Patent GB-812367-A (GB)
Title: Improvements in or relating to device and method for monitoring chemical etching processes
IPC: C23F1/02
Filed: 1955-08-24
Abstract: 812,367. Pickling metals. NORTH AMERICAN AVIATION Inc. Aug. 24, 1955 [Sept. 2, 1954], No. 24342/55. Class 82(2) A device for monitoring the attainment of a desired depth of chemical etch comprises a sample piece or "fuse link" (6, 23) adapted to be exposed to the same etching action as a workpiece 2 the link having a critical dimension 62 (Fig. 8) dependent upon the depth of etch desired, and detecting means responsive to a discontinuity in said link caused by the etching action at said critical dimension to determine when the said desired depth of etch has been reached. The detecting means may be electrical, mechanical or pneumatic. Schematic Fig. 1 shows the link 6 in the same etching bath as the workpiece 2, the link and workpiece being normally of the same material and the critical dimension 62 equalling the desired depth of etch. An etch-proof film 3, (59 in Fig. 8) may be supplied to parts of the workpiece and/or the fuse link. An electrical current passes through the link 6 via leads 7, 8, whilst a detecting unit 9 senses the discontinuity when such occurs and conveys a signal to a device 10 which gives a visual or audible warning, and/or to means 12 which actuates equipment 5 to remove the workpiece from the bath. Fig. 2 shows typical electrical means for detecting and signalling the attainment of the desired depth of etch, in which the "fuse link" 23 is shunted across the low voltage coil of a step-up transformer 25 and is in series with the low voltage side of the step-down transformer 17. When the discontinuity in 23 occurs its shunt action is reduced or ceases, with consequent rise in voltage supplied to transformer 25 the output of which actuates sensing relay 28 to pull down switch blade 29 and close the circuit between transformer 17 and power relay 35. Energization of 35 thus closes the ganged switch 30 closing contacts 32, 33, 34 to actuate audio signal 37 and light signal 36, and also power means at 38 to operate handling equipment such as 5, 12 (Fig. 1). The circuit of switch 30 is of the holding type which energizes relay 35 until the feed circuit switch 15 is open. Lamp 22 indicates activation of the circuit and goes out when the fuse link is severed. In Fig. 3 (not shown) is illustrated a multiple channel circuit developed from that of Fig. 2. In Fig. 6 the "fuse link" comprises the thickness of 57a of the article 52 itself and an etch proof circle 54 of diameter twice the desired depth of etch the backing 53 being also etch proof. This link is connected at 55, 57 into a circuit exactly as the link 23 is inserted in Fig. 2. Fig. 9 represents a mechanically actuated device, in which a discontinuity in link 23 results in the upward movement of a flag 66 due to the counter-weight 67. The lever 65 may bridge contacts 33a to actuate bell 37. Alternatively the fuse link may be horizontal (Fig. 11, not shown) and have a flag rod resting thereon; or it may close an air tube 88 (Fig. 10) which is pressurized by means 92, so that severance of the link releases the pressure to actuate a valve 90 leading to a visual or audiosignal operating line 91, or moves a diaphragm 94 to actuate a visual float 93.