Vertical holes - Test 2
I've changed the set up, now using a thick piece of wood whose top and bottom are probably not parallel. 

I've then used long metal shafts so that I can measure the variation over a reasonable height.

However as the shafts were not glued in place, there was some flexing at their top ends.

I did not have a Bad drill of the correct size, which may have made a difference to the results.

I measure the distance between the shafts at their base and at the brass collars at the top.

 

Results

               
wood 37.6mm deep shaft tops at 117.6mm (approx 4.75") above wood          
shaft diameter 6.26-6.34 depending on pressure on micrometer        
so measurements to nearest 0.05 as slight flexing of shafts at the top depends on pressure      
Measure 1     Measure 2     Measure 1-2  
top bottom top-bot   top bottom top-bot   top bot top-bot
S12 40.8 41.55 -0.75   40.9 41.55 -0.65   -0.1 0 -0.1
S23 40.85 41.7 -0.85   40.9 41.7 -0.8   -0.05 0 -0.05
S34 42.5 42.8 -0.3   42.6 42.85 -0.25   -0.1 -0.05 -0.05
S41 41.15 41.85 -0.7   41.2 41.85 -0.65   -0.05 0 -0.05
Spread     0.55       0.55      

0.05
                     
Reverse measurements with shafts pointing out the bottom of the wood          
Measure 1     Measure 2     Measure 1-2  
top bottom top-bot   top bottom top-bot   top bot  
S12 41.7 41.65 0.05   42.05 41.7 0.35   -0.35 -0.05 -0.3
S23 42.65 41.95 0.7   42.6 41.95 0.65   0.05 0 0.05
S34 42.75 42.85 -0.1   42.7 42.85 -0.15   0.05 0 0.05
S41 42.55 42.05 0.5   42.6 42.05 0.55   -0.05 0 -0.05
Spread     0.8       0.8      

0.35
I feel the bottom measurements are accurate as there is no flexing - so the difference from both sides are  
  bot difference     bot difference   Bot diff 1-2  
S12   -0.1       -0.15     0.05    
S23   -0.25       -0.25     0    
S34   -0.05       0     -0.05    
S41   -0.2       -0.2     0    

I think this indicates that there is a wobble over 117mm of around 0.5mm - 0.4% or 0.25mm over 37.6mm giving - 0.6%.

This indicates that I haven't a hope in hell of drilling perfectly vertical holes and as the spacing of arbors is critical, having them adjustable is a very good idea.  I read on some wooden clock page, dealing with gears, that 15thou can make all the difference - and that is less than my wobble!

 

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