mini lathe 3 jaw chuck problem

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Hi,
I have had a mini lathe for a few years and recently the chuck hasnt been holding stuff centered, i noticed it when I was measuring the wall thickness of a drilled hole one side was .02 inches thicker than the other side, also when turning it turns things lopsided. What are possible causes of this :confused: ive tried cleaning out the inside of the chuck which seemed to help a little but not much. Help would be appreciated.
Thanks,
Matt
 
There may be nothing wrong with your chuck , an off-centre drilled hole may be your tailstock out of alignment. Try turning a test piece , that will give you a better idea.
 
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When you reinstalled the chuck jaws after cleaning, did you put the jaws back into their correct slots? My chucks for my mini lathe, including the 4 jaw self centering, purchased off e-bay, have the jaw # stamped in the outer end of the slot. If the jaws aren't in their correct slots they won't line up correctly in sequence with the scroll. If the slots are not numbered, check that the jaws are in the correct sequence. When fully closed the jaws should meet evenly.

Chuck
 
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Another possibility is that the jaw scrolls are worn. I just went through this with a 5" three jaw I'd been using for 6 years. The maximum recommended diameter workpiece recommended for this chuck is 2" but I often chucked stock as much as 2.5". By doing this I was putting a lot of pressure on the end scrolls and they became worn out. Check the jaw scrolls for wear

Cheers,
Phil
 
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Try putting the 1st jaw in a different slot. You still need to put them in in the right order. I ended up marking my chuck so I always got the jaws in the same slots.
 
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I checked that I had the jaws in the right slot, they were. The scroll looks like its broken closest to the center hole i dont know if its just bad machining though.
 
3 jaw chucks are not really precision things.

They get bumped, banged, abused and go out of round.

There is a way to fix that.
Even in an out of round 3 jaw chuck, the tool will cut perfectly round.

Chuck up a piece of scrap stock and turn it to any size you wish.
Cut off a short section of that turned piece then chuck it up in the
back of your 3 jaw chuck behind the the gripping teeth edges.
Then take a boring bar and lightly skin the gripping area of the jaws.

Your 3 jaw will be perfectly round..... for awhile.
Hang on to that turned slug you used to true up the jaws.
You'll be needing it again. ;)

Rick
 
I tried truing jaws by boring them and it resulted in jaws that only held true for the diameter that was bored. Just bite the bullet and get a new 3 jaw chuck. They do not last forever. I'll bet that the scroll is worn but that said taking it apart and cleaning it throughly may restore accuracy. It didn't work for me. I even tried marking the stock in the chuck with a centre punch so that if the part had to be taken out of the chuck I could line it up again with one of the numbered jaws. This too was a temporary fix for the real solution, a new chuck.

A new chuck will bring a smile to your face. It really is worth it.
 
Like Rick stated three jaw chucks are not a precision holding device. That being said you can purchase very precise three jaw chuck if you want to spend the money.
There are at least ten things that will effect the hole you are trying to drill. Headstock Alignment, Headstock Bearings, Threads on Headstock Spindle, Chuck Backplate, Chuck it's self, Tailstock Alignment, Tailstock Chuck Mount, Drill Chuck in Tailstock and the Drill Bit itself. Any of these will effect the accuracy of the hole you are drilling.

I would not try and center a hole in the end of a piece of stock held in a three jaw chuck unless I had turned down the outside of the piece to the diameter I wanted before drill. That way, if all the other items listed above are in close tolerances, the hole will be centered in the piece.
 
Have you ever had the chuck off the spindle? You can check the back of the chuck and the spindle face. When swaping between chucks I have found telltail slivers of swarf stuck and compressed between the two. So when I change between 3 and 4 jaw chucks I check and wipe the mating surfaces before mounting.
 
Hopefully you are using a centre drill to create a pilot hole. The heavy shank keeps it rigid while it starts the hole position. A twist drill that is poorly sharpened will not bore a straight hole so try a new bit. You will know if the chuck is bad without boring a hole. Aligning the tail stock may be all that your machine needs.
 
I found that the jaws over the years may flatten a bit or wear on the edge. I check them with a dial indicator and true them up with a diamond grinder bit in a high speed grinder attached to my tool holder. Don't forget to lock the chuck from opening while grinding.. Hope this helps!!
 
You can always drill and tap 4 holes 5 mm at 90 degrees on the outer diameter close to the spigot register recommend 4 mm from the face.
Put in 4 set (grub) screws with brass ends so as not to bruise the spigot register and adjust the concentricity of the chuck holding a piece of ground bar (end mill shank for example).similar to the 4 jaw techniques.
When assembling the chuck to the spindle,lightly nip up the 8mm x 3 studs,and adjust the concentricity
This is known as a "floating chuck."
I did this many years ago,and fairly seldom do I need to re-adjust when putting on the chuck.I always locate the chuck in the same position relative to the spindle.

John
Spain
 
Bought a M.I.C. mini scroll chuck in Hongkong. Made a mistake and took for granted it would as good as a 4" Self Centering Chuck. Used it to mill a 25 T spur gear. I had uneven height/depth
teeth. Check dial gage and found reading as-----0-------8--------12 thou readout. Had to add shims to centre gear blank. Now fingering out how to grind true chuck tooth to get chuck centered.

IMG_1980.jpg
 
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