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jaw design-sub spindle handoff accuracy

Stirling

Hot Rolled
Joined
Dec 11, 2013
Location
Alberta canada
taking my first crack at a job with a sub/Y axis lathe and have a few questions, most notably to handoff accuracy and jaw design. honestly im likely overcomplicating this job, but I wanna play with the new toy :/ :p

oiginally i would use a 2x lathe to turn the shaft in one full length then part off. (shaft 9.5" long 1"-2" dia 4140 pre harde stress relived, 400 units)
with a sub if i did each side i could save 10% material use not haveing a wasted slug to grip on. Its a slow rpm shaft and 0.002" runnout is acceptable, but i prefer to keep things as tight as possible targeting 0.0005" max shaft runnout when installed.

With a sub spindle lathe (nak as200l) i was thinking of turning the long front section with a center in the sub spindle, (turret install and pickoff for a support center in the sub) swapping to the sub and gripping along the miltiple diameters availible (2.5, 1.1, 1.0) for a deep grip on the shaft. but maybe it would be better/accurate to simply grip 0.5" deep on the 2.5" diameter (what will later be turned eccentric)

Is it better to grip along multpiple diameters or does this introduce potential of error? better to simply grip on one dia? (this question goes both for the shaft and for a removable sub spindle center support in the jaws
 

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Is it better to grip along multpiple diameters or does this introduce potential of error?
In a perfect world the more you can grip on the better it is. But that's just not reliable. Unless the diameters AND your jaws are all held identical, every time.
So realistically, gripping on 1 diameter is the way to go.
 
In a perfect world the more you can grip on the better it is. But that's just not reliable. Unless the diameters AND your jaws are all held identical, every time.
So realistically, gripping on 1 diameter is the way to go.
what level of accuracy would you anticipate having a handoff on a shaft like this? (gripping on the 2.5" mid section 0.875" can be griped for length on both ends)
lathe is near new

ill have to sping off a few samples and try i suppose. i can save around $1000 i material each time this job comes up by buying shorter sotck, but man would suck to find out its not very concentric!
 








 
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