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Centering large stock
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I am working on a 3/4 SS 303 rod and trying to get it centered in the chuck of my HF 7x
mini lathe, and it seems to always be off a little. I used my mag base dial caliper and can get it close but not close enough. Any ideas on this? The rod is 4". I have a QCTP, steady rest, and follower rest on the way from littlemachineshop but need ideas until they get here please. Thanks Mark |
¿ªÔÆÌåÓýI am assuming you are?using a 3-jaw
chuck?? It would then be normal to be off a little.? This is due to
the accuracy of how well the chuck can work.? (Mainly, the scroll in these
chucks just isn't all that precise.)
?
If you need accurate centering, you should consider
a 4-jaw independent jaw chuck.? You can move each jaw independently and,
while measuring with an indicator, adjust opposite jaws until it runs
true.
?
With only a 3-jaw, you can usually get pretty close
by putting very thin shim material between a jaw(s) and the work piece as
needed.
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Chuck K.
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John Lindo
How close do you want the concentricity.? The group can help from there. John L Spain
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It is within a few thousandths. I think the issue is centering the 4"
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long rod with the tail post center because when I use it to hold the end of the rod it throws it off more. Can someone show me a link on centering the tail and chuck please? I have seen a few but using a metal strip to do it doesn't seem to work all that well. Also I need to get some books on machining with the mini lathe so If anyone can respond to me on their favorite book that would be great. Most of the books I've found online and at my schools engineering library are for large machines or really old. Also is anyone near Dallas tx? I would like someone to learn from? Sorry for the long post! Mark --- In 7x12minilathe@..., John Lindo <bechetboat@...> wrote:
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There are several ways, depending on what measuring tools you have. The easiest to visualize is simply attaching an indicator to the chuck & indicating on the TS ram. When you rotate the chuck, the indicator will show TS misalignment. The problem is that the indicator face is difficult to see during most of the rotation.
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Another way is, "Rollie's Dad's Method", it's in the "files" section, under that name. The classic method is turning a test bar between centers & measuring the diameters at both ends - TS misalignment is 1/2 the diameter difference. My sleazy version of that uses a piece of 1/2" threaded rod, 2 fender washers & 4 nuts. Clamp a fender washer between 2 nuts at both ends of the rod. With the rod mounted between centers, take cuts on the fender washers until they're both cut without moving the cross slide. Measure the resulting diameters - the difference is twice the misalignment; if the washer on the TS end is larger, it means the TS needs to be moved towards the operator's side of the lathe. With the stock TS offset adjustment method, I found it was easier to remove the TS from the lathe, measure & adjust the offset with a depth mic, clamp everything down & return the TS to the lathe. There's a lot of useful set up & improvement info at: Going back to your original issue, odds are that your chuck isn't exactly concentric with the spindle axis. First, wiggle the rod slightly as you tighten the chuck; that ensures it's truly seated on the jaw faces. Indicate on the rod as close to the chuck as possible & as far away as possible. Differences between the near & far readings indicate either a bent rod or angular misalignment of the chuck. If the rod is truly straight, you'll probably need to resort to the Varmint Al jaw grinding method. OTOH, you'll probably find the chuck axis is parallel to but slightly different than the spindle axis. That's relatively easy to fix. First, groom the chuck's internals, then reverse the chuck on the spindle & open out the mounting alignment diameter by a few thousandths, at least as much as the indicator readings showed. When you remount the chuck, you can align it for zero runout before fully tightening the mounting nuts. It'll hold alignment over a surprisingly large work diameter range - most of the misalignment seems to be from the body machining, not the scroll accuracy. Roy --- In 7x12minilathe@..., "markust74" <markust74@...> wrote:
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tdhofstetter
The ideal is to start with stock slightly larger in diameter than your target diameter, chuck it however it chucks up, and turn it to size. Then it's guaranteed centered. Just don't disturb its mounting until you're finished machining all the features.
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Or if you MUST start with stock at finished size, determine which direction it's nonconcentric and how much. Shim it (paper or aluminum or brass) shims between chuck jaws and stock to center it. Or make up a new set of aluminum jaws for your chuck, lock the chuck's internal jaws around a washer that holds the new soft jaws just less than 3/4" apart radially, and turn the new jaws to 3/4" ID. That MUST be done with the jaws locked tight, so preload 'em just as they'll be preloaded when they're gripping your stainless rod. -- Tim -- --- In 7x12minilathe@..., "markust74" <markust74@...> wrote:
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I would love more info on making a new set of aluminum jaws for my chuck if you could provide specific details please?
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I have to start wit stock at 3/4 finished size and turn the end to 11/16 to fit inside a plastic tube (I also have to make a groove for an o-ring to fit to seal it in the tube....etc --- In 7x12minilathe@..., "tdhofstetter" <t.d.hofstetter@...> wrote:
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John Lindo
Hello ?Mark Take a look at the links below.hope they work for you. I converted a set of old soft jaws that screw into the standard chuck scroll and used as a master set so you can make? up new jaws that just bolt on in various positions and with dowel pins for?repeat-ability.? The brass jaws I have just recently used to turn alloy blanks of 120 mm dia. The photos may help you in your quest. https://plus.google.com/photos/112848589944601328801/albums/5782176773834726689 https://plus.google.com/photos/112848589944601328801/albums/5777133607405420353 John L? Spain
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