Saving a Casting Blow Out
The client is coming Monday morning to sample the prototype. Therefore, we're casting on Friday just for the sake of making the whole process more stressful. Who needs margin for error?
Freeing one cast from the shell reveals a backplate that is absolutely perfect! Remove the sprews and vents with an angle grinder, a little pneumatic sanding action, wire wheel, and buffer, and this piece is ready for plating. A photo of the good piece is shown below alongside of the ceramic shell it came out of. You can see that detail of ceramic shell is pretty amazing, capturing the surface texutre concentric circles of the original 3D printed model. Just wanted to put in a good mention for ceramic shell. It captured the fine beaded detail on the backplate beautifully.
But the second cast is another story. Brass casting can be gassy, and chipping away the shell revealed air bubbles trapped during the pour. The piece was sprewed with both top and bottom channels for back pressure. Nevertheless, it wasn't vented enough to allow air to escape. As Murphy would have it, flaws never happens on the back of your item. They occur on the faceplate just so everyone can see.
Sanding only reveals a deeper crevass from one of the plate to the other. It's too late to reschedule. The clients have purchased tickets from overseas and they will be on their way.
To buy time to think, I went ahead and did what I knew to do - prep the good piece. Then the idea surfaced about cutting out a faceplate from brass sheet to reface the flawed one. This could work theoretically, but real life is often another thing entirely. I sanded down the cast faceplate another 1/16" to match the patch metal. I then took 1/16"
sheet brass, drilled a through hole large enough for a rod to pass, and wedged the rod in the gap on the table of my band saw to keep the center stable. Measuring out the radius, I kept the rod upright while turning out a circle cut on the plate. It worked. Almost. The first try was about 1/16" too small, leaving the original outer edge. The next try almost worked - a little closer with just 1/32" shy of the goal. The next one would have been perfect. But I over sanded the edges by twirling the circle patch from the rod in the center against a 120 grit belt sander (this will help ensure your piece is perfectly round). The next one worked perfectly.
After cleaning both pieces thoroughly with ScotchBrite, I fluxed both sides and tinned the sheet. Then I clamped the heck out of it and heated the piece from behind, drawing out the solder to the sides until that silver hair line appeared all around. Slow and steady wins the race. Getting your soldering right saves hours of work by not having to sand excess on those sides.
And it worked! The edge angles on both pieces were matched with an orbital sander taking the work to 800 grit until the difference was instinguishable. The only perceivable problem would be for the client to notice the hair line of solder around the face plate once the finish plating was complete. We held our breath.
The client loved the pieces - with one
exception. The surface was too perfect. They really wanted pock marks to give the impression of age. So laughingly, I took a punch and hammered crevices back in.
Just wanted to show how one nightmare was turned into a save. What are your stories?