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.

Tips For Buying Your First Milling Machine

If you’re interested in making things (and since you’re reading this, we’re going to assume you are), you’ve almost certainly felt a desire to make metal parts. 3D printers are great, but have a lot of drawbacks: limited material options, lack of precision, and long printing times. If you want metal parts that adhere to even moderately tight tolerances, a milling machine is your only practical option. There is, after all, a very good reason that they’re essential to manufacturing.

However, it can be difficult to know where to start for the hobbyist who doesn’t have machining experience. What kind of milling machine should you get? Should you buy new or used? What the heck is 3-phase power, and can you get it? These questions, among many others, can be positively overwhelming to the uninitiated. Luckily, we — your friends at Hackaday — are here to help give you some direction. So, if you’re ready to learn, then read on! Already an expert? Leave some tips of your own in the comments!

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Removing a Broken Tap From Something Really Really Expensive

What happens when you break a tap or a bolt in a component whose price tag sits in the tens of thousands. Just drilling it out and throwing in a nut insert stops being acceptable. Is there a way to remove the tap without damaging the master part at all?

Broken tap stuck in the hole it was threading
Broken tap stuck in the hole it was threading

Well, that’s where [Tom Grafton] of Jerry’s Broken Drill and Tap comes in. He’s here to remove taps and chew bubblegum, and he’s definitely chewing bubble gum loudly the whole time. His primary work horse is a Metal Disintegration Machine.

A MDM is basically half of a typical wire EDM set-up. In EDM you used an electrode to punch a hole through the material. Then you thread a wire through the hole, thread it through a sometimes startling array of pulleys, and get going.

[Tom] used the MDM with an appropriately sized electrode to precisely disintegrate the middle of the tap out. After that it’s some careful work with a specially machined magnetic chisel. A quick chase of the threads with a tap and it’s back to the customer.

As you can see in the video after the break, the end result is a threaded hole that’s so indistinguishable from the rest he has to mark which one it was; presumably so the customer doesn’t forget why they’re paying him.

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Micro Tesla Turbine is an Engineering Tour de Force

A corollary to Godwin’s Law ought to be that any Hackaday post that mentions Nikola Tesla will have a long and colorful comment thread. We hope this one does too, but with any luck it’ll concentrate on the engineering behind this tiny custom-built Telsa turbine.

For those not familiar with Mr. Tesla’s favorite invention, the turbine is a super-efficient design that has no blades, relying instead on smooth, closely spaced discs that get dragged along by the friction of a moving fluid. [johnnyq90]’s micro version of the turbine is a very accomplished feat of machining. Although at first the build appears a bit janky, as it progresses we see some real craftsmanship – if you ever doubt that soda can aluminum can be turned, watch the video below. The precision 25mm rotor goes into a CNC machined aluminum housing; the way the turned cover snaps onto the housing is oddly satisfying. It looks like the only off-the-shelf parts are the rotor bearings; everything else is scratch-made. The second video ends with a test spool-up that sounds pretty good. We can’t wait for part 3 to find out how fast this turbine can turn.

Size matters, and in this case, small is pretty darn impressive. For a larger treatment of a Tesla turbine, see this one made of old hard drive platters.

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Fixing a Broken Bandsaw with a Custom Steel Part

When a large bandsaw broke down due to a cast iron part snapping in two, [Amr] took the opportunity to record the entire process of designing and creating a solid steel replacement for the broken part using a (non-CNC) mill and lathe.

For those of us unfamiliar with the process a machinist would go through to accomplish such a thing, the video is extremely educational; it can be sobering both to see how much design work happens before anything gets powered up, and just how much time and work goes into cutting and shaping some steel into what at first glance looks like a relatively uncomplicated part.

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Tales of Garage Design: Achieving Precision from Imprecise Parts

Designing parts to fit perfectly together is hard. Whether it’s the coarseness of our fabrication tools or the procedures of the vendor who makes our parts, parts are rarely the exact dimension that we wish they were. Sadly, this is the penalty that we pay by living in a real world: none of our procedures (or even our measurement tools!) are perfect. In a world of imperfect parts, imperfect procedures, and imperfect measurement techniques, how on earth are we supposed to build anything that works? Fortunately, we’re in luck! From the brooding minds of past engineers, comes a suite of design techniques that can combat the imperfections of living in an erroneous world.

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Machine Shop Soaps Are Good, Clean Learning Fun

At first glance, it’s easy to dismiss the creation of custom bath soaps as far outside the usual Hackaday subject matter, and we fully expect a torrent of “not a hack” derision in the comments. But to be able to build something from nothing, a hacker needs to be able to learn something from nothing, and there is plenty to learn from this hack.

On the face of it, [Gord] is just making kitschy custom bath soaps for branding and promotion. Cool soaps, to be sure, and the drop or two of motor oil and cutting fluid added to each batch give them a little machine shop flair. [Gord] experimented with different dyes and additives over multiple batches to come up with a soap that looked like machined aluminum; it turns out, though, that adding actual aluminum to a mixture containing lye is not a good idea. Inadvertent chemical reactions excepted, [Gord]’s soaps and custom wrappers came out great.

So where’s the hack? In stepping way outside his comfort zone of machining and metalwork, [Gord] exposed himself to new materials, new techniques, and new failure modes. He taught himself the basics of mold making and casting, how to deal with ultra-soft materials, the chemistry of the soap-making process, working out packaging and labeling issues, and how to deal with the problems that come from scaling up from prototype to production. It may have been “just soap”, but hacks favor the prepared mind.