Inherent Precision Of Mounting A Work Piece

grepper

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Another noobie question!

With a HF 7x10 lathe, if I machine something, remove it from the chuck and then remount it, how accurate is the centering? With these inexpensive lathes and chucks, can I expect accurate repeatability in mounting/dismounting/mounting a work piece? Anyone ever do that a bunch of times and measure it?
 
If you have a previously machined surface you can indicate off that to see how much run out has ocurred.

Generaly if you need concentricity from end to end then flipping a part in a three jaw chuck isn't going to do that.

If the concentricity isn't critical for the part it's fine though.

The limiting factor as you say will be the repeatability and acuracy of the self centering chuck your using.

(I didn't mention 4 jaw independant chucks as thats one of their purpouses and they don't as such locate a part for you)

Stuart
 
Grepper, that is a great question you have asked. You can see from the responses above that there are things you can do to get pretty good positioning. Also, sometimes it does not really matter - one aspect of machining is to know when you need something to be really good and when to say good enough (whether it is a finished part, or some aspect of your set up) - the official term is "tolerances".

There is no such thing as perfect. Depending on how good the machine is, how good the work holding device is and the skill of the operator will determine how the accuracy of the result. Taking your example of how accurately a chuck will hold after re-gripping a part? Well, the old 80/20 rule applies. You get 80% of the result, with 20% of the effort. Even a very modest old worn lathe and chuck and operator who has not gone through and set the machine up should be able to remove and reinstall a part and end up with a runout of 0.01". To get down to 0.001" with a 3 jaw - that takes some doing. Now if you wanted to get down to 0.0001" runout after repositioning - that takes some serious effort ($$).

I encourage you to keep working on the concepts of accuracy and precision (it'll drive you crazy). Even cheap kit can get pretty good results.
 
it's very difficult to retain concentricity when removing parts from a chuck then re-chucking them.
a 4 jaw is slightly easier to get the work concentric again, but a 3 jaw will cause issues-
unless you are a lot luckier or better than i.
working between centers is your best bet, if it's gotta be gnats nuts fine.
 
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