Introduction: Spun Steel and Waterjet Cut Sculpture

About: I'm a sculptor working in bronze, stainless steel, and steel.

This instructable will guide you through the process of fabricating a steel sculpture comprised of spun hemispheres and water jet cut flat patterns. I've inluded a dxf file of the geometry if you want to give it a try.

Step 1: Modeling

I began this piece by modeling a sphere which was rotated 270 degrees, leaving a 90 degree void. The form was then copied and after screwing around with it for hours (I know it looks simple), I arrived at a nice composition. The shapes individually have very little emotional appeal but when they interpenetrate, an entirely new form is created that has a shared component and dynamism. I equate it to a covalent bond in which atoms combine, sharing electrons to create a molecule.

Step 2: Parts

Much of my work over the past few years has been comprised of spherical sections of metal that are fabricated to create forms evocative of nature, the cosmos or anything that speaks to me. Pre formed metal hemispheres are available as stocked items in a variety of sizes from Wagner Companies - or Commercial Metal Forming - . The parts provided by these manufacturers are die formed and are mostly free of tooling marks with the exception of a small centering hole. The drawback is that they do not manufacture hemispheres between 12" and 30" in diameter. For dimensions between 12" and 30" the parts need to be spun. I've been working with RW Metal Spinning in Hayward, CA for several years and contracted them to make three 16" hemispheres for this piece. The video in this step shows Roy Gonzalez Sr. spinning an aluminum part in his shop. Great guys.

Step 3: Cutting a Hemisphere on a Band Saw

This sculpture consists three hemispheres, cut and assembled into two interpenetrating spheres rotated to 270 degrees (that's a complicated sentence but I'm not sure how else to describe it) and four flat patterns. One of the hemispheres needed to be cut in half and the band saw in the Pier 9 workshop was the perfect tool. The saw allows a 16 1/4" long cut with a throat depth of over 8". I built a jig of angle iron, tack welded the hemisphere to the angle and clamped the piece in place. A laser cut pattern was used to help align the blade to bisect the piece. It was a perfect "cowboy cut".

Step 4: Cutting a Hemisphere With a Plasma Cutter

I had to cut one small wedge section of a hemisphere to fit and my skills on the Omax Water Jet were not yet up to the task a making a 5 axis cut. So, I had to resort to the plasma cutter to make it work. A laser cut pattern was made, slipped over the hemisphere and using an angle finder, the pattern was adjusted to the angle dimension of the computer model. The wood pattern acted as a guide and the plasma torch traveled around the dome to make the cut. After a bit of grinding to clean the cut, the pieces were fit together.

Step 5: Water Jet Cutting

Now that the hemispheres were cut, it was time to cut the flat patterns on the Omax Water Jet Cutter. To create the patterns, the computer model was exploded and the flat shapes were saved as .dxf files, a file type that Omax Layout reads. The patterns were imported, cut in short order and then the final assembly started.

Step 6: Fitting and Tack Welding

Now that all the parts were cut, it was time to fit the piece together and begin welding . The hemispherical sections were registered by carefully measuring between known points and then tack welded. Flat patterns were tacked in sections and the whole piece was then assembled. All of the joints needed to be prepared for welding so the seams were beveled with an angle grinder. I really should have done this before assembly but I was anxious to see the piece together. I then started skip welding the entire piece. Skip welding is a technique used to evenly distribute the stress of welding over the work to minimize distortion. Small "stitches" about 1/2" long were welded at consistent intervals along all of the joints. When skip welding turn the piece often to minimize hot spots and prevent the metal from deflecting. Given the relative strength of the hemispherical forms and the forgiving nature of mild steel, I wasn't worried that the metal would distort too much.

Step 7: More Welding

I continued skip welding the piece, filling in the spaces at even intervals until the sculpture was completely welded and ready for finishing. Next, all of the welds will be dressed, welded some more to build up edges and fill low spots and then finished real pretty.

