Another obscure FP3NC assembly question - Y axis chain

The FP3NC (and 4) has a chain to drive the Y slide for adjustment. When the slide is forward this leaves an opening for chips to pack into, if your housekeeping is not good - and the housekeeping of the previous owner of this machine was decidedly not good. I took it apart to clean it and discovered that the drive sprocket is missing one tooth, the idler sprocket is missing two teeth. At first I put this to machine abuse - Lord knows I have seen enough of it. But the teeth are neatly machined off. It became apparent that a missing tooth is necessary so that when the sprocket rolls over the dog that fixes the chain to the Y axis the tooth and dog do not crash. When assembled, you must make sure that the sprocket is phased so that the missing tooth will be clocked correctly when the dog is encountered.

Now the question: why does the idler have two teeth missing? They are not adjacent, but have one tooth between the missing locations. At first I thought maybe it was necessary if the slide was removed to the rear, as there is another chain dog there that the idler would also have to go over. But the dogs are 48 links apart and the sprockets are 12 teeth, so one missing tooth would be all that is necessary to miss both.

Only thing I can think of is that the FP4NC may be different, and uses the same par number. Anybody know the real answer?

Been long time since i was in there but i recall there being one missing tooth on the rear sprocket and two on the front.
The front sprocket axle is made eccentric to adjust the chain tension via the allen in the axle center.
My stupid guess is that the two teeth missing on the front is a timing aide when mounting the chain...if you run the ran fully forward all the way to the stop pin the rear sprocket will be engaged with the cutout over the peg on the "Y" base casting. If the front sprocket is timed correctly on the chain both cutout teeth will be visible pointing to the rear, equal in spacing to the chain above and below.....
Pretty thin but there is no mechanical reason you need both teeth cutout on the front.


Remember that when clamping the ram to the casting below it that only the very front clamp bolt on the "Y" and the rear clamp bolt on the ram are in play.....The center clamp bolt is left completely loose. The only time the middle bolt is used is when mounting accessory heads like the slotting head or high speed head.....
Cheers Ross