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Old 04-03-2011, 01:29 AM
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kenb kenb is offline
MetalShaper of the Month Aug. '09 & Mar. '11 Rest In Peace Ken 1960-2011
 
Join Date: May 2009
Location: Flamborough, Ontario, Canada
Posts: 418
Default 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.

Ken
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Last edited by kenb; 04-07-2011 at 10:15 PM.
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