Notes on machining bismuth

Cast and Machined Bismuth, Kinda

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MACHINING OF BISMUTH

1. Safety Precautions

There are no particular safety precaution* necessary when bismuth is being machined other than those observed in good machine shop practice.

2. Recommended Coolant

Bismuth may be machined dry. If a coolant is desired, a solution of powdered oil and water in recommended proportions (Henry Sanson & Sons, Inc., Manhasset, L.I., N.Y., manufacturer) is satisfactory. However, plain tap-water is equally effective.

3. Machining

Bismuth is not difficult to machine. The conventional machines and tools used for cast iron are satisfactory. The metal in its natural form is hard, crystalline and extremely granular in structure. Bismuth, as cast, has a resemblance to lead but is harder and extremely brittle.

4. Tools

All cutting tool angles should be similar to those employed on cast iron. Tools with a more positive rake - as those used for mild steel - also work with good results. Any of the conventional materials used for tool bits, cutters, etc., can be used satisfactorily.

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a. Turning

Bismuth turns very easily but has a tendency to chip and uproot the crystals. Ordinary high-speed tool bits are satisfactory. Tools may be ground with rake angles similar to those used for cast iron or mild steel (1-1/2° to 5° positive).

b. Milling

All milling operations are simple. The only difficulty that will be encountered is the crystalline and granular structure of the metal. Sharp edges are difficult to maintain since the metal will chip and flake off wherever the tool leaves the work. The metal is so brittle and weak that it barely holds together during the cutting operation.

c. Drilling

Drilling bismuth is not difficult if care is exercised when starting the drill. The drill has a tendency to go off center and follow the crystals. The "creeping" is very noticeable whenever small holes are being drilled.

d. Tapping

Tapping bismuth is difficult. The metal crumbles ahead of the tap and crushes and uproots the thread. The crystalline and granular nature of this metal makes it almost impossible to tap or thread, i.e., tapping, threading with dies, turning on a lathe (either external or internal).

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e. Grinding

Bismuth can be ground easily with no particular problem. Any of the conventional methods of grinding, i.e., cylindrical (internal or external), surface, thread and centerless are simple. If bismuth must be threaded and the size of the thread will permit, thread grinding is recommended. However, threading should be avoided if possible because the thread will usually be too weak to be of any normally practical use, Norton or equivalent wheels of a 38A60 E-12 VBEP or 32A60 E-12 VBEP grade do excellent finishing operations if a soluble powdered oil and water solution as recommended by the manufacturer (Henry E. Sanson & Sons, Inc., of Manhtasset. L, I., N. Y, )is used.

f. General Machining Notes

1. Bismuth may be machined to any shape or form desired by using regular cutting tools, 2. Bismuth is not adaptable to threading. It is too brittle and granular to withstand any shear action and has a tendency to separate the crystals and crumble. If it must be threaded, a coarse thread is preferable. (27)

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27. Report From Brookhaven National Laboratory - Received 11/1952 from J. Nicholson.