Determining starting blank size for beaten copper alembic dome?

[Hefty]

I've done a search here on IFI and elsewhere and I've got nothing:

How can I determine (at least roughly) how big to make a circular blank to create a dome to fit on a boiler of a specific diameter?

And, because I will be using a combination of sinking and tuck shrinking, do I need to adjust further to allow for stretching and/or shrinking of the overall blank to final?

Any help or advice would be greatly appreciated!

Cheers,

Jono.

[Frosty]

 I'd cut the blank maybe 10-15% larger than the desired finish diameter as the departure point and trim any extra. Of course you could just keep shrinking until it fit. 

My first thought is to spin it and trim the excess but I don't see pics in a quick search to see how complex one of these things is supposed to be. Lots of marketing talk and we all know how "informative" that is. 

Remember for as soft and easy to move as copper is it work hardens abruptly and cracks, splits, etc. You will get at MOST three passes before you need to anneal the part. A pass is ONE round of tuck and planish or once around the circle sinking. Be conservative and anneal every TWO passes.

Barely showing red in dim light is hot enough, you can cool it water or start working while it's hot but NEVER expect it to hold together for more than three passes. 

Copper is fun to work, have fun. (Cut a couple extra blanks just because)

Frosty The Lucky.

[JHCC]

I read somewhere that the standard silversmith's formula for sizing a blank is to add the widest horizontal dimension (the diameter for a round piece; the length, for an oval) to the height. In other words, if you are making a hemispherical dome that's 8" in diameter and 4" high, you'd make a blank that was 8" + 4" = 12". If you were making a shallow dome that was only 2" high, your blank would be 8" + 2" = 10". If you are making a high dome that's 6" high, your blank would be 8" + 6" = 14". Make sense?

(Or if you're using metric and are making a dome that's 20cm in diameter x 10cm high, that's 20cm + 10cm = 30cm; 5cm high, 20cm + 5cm = 25cm; 15cm high, 20cm + 15cm = 35cm.)

[Frosty]

OR if your dome was 4" dia x 8" high your blank would be 12" dia.

Frosty The Lucky. 

[JHCC]

From which we deduce that the Commutative Law of Addition is still in effect.

[George N. M.]

The problem, of course, is to allow for the stretching of the metal as it is being raised into the dome.  You want enough metal that the metal at the top is not too thin when completed and the rim has been "sucked in" enough that the rim is the diameter you want when the dome is raised to the desired height.  I would try John's formula and see how it works.

You can also work from the outside over a form and thicken the edge of the rim while leaving the top of the dome the original thickness.

There is an armoring technique for making the crowns of helms which moves metal to the center of a disk so that the top of the crown is thicker than the rim.

"By hammer and hand all arts do stand.:

[Scott NC]

I googled alembic dome and you might have some left overs from a hemispherical dome formula.  It looks more like a catenary dome to me.  It might get you close.   I've been wrong before though.  Many times.....

[Hefty]

12 hours ago, Frosty said:

My first thought is to spin it and trim the excess

That would be cool and I'd love to get into metal spinning but I need to be honest with myself and show some restraint. It's a whole other rabbit hole I could follow forever and chasing my current hobbies keeps me poor enough as it is, in money and in time! 

George, yes, I think I'm going to practise different forming methods in sheet steel to see the different effects on the thickness in different areas. Then I'll have to also, sparingly, do some practise in copper to compare the work hardening as Frosty mentioned.

Scott, yes, I'm going to have to figure out just how tall/deep I can form by hammering.

Thanks for the info so far, gents!

Any other tips for the forming? I haven't done much sheet metal work in my time. Mostly little bits and pieces in high school.

[George N. M.]

Check out references, books and videos, on armour making.  Armourers are the masters of putting complex, 3D shapes into sheet metal.  

I have found that sometimes it is better to do some things hot and sometimes it is better cold.  If you don't have a swage block you can use a block of wood but it does get smokey if you are doing it hot.  Also, be aware that any imperfection in either your hammer face or the backing, swage block, etc. will transfer to the metal you are working.

GNM

[Hefty]

Yes, I am still deciding on what sort of form to sink into or work down over. I've done some practise sinking and shrinking using the end of a pipe that has been filed to a rounded edge. I saw Roy from Christ Centred Ironworks do this in a short on Youtube. My practise piece has some marring from the edge of the pipe, but for a still, while a perfectly smooth would look really nice, it's not critical for the function.

I'm also thinking about getting an old trailer hitch (or something larger if I can find it) and trying working down over that.

[swedefiddle]

Good Morning,

Take a piece of log, stand it on it's end (about 24-30" high). Put your ugly wheel on your angle grinder and form a few dish shapes. Turn the log end for end and put different shapes in the other end. Using wood for the anvil won't stretch the material too fast. Be sure to anneal your work piece, a LOT!! Using the log technique, you should be able to sit the bum on a seat or stool. Saves hurting your McBack!!

Another simple sunny day, regardless what the weather is doing.

Neil

[Frosty]

It's really hard to sink a very deep form but you can crimp and shrink a flat blank into s drinking straw with practice. Of course if you can hit it hard enough you can closed die form copper in one smack and only have to worry about forming the rim. 

Frosty The Lucky. 

[Hefty]

My practise has been with sinking the edge of the of the blank into the pipe end to form a tuck and then shrinking the tuck, but I have only worked with small, sheet metal blanks. Do people normally form tucks over stakes, or into forms? Frosty, I've seen some videos you have suggested in the past involving using a tucking fork so I guess I could try that sort of thing too.

