How to dial in part in 4-jaw chuck that doesn't protrude jaws

Friendly non murdering Sword

Friendly non murdering Sword

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Hey everyone,

So I'm owner of a lathe for maybe 1 month now and I'm still getting used to using it. I now have to turn a small disk and I am scratching my head on how to dial that part in...

So the disc is 4mm (0,15") thick and 90mm (3,5") in diameter, I turned the outside diameter by putting the disc on an arbor. I now wish to bore out the center and only my 4-jaw has a wide enough opening to hold that part, so I have to dial it in... but how?

The problem is, that the part doesn't protrude the jaws so there is no way to reach the outside diameter with an indicator and then turn the chuck. I also can't shim the part so it protrudes the chuck slightly because there would maybe be 1-2mm (0,04"-0,08") of part to grip to left.


Another possibility would be to dial in at the bore with a dial test indicator. Bu then again the bore is only 10mm (0,4") and rough drilled so I don't know how accurate that would be. Any suggestions?
 

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You could use a gauge pin or gauge block on the perimeter to extend the edge out passed the jaws, and indicate on that. Or you could indicate on the outside of the jaws themselves, after measuring them to ensure consistency.
 
Hi,
For starters, that indicator isn't the best for this a finger style is better, but if you can get the plunger into the bore, you may be able to indicate it in that way. Another thin to try, although maybe not the most accurate, is to indicate the outside of the jaws. They should be uniform thickness, just be sure to find the bottom of the arc, as they usually are curved.

Another thing you could do if your indicator holder is rigid enough, use the carriage to move the indicator out of the way of the jaw as you rotate the spindle.

Richard
 
You are going through a learning process here, it takes time to learn the tricks of the trade. :) Order of operation is important and it takes time to figure out the best way to make parts.

In this case you really want to machine the ID first, then machine the OD. Doing it this way will make the ID and OD perfectly concentric and you won't have to dial in anything.

I would dial in the bore first, as close as possible, then bore the ID to size.
Then make a stub arbor out of some 25mm shaft.
Then bolt the disk to the stub arbor to turn the OD.

Here is a stub arbor I made for turning the OD of some UHMW

In this case it required the screw in the end to drive it because of the low friction of UHMW, you would not have to do this with your project. Just the bolt in the end would hold the part.
1626716539941.png

The end of the stub arbor drilled & tapped for a bolt.
1626716576795.png

And the part secured with the bolt.
1626716605443.png

And secured in place ready for turning.
1626717148652.png

Another stub arbor I made for turning the OD of some washers.
1626716959100.png
 
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Only idea I can think to add here is dialing on the inside of the jaws. Eliminates the thickness of each jaw variability and should be easy enough to find the bottom of each arc and easy to hold your dial plunger back to get to the opposite jaw. This should get you to within a couple thou. How accurate does it need to be?
 
Doug Gray said:
If your centre hole from the arbor is true, you could use the double centre methode.
Click to expand...

Yep. That's the way I do it. And if your dead center doesn't have a pilot hole in the small end just make one up. 30 or 45 degree taper on one end and a pilot hole in the other on a piece of 1/2" or so scrap about 3 inches long.
 
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