Introduction: Using Bismuth to Hold Parts for Machining.
Often the most difficult part of machining is figuring out how to hold the part securely. Below is a relatively easy method to make a variety of parts- small, irregularly shaped and/or delicate.
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Step 1: Clamp Stock in Vise.
This step is self explanatory. If starting with an irregularly shaped piece, you can skip to step 3.
Step 2: Machine the First Operation
I typically use this technique for CNC fabrication, but it can be useful in Manual Machining as well.
- material thickness should be at least 1.5mm larger than maximum thickness of eventual part. This extra material holds piece(s) together and will be machined off in the 2nd op.
- include enough flat coplanar surfaces so that part will lie flat on 2nd op tooling plate.
- include 2 widely spaced, precision holes (I drill and ream for tight clearance on a 1mm pin). These holes will correspond to locating pins on tooling plate. They can be located in your part (as above) or in sacrificial posts off to the side.
Step 3: Place Part in 2nd Op Fixture.
- Cavity that is deep enough to contain liquid Bismuth at a height that will support your part(s).
- Removable walls (or other feature) that facilitate removal of piece after machining (theoretically you can re-melt the Bismuth in the fixture, but I don't like using heat at my machines).
- Accurately placed pin holes that correspond to holes machined during first op. I use one of these pin locations as my 2nd op X0, Y0 machine coordinates.
- Be sure to establish your Z0 position relative to the fixture bottom (you will not be able to accurately determine it after the Bismuth is added).
Step 4: Adding Molten Bismuth
- Important- Make sure all surfaces (fixture and part) are clean and dry (you will be pouring molten metal against them).
- Clamp part down tightly once it is located on pins and lying flat.
- Which Bismuth to use? There are a number of Bismuth alloys to choose from. Their composition determines the melt temperature. Though the common 158* F alloy (sometimes known as "Woods" or "Cerrobend") is nice because you can easily melt and remelt it, it also contains Lead and Cadmium (nice elements to avoid). As a result of this, for all our work, we use the 281* F alloy. Its composition is 58% Bismuth, 42% Tin, both relatively safe metals. It is available many places, but I've found that Rotometals.com consistently has the best prices (Bismuth isn't cheap, but it is reusable).
- Melting the Bismuth- Here you must be careful! Make sure you have good ventilation, carefully plan your moves and of course use proper body, face and eye protection. You do not want to have molten metal spill or splash on you or others. I use a simple electric hotplate and small, thick walled pot with pouring spout. It is also advisable to use an electronic thermometer with alarm so that you don't accidentally overheat metal (a quality cooking thermometer will work).
- Once molten, carefully pour Bismuth into fixture. If you have trouble getting metal to flow around your part(s) before it cools, consider preheating fixture with heat gun.
- Let Bismuth cool slowly! There is a tendency for parts to "lift" if you try to cool them fast.
Step 5: Machine the 2nd Side of Part
For the 2nd Op, I start by facing off the top material. This exposes your part(s), and sets them up for any backside machining. If you've done a good job of pre-setting your work coordinates, have accurate locating pin/hole positions, and of course, wrote a good CNC program, these features should align nicely.
Step 6: Re-melt Bismuth and Remove Finished Parts
When all machining is done in 2nd op, the solid Bismuth piece containing the individual parts is ready to be removed from fixture and re-melted. Because of Bismuth's high density, your parts will float to the top and be easily removed with tweezers.
Step 7: Assembly!
Remove any lingering Bismuth from parts (scrapes/brushes off easily), do any pre-assembly preparation (we do a light tumble to knock off sharp edges), then assemble! In the case described in this Instructable, we've now made a cool, hinged, titanium wedding ring.
I hope you found this article interesting and/or helpful.