I've spent a couple of hours with a bar in a lathe convinced that there was someting wrong with the lathe on the assumption that the bar wouldn't be so inaccurate. In fact, it was the bar. So, given an unknown bar, what is the best strategy to use to ensure that the bar is true independent of the trueness of the lathe?
How about this:-
Ensure that there is no give in the work when held in the chuck and skim it until it is true. Centre drill the end. This now means that if it is transferred to another lathe and aligned in the chuck so that it's centre-hole lines up with the tail stock centre, the work is still true.
Is there a simpler and as an effective a procedure that should always be carried out? jjj
Basically you chucked up a potato chip, and got it to run true.
There are a couple of ways of workholding in a lathe.
1) in a chuck, single-ended. As long as the chuck holds the stock solid, you can work on a part and turn true concentric diameters - to the spindle axis. Every diameter you turn will be concentric with every other one *until* you remove the part from the chuck. Once you take it out, you loose some of the goodness in the setup. So often if one is starting with rough finish stock, or has a crappy three jaw chuck that won't center will, the drill is:
A collet is a generic term for a workholding device which is nothing more than a hole bored in a sleeve, and the end of the sleeve has one or more radial slots which allow the collet to close down on a piece of stock that is in the hole.
This is accomplished by either screwing a ring down over the outside end of the collet, or by pulling it back into a tapered socket, by means of a draw bar.
New collets typically give centering of round stock within a few ten thousanths of an inch.
Collets:
These were manufactured by the folks who basically invented collets, Hardinge. Some lathes like this one accept them directly in the nose of the spindle:
A view of what this looks like with the collet in place, but without any stock present:
That last one is a different, larger machine, and the collet is a "5C" type, which is pretty much the most common kind found in industry today. Because they are so common, they are the least expensive, and used collets are quite available.
That last machine has a lever-acting draw-bar to clamp the collet down on teh workpiece:
Smaller machines typically use a smaller collet, the
3C size is typical in home shop lathes. Because these are almost never used in industry, and there are about a bazillion home shop guys trying to get ahold of them, even badly worn examples often fetch high prices.
It's a pleasure to behold a person who is used to either using a battered, worn three jaw chuck, or one who has learned to center up stuff in a four jaw, when they are suddenly exposed to the speed and accuracy obtained (trivially) with a collet setup.
Jim
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