Aperture Drops Blank Sizes

As the glass drops through the aperture, it stretches, but the whole substance of the piece is drawn toward the hole. If there is not enough spare glass around the hole, the whole piece will be drawn through the aperture.

There is a minimum size of the glass in relation to the size of the drop out hole. Up to some maximum size, the greater the diameter of the hole the 

greater the amount of spare glass there needs to be.

Also relevant is the depth of the drop. A shallow drop needs only a few centimetres larger than the hole. While a deeper drop needs a greater amount of glass surrounding the hole.

I have found that for a 300mm diameter hole, with a 150mm drop the glass needs to be 35mm larger all around. Thus an aperture of 300mm needs to be at least of 370mm diameter for this 150mm drop. I have done drops with 550mm diameters with only a 650mm diameter blank. This indicates to me that there is an amount of spare glass that will be sufficient even for larger diameter drops, but I have not found it yet. For a large drop with an aperture of 500mm and a depth of 350mm, I used a 100mm margin.  This gave a disc of 700mm.  It successfully dropped with the rim moving only about 20mm.

One element that can reduce the size of the blank is to make an inclined collar around the aperture of the drop mould.  

This idea is based on the observation that as the glass begins to fall through the aperture, the outer edges of the glass rise from the mould surface so the glass is resting only on the inner edge of the drop out mould. 

This inclined drop out mould will be like a shallow bowl rim, but without a bottom. The glass blank then rests with only its outer edge on the collar.  When the temperature increases to the point that the glass begins to slump, the glass will conform to the slope and so create enough friction to restrict the glass from falling through the aperture, although it is with a smaller than normal rim.  The actual size of the rim for each size and depth will need to be determined by experience. 

More information can be found here