 Nominal section feed

Nominal section feed is the intended advance of specimen relative to knife between successive cutting strokes of a microtome. It specifies a machine setting or commanded increment. It is distinct from the recovered physical section thickness, which can change through cutting and handling, and from the separation of sections selected for an image stack.

JEOL describes presenting the block to a diamond knife by a preset advance and then making the cutting stroke. It also notes that material hardness and environmental temperature can cause recovered sections to differ from the preset thickness.^[ref] Consequently the feed record is useful acquisition metadata but does not independently verify thickness.

An original acquisition ledger illustrates a changing feed. Let the ideal block face before cutting be at $z_0=0$, and stipulate successive feed commands of $60,60,100,60\mathrm{nm}$. If motion follows each command exactly, successive cutting planes lie atIf images represent the newly exposed faces after cuts 1 and 4, their separation is $f_2+f_3+f_4=220\mathrm{nm}$, not three times the initial $60\mathrm{nm}$ setting. This statement concerns block-face positions. For images assigned to centres of collected slabs, the coordinate convention must also account for their thicknesses: in this ideal example, centres of slabs 1 and 4 are $30$ and $250\mathrm{nm}$, a separation of $220\mathrm{nm}$, while centres of slabs 2 and 3 are $90$ and $170\mathrm{nm}$, separated by $80\mathrm{nm}$ rather than their face spacing of $100\mathrm{nm}$.

Retain feed changes, cutting order, omitted images and the coordinate convention used by the image stack axis calibration. Independently measuring specimen advance tests the feed mechanism; measuring recovered physical section thickness tests a different outcome. Backlash or thermal expansion can affect the first, while section compression and other deformation can affect the second. The ideal ledger assumes neither kind of discrepancy.