Quickly Aligning A Four Jaw Lathe Chuck

randyc

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Many new hobbyists seem to have little use for this type of work holder, preferring devices that tend to self-align, like a three jaw chuck or some form of collet system.

A typical 4-jaw alignment experience might go like this:

  • Work is installed in chuck and the jaws tightened loosely while visually centering the work
  • Chuck is rotated by hand, checking work eccentricity with DTI and adjusting as required
  • Repeat , over and over and over and …

Does that seem about right ? OK, that procedure will get shorter and shorter with practice but it can be improved considerably with just one simple change:
  • Make another chuck key.
I didn’t take any photos when I made the small chuck key shown at lower right but making the tool is a simple process. The exterior features are not critical but the same square interface dimensions as the original chuck key are obviously required. The part is blued using my normal hillbilly process: position the parts over the gas burners on the kitchen stove and dunk ‘em in used motor oil when the correct color has been reached.

P1040996_zpsmk3xbcvt.jpg

For initial alignment in a four jaw chuck, I prefer a travel indicator rather than a DTI. This is because the run-out of the work piece is likely to be excessive, at least initially, and would probably exceed the travel limits of a typical DTI.

This one is permanently mounted to a magnetic stand and is parked on the back of the lathe taper attachment when not required.

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At this point, for safety sake, disengage the drive system of the machine by loosening the belt drive or placing the spindle drive in neutral. In this old Sheldon, lifting the lever loosens the belt as can be seen in the photo.

P1050003_zps6ys0gxnp.jpg

Rotate the chuck (and work piece, of course) so that one of the jaws is aligned with the travel indicator. With the spindle of the travel indicator touching the center line of the work piece, note the reading (or zero the reading). Rotate the chuck 180 degrees and note the reading.

2015-04-12_zpspylagy8k.jpg

Insert the two chuck keys in the near and far side adjustment screws. NOTE: This is a posed photo with the lathe drive train disconnected. NEVER leave a chuck key in a lathe chuck. If the key is in the chuck it needs to have a hand attached to it !

P1050004_zpshjczpyri.jpg

Using the two chuck keys, one to loosen and one to tighten, watch the indicator while gently adjusting the chuck keys until the work is moved ONE-HALF of the error previously noted.

Rotate the chuck ¼ of a turn so that the other two jaws are now aligned with the indicator axis. Note the indicator reading then rotate the chuck 180 degrees and determine the difference between the highest indicator and the lowest one.

Using both chuck keys, tightening and loosening simultaneously, move the work one-half of the error distance.

Repeat this one or two more times until the run-out is about .001 or .002. At this point, we need to do some tightening to secure the work in the chuck. Continue the process of rotating the chuck, checking the run-out and then rotating 180 degrees and correcting by one-half of the small error.

You might try using only one chuck key to “push” the work away from the high point of the run-out. Do this a couple of times and the result should be a tightly secured work piece with minimal run-out.

OK, so the work is centered at the location of the indicator, near the chuck, but it’s highly unlikely to be centered along the entire length. Typically the next step is to move the indicator near the end of the work. Rotate the work until the high point is found.

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Move the indicator off of the work then lightly tap the high point of the work piece with a soft-faced hammer. Check the run-out and repeat the process as required.

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When the run-out is within desired limits, carefully drill a center hole and then use the tailstock center to keep the work properly aligned with minimum run-out.

P1050008_zpsg5dtrtno.jpg

If for some reason it’s not practical to center drill the end of the part, a steady rest can provide the support required to keep the work aligned.

This simple process can reduce the time required to align a 4-jaw chuck from minutes to seconds. When I aligned the test bar for the above photos, I timed the alignment process. It took less than one minute to align the bar to within .001 !
 
I have been using 2 keys on my lathe at home for about 10 years. The 4 jaw I use at work every day is 20"dia. My arms arn't long enough to reach around it and the work.:D
 
I have been using 2 keys on my lathe at home for about 10 years. The 4 jaw I use at work every day is 20"dia. My arms arn't long enough to reach around it and the work.:D

I don't recall when I started using two chuck keys, it's been a few years but definitely not 10 years ... maybe we read the same suggestion on another forum ?

Fortunately, most of us won't experience your problem of reaching around a 20 inch chuck since this is a hobby machinist forum, right ? (My shop slab couldn't possibly even support a lathe of that size, LOL !)
 
The 4 jaw I use at work every day is 20"dia. My arms arn't long enough to reach around it and the work.:D

20" chuck, well shoot, then the lathe should be large enough that you could just lay on the bed, after all it is a "bed", then you could use 2 keys. And by laying on the bed, you could also get close up to the workpiece, close one eye & eyeball that sucker to get dead nuts. ;) I would love to see the look on you're boss' face if he caught you doing that! :big grin:


Rotate the chuck (and work piece, of course) so that one of the jaws is aligned with the travel indicator.

When I first started using the 4-jaw I use to chase that sucker around for days cause I was chasing the absolute highs & lows. And if it was round stock that wasn't very concentric it would be weeks. Well I didn't know any better then & I had no one around to teach me. Then one day it dawned on me, just tighten the highs at each jaw, moving half the difference of what the opposing jaw indicated. I was no longer afraid to use the 4-jaw anymore & like using it. I still only use one key though.

I prefer to use just a dial indicator for indicating the 4-jaw too. If I get down to 1/2 thou or less it's good enough for me. I haven't needed to dial something with accuracy better than that so I never use a DTI for that.
 
The 20" diameter sounds like the 36" vertical one I ran sometimes as a operator after high-school. That old Bullard model might fit in the average size garage. For its size I recall it spin pretty easily when in neutral position.
 
The 20" diameter sounds like the 36" vertical one I ran sometimes as a operator after high-school. That old Bullard model might fit in the average size garage. For its size I recall it spin pretty easily when in neutral position.

That brings back some memories ! Those Bullards are very cool (unfortunately I never even got to touch the controls on one) ! When I worked at Westinghouse, there was a line of those "small" Bullards in one building and that line of machines must have been 100 yards long !
 
That brings back some memories ! Those Bullards are very cool (unfortunately I never even got to touch the controls on one) ! When I worked at Westinghouse, there was a line of those "small" Bullards in one building and that line of machines must have been 100 yards long !

I should say I only used it for one project. The shop had the job of making forging dies for the old International Harvester plant in Louisville. They assigned me the job of roughing the forged blanks of steel into useful mold sections for the lathe machinist. The shop had a hydraulic controlled tracing lathe that he used. I remember breaking many cemented carbide tool bites. I could never talk and run a machine without problems back then. I was in my teens. That was the only time. That old machine wasn't used much to begin with but I did like it. Those were the days.
 
I honestly never even thought about two chuck wrenches, and I've dialed in thousands of parts. One important note, Never take your hand off the chuck wrench when in use. I know this was probably just done for the picture, but I think a note should be added. I've seen some nasty gouges in ways due to the chuck wrench being left in, not to mention some stiches in side of one lucky apprentices head (lucky it wasn't worse).
Bryan
 
For anything over 13 mm diameter, I only have a 4 jaw independent, and have always used two keys. But for most stuff I don't use an indicator but rather just place my tool holder loose in the QCTP rotate the chuck backwards and ease the tool in to the close spot then proceed to rotate and split the difference. I can get pretty close quickly for most stuff that I start turning from scratch.

David
 
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