstrate the process. I like round blanks for learning on, because it provides a nice consistent test-bed for trying out a few tucks for the first time. If you screw up and wreck one of your tucks somehow, you can just rotate the sheet a few degrees and try again with a clean slate in the same identical circumstances as the last one.

The only two dimensions that really matter to us are the circumference, which is represented in red, and the surface area of the sheet in square inches, which is represented in blue. We need not concern ourselves with the specific numbers, it's the nature of the relationship between these two dimensions that will have an effect on the shape you are trying to create.

Since we haven't altered this flat piece of sheet in any way yet, the surface area represented by the blue grid is an absolutely perfect fit to live within that red circle that highlights the circumference of the panel. This is why it sits perfectly flat on the bench.

Let's mess around with that relationship a bit, and see what happens!

To give us some reference points, I've divided this round blank into an eight slice layout, with one zone highlighted in red. The two red arrows at the outside edge highlight two marks I made with a sharp punch at the bottom corners.

These punch marks provide a place for my dividers to grab onto, so we can capture a measurement of the distance between these two points. We'll put these dividers aside for now, and grab them again later for reference to see what happens after we do some tuck shrinking.

Here's the tucking forks we'll be using. Note the slender taper to the forks and the parallel gap between them. Rounding and polishing the tips of the forks is critical, because even then, they'll still manage to gouge the metal up in a serious way if you aren't careful.

I have a preference for solidly mounted tucking forks, which give you something to twist against while forming the tucks. I've tried using hand held tucking forks with a T handle on them before, but I think you'd need a set of arms like the gladiator type chariot racers of 1500 years ago to use them with any degree of regularity. :)

Tucks are formed by inserting the metal between the forks, and rotating the sheet back and forth with your hands until the tuck is formed in the place you want it. If you want the shrink to extend deep inside the panel, you would drop the material all the way down to the bottom of the forks when forming the tuck; if you just want to shrink closer to the edge, only drop the material part of the way down into the forks to produce a shorter tuck.

Don't be embarrassed if it takes you a few tries to figure out which way to turn the metal to produce the tuck on proper side of the metal, I've been doing this for years and I still get mixed up sometimes.

I made this tuck really deep into the sheet to ensure I got some good shrink out of it, but when I get the bulk of a tuck formed, I always raise the part up and define the shape of the open end of the tuck with the last 1/2" of the top ends of the forks. This makes it very easy to get the exact shape you want at the open end, without bothering to fight with the rest of the material further into the panel.

Here's our panel with a tuck formed in the pie slice with the red markings. The red circumference line is exactly the same length as before, but some of it has been gathered up into a very small area so we can do some work on it.

Note that we haven't changed the shape of this sheet in any way yet (although we did buckle it a bit), as you could still grab a rubber hammer and pound it back out absolutely flat again with minimal effort. So far, we have only changed the arrangement, and we still have a sheet in a state where the outside circumference and the surface area contained inside it can agree on things.

Next, we'll take a hammer to that tuck and see if we can't shorten that red line a bit.


kenb 04-03-2011 01:29 AM

Pounding down a tuck
Before we take a hammer to this tuck, let's step back for a second and take a closer look at what we are about to do.

We'll be pounding these tucks down on a 3 foot tall wooden log I keep around specifically for this purpose, as solid lumber like this makes an excellent support structure and it's gentle on the surface of the metal too. To pound down the tucks, I'll be using a tucking hammer with a dome shaped UHMW hammer head.

To shrink this edge, the plan is to hit the top of that tuck with a sharp hammer hit so that the metal along the top and side surfaces gets crushed into itself before the bottom corners of the tuck can escape to the sides, as shown by the larger red arrows. When you crush the tuck in this fashion, a small portion of the circumference gets converted into additional metal thickness in that area, resulting in a slightly shorter, but slightly thicker edge.

The secret to making this work is friction, specifically at the bottom corners of the tuck where it contacts the surface if the wooden stump. Tucks with shallow slopes on the sides will escape easier than tucks with steeper sides, so tucks with steeper sides are generally more productive per hammer swing in the long run, although the risk of an accident increases the closer to vertical they are. You will see more about this issue in the next post in this thread.

To prepare a tuck for a beating, it's always a good idea to knock over the top of the end opening first, as this provides some resistance to the natural tendency of a tuck to open up at the end when you start hitting it. It just takes a quick wunk with a hammer to create it, just enough to reverse the arch at the open end of the tuck.

People have different opinions about where to hit a tuck first, some start at the inside, some in the middle; it sounds to me like it isn't that big of a deal, although I usually save the open end of the tuck for later. My usual approach is to start about a quarter of the way in from the outside and work inwards, and then double back and work outwards again. Often times though, I'm working in some pretty tight areas (like inside necks of vases), so I'll often settle for anywhere I can get a decent hammer shot at them. :)

Here's the same tuck after the first ten or so hammer hits. You can see the general shape and structure of the tuck is still pretty much intact, but I've managed to crush the top surface until it started to reverse on itself, and the metal in that area was warm to the touch for a few seconds after I finished hammering it. This is the true litmus test for determining if you actually shrunk the metal or not; if it's warm to the touch after beating it with a hammer, you just moved some molecules around and altered the shape of your sheet. Success!

The next twenty or so hits will be concentrated on pounding down the ridges I created along each side when I pounded the center down, and the hammering will continue until the surface is somewhat smooth again. Remember, the object isn't just to make the tuck sit flat again, but to encourage it to sit flat again in the smallest possible footprint.

