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Use a center or not?

 

I¡¯m making a kind of reference rod on my U3, with the aim of making it as concentric to the spindle as possible.


So (following the procedure in Doug Collinge¡¯s ¡®Make an ER-16 Collet Holder for your Unimat¡¯, in the Files section) I have bored and threaded one end of some brass rod to M14, bored clearance for the register, and used a parting tool to true the end where it nips up against the shoulder of the spindle. It fits the spindle well. So far so good.

Now I am going to turn a 30mm length at the other end down to 10mm (the piece is 56mm long). My question is - should I turn it ¡®as is¡¯ without any additional support (relying on the robustness of the fit on the spindle to hold the piece ¡®true¡¯) or center drill it and use a center on the tailstock (at risk of introducing inaccuracy from the item - collet or chuck - holding the center drill)?


Or maybe the countersink created by the center drill will always be truly concentric, even if the center drill is slightly off center? The brass rod won¡¯t bend (I guess)? ?I just don¡¯t know, but don¡¯t want to spoil the work I have done!


I haven¡¯t had so much fun for years - what a wonderful little machine!
 

For the BEST accuracy and concentricity, you're supposed to turn between centers on a lathe. If you want the ultimate possible concentricity, you grind the centers on the lathe immediately before the job, according to all the old books on lathe work.?

Funny thing, I took a class on machining a few years ago, and they did not even teach turning between centers. Of course, my class was the prerequisite for the CNC Machining class, where they don't need such things! ;) I'd also taken a course in high school, 50 years ago, and they did teach machining between centers there. My, how times change!

Bill in OKC

William R. Meyers, MSgt, USAF(Ret.)

Aphorisms to live by:
Good judgement comes from experience. Experience comes from bad judgement.?
SEMPER GUMBY!
Improvise, Adapt, and Overcome.
Physics doesn't care about your schedule.
The only reason I know anything is because I've done it wrong enough times to START to know better.
Expect in one hand, expectorate in the other. See which one gets full first.



On Monday, July 17, 2023 at 01:08:02 PM CDT, Peter Brooks <peter@...> wrote:


I¡¯m making a kind of reference rod on my U3, with the aim of making it as concentric to the spindle as possible.


So (following the procedure in Doug Collinge¡¯s ¡®Make an ER-16 Collet Holder for your Unimat¡¯, in the Files section) I have bored and threaded one end of some brass rod to M14, bored clearance for the register, and used a parting tool to true the end where it nips up against the shoulder of the spindle. It fits the spindle well. So far so good.

Now I am going to turn a 30mm length at the other end down to 10mm (the piece is 56mm long). My question is - should I turn it ¡®as is¡¯ without any additional support (relying on the robustness of the fit on the spindle to hold the piece ¡®true¡¯) or center drill it and use a center on the tailstock (at risk of introducing inaccuracy from the item - collet or chuck - holding the center drill)?


Or maybe the countersink created by the center drill will always be truly concentric, even if the center drill is slightly off center? The brass rod won¡¯t bend (I guess)? ?I just don¡¯t know, but don¡¯t want to spoil the work I have done!


I haven¡¯t had so much fun for years - what a wonderful little machine!
 

Bill, I just realised I hadn't thanked you for the reply - thanks for the advice!

I did use a center drill and then a live center, and all worked out well.

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