I also have tried anything too deep yet. i guess that will be my next challenge.

[Frosty]

I've never seen let alone tried to form the tuck in the end of a round like pipe or hole in a swage block, I'd call it a dimple rather than tuck. 

I've done it by bending a piece of round bar in half like a bobby pin then bending the creased end 90* so it rested in the hardy hole. If you make the ends long enough it doesn't need to "fit" the hardy hole to stay in place AND it lets you reposition the creasing swage and access it on an edge of the anvil.

Take a straight edge and protractor small enough to center on the blank with the marked edge exposed o n the blank so you can mark it. I eyeball this and you can easily enough by splitting each ray in half. 

Anyway mark rays from center to edge of the blank, the number of divisions and marks depending on the blank diameter and partially how much you want to raise it. I started with 36 rays on 10" blanks, 24 on 8" blanks, etc. You get the idea, yes?

How sharply and deeply I creased depended on how much I wanted to shrink the piece, a shallow bowl got shallow creases spaced farther apart towards the "flatt-ish center and increasing in number as I approached the rim.

The process is, Crease, Plannish, crease, plannish, rinse repeat, until you have what you want. I've never raised a cone but my intuition says, consult the books, the voices are muttering Uh . . . over and over. I'd cut a few ( probably sacrificial) extra blanks. Education costs and self education costs more. Eh?

The top tool to make the creases tends to depend, I've done shallow bowls with an old top fuller and if I were to raise more I'd profile a curved fuller and really round the ends. 

I've made the top tool from round stock for shallow rounded creases and ground round edges on flat bar from 1/2" down to 1/2" strap stock, Again I'd grind a curved profile for the same reasons a curved profile works so much better than straight on chisels, fullers, etc. Not an exaggerated curve just a gentle one. (the voices are clamoring, Yeah, YEAH, that's the ticket, you can always sharpen the curve if necessary!)

Remember to keep your planishing tools polished and the anvil die might need a conic or wedged shaped. Polishing a rawhide mallet probably isn't necessary but be CAREFUL WHERE YOU LAY IT, ANY HARD BITS IT PICKS UP WILL MAR THE PIECE!

I have a busy day planned, more later if I can stay awake.

Frosty The Lucky.

[JHCC]

I make a lot of bowls, and my favorite forms are made from decommissioned welding cylinders. The simplest version 6” or so cut from the bottom of a cylinder, flipped over, and with the hollow in the bottom ground smooth.

I also have a couple (which I prefer) made from the top end, cut off, flipped over, welded back into the cylinder (or at least a part thereof) and onto some kind of base, and with a rim of 1/2” or 3/4” round bar welded around the edge. 

44 minutes ago, Frosty said:

The top tool to make the creases tends to depend

Here’s one I made from some 1/2” round bar:

49 minutes ago, Frosty said:

my intuition says, consult the books

Here’s the relevant section from Moving Metal by Adolf Steines:

[Hefty]

This is all great info! Thanks all!

Hopefully I'll get a little time to make some tools and practise today.

I'll report back how I do.

[Hefty]

Ok, I had a U shaped piece already made that I could use to start some tucks with a small coil spring top fuller. (I made a smaller U out of garage door spring too, but I forgot to take a photo)

Then I hammered these in with my rounding hammer on the anvil. I alternated this with hammering into the axle side of a small trailer wheel hub that I smoothed with a flap disc.

I had forgotten just how soft fully annealed copper sheet is! After a few rounds of tuck shrinking, I started focussing more on just sinking into the round form. I got to a point where I was stalling a little and not getting any deeper, so I made something like a ball-end punch out of a piece of coil but used it more like mini rounding hammer so I could focus more in the centre of the round form from dead-soft to go deeper a little faster (still regularly annealing, though).

This first attempt in copper was 115mm diameter (~4 1/2") flat and I got it to 93mm diameter (3 5/8") and 42mm (1 11/16") deep. Including messing around experimenting with it and thinking it through, it probably took me 45 minutes or so. This next photo shows the tin lid I traced to start with for comparison. The second photo shows the depth I was getting, but this was just before I switched to the ball end punch/hammer for more depth/height in the middle.

It's fun and really satisfying to see the shape transform. Yet another rabbit hole to get lost in!

Cheers,

Jono.

[JHCC]

Looking good. Nice job keeping the curve regular. 

[Frosty]

Nice job.

Frosty The Lucky.

[JHCC]

Here’s another approach to tucking, from a batch of bowls I started this evening. 

First, set up the fly press with a ball fuller and a pair of angled bottom fullers:

Bump in the disc from one side at 60° intervals around the circumference:

Flip the blank over and bump it in from the other side, also at 60° intervals, centered between the hollows from the previous step:

This will pull the first set of hollows back into the original plane of the blank and give you deep tucks on one side:

Then, using a form from an old welding cylinder with a hexagonal base, deepen those tucks, first with a few strikes of a cross peen hammer:

 And then with the straight peen of a raising hammer:

 The blanks are now ready for hot forging:

[Hefty]

Thanks JHCC. That looks like a larger scale version of what I did with the U shape in the vise and the smaller fuller. Especially flipping the piece to define the tuck from both sides.

I've finally found some good information and videos on proper methods for raising, too. I didn't have the full/correct idea initially, but I do now and it's a little harder, and slower, than I thought to do it right. I'm getting there, but I'm still only practising on small pieces before I bite the bullet and go full size.