[nateman_doo]

So I started playing around with my lathe out of the box & I turned a piece of aluminum rod. looked pretty good. Then I turned it around, and it wobbled even after putting the finished side in the chuck.

Why is this, and how do I fix:


You can see sort of a "step" along the rod.

[knudsen]

If I understand what you are saying, and what I see in the pic, you have a huge amount of chuck runout. Best bet, IMHO is get a 4 jaw and learn to crank it down to .001" or less runout.

[nateman_doo]

Can you clarify what runout means? I just put this chunk in the lathe-turned it, flipped it, turned it again and thats what I got. Obviously something on this lathe is not right.

[grg12]

"Runout" in this case means "how much out of center my chuck holds" - and should be as low as possible. Good quality chucks gaurantie low rounout - cheap ones... are cheap
My 3 jaw chuck "cheap china" quality has 0.07mm rounout - that means, if i machine a rod the same way you did - 0.15mm step could be expected. If you want perfect surface - machine whole surface in one mount. If it's impossible - you can try to use chuck with independent jaws, indicator and a lot of patience, or use collects, or turn the part between center.
Here is something to read about lathes for beginners: Getting Started

[knudsen]

independent jaws, indicator and a lot of patience

I would argue respectfully that a 4 jaw (independent jaws) only takes a few seconds longer to setup than a regular 3 jaw scroll chuck. I can set mine to .002 or .003 in just a few seconds longer than a 3 jaw, .0005 or better in one or two minutes. And I am not fast, just experienced with a 4 jaw, and I know the proper technique. I only use a 3 jaw for hex stock. I certainly respect your opinion and answer, grg12, but I also want to convey the fact that a 4 jaw is not hard to setup with the proper technique and a little practice. IMHO, the 4 jaw is superior in all ways to a plain 3 jaw scroll chuck, and should be used as the regular chuck, especially when we are dealing with imported stuff. If doing production and making many of the same part, I can see the need for speed , but in that case, a collett is the correct tool. :cheers:

[sansbury]

The stock 3-jaw on my 7x gets me to +/-.001" very reliably. If you're getting a lot worse, I'd remove the chuck and give everything a good cleaning and make sure there's no chips or debris in there preventing a proper mating of things. Especially if it's brand new--you need to get all of the gunk out of all the places, not just scrape it off the surface.

Another thing is to measure it when mounted in each of the three possible positions (i.e. mount, measure, unmount, rotate 120 degrees, remount, etc.) and see if it's any better in one position. If it is, use a punch to make a little alignment mark.

For the vast majority of everyday low-precision parts, the 3-jaw should be totally sufficient. With respect to knudsen, if the 4-jaw really takes him only a few moments more than the 3-jaw, he is either the fastest key in the west with the 4-jaw, or the world's slowest machinist with the 3-jaw He is right that collets are king for production, but a collet set and chuck that's better than a decent 3-jaw isn't going to be cheap, and if you're doing a lot of mixed sizes, it's a lot less convenient.

[Dano523]

Dail indicator on a mag base, a piece of stock, and razor blade will solve most of our problems.


remove the chuck off the machine, pull it apart fully and clean it.

Now when you put it back on the machine, chuck up a short piece of straight stock (just need about a inch protruding out the jaw), and while tightening the chuck to the back plate, dial indicate the stock so the jaws are running true (forget about the body, it the jaws that you want to run true). Do it right, and you should have the three jaw chuck running in the .0003 range without problems.


Now that you have that end of the machine running true, turn that piece of stock to a point (about a 20* angle), and let adjust the tail stock.

Pointed end stock in the chuck running true, dead center in the tail stock, bring the two together with a razor blade between them. As you slowly bring pressure of the dead end center into the mix, look at the razor blade to see which way it's canting. When the tail stock is set true, then you should be able to turn on the machine with the blade running true a well (no cant as it spins).


Now for the final test, chuck up piece of stock a few inches using the center in the tail stock, and make a cut all the way across to check the OD of the stock the full length of the cut. The chuck you are going to leave alone, but depending on the other end of the piece you just turned, if it larger of smaller, then adjust the tail stock the needed amount to fix the problem.


Lastly, depending on the distance you are trying to turn a piece of stock (even with the both sides supported, you may need a follower rest to keep the stock from center deflecting). Most larger stock piece shouldn't deflect much (may take a few finish passes to get it unified), but on small OD pieces, deflection can be a monster.