Machining and Dimensional Tolerances for Molybdenum Crucibles

A sintered crucible is only a blank; whether it can actually be installed in a furnace is decided by machining. Turning, grinding and end-face machining each solve a different problem, and roundness, perpendicularity and wall thickness variation are largely fixed at the workholding stage.

The machinability of molybdenum lies between cast iron and the refractory metals; it is noticeably brittle at room temperature and prone to edge chipping during cutting. With the same tool and parameters, a new batch of blanks can produce chatter marks, which is related to blank density and allowance distribution. Tolerance ranges for different sizes can be checked in molybdenum crucible product parameters.

Workholding Comes Before Cutting Parameters

Thin-wall cylindrical parts deform elastically easily during clamping; excessive clamping force lets the workpiece spring back after machining and become oval. A common practice is to increase the contact area, spread the clamping force with soft jaws or dedicated fixtures, and schedule stress relief between rough and finish turning where necessary. The stress relief step depends on the blank allowance; when the allowance is large, a single stress relief may not release all the deformation.

The clamping datum must be consistent with the design datum. When the outside diameter is used to locate the bore, wall thickness variation is directly affected by the accuracy of the outside diameter; when the bore is used to locate the outside diameter, bore roundness must be secured first. Frequent datum changes accumulate error in the wall thickness, a principle also explained in machining and forming processes.

Control of Roundness and Wall Thickness Variation

Out-of-roundness usually comes from three sources: workholding deformation, spindle runout and tool wear. In rough turning the depth of cut is larger, and tool deflection caused by cutting force makes the wall thickness vary periodically around the circumference; in finish turning the depth of cut is small, and if the machine tool lacks rigidity, chatter marks appear in another form.

• Leave a uniform allowance in rough turning and re-measure the wall thickness before finish turning;
• Use a smaller depth of cut and feed in finish turning to avoid tool deflection;
• Keep the cutting edge sharp, since a dull tool increases cutting force;
• Measure in stages during machining instead of checking only after completion.

Wall thickness variation can be checked with an ultrasonic thickness gauge at points around the circumference, together with a caliper for the outside diameter. Only when the data are recorded can it be judged whether the deviation is random or fixed in orientation; the latter usually points to workholding or the geometric accuracy of the machine tool.

End Faces and Perpendicularity

The end face of a crucible is the datum for seating and sealing, and a perpendicularity deviation tilts the crucible inside the furnace and disturbs the symmetry of the thermal field. When turning the end face, the workpiece overhang should not be too long, and the tool overhang must also be controlled; otherwise deflection produces a convex or concave face.

The distribution of end-face allowance has to account for blank deformation. If the rim is already oval after sintering, cutting directly to nominal size leaves insufficient allowance in places, so the depth of cut should be set after measurement. For large-diameter crucibles, contact can be checked with a flat plate and feeler gauge after end-face machining, with lapping correction where necessary.

Traceback After Out-of-Tolerance

For a dimensional deviation, check the five stages in turn: blank allowance, workholding, tooling, machine tool and measurement. Insufficient allowance usually shows up as local unmachined skin or a thin wall; workholding problems tend to produce a regular oval; tool wear worsens as the number of machined pieces increases.

On surface roughness, very low values should not be pursued on molybdenum parts: they require smaller feeds and higher speeds, which increases the risk of edge chipping. The machined surface should be free of burrs and embedded particles, and edges should be chamfered; the inspection criteria are given in the dimensional inspection specification. Aligning the datums for machining, inspection and assembly removes many dimensional problems.