Molybdenum- Machining

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The main property of pure molybdenum which has influence on its machinability is its grain structure. In the "as sintered" condition, it can be machined relatively easily, but it is somewhat more difficult to machine after working. Molybdenum that has undergone a considerable amount of working is best machined if it has a uniformly fine grained and fibrous structure. Molybdenum machines with the crumbling chip which is characteristic of hardened SAE 1040 steel. While it is possible to machine molybdenum with high speed steel tools, tungsten carbide tools are generally recommended for better tool life. Very satisfactory results are obtained with Grade 2A5 Vascoloy Ramet tools.

Turning and Milling Molybdenum:

For inside and outside turning, tools should be ground to angles and rakes similar to those used for cast iron. Speeds up to 200 feet per minute, with a depth of cut up to 1/8", are satisfactory for rough turning. The feed should be 0.015 in./rev. For finishing work, speeds up to 400 feet per minute, with a depth cut of 0.005" to 0.015", and a feed of 0.005" to 0.010", should be used.

It is very important, in turning, that the depth of cut always be greater than 0.005". If depth cut is less, tool wear will be excessive.

Sulphur base cutting oil can be used as a lubricant for roughing cuts, and kerosene or sulphur base cutting oil can be used for finishing work. If lubricants are not used, tool wear will be excessive. Sulphur base oils cannot be used for machining electronic parts. Chlorinated oil and solvents have proved very satisfactory as a machining lubricant.

Molybdenum has a tendency to chip while being machined, and care must be taken to prevent this. Work should be firmly chucked, tools rigidly supported, and machines should be sufficiently powerful and free from chatter or backlash. A copious supply of coolant is essential.

Face milling is not generally recommended. It may be accomplished when necessary, however, by the use of carbide tipped cutters. The speeds and depth of cut should be similar to those used in lathe turning, except that the depth of cut should not exceed 0.050".

Molybdenum plates can be edge machined. In fact, plates thicker than 0.050" should be edge machined rather than sheared to finished dimensions. This work can be done either on a shaper or milling machine, and the machining should be done along the edge, rather than across the edge. The molybdenum should be clasped between steel plates while being machined to avoid chipping the edges.

Drilling, Threading and Tapping Molybdenum:

Molybdenum can be drilled with high speed steel drills, although carbide drills are recommended for very deep drilling. When using high speed steel drills, the speed should be 30 to 50 feet per minute with a feed of 0.003 in./rev. A cutting oil should be used for all drilling, tapping or threading.

Some difficulty may be experienced in threading or tapping. The thread depth should not be more than 50% to 60%, because of the tendency of molybdenum to chip. Rethreading or retapping should not be attempted at any time.

Molybdenum can be roll threaded. In this operation, the molybdenum stock and the die should be heated to approximately 325 oF. It is neither necessary nor desirable to heat molybdenum beyond this temperature, since it attains ample workability at that point. Molybdenum can be heated to 325 oF in air without danger of oxidation, but must not be heated to temperatures above 500 oF except in hydrogen or other approved protective atmosphere.

Sawing:

Power hacksaws and band saws are used effectively to cut mill products of molybdenum to suitable lengths for subsequent machining or to produce rough shapes with plane surfaces. Both types of saws are widely used by ships that machine molybdenum parts. The most effective blades are made of high speed steel with only the tooth area hardened. It is not necessary to use a cutting fluid, but a soluble oil coolant flowing through the hacksaw cut will remove the chips and extend blade life.

Electrical Discharge Machining:

Electrical discharge machining (EDM), or spark machining, may be used to produce small and irregular shaped holes and slots in molybdenum. The process is also useful for producing cavities of complex shapes in molybdenum bars or forgings. EDM is particularly well suited for machining intricate parts for the electronic industry, especially 'cut-outs' in thin molybdenum sheet. EDM wire machining can be used effectively for straight edges of internal openings or slits too thin for sawing or milling operations.

Electrical discharge machining is accomplished by rapidly recurring electrical discharges between an electrode and the workpiece in a dielectric fluid. The electrode is always negative and the workpiece is positive. The electrode for machining molybdenum is usually graphite, but equally satisfactory results can be obtained with copper, brass, or molybdenum electrodes.

The spark discharge, as it accomplishes the removal of metal from the workpiece, forms small craters on the surface that is generated. The tiny craters comprise part of a surface layer of disturbed metal that may be 0.0001" to 0.001" thick; surface roughness may be in the range of 25 to 250 microinches (0.625 to 6.25 microns). The thin surface layer may also contain microcracks that could affect the performance of molybdenum parts adversely if the surface is stressed in tension. For many applications, therefore, the surfaces formed by electrical discharge machining should be ground or polished with abrasive cloth to remove the damaged layer. The layer may also be removed by chemical milling or electropolishing.

Electropolishing:

Molybdenum can be electropolished in a number of different solutions. Commercially the two most commonly use are:

              (a)         phosphoric acid-sulfuric acid

              (b)         straight sulfuric acid

The first solution requires a much higher current density than the latter but also gives a better finish. Both baths are used at room temperature with the molybdenum as the anode.

                                               (a)                                                  (b)

Solution:               4 gal phosphoric acid                       2 parts sulfuric acid

                                     1 gal sulfuric acid                                1 part water

                                        4 gal water

Current:                10/14 amp/sq in                             0.7 - 2.0 amp/sq ft

Time:                            30 sec                                                --

Final Treatment:        Dip in denatured                                Film of blue oxide

                                         alcohol prior                                 forms on workpiece.

                                       to water rinsing.                             Removed by immersion

                                                                                              in alkaline cleaner

                                                                                         or caustic soda solution.