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Wobbly work piece in the lathe's chuck

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Pcmaker

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#1
Are there any tips on how to make your workpiece inside the chuck of your lathe not wobbly? I'm trying to get it as straight as possible, but having no luck. I tried outer and inner jaws. I don't want to just "eye" it.

I'm trying to smooth a small aluminum flashlight's handle. Get rid of the knurling on the aluminum as well as the paint coating.

Also, what's the proper machinist term to use if a cylinder is "true" and if you indicate it, you get almost no movement on the needle? Is it conecentric?
 

P. Waller

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#2
Move the chuck until there is as close to 0 runout as possible with an indicator, do it at the jaw face then at the beginning of the part, this could take time with an existing part.
Concentricity refers to the positional error between the center positions of 2 or more circular features on the same part.
 

ttabbal

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#3
I would put it in a 4 jaw and use an indicator to dial it in using the independant jaws. If it's remotely round, you should be able to dial it in to the limits of your measurement gear and/or patience. If you don't have an independant chuck you can probably tap it lightly checking as you go. This might mark the material, but any workholding has that possibility.
 

Tony Wells

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#4
Since you are taking only light cuts, if you have a suitable ball bearing center simply turn up a center out of some stock that will drive it from the chuck. Between centers will give you the best concentricity but watch out for any ID threads that run out to the ends. If there are threads, the runout probably won't be good, but you could 1) after mounting the workpiece, dial in the OD with your indicator and 4-jaw, or 2) turn and thread a close fitting nipple and screw the body onto it to drive it. You still will need a "bull nose" or pipe center for the tailstock end.
 

P. Waller

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#5
As an example I turn 1 or 2 of these parts per year, so far fortune has shined on me this year but there are still 4 months to go (-:
The diameters on each end have a concentricity call out of .005", I am quite sure that I have never achieved this but they always worked in service and have never been tagged.
It is a cam with a 3/8" offset held by bearings on each end and one end accepts a keyed drive component, either gears or belts, I have no idea what it does.
 

mikey

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#6
Make yourself a centering tool like in this video:
 

savarin

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#7
As Mikey showed above. I use mine all the time. I used a roller blade bearing as they are cheap.
 

Pcmaker

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#8
I knew my fidget spinner would come in handy one of these days...
 

BaronJ

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#9
Hi Guys,

Those things are called a "Bump Center" They are very easy to make.

As far as the original poster is concerned if the flashlight (Torch) in the UK, is basically a hollow tube, then an expanding mandrel is the device to use. Held at one end in the chuck and located by a live center at the tail stock end.

I have one somewhere that is basically a length of threaded rod with a thick, maybe 12 mm, rubber sleeve in the middle of it, a nut and large washer at each end. The tube is placed over the rubber sleeve and the nuts tightened to force the rubber to expand and secure the tube.

Its been a while since I used mine, if I can lay my hands on it tomorrow, I'll take a photograph and post it.
 

Pcmaker

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#10
I just made one. Works awesome. Had to destroy my fidget spinner, though. I'm gonna get me an expanded mandrel set in the future. Seems like it'll come in handy

 

BaronJ

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#11
Hi Pcmaker, Guys,

A little bit horses for courses with expanding mandrels ! The type I described is a simple home brew way of making one to do a particular job, and for simple tubes works very well, but it is not a precision item. You can buy or make expanding mandrels that are far more precise, which essentially are two tapers pressed together, one inside the other, to expand and grip the work.

For instance, I made a mandrel handle to fit inside the bore of my lathe spindle, it works by expanding the end, by tightening a nut on the exposed end it pulls a cone into a split tube. This causes it to grip the inside of the bore.

Actually a great tool for tapping holes using the lathe.
 

PT Doc

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#12
As an example I turn 1 or 2 of these parts per year, so far fortune has shined on me this year but there are still 4 months to go (-:
The diameters on each end have a concentricity call out of .005", I am quite sure that I have never achieved this but they always worked in service and have never been tagged.
It is a cam with a 3/8" offset held by bearings on each end and one end accepts a keyed drive component, either gears or belts, I have no idea what it does.
Sweet!
 

macardoso

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#13
As stated above, concentricity is the measurement between the centers of two features on a part, Total Indicated Runout or Total Indicator Reading (TIR) or Full Indicator Movement (FIM) is the measurement of how far off axis the centerline of your part is to the rotation axis of the lathe or mill spindle. On a perfectly round part, it is double the distance which the axes of the part and the spindle are in error. TIR is influenced by the roundness of your part, as well as the quality of your centers (if your part has drilled centers).
 
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