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Lathe Machining of Bronze Coin Flans
 
Continued (p.7)

 Machining the Flan Faces 

For machining the flan faces it would be necessary to use a tool that would not attempt to cut the entire radius of the flan at once, in order to keep the forces and torque involved under control. Those who have experience in turning and facing metal parts on a lathe know that the tools used for this purpose always employ a relatively sharp point which continuously shears or skives a small amount of metal (the chip) from the workpiece. This tool is gradually fed across the area to be machined so that the entire surface is turned or faced, but at any given moment only a small part of it is being cut. A tool of this type cutting a chip during a facing process is shown in Fig. 16


Fig. 16
Facing tool

When the area of metal being cut is kept small, the very high pressures required to shear the metal can be provided without transmitting large forces to the cutting tool or to the workpiece. This is the principle on which all modern metal lathes work, and exactly the same problems would have been encountered in cutting metal in antiquity. The ancients clearly understood this approach, because they engraved metal with hand held tools designed on this principle. 

In considering whether flans were turned on a lathe, numismatists have been led astray by assuming that flans had to be turned between centers – i.e. with centering pins pressed into the dimple on each side defining an axis of rotation. This clearly could not have been the approach used because the dimples on obverse and reverse are almost never aligned. Moreover, there would be no reasonable way to apply torque to the flan to drive it while it was being cut. Parts that are faced in modern lathes are never turned between centers, but are held in chucks or clamped against the faceplate. Evidently, the process used to face ancient flans must have acted upon one surface at a time. It is logical to assume that the other side was held against a flat surface during the facing process and that the pressure applied prevented it from slipping while it was being cut. 

The Centering Pin 

The reason for these dimples becomes clearer if we postulate that a pin ending in a conical point was forcefully impressed into the flan while it was being cut. The resulting dimple would have prevented the part from sliding sideways under the cutting pressure, while the force impressing the pin into the flan held it against a flat surface and prevented it from spinning relative to that surface, as shown in Figure 17.


Fig. 17
Centering pin

The Rotating Platen 

Rotating the workpiece while the tool was held fixed (except for the advance required to feed it across from the edge to the center of the flan) was a method the ancients were quite familiar with. Pottery had been made for millenia on a rotating potter’s wheel, and stone objects such as column drums and large vases were shaped on rotating turntables. It is therefore quite reasonable to extend this well-known method to visualize a rotating turntable or platen against which the flan was held, with the centering pin aligned with the axis of rotation. Such an arrangement is shown in Figure 18, where the flan F is held against a rotating platen PL by the centering pin R, while the tool T that is cutting chip C is swung across the flan by a lever.  


Fig. 18
Flan being faced on a rotating platen 

This lever (there are other ways to move the tool across the work, but this is the simplest) pivots about an axis to move the tool. It also has to be capable of moving in the direction of the axis of rotation, i.e. vertically in the figures, to adjust the depth of cut.

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