The lowest standard feed/rev for a BH600, (and for the BH900, Chester Craftsman etc presumably) is 0.0047inch.
On mine, I reduced this by replacing the 40T input gear to the Norton box, with a 80T one.
(The cutter used to cut the gear was a No.7, Mod 1.25, 20PA , and a Peter Robinson Slotting Attachment to cut the keyway. Casting from College Engineering Supply, other material from stock, and now on a raising block to suit the BH600)
As you might guess, the new gear would not fit because of the stud used to close the gear cover, so the the gear cover closure needs to be repositioned. Removing the securing "nut" from the Gear Cover resulted in the destruction of the Bakelite/Composite knob. Fortunately a suitable replacement was available, otherwise a knurled aluminium item would have been the end result.
The counterpart to this, entailed the stud being removed and repositioned in the rear of the Headstock, after the gear cover had been redrilled in a position that cleared the new gear.
Moving the securing "nut" from the gear cover resulted in the Bakelite/Composite knob being destroyed. Fortunately a replacement was available, or the end result would have been a knurled aluminium one!
When all this was completed, I calculated that with the 120/127T compound gear suitably arranged, the finest feed obtainable was now 0.0022"/rev.. Using a tipped tool with a 0.005" cut, at minimum speed, lubricated with neat cutting oil, an extremely fine surface finish can be obtained when facing Mild Steel.
A 1:1 tooth belt drive off the 120/127T compound gear should produce a feed rate of 0.0016"/rev., or 0.0015"/rev with a gear train of 40/127/120, and the 1:1 belt drive. AS long as it all fits inside the gear cover!
As has already been said, an even finer feed could be obtained by using extra gears, (and a spindle), to compound the gear train, (or an epicyclic cluster?) but I am lazy and looked for what seemed to be the easy way at the time!
Howard