Step 8: Metal Finishing-Step 1

After a lot of TIG welding, the piece was finally ready for finishing. Nothing can ruin a nice fabricating job like poor metal finishing. Some general (and obvious) rules to follow when metal finishing:

  • Start with large tools and move to smaller tools.
  • Start with coarse abrasives and move to progressively finer abrasives.
  • Work away from edges. That is, make sure your grinder wheel is cutting away from and not into and edge.

I started with an 80 grain sand paper to dress all the outside corner welds I could reach with a 7" angle grinder. To dress the inside corner welds, I stepped down to an 80 grain flexible grinding disc which produced a clean joint.

Step 9: Metal Finishing-Step 2

4 1/2" and 7" angle grinders make quick work on a small piece like this. Inside corners however, require different tools. To get into the corners and other tight area, I used a few different tools:

  • A Dynabrade Dynafile III is a sweet tool. It is basically a miniature belt sander that can be used with multiple attachments and varying belt sizes. For this piece I used 1/8" wide 80 grain belts followed by 1/4" wide scotch brite belts.
  • A right angle die grinder with a 1 1/2" 120 grain disc was used to get farther into the corners.
  • A pencil grinder with a round burr was used to dress the corners no other tool could reach.
  • Finally, a triangular file was used to get a straight, clean edge.

Step 10: Metal Finishing-Step 4

After all of the welds were dressed, a 5" random orbital (DA) was used to unify the piece and remove the directional grinding marks. I started with 80 grain and moved up to a 120 grain disc. After the entire piece was uniform, I hand finished the sculpture with an 80 grain hand pad. Hand finishing with a coarse abrasive over finer scratches creates a nice surface depth on the metal and really gives a sculpture a rich, hand made feel. It can be a rabbit hole however, so know when to stop or you could spend days rubbing the thing!

Step 11: Metal Finishing-Step 5

There wasn't very much mill scale left on the steel but what was there needed to be removed before a patina could be applied. If you've ever tried grinding the scale off, you'll know that it is hard, tedious and slow going. Muriatic acid will strip it quickly and leave a rough grey surface that can then be sanded. Muriatic acid is Hydrochloric acid and is used mostly to etch and clean masonry and concrete. It is extremely hazardous, and can cause serious chemical burns so be safe when using it. A diluted solution makes it a little less gnarly to work with, so I added a couple ounces of acid to a quart of water. Always add acid to water, not water to acid. The ingredient you're adding to will splash when the second ingredient is introduced. Better that water splashes out than acid! Use rubber gloves and use in a well ventilated area as the fumes are strong. The acid solution can be applied with a natural bristle brush or some sort of hand pad. I lightly scrubbed the piece with a scotch brite pad to help loosen the mill scale as the acid dissolved it. When the scale is gone, thoroughly rinse the piece and dry it immediately to minimize oxidation. Store any unused acid solution in a plastic or glass container and either use it at a later date or dispose at a hazmat location.

Step 12: Metal Finishing-Step 6, Patination

Almost done. I prepared two solutions before applying a patina:

  • 1 quart of water to 16 ounces of bluing.
  • 2 tablespoons of baking soda to one gallon of water.

The steel will begin to darken immediately when the bluing solution is applied so be ready to work quickly. You can apply the solution by dipping, brushing, spraying, or any combination that works for you. I used a brush and fine scotch brite and worked the entire piece to try to get an even patina. Before starting, flood the sculpture with water or alcohol: this will allow the bluing to sluice over the piece to minimize streaking. When you arrive at something you like, rinse the piece well with your baking soda solution. This will aid in stopping the oxidizing reaction caused by the bluing. Rinse again with clean water and dry immediately with a soft cloth.

Step 13: Waxing

There are many techniques for applying a protective coating to a sculpture, including oil, lacquer and wax. I generally wax sculptures because it's fast, easy and can be easily stripped if necessary. Carnuba based waxes are preferred because they dry clear and hard. Do not use auto wax as it leaves a filmy white residue in recesses on the piece. Before waxing this piece, I heated it gently with a patina torch to drive any moisture out of the metal. A thin coat of Trewax was applied with a soft brush (pastry or chip brushes work well), allowed to dry for 2 minutes and then rubbed off with a soft cloth. One more coat of wax and done.