ARMATURE LAMINATIONS 35 



there will be an undesirably high percentage of space lost length- 

 wise between the core flanges. 



Formerly it was thought desirable to anneal the plates after 

 punching, and this is doubtless the ideal method ; but it involves 

 considerable expense, and is nowadays rarely regarded as necessary 

 in the case of armature core plates, and the sheets are generally 

 purchased already annealed. 1 The annealing, as has already been 

 stated, should be from as high a temperature as practicable with- 

 out causing the plates to stick together. 



In the operations of punching out the discs and the slots from 

 the plates as delivered, the surface should be moistened in advance 

 of the punch (in the case of index die-work) by applying turpen- 

 tine with a brush. In the case of a compound die, the turpentine 

 must be applied over the portions to be punched out. Turpentine 

 has great advantages over oil and over soap-and-water, since no 

 subsequent cleaning of the plates is required, as would be necessary 

 were oil or soap-and-water employed. In former times, when 

 these latter materials were used, the plates were subsequently 

 cleaned by immersion in a hot bath of dilute caustic soda. Of 

 course, when the plates are annealed after punching, they are 

 cleaned in the process. Some firms coat the sheets, prior to 

 punching, with thin paper, to insulate them from one another ; 

 but it is generally conceded that the application, after punching, 

 of coatings of insulating varnish, is fully as satisfactory a method 

 and involves less expense. In some cases of paper-insulated arma- 

 ture cores which were taken apart after a few years of service, 

 the paper was found in an advanced stage of disintegration, 

 and the core discs had become oxidised under the influence of 

 exposure to the paste with which the paper had been applied. 

 The crumbling of the paper would appear to be due to the com- 

 bined influences of vibration, heat, and pressure. 



Let us now return to the consideration of the 1 metre x 2 metres 

 steel sheet (Fig. 16), as originally received. Unless it is to be used 

 for discs of nearly 1 metre diameter, or for segments nearly 1 metre 

 wide, it will often be found convenient to first cut it up by shears 

 of some such type as the tool illustrated in Fig. 22, into smaller 

 sheets of suitable dimensions. Whether these sheets shall next be 

 punched at one operation by a compound die, or whether this shall 

 be done in several steps, is a question involving the disadvantage 

 of the very expensive compound die on the one hand, and the 



1 Plates for transformer cores are generally annealed again after punching, 

 owing to the greater importance of minimum core loss. 



