The term 'Fabrication' applies to operations, such as: cuttings, stamping and punching of natural sheet mica to specified size, shape, thickness and design within close dimentional tolerance for electrical and electronic end-users.
Natural Mica being available only in flat sheets, fabrication of mica are performed on a production basis of using foot or power punch presses by the help of a compound die similar to those used in metal stampings. Where the quantities are not large enough to permit the cost of a die, mica may also be hand-sheared, drilled or lathe turned on any of the common tools used for machining metals. In hand-shearing the general procedure is to cut all the four sides of the sheet mica using a master block as the gauge. Minimum tolerance possible in hand shearing is 1/64 of an inch. In drilling or lathe turning the main trick is to place the mica under maximum possible compression and hold it firmly during the operation. A template is used for accurate dimentional measurement. Machine speeds should be similar to those used for machining brass.
Normally, mica is fabricated with compound dies by foot or power punch process, especially if the finished piece contained perforation of any kind. A compound die punches the outside shape and all the inside holes with one stroke of the press most accurately. The compound die necessary for stamping natural sheet mica may be grouped in two general categories. One category comprises the highly complex dies required to produce Mica Spacers containing a number of perforations in various sizes and shapes for use in electronic tubes. The extremely critical dimensions and spacing of the perforations can be obtained only with fully designed and accurately constructed dies. The other category of the compound dies is used in the production of not very complicated mica parts, such as: Condensor Plates, Transistor Mounting Washers, Backing Plates, Discs, Washers, Joints or Shields, Stove Windows etc., of a simple dimentional shape with or without a few perforations. These dies require less complexity and precision as compared to those demanded in the fabrication of spacers.
Each mica patterns requires a special precision die to produce clean edged punch parts free from burrs, delaminations and cracks, etc. Even for the simplest pattern the die must be made of special steel (HSS) suitably hardened (in oil) and tempered, with expert workmanship and precision equipment. Jig boring machine, high precision grinders, lathe machine, optical measuring equipments, tool makers, such as: remers hand filling equipments, intricate contour milling hands, master templates, die makers, etc. are some of the important equipments required for hand filing and finishing of a mica die. The more intricate phase of building mica dies centres around the cutters, shaving and piercing punches and female die. It is a known fact that making an accurate mica die is a most difficult task as they are absolutely different than metal sheet, or any other known product.
A different die is required not only for each change in the pattern, but also for each change in thickness, even for simple shapes for precision work. The mica is fed to the press by hand. Automatic feeding devices have not been practicle because of irregular shape of the pieces of mica. Mica pieces whether in the form of block, thins, or films, are usually caliberated to close range of thickness for precision punchings, as it is an important variable in the process. An uniformly Thick Mica fed to a die improves the cutting quality of the die and produce parts having contour and holes. Mica films for capacitors are usually fabricated within a very close range of thickness tolerances.
Natural Mica Washers and Discs are punched from the smallest to 9 inch outside diameter, both in random and calibrated thickness. The sharpness of the die is well maintained to give punch parts with clean cut edges. Bonded Mica Washers are made by placing fully punched Natural Mica Washers on a mandrel, immersing them in a shellac or silicon binder and curing the complete assembly to form a solid tube of washers and finally split them to specified thickness, but close tolerance are difficult to obtain.
Production of dies suitable for Fabricating Mica Parts accurately is highly specialised operations. Although design problems usually vary with each pattern, all designs are governed by a few fundamental rules. Inherent limitation of mica imply the following consideration in die-designing which has a direct bearing on the cost of the fabricated part:
a) Mica is hard and to minimise possibilities of breaking the steel punches, maximum thickness should not exceed 50 per cent of the diameter of the smallest hole.
b) Holes should not be placed too close to outside edge, as otherwise the material will break down through cracking and delamination. The distance depends upon the thickness of mica and the size of the hole.
The minimum distance between the hole and the edge or between two holes should be 80 to 100 mils for condenser plates for capacitors and not less than 20 to 25 mils for other parts.
c) For clean cutting of mica, 0.0002 to 0.0003 inch side clearance is held between the piercing punches and the openings.
d) Mica is highly abrasive and when worked it gives off dust particles injurious to the die. Air jets are, therefore, built into the die-set to disperse the dust particles.
e) Lubrication in any form is not permitted in mica dies, because mica must be kept free from contamination. Lubrication is, however, used on die posts because they have been adequately protected with accordian boots.