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Steam Stationary Engines, Traction Engines

Case Bunker Bends

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Old 06-23-2019, 08:12:58 PM
JLee JLee is offline
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Default Case Bunker Bends

When you make the Case steam engine contractors bunkers, what is the best way to make the bends for the corners? How do you bend them accurately so you get a nice tight fit?
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Old 06-24-2019, 01:27:30 AM
sawiley sawiley is offline
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Default Re: Case Bunker Bends

I assume you're asking about how to go about flanging the corners for the top and bottom heads. I made a set of bunkers for a friend's 40 hp Case some years back. As I recall, the plans called for a 1.25" flange, with corners nominally 2.625" radius.

I had a local metal shop waterjet cut six outside and two inside corners from 6" squares of 1" plate (should have used at least 1.25" plate), with radii adjusted for metal thickness offsets. I then built a frame to hold the corners, along with square tubes on the straight sides, making a full size die to bend on. The head blank was cut to shape with a plasma arc cutter, with the 1.25" for the flange added on to the basic shape. Blank was centered and securely clamped to the die frame, and I started bending. The straight sides bent fairly well cold, working back and forth along the side with a large hammer, using a piece or heavy stock to clamp along the edge just back of the bend. The corners i bent hot, clamping a piece of heavy angle diagonally across the corner both as a clamp and a heat barrier, then heating with a torch and hammering. Care must be taken to not allow the metal to form bottlecap wrinkles, they're hard to correct later.

The inside corners have to stretch, it may help to flatten the curve of the blank and leave some extra metal width to work with. If the die corners are not taller than the flange, the corners will wrap around the die and make it hard to remove the die. Die corners could also be made on a mill with a rotary table.

Drawings for many of the bunkers do still exist, Case kept much of the archive. I think most of the drawings are now in a museum in Racine, other threads will have the contact info. The drawings can be hard to read, and the one I was working with alternated between fractions and decimal measurements, even on a single side of a part.
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Old 06-24-2019, 04:53:47 PM
b7100 b7100 is offline
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Default Re: Case Bunker Bends

I bent the radius on my 3d scale case bunkers in an H press. I experimented with diffrent radius dies (pieces of pipe/rod) on the upper part to get the radius I wanted. The lower die was a good stiff piece of angle iron. I also made a gauge to get accurate dimensions. Of course this was on a 3d scate with 16 gauge steel. You would have to scale up. Another thing I highly recommend is a coating for the inside called coal tar epoxie. Bunkers are 3 years old and not a spec of rust inside. I have posts with pictures in the scale model engineering forum.
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Old 08-12-2019, 04:33:04 PM
sawiley sawiley is offline
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Default Re: Case Bunker Bends

Finally had some time to post a bit more on this. At first, I thought the OP was asking about the lower tank heads, which are flanged on both inside and outside corners. I later realized he might have been asking about the sides, especially getting the upper fuel bunker sides to match the lower water tank sides.

There are three main ways to bend sheet metal -

Press brake - a hydraulic press is used to squeeze sheet metal between male and female dies to form a corner. A round male die will form a smooth radiused corner, but aligning the center of the die with the center of desired curve can be a challenge.

Progressive bend - sheet metal is clamped into a bending brake, bent slightly, then advanced slightly, bent, advanced, bent, advanced, etc. The resulting curve can end up looking segmented, but no specialized dies need to be made.

Swing frame - a jig is made to hold a die clamped to the workpiece, and bend the sheet metal around it. Think of a Hossfeld bender, only much wider.

In all three methods, what really matters is how you handle offsets for material thicknesses, and how you lay out beginnings and endings of bends. When metal is bent, you never compress the material to the inside of the bend, you stretch the material on the outside of the bend. Thus, when laying out locations of bends, you always measure inside-inside, even if that means flipping to the other face of the workpiece.

In the instance of a set of Case bunkers, the nominal radius of the corners of the sides is 2 5/8", if i recall correctly. That is the inside radius for the tank side, and the outside radius for the bunker side, if they are to fit together neatly, with the bunker side fitting inside the tank side.

When I did the set for my friend's Case, I used progressive bend. With 2 5/8" being the inside radius, the math for the tank looked like this:

Radius x 2 = Diameter (2 5/8" x 2 = 5 1/4")
Diameter x Pi = Circumference (5.25" x 3.14 = 16.485")
Circumference / 4 = length of 90 degree bend (16.485" / 4 = 4.12125")
4.12125 = roughly 33 increments of 1/8" (for progressive bend)
90 divided by 34 bends equals 2.647 degrees per bend to form a 90 degree corner. (33 segments, plus 1 starting bend)

The math for the bunker, where the 2 5/8" was for the outside radius, would be different, so I subtracted the 1/8" material thickness and ran the math again.

Radius x 2 = Diameter (2 1/2" x 2 = 5")
Diameter x Pi = Circumference (5" x 3.14 = 15.7")
Circumference / 4 = length of 90 degree bend (15.7" / 4 = 3.925")
3.925" = roughly 31 increments of 1/8" (for progressive bend)
90 divided by 32 bends equals 2.8125 degrees per bend to form a 90 degree corner.

A bend or two fewer, with each slightly tighter, to make the smaller curve. The big thing to notice is that the overall length of the 90 degree bend was about 3/16" different. So, regardless as to which bending method you use, you lay out a straight side (same for both bunker and tank), use the correct length of bend, and layout the next straight side.

I think the outer left and right sides are about 19" along the straight face. Thus, the bunker would be laid out 3.925" bend, 19" flat, 3.925" bend, and the tank would be laid out 4.12" bend, 19" flat, 4.12" bend. Once properly bent, they should fit tight.

If the tank side goes from the center rear, around a corner to the front, around another corner back towards center, then a reverse corner to the front, you would lay out rear flat, 4.12" bend, side flat, 4.12" bend, front flat, 3.925" bend, side flat. Don't forget your overlaps if you're riveting the seams.

If you use a press brake or swing frame, your die will need to be sized for the inside diameters, so two different dies will be needed to get the right radii for matching inside/outside curves, and you'll need to figure out how to locate the workpiece in the press so that the bend happens in the right spot. Some trial and error may be needed.

Most bends need to be overbent a trifle, since metal tends to spring back a bit. How much is dependent on material, thickness, width, and ambient temperature, so you might want to make sure you have a couple test pieces available before bending the final workpiece. Pieces bent too tight can be flattened back out a bit by laying them on a large diameter pipe or radius cornered square tube, and smacking them with a hammer a bit. Use a block to protect the workpiece from hammer dings. Waving a torch briefly across the outside of an overbent piece can sometimes help it spring back a bit farther. Bends that are close, but not perfect, often can be mechanically pulled into place during final assembly.

Drawings for Case bunkers are still available, I believe, from a museum in Racine (other threads will have info on this). The Case drawings I worked from alternated between fractional and decimal dimensions on the sheet metal layouts, it was a bit hard to interpret at first.

One last word - don't get too wrapped up in perfection, a close look at many manufacturers' original tanks will show all sorts of imperfections. They were trying to kick product out the door, they weren't worried about perfect fits and finishes.
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