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| Chuck runout question... |
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| AdeV:
With a typical 3-jaw chuck, what would you consider to be the maximum acceptable "repeatability"? i.e. if you put a piece of straight & true round-bar in your chuck, how many thou of runout would you consider "normal"? I ask because all my 3-jaw chucks (3 of them now) seem to suffer from major "non-repeatability". e.g. I can take a piece of bar, mount it in the chuck, and get it running dead true (because the 3-jaw is mounted in a 4-jaw...). Undo the 3-jaw & take the bar out. Now put it back in & tighten, and I may have as much as 0.010" total indicated runout. In my mind, this is totally unacceptable. I'd stick with the 4-jaw, but that's got wear issues as well - and unless I'm really careful - and lucky - it clamps up at a slight angle, so I end up cutting tapers... |
| Bluechip:
Hi Adev Last time I checked about a year ago .... IIRC My newish TOS 4" SC3-jaw is about .002 TIR 1" out from jaws. New TOS 4" SC4-jaw a bit better, about .0015" Ancient Pratt-Burnerd was some .008 on a good day .. :( What are you using as a test bar. Some 'round' bar ain't all that straight. I used a 1/2" new dormer drill bit with a long shank, gripped reversed with most of the jaws on the parallel part.. All over the shop with commercial MS round. Try a bit you've turned yourself, so you know its circular, and straight. BC |
| Ned Ludd:
Hi AdeV, Have you tried the repeatability test using the same chuck key hole each time. I have mentioned this before elsewhere that there is only one key hole that is accurate. On the outside circumference of the chuck there will be a "O" stamped, if not use the hole in-line with the label on the face of the chuck. I know some will poo-poo the idea but it is true and does work, well it will till the chuck gets worn out. The theory is that the chuck was set up with the labelled hole to allow for play in the scroll. Ned |
| Bogstandard:
Ade, Anywhere between 0.001" to 0.003" is about average on a new reasonably priced chuck. Once the scroll and jaws become worn, as yours surely are, then the scrapman cometh. Unless you are forever going to be doing roughie jobs, and always truing up the piece first using larger material. As Ned has stated, I broached the idea a few years back and people didn't believe me, until they tried it for themselves and couldn't fathom out why it worked, but it does. In fact the first thing I do when fitting a new chuck is try it in all bolt positions first to get the least runout, then try to find which keyhole gives the best results then mark that up. Invariably, if the hole has a zero on it, that usually is the best one. I think it is something they do in the factory during manufacture, they always use the same keyhole as they set the parts up for machining and assembly. In your case, I think new chucks might be in order, they have had their day. John |
| AdeV:
Thanks everyone for your replies. Looks like I have duff chucks.... --- Quote from: bogstandard on February 25, 2011, 09:17:12 AM --- In your case, I think new chucks might be in order, they have had their day. --- End quote --- I think you're probably right.... I keep trying to persuade myself that I can pick up a bargain chuck and "it'll be fiiiiiiine"; but I guess not. I know both my 4-jaws hold at an angle, even the one we tried to fix; the 3-jaws are just all over the place. I'll look for a stamped "O" on my latest acquisition, but it would seem that someone's taken the rust off it with an angle grinder, so who knows if any markings have survived... |
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