mechanical clearances: good or bad?

To investigate why the lock on the fixed long pinion was so important in making number transfers reliable, I slowed down and single-stepped the copy operation, and watched carefully. What I discovered was that in "giving off" the first non-zero digit, the source digit wheel was not turning a full digit position -- it was only moving about 80% of the way. 

Why? Because there was too much free play between the finger on the axis and the nib on the digit wheel. It turns out to have been intentional, because the design provided clearance to allow for the elevation of the finger to the level of the nib without any possibility of interference. The digits are spaced on the wheel by 18^o. The finger width was 8^o and the nib width 8^o, for a total of 16^o, which provided 2^o of clearance.

In this case clearance is a bad thing because it produces a dead zone of movement. I changed it to instead have negative clearance of 0.5^o by increasing the finger width to 10.5^o, and using the Piers-Plummer-style straight angled chamfer on the finger to deal with any slight misalignment. 

Now copying digits works perfectly even without the fixed long pinion locks in action! But of course I'll normally use them anyway, just in case.

The moral is: think carefully about clearances. Sometime they should be positive, sometimes zero, and sometimes negative, depending on the motion required.

Another case where negative clearance will probably be needed is in the chain of fixed and movable "wires" in the anticipating carriage mechanism. In that case there must be no accumulation of clearances that would produce a gap in the chain, even if it goes through all 30 or 40 digits of the sum.