THE SCIENTIFIC PAPERS OF 



of that plate it would be found that this piece was not as thick as 

 the plate, but on the contrary considerably thinner. What then 

 had become of this metal ? It had not gone to increase the 

 diameter of the burr, although its lower part might be a little 

 bulged out in that way ; but while the punch was being driven 

 into it that metal had accumulated somewhere in the plate. 

 There was thus a ring of highly compressed material formed 

 round the hole, and this compression caused the surrounding 

 portion of the plate to be under high tension. The result must 

 be that, if pressure was applied, the portion already under high 

 tension received additional tension, and tearing action began. 

 Hence the great loss of strength in punching. He held in his 

 hands the results of some experiments, made with great care by 

 Mr. James Riley at the Landore works, which proved precisely 

 what he had stated. There however plates, which before punch- 

 ing bore 29'33 tons per sq. in., bore after punching 30*7 tons per 

 sq. in. without annealing. The reason of this anomaly was, the* 

 die in these cases was made considerably larger than the punch, 

 and thereby the compression was practically avoided.* Another 

 kind of punch had lately been introduced from America, which 

 was well worth the attention of the meeting. It was a helical 

 punch, which, instead of driving out the metal before it, cut it out 

 in a spiral form. This mode of punching he thought very pro- 

 mising, and he believed by adopting such a method the weakening 

 effect that had been experienced in punching thick plates would 

 be entirely got over, without unduly increasing the size of the die. 

 In punching thin plates the author found there was but very 

 slight reduction of strength, and that the punched plate when 

 annealed resumed its former strength exactly. It had also been 

 found that by riming out a punched hole the strength of the metal 

 was entirely restored : showing that the cause of weakness was in 



* Mr. J. GL Mair has since communicated to the Secretary the results of some 

 experiments made on this point by Mr. Henry Sharp, of Bolton. Four pieces of 

 \ in. steel plate, 2^ in. wide, were punched with an \ in. punch, but with a die 

 g in. in two cases and f in. bare in the other two. The results were as under, 

 showing 25 per cent, more strength for the greater clearance : 



I in. die. f in. bare die. 



1st experiment . . . 32 '30 25 '92 



2nd experiment . . . 32 '85 26 '08 



32 '5 tons per sq. in. 26 '0 tons per. sq. in. 



