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     After the commutator has been satisfactory resurfaced, the mica insulation separating the copper segments must be undercut.  Undercutting is one operation that is most easily accomplished with the armature removed from the machine.  Various tools are available, however, that enable undercutting to be preformed on a commutator "in place" without undue hardship.
     Of the various undercutting practices used, only the two most common methods will be discussed herein.|
There are three basic types of slots that can be produced by the use of circular cutters.  The U-slot, the V-slot and the Compound-angle slot.


     The U-slot (as shown in Fig. 1)is generally preferred if the slots are accessible for easy cleaning.  These slots have the advantage, if done carefully, of being effective until the commutator has worn down the full depth of the undercut.  The slot should be cut to a depth of 1/32" (0.032"), or not more then 3/64" (0.046").  If cut too deep, accumulated dust will not be thrown out by the centrifugal action of the rotating commutator.

     When using a circular cutter, the width of the cutter is chosen to exceed slightly the thickness of the mica.  It is recommended the the SAW THICKNESS be figured on the basis of the mica thickness plus 0.003" (0.08mm).  This will allow the saw to remove the full width of the mica plus 0.0015" (0.04mm) of copper on each side of the mica slot.  If unable to determine the mica width, the use of a feeler gauge can best determine the required saw thickness.  Consequently, some copper is cut or dragged off the bar during undercutting, (as shown in Fig. 2). 

     In addition to leaving a jagged edge projecting from the commutator bar, the edge of the bar becomes somewhat work-hardened and hence will not wear down uniformly.  Therefore the edges of the bars must be chamfered by using a suitable slotting file or a specialty shaped scrapper.  **See Martindale Slotting Scrappers
     A chamfered face of approximately 1/64 inch is usually adequate to remove any roughness or edge hardening that could be disturbing to the brush faces.


     V-slots keep slots free from dust accumulations at low speeds, and do not require a separate operation for chamfering of the bar edges.  V-slots are usually made with either a slotting file, or a "V" tooth circular cutter.
     Usual practice is to use a circular cutter having an included angle between cutting edged such that a cut made 1/16 inch deep will also leave 1/32 inch free copper above the mica.  The "V" tooth circular cutter is available with 40░, 50
░, or 60░ angle between the cutting edges.

     To obtain a 1/16 inch deep cut with a 1/32 inch free copper above the mica, the following table may be used:
                                 Thickness of Mica        Angle of "V"-Cutter
                                           0.023 inch                                40 degrees
                                           0.029 inch                                50 degrees
                                           0.036 inch                                60 degrees
     The necessity of accurately centering the circular cutter on the mica is readily apparent.  Mica fins in V-slots being wedge-shaped, are more difficult to remove than the fins of uniform thickness left at the sides of the U-slots by inaccurate centering of the circular cutter.


     The teeth on the compound land mica saw are alternately ground to a special taper which reduces the impact on each individual tooth and produces chips of just slightly over half the width of the mica slot thereby eliminating the tendency to clog.  When undercutting with a compound land saw the bottom of the slot will appear to be flat.  However, as a result of the reverse taper on alternate teeth, the slot will have a slight pyramid or convex surface.  this type of a saw operates cooler and cleans better thereby prolonging the saw life with resulting economy to the user.

     After a commutator has been undercut, it should be very carefully inspected to assure that all copper particles have been removed, that the bars have been carefully chamfered, and that all sharp edges and burs have been eliminated.  Then each slot should be individually checked and reworked as necessary to remove any traces of fin or side  mica.

     Finally, the surface should be lightly polished with a fine-grain commutator stone.  **A more popular method is the use of rubber bond cleaning stone, which will properly finish the surface and leave the proper filming required.

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