Introduction: Big Stainless Steel Sculpture Part 3
Here's part 3 of the big stainless steel sculpture odyssey. If you're just tuning in, this is a commissioned sculpture for Essex Properties in San Mateo, CA. It was designed in 3D Studio Max and is constructed of 7 gauge 304 stainless steel. Read on to see how the piece was finished. If you want to check out the prequels, they can be viewed here:
Step 1: Welding
I would generally MIG weld a sculpture of this scale, but given that it was stainless with a fine finish I decided to TIG weld it. For the most part, I pulse welded to have greater control, especially in the tight areas that would be difficult to finish later. Check out the really boring movie above if you're curious about the pulse. Not especially instructive, but at least you can hear and see the pulse. More welding details: 308 L welding rod was used.
The sculpture was initially tacked together and was then "stitched" until the piece was fully welded. There were also about 20 plug welds, some of which are evident in the 4th photo in this step. The sculpture has a series of interior structural elements that are connected to the skins of the hollow forms by plug welding. These ended up being kind of a nightmare, but more on that later. The last photo shows how I set up my TIG torch so I could drag the cup on the metal and maintain a consistent arc length. This was the end of the fun part: on to grinding.
Step 2: Tools and Abrasives
Finishing stainless steel is tedious work. Stainless is hard and tough, easily work hardened and has a high coefficient of expansion which means that it moves a lot when heated. Some general tips when grinding: use large disks when possible, keep your tools moving to prevent hot spots, always work from coarse to fine.
Advances in abrasive technology has made the experience less painful and 3M in particular has some great products (no, I'm not on the 3M payroll). The ridiculous list of abrasives can be found in the .pdf file below if you really want to geek out on abrasives.
All of the corner welds were initially dressed using a 7" 3M GreenCorp 60 grain flexible disk followed by fiber disks, non-woven disks and random orbital abrasives. When possible, I avoided grinding on the clean mill finish (2B) and instead used only a random orbital sander. Most of the piece however, required a sanding pass with at least a medium or fine scotch brite to get rid of scratches from forming and fabricating.
All the finishing was done with the following tools: Makita 7" angle grinder, Dewalt 4 1/2" angle grinder, 3M 6" random orbital pneumatic sander, Dynabrade Dynafile 3 air powered abrasive belt machine (best tool ever!), die grinder, and pencil grinder.
Step 3: Grinding and Finishing
The plug welds, as mentioned in the first step, were a huge pain in the ass. The problem is that the plug weld deflects the metal creating a dimple that needs to be ground flat. So, that meant screwing up the nice 2B finish with a 120 grain 7" fiber disk, floated over a large surface area to blend in the dimple. Good times. From there, I worked the area using progressively finer abrasives until I was able to use a 6" random orbital tool on the entire piece.
Step 4: The Dynafile Is Your Friend
This may be the best specialized hand held tool for metal finishing ever. The Dynabrade Dynafile (no, I'm not on the Dynabrade payroll either) allowed me to grind fillet welds that couldn't be reached with an angle grinder and it's way better than a die grinder. The tool has many attachments for different abrasive belts, but for the most I used a 4" wheel with a 1/8" x 24" 120 grain belt. If you do a lot of metal finishing, save your pennies and buy one of these.
There was one area that the Mighty Dynafile could not even conquer. There, I used a die grinder with a conical shaped 120 grain cartridge roll to dress a weld and remove the heat tint. At this point, all the welds have been dressed and it's on to the random orbital (DA) tool.
Step 5: More Finishing
The goal now is to finish the surface to a 240 grain non directional surface. I began with a 120 grain 6" disk and worked up in increments to a Very Fine Scotch bright pad. The DA steps were as follows: 120, 150, 180, 220, 240, VF Scotch Brite. You can see the finished and unfinished surfaces next to each other in the last photo.
Step 6: Fine Tuning
After the final DA pass, I dressed up the edges and fillet welds with 3M unitized wheels which de-bur and polish at the same time! I used an 8" x 1/8" disk on a low rpm pneumatic angle grinder to reach the fillet welds that were dressed with the Dynafile. On the outside edges I used a 2" unitized wheel to give the corners a slight radius and make the thing less dangerous! Finally, the entire piece was hand finished with 240 grain sandpaper followed by a VF Scotch Brite hand pad.
Step 7: Preparing to Weld the Base Plate
The piece is now done (mmmm, shiny) and now needs to be welded to a 1" stainless base plate. The base plate was clamped and the sculpture set inside a water jet cut hole to allow for welding on the top and bottom of the plate. I was a little worried about deflecting the metal in the sculpture when welding, so I clamped some heavy aluminum plates just above the joint to act as heat sink and stiffener.
Step 8: Welding the Base Plate
I beveled the top of the base plate to prepare the joint, and then tacked the sculpture to the base. Then it was stitched and completely welded. The piece was then turned and welded on the bottom leaving a 1" gap to allow condensation to drain. With the base, the sculpture weighs in at 1,650 pounds. Finally, I signed the piece before the last step: cleaning.
Step 9: Cleaning
Stainless is a great metal unless it's contaminated. To make sure no free iron was on the sculpture, I rinsed it and cleaned it with Naval Jelly. The active ingredient in this stuff is phosphoric acid which converts iron oxide to ferric phosphate which is relatively stable. Brush it on, lightly scrub and let it sit for 5-10 minutes before rinsing thoroughly.
Step 10: Presto
Zip, zip, it's done. I'll be back with photos from the install in another month or so.