Is This True

From the Sherline website.
"Due to the nature of the design of a 3-jaw chuck, it cannot be expected to run perfectly true. Even 3-jaw chucks costing five times more than the one
made for this lathe will have a 0.002" to 0.003" runout. If perfect accuracy is desired in a particular operation, the use of a 4-jaw chuck or a collet is recommended. Both are available for your Sherline Lathe."
I don't have a Sherline lathe, but I was considering mounting a small Sherline chuck on a stepper driven spindle for a fourth axis on one of the mini CNC mills in the shop.
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Bob La Londe wrote:

The Adjust-Tru chucks have provisions to allow them to be adjusted to run true, but I expect that trueness only applies at the particular part diameter that they are adjusted for.
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Kinda like polishing the jaws of a chuck? Only works at a particular diameter?
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No, there are 4 adjustment screws on the mounting plate that allow you to after loosening the clamping bolts to center the chuck body on the base plate then secure the clamping bolts.
All the eaze of a 3 jaw with the accuracy of a 4 jaw independent. Very nice setup, I use one at work from time to time.
Wes -- "Additionally as a security officer, I carry a gun to protect government officials but my life isn't worth protecting at home in their eyes." Dick Anthony Heller
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You may not get perfect repeatablity on a single diameter but you can dial in any chucked up part to your level of perfection.
Wes
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Wes wrote:

You have to use the same chuck tightening position or the scroll position will not repeat position. I mark one of them with a magic marker so the same one is used every time. It makes a big difference especially if the chuck scroll is worn.
John
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Wes wrote:

No, it's as I indicated, the adjustment only works at the diameter part it was adjusted on. If you chuck up a part of a notably different diameter you'll get runout until you once again adjust the Adjust-Tru. i.e. it can be accurate with any diameter part, but not at the same time, it has to be readjusted for a different part diameter.
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A new, unworn Bison Set-Tru does that. I use the 3-jaw for speed and convenience, the 4-jaw for accuracy, the Set-Tru feature very rarely for my one-off type of work.
jsw
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On 11/12/2010 07:51 AM, Bob La Londe wrote:

That's certainly been my understanding of 3-jaw chucks all along -- if you want perfect accuracy you use a chuck with individually adjustable jaws, and you run the part in with a dial indicator to establish runout before you cut.
--

Tim Wescott
Wescott Design Services
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Or a collet, depending what you are doing and how fast you need to be doing it (especially if it's over and over again on parts the same size.)
--
Cats, coffee, chocolate...vices to live by

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On 11/12/2010 09:01 AM, Ecnerwal wrote:

Or you plan your work so that you do it all in one chucking-up operation.
I've found that I can mark the piece and my chuck, take it out, and put it back in so the marks line up, and get decent repeatability -- so far. I certainly wouldn't count on that, though.
We both forgot to mention turning between centers.
--

Tim Wescott
Wescott Design Services
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There aren't many adjustable workholding chucks that are perfectly true to the center axis. The more moving parts, the less accuracy.. generally.
Variations in the overall (in)accuracies of individual parts will dictate the accuracy of a chuck (lathe, drill, etc).
Collets are generally considered to be much more accurate than adjustable chucks. The adjustability feature of chucks is more for convenience.
Turning a simple part such as a bushing with a shoulder, will most likely be most accurately machined/turned, when the bushing dimensions are cut to desired sizes, then the part is cut off. This is a common practice for very good/excellent concentricity.
Having chucks with soft replaceable jaws allows the user to bore an axis-concentric hole/bore in the jaws for a particular sized workpiece, and re-bored for a different size for best accuracy.
The jaws themselves, the guides that position the jaws and a scroll that engages the jaws for adjustment, are all involved in the overall accuracy of adjustable lathe or drill chucks.
The jaws typically aren't identical in adjustable chucks (the gear-type teeth that engage the scroll differ from jaw 1, 2, 3 etc), so it's not as if one could build a very accurate chuck with only a couple of sets of jaws.
The big difference in accuracy between chucks, is cost. Accuracy and quality control are more expensive. China and India typically make consumer grade products that looky-like a real item, but quality control and accuracy generally aren't priorities.. just filling up the cargo containers for the next shipment is what matters.
Accuracy of chucks from other locations will vary, depending upon the cost and the intended application that they were manufactured for.
--
WB
.........


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+1 on the soft jaws! I think it's important to plan a job around the limitations of your tooling. I only see where the accuracy of a chuck come into play is on a secondary operation or poor planning. If I have to, I make soft jaws but I try to avoid the extra work with forethought.
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Yep, high precision lathe work was done for generations without such versatile or specialized tooling.
Expectations of perfect tooling are unrealistic.. especially with all the low priced accessories available today for home shops. The premium, high grade tooling doesn't guarantee that the work will be perfectly accurate anyway.
I agree with the poor planning and forethought comments.
--
WB
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    --I've got a 6-jaw Buck Adjust-tru chuck on a Myford Super 7B and I can dial it to within .0001" if I have to. If you're needing accuracy that's what you need to do. I don't often have something so kattywompus that I have to mess with the chuck tho; it's repeatable accuracy is less than .001" fer sure. If Sherline can't maintain that sort of accuracy it sounds like a badly designed spindle assembly, not a chuck problem. There's a reason the Sherline is inexpensive and that's gotta be one of 'em..
--
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Bob, I don't know where you got your accuracy numbers, but all of my 3 jaws are accurate to less than .001". Keep them clean and oiled and don't over torque them and they are accurate enough for 90%+ of your work. Steve

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Bob La Londe wrote:

My understanding is that good 3 jaw chucks are marked with a master tightening socket, marked 0 in all the cases I've seen, and that socket when used to tighten the new chuck gives the minimum runout. IIRC this is intentional and due to necessary internal clearances as the scroll and the diameter it bears against are made so that the thrust from the master socket pinion take up the internal clearances to provide the most true running.
I have a couple of Bison 3 jaw chucks and a TOS 3 jaw chuck and that holds true. The last Bison chuck I bought about 6 years ago was a bog standard accuracy chuck and had 0.01mm TIR runout on a 1" diameter test bar at the chuck after mounting on the backplate, The back plate mounting procedure being which has been mentioned on RCM before with a slight interference fit register machined on the mounted backplate to suit the chuck register diameter and no adjust-tru provison on the chuck. Due to its accuracy class I wouldn't expect that to hold throughout the clamping range or that the runout away from the chuck was great but it's way better than some Chinese chucks I've seen, and has always been more than sufficient for the vast majority of the work I do.
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"Bob La Londe" wrote in message
From the Sherline website.
"Due to the nature of the design of a 3-jaw chuck, it cannot be expected to run perfectly true. Even 3-jaw chucks costing five times more than the one made for this lathe will have a 0.002" to 0.003" runout. If perfect accuracy is desired in a particular operation, the use of a 4-jaw chuck or a collet is recommended. Both are available for your Sherline Lathe."
I don't have a Sherline lathe, but I was considering mounting a small Sherline chuck on a stepper driven spindle for a fourth axis on one of the mini CNC mills in the shop.
Yes, it's true!
Steve R.
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