Fail Of The Week: Machining Bismuth

[David Cook]’s summary below the write-up of his experiences working with a bismuth ingot is succinct.

I wasted a weekend learning why elemental bismuth is not commonly used for metal parts.

It’s a fair assessment of his time spent growing unspectacular bismuth crystals, casting a bismuth cylinder that cracked, and machining bismuth only to be left with a very rough finish. But even though he admits the exercise was unsuccessful, he does provide us with a fascinating look at the physical properties of the element.

This is what [David] wanted to make. Alchemist-hp + Richard Bartz with focus stack. (Own work) [CC BY-SA 3.0], via Wikimedia Commons
This is what [David] wanted to make. Alchemist-hp + Richard Bartz with focus stack. (Own work) [CC BY-SA 3.0], via Wikimedia Commons

Bismuth is one of those elements you pass by in your school chemistry lessons, it has applications in machining alloys and as a lead replacement but most of us have never knowingly encountered it in the real world. It’s one of the heavy metals, below antimony and to the right of lead on the Periodic Table. Curious schoolchildren may have heard that like water it expands on solidifying or that it is diamagnetic, and most of us have probably seen spectacular pictures of its crystals coated in colourful iridescent oxides.

It was a Hackaday story about these crystals that attracted [David] to the metal. It has a low enough melting point – 271.5 °C – that it can be liquified on a domestic stove, so mindful of his marital harmony should he destroy any kitchen appliances he bought a cheap electric ring from Amazon to go with his bismuth ingot. and set to work.

His first discovery was that cheap electric rings outdoors aren’t very effective metallurgy furnaces. Relocating to the kitchen and risking spousal wrath, he did eventually melt his bismuth and pick off the top layer once it had resolidified, to reveal some crystals.

These are the bismuth crystals he made.
These are the bismuth crystals he made.

Unfortunately for him, instead of spectacular colors and huge crystals, the sight that greeted him was one of little brilliance. Small grey crystals with no iridescence. It seems the beautiful samples are made by a very slow cooling of the liquid bismuth, followed by a quick pouring off of the remaining molten metal. Future efforts, he assures us, will involve sand-insulated molds and careful temperature monitoring.

Undeterred, he continued with his stock of bismuth and embarked on the creation of a cylinder. Early efforts with a clay mold resulted in cracked cylinders, so in desperation he cast the entirety of the metal in an aluminium baking tray and cut the resulting ingot to a rough piece of stock for turning.

Poor finish on machined bismuth.
Poor finish on machined bismuth.

With the bismuth in the lathe, he then came face to face with what he alluded to in his conclusion above, why machined bismuth parts aren’t something you’ll encounter. His cylinder came out with significantly rough patches on the surface, because bismuth is both crystalline and brittle. He suggests improvements could be made if the metal could be solidified with fewer crystals, but it’s obvious that elemental bismuth on its own is not a winner in the turning stakes.

We suggest you take a look at [David]’s write-up. It may be presented as a Fail of The Week here, but in fact it’s more of a succession of experiments that didn’t work than an unmitigated disaster. The result is an interesting and well-documented read that we’re sure most Hackaday readers will gain something from.

Aside from the bismuth crystals linked to above, we’ve featured bismuth a few times here at Hackaday. A low-temperature soldering process used it in an alloy, and we’ve even featured someone using it in another alloy to print using a RepRap.

Thanks [nebk] for the tip.

Pewter Casting with PLA

Over on Hackaday.io, [bms.had] is showing his technique for 3D printing molds that he uses to cast (lead-free) pewter objects. The process looks simple enough, and if you have a 3D printer, you only need some lead-free pewter, a cheap toaster oven, and PLA filament. He’s made two videos (below) that do an excellent job of showing the steps required.

Even though the pewter is hot enough to melt the PLA, it doesn’t appear to be a major problem if you quench the piece fast enough. According to [bms.had], a slower quench will melt some PLA although that creates a smoother surface. You can see the 0.31 mm layer lines in the cast, though, although you can use any layer height you like to control that. Creating the mold is simple (the videos use Tinkercad, although anything suitable for creating 3D models would work). You essentially attach a funnel to your part and make the entire part a hole inside an enveloping shape.

Continue reading “Pewter Casting with PLA”

Casting Engagement Rings (Or Other Small Metal Parts!)

[Paul Williams] wrote in to tell us about his most recent and dangerous endeavor. Marriage.

As a masters student in Mechanical Engineering, he wanted to give his wife (to be) to be a completely unique engagement ring — but as you can imagine, custom engagement rings aren’t cheap. So he decided to learn how to make it himself.

During the learning process he kept good notes and has produced a most excellent Instructable explaining the entire process — How to make the tools you’ll need, using different techniques and common problems you might have. He even describes in detail how to make your own mini-kiln (complete with PID control), a vacuum chamber, a wax injector and even the process of centrifugal casting. Continue reading “Casting Engagement Rings (Or Other Small Metal Parts!)”

Making keys after the apocalypse

Making keys is an amazing art with a lot of skill and technique involved. For those of you living in a post-apocalyptic world, [Dan] has a much simpler solution to the problems of having one too few keys for your locks and deadbolts – just cast them out of scrap with the power of the sun.

To make the mold of the key, [Dan] is using a two-piece plaster of paris mold. First, a thick layer of plaster is laid down in a small container and the key floated on the surface. After drying, sprues are put in with clay and the key embedded in a curing plaster block. After a few hours, a proper mold is created ready to receive molten metal.

The casting material is zinc – not as hard as the original steel key, but more than strong enough to turn a lock. This zinc is melted in a steel and plaster crucible with a gigantic fresnel lens.

As for the utility of this method of copying keys after the apocalypse, we’ll have to wonder how practical this method is. A giant fresnel lens isn’t just something you randomly find unless you’re going house to house looking for projection TVs, and finding a can of mold release after the end of the world is beyond credulity. That said, it’s a cool demonstration of metal casting that can be easily accomplished at home or at any hackerspace.

Continue reading “Making keys after the apocalypse”