Wednesday, October 23, 2019

New Shop Tool - Old, Obscure Dividing Head

I've needed to create a gear to finish the hand lever collet closer attachment.  This meant I needed a good dividing head (well, I had the small rotary table, and could have done just fine with that one, because a 40 tooth gear is perfectly divisible on a rotary).  I ordered an old dividing head off of eBay. It's time to get it identified.





It appears to be labelled as a "Republic Tools", and the "1161" prefix stamped into the spindle was also stamped into the bottom of the casting (matching numbers... that's a good thing).  It rotates very smoothly, so it's been well cared for.

Typically, a dividing head is a 40:1 ratio, meaning you turn the handle 40 times and the spindle turns once.  This one looks different.  It felt like a 48:1 with the brief check I ran.  I will have to verify this, but it felt less like a 40:1.

The spindle through hole is about 1" (0.923" in the back, probably just an awkward angle and not accurate in any form of the word, and about 1.050" in the front).  There is no real taper on the bore of the spindle, so it's not 3C compatible.  It is definitely not 5C, or a morse taper.   That is going to force me to use the threads for any indexing purposes.




The spindle thread is 1 3/4-8 TPI, so it's an older thread that I might have to build an adapter or a backplate.  I'd like an adapter do I could potentially move from the lathe to this without removing the workpiece from the chuck itself, then things are more likely to be concentric if I need to change machinery between turns.



My brain went immediately to trying the infamous 127 tooth gear so I could potentially cut metric threads on the lathe, and that lead me to check the diameter of the hole plates :



At 4.875" on the outside diameter, if I ran a hole pattern around a 4.5" diameter circle (14.137155" of travel along that diameter) and divided that 127 times, we'd have 0.11131618" between holes along that path.  Seeing as that hole pattern would not have enough material between holes (let's call that "meat"), it could not be a 127 straight-line sequence.  I would have to offset the holes to get a good pattern into at least two rows, but considering 127 is a prime number, it should be three rows (or you end up with two holes on the same path right at the end).  Is that doable?  Absolutely.  I might just have a shop put the holes in the plate for me, though the accuracy isn't as critical for them because I'd be using that to bootstrap another 127 hole plate.  If another shop did it for me via CNC, I'd not need to bootstrap it - it would be accurate enough.  If I do it myself, I would use the index plate to make a second index plate (causing it to be much better in accuracy on the second one at 48 times, if the ratio is 48), and then I'd use the second index plate to create a third index plate.  This would improve the accuracy 48*48, by 2304 times.

This looks like it is going to be a fun addition to the shop!

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