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Indicating a Part in a Lathe

G

glbreil

Cast Iron
Joined
Feb 10, 2008
Location
McLeansboro, IL
On my lathe when indicating a part in my four jaw chuck to run true when I get down to say .0001" TRO it will repeat that every other round of the chuck, but the round in between is .0003" TRO. That doesn't seem possible but it is always the same. I am indicating with a .0001" Starrett indicator using a ground mandrel in the bore of the part. It does the same thing every time.

What is the deal?
 
Seems to me a spindle or chuck issue would be every revolution, but a gear, or something in the gear teeth could put pressure at every other, every third, every...etc. revolution, especially a number in the tenths...
 
If your lathe is a plane bearing machine, the oil "wave' between the spindle and bearing goes around with the spindle about every other time.

Regards,

Stan-
 
glbreil said:
On my lathe when indicating a part in my four jaw chuck to run true when I get down to say .0001" TRO it will repeat that every other round of the chuck, but the round in between is .0003" TRO. That doesn't seem possible but it is always the same. I am indicating with a .0001" Starrett indicator using a ground mandrel in the bore of the part. It does the same thing every time.

What is the deal?
Click to expand...

How much closer do you expect to get it for general Lathe work?
 
Unless you are building a Swiss watch I doubt that I'd worry too much about it. If the machine is brand new and a very accurate high end lathe, maybe worry about it, but in a used machine that's pretty close.:codger:
 
In addition to Stan's observation, I have caused that much error just by pulling on the chuck when rotating a plain bearing lathe by hand. Try running it at very low speed and see what you get. That will establish the oil film, or it will have pressure, if a pressurized bearing. It will also take any outside source (you pulling on the chuck) out of the picture.
 
Thanks Guys, I know that is close enough, I expected it to do what ever it does every time the chuck goes round not every other time.

I bought the lathe new 10 years ago or so and it has always done the same, I just did not know why. It is a Jet tool room lathe, I don't know what kind of bearings it has.

Thanks Gary
 
I have seen the same thing in 5C spin fixtures and Stan's theory is the one I have always used. Your lathe probably has greased ball bearings? Possible the same explanation with the grease or out of round balls? What ever the cause it is an irritation when straigtening parts to tenths.
 
If you are indicating parts in a lathe to .0001 on a regular, fast basis, then you, sir, are a kick butt lathe operator. Bill S
 
If the spindle has ball bearings, one probably has an oversized ball which is making almost a turn every two turns of the inner race. That is why expensive lathes have expensive bearings. Since it will not be making a full half rotation for each spindle full rotation, if you are sufficiently patient, you can probably track the high point as it moves around.

Bill
 
glbreil said:
On my lathe when indicating a part in my four jaw chuck to run true when I get down to say .0001" TRO it will repeat that every other round of the chuck, but the round in between is .0003" TRO. That doesn't seem possible but it is always the same. <SNIP>
Click to expand...

Now there's a problem I wish I had ...

.. Gregg
 
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