How important is lathe chuck run out?

Pcmaker said:
I've wanted to get 4 jaw self centering chuck anyway. This is my lathe:

http://www.grizzly.com/products/Grizzly-7-x-12-Mini-Metal-Lathe/G8688

It comes with a 3" chuck with 3 jaws.

Can I upgrade my lathe to use a 4" or larger chuck? How do I know it'll fit my lathe? I see some chucks on Amazon, but not sure which one would fit my lathe and also which one would be an "upgrade" with less runout
Click to expand...
Littlemachineshop.com sells a 4” 3-jaw scroll chuck with adapter to mount on your 3” spindle flange for $132 + shipping. They claim .003” max radial runout. No affliation, just a satisfied customer.

Tom
 
Pcmaker said:
How important is chuck run out?
Click to expand...

It's not.
As stated above well planned work obviates the run out.
In addition any run out means reversing work will have a visible artefact in the finish so anything but perfect is a pointless ambition :D
 
Hi Pcmaker,

Why not measure the run out at say three different diameters and see what you get. You should find that at one particular diameter you have a minimum figure.
 
So in that video, he is getting 8 thou ! No big deal !
 
I hope that endmill is a good enough indicator for that
 
The run out on the 6” 3 jaw on my 13x40 had .012”TIR . It was a PITA to deal with. A lot of my lathe work is second ops. I just bought a new 6” 3 jaw with integrated D1-4 Mount from Shars and it has .0015”TIR. It’s taken the dread out of lathe work and made it fun again.
 
PCMaker, here is some basic info that might give you some insight.
  • Run out on a 3 jaw chuck has numerous sources - the accuracy of the spindle, how the chuck mates to the spindle register, how tight your bolts are that tighten the chuck to the spindle, the accuracy of the machining of the chuck body/jaws/scroll and probably several others that I can't think of right now. They all add up to your TIR, or total indicator run out. The way to check how much is there is detailed in RJ's post.
  • All scroll chucks have some run out, although high end chucks typically have less than cheap ones. Now, the very first time you chuck up a work piece and turn it is called a First Operation. In a first op, no matter how much run out your chuck has the work piece will turn on the center line of your spindle and the work will be accurate as long as you do not remove the work piece from the chuck. What matters is that the jaws hold the work solidly and that you do not remove it from the chuck. This is a good thing because much of the stock we start with is not truly round to begin with, but it is once we turn it.
  • It is only when you remove the work from the chuck for some reason and then try to re-chuck it that your run out matters because the likelihood of being able to get it re-centered or aligned with the spindle centerline are nil unless you have a chuck that allows you to dial it back in. You can use a 4 jaw chuck or a scroll chuck with an adjustable position (set-tru, adjust-tru, whatever) or possibly a collet chuck to get it close to or back on the spindle centerline. When you re-chuck a piece (that has already been turned) for further work on the lathe, this is called a Second Operation and the accuracy or run out of the chuck used for a second op is where you need minimal TIR. This is really where a 4 jaw comes in handy. It is the potentially the most accurate chuck you can have; it is also the slowest to use.
The bottom line is that it is pointless to check run out on a 3jaw scroll chuck provided the jaws are intact, in good shape and grabs the work tightly. For first op stuff just use it. For second op stuff, get a 4 jaw.

As for how big a chuck to get, use the smallest chuck you can that will do the work you need it to do. Big chucks hold bigger stuff but weigh more, wear the spindle and spindle bearings more and the jaws have the potential to contact the ways if extended far enough. I would think the largest you should go is a 4" without worrying about hitting the lathe with the jaws. A 5" will work but will not gain you much more capacity (because of the jaw hitting thing).
 
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You can grind the inside of the jaws with a die grinder on the tool post......I have a friend that did this to his chucks he said he got better results using a carbide burr instead of a grinding stone...


 
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