I’ve measured all the spacers I have for US Monotype Display mould blade kits. There are two styles (long and short) which correspond to the style of blade front (thin or thick respectively). This drawing shows both superimposed:
There seemed to be quite a bit of variation between spacers, so I chose to set many of the dimensions to round fractions of an inch, to place the hole exactly on the vertical midpoint, and to have 1/16″ radius on all corners. I couldn’t quite justify making the height ⅜″ though.
As to the thickness of the spacers, I measured each one by comparing with standard gauge blocks and charted actual thicknesses against nominal pointsize. Once the gunk of ages was cleaned off, the spacers appeared to have flat and parallel faces to better than a ten-thousandth of an inch. I calculated a linear fit between the nominal point sizes and measured thicknesses, except two of the 14-point spacers that seemed to be at least a thousandth of an inch too thin which were not includes in the fit. The formula I came up with was:
Thickness (inches) = 0.0138887 × Pointsize + 0.0001602
The formula results matched the actual thicknesses of the spacers within three ten-thousandths of an inch. These constants can be justified: 0.0138885 is 0.4% larger than the actual size of a point, and this gives an allowance for shrinkage of the type as it cools from solidification temperature to room temperature. The 0.0001602 provides compensation for the thickness of the oil film lining the mould cavity (which does not vary with point size). That’s my story and I’m sticking to it.
All the blades appear to be about half a thousandth of an inch narrower than the corresponding spacer thicknesses. This allows enough clearance for the blade to move smoothly in the assembled mould.
My partial 18-point kit has a thin-front blade so I will be making a long spacer for it, and the thickness according to the formula should be 0.25015″ but I’ll probably make it a shade thicker and try it out. The size of the cast type is after all the true proof of the spacer thickness. I have never made a part to that accuracy before, and I don’t have a good feel for how much metal can be removed by various lapping and polishing steps.