Here's the tuck after it's been flattened back out with my UHMW tucking hammer. A quick check with the dividers reveals that we have succeeded in shortening the circumference of the sheet by almost 1/8"! This might not seem like much, but the cumulative effect of multiple tucks done the same way adds up after a while.

Here's the same sheet after I repeated the same process on the other seven pie slices. As you can see, it has assumed a bowl shape because the surface area is now too large to be contained within the newly shortened circumference. Because of this disagreement, the extra metal inside the circumference has to go somewhere, resulting in the shallow bowl shape. Not a bad one for 8 tucks either!

To be completely accurate, the surface area was also reduced somewhat by the tuck shrinking, but the circumference decreased at a faster rate causing the dome to form. It's all in the mathematics, which I don't have a very good grip on.

It's also worth noting that some of this apparent shrinkage will be lost when the part is planished out in an english wheel or with a slapper, when all of the fine creases and tuck marks are completely flattened back out again.

Next, things to avoid when tuck shrinking.


kenb 04-03-2011 02:19 AM

Things to avoid
Some things to watch out for.

Tucks like this one are too shallow to be effective, as there isn't going to be any resistance at the bottom corners to prevent them from spreading out when the hammer blow arrives.

Here's the opposite extreme! This is one of the ugliest tucks I've ever made, and I crafted it up special to show how not to make a tuck. This tuck is just asking to be knocked over to the side, which I am now going to do to demonstrate how bad things can get.

This is the last thing you want to see after you've been working on a part for a while. This tuck got knocked over to one side and crushed flat, which can happen a lot faster than you might think. It only takes a couple of fast hammer blows to make it happen, and you usually only notice that it's happening after the first strike, just when your hammer arm is likely on its way down already to deliver the next hit. If you see a tuck starting to lean over to one side after a hit, it's a good idea to stop your swing immediately (if you still can!) and straighten the tuck out manually before resuming.

Knocked over tucks like this can be saved sometimes, as long as a sharp crease hasn't been formed yet and you can find a way to peel it back open. Once a sharp crease has been established though, a crack will often form in that spot later on when you resume working in that area.


kenb 04-03-2011 02:48 AM

More on Thickness vs Circumference
Hi Guys,

I thought I'd show you a couple of pictures I have around that do a very good job of illustrating the thickening process that occurs when shrinking the edges of metal sheet.

This part is the day cabin for my PT Boat project, and it was created from flat sheet (3003 aluminum, .063" thick) by shrinking the corners again and again until it assumed the shape of a box. This task took several annealings along the way to prevent the aluminum from breaking up.

Here's it is after starting to tuck shrink the corners in exactly the same way as I described the shrinking process earlier in this thread. Basically, you are just looking at a round bowl with square corners on it, where the shrinking effort has been concentrated into four specific areas instead of being equally distributed around the perimeter.

Here it is again after being raised all the way up. Note how much smaller the circumference of the original outside edge is now, and also observe where all the extra material ended up. The aluminum in the shrink areas in the corners has swelled up from .063" all the way up to .120" in some places, almost double the thickness of the original source material.

You'll never see these extremes in thickness when working with CRS, but the concept is pretty much the same with both materials.

I'll get to work taking some pictures of the stump shrinking process when I have a couple of days off early in the week. Then I'll post a few words about tuck shrinking with a stump.

In the meantime, feel free to chime in with any questions, comments, or additional details that you feel would benefit the discussion. I only ask that you keep the conversation focused on tuck shrinking methods only, with a view to coaching newcomers to the craft.


Kerry Pinkerton 04-03-2011 06:04 AM

EXCELLENT tutorial Ken. I've made it a sticky!

chuckdeville 04-03-2011 06:23 AM

Very well written tutorial, I bet this will help a lot of people.

HEATNBEAT 04-03-2011 05:45 PM

Very nice Ken!!!!!:)

jhnarial 04-03-2011 07:48 PM

What A treasure!!! Thanks as always Ken.

The only thing I think we should add is the placements of the tucks, when, where and why.

You need to be able to know when the panel we are shaping needs shrinking. We also need to know where to place the tucks we need to accomplish our panel.

For example say if I tuck shrink this depth all the way around several times, what will my end result be.

You will end up with a coned shape bowl, why is that. It is because we only shrank the metal to one point.

If you were looking for a deep bowl, that one point could actually have been the starting point of a radius. Now we need to know where the ending point of the radius is so we can start our next round shrinking on that point.

This is so hard to explain, for me but it is really not that difficult but if you can figure out where when and why you our almost a metal shaper.

Hopefully someone that has a better understanding and is a better writer will chime in.

bigbob 04-05-2011 08:01 AM

Thank You ! very good demonstration.. I've never seen a tucking tool!! Very interesting I use an old sassafrass tree trunk I make domes for breast an such on my sculptures but I guess I do it wrong I always start at the center and spirle out to the edge then start on the edges I will know how to do it the right way! Thanks and hope to see more!!... Good pictured too!

ShawnMarsh 04-05-2011 09:43 AM


Originally Posted by bigbob (Post 32304)
Thank You ! very good demonstration.. I've never seen a tucking tool!! Very interesting I use an old sassafrass tree trunk I make domes for breast an such on my sculptures but I guess I do it wrong I always start at the center and spirle out to the edge then start on the edges I will know how to do it the right way! Thanks and hope to see more!!... Good pictured too!

There's no right or wrong way Bob, if you get the results you are looking for then you are doing it the right way. This is just one way of doing it.

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