Trying to 'true' a 3 jaw chuck

[aviatakl]

To the collective.

I have an 'old' CZ300C lathe made in the ROC. The 3 jaw chuck is worn out, I have owned this lathe for 12 years and it has done a fair bit of work.
I have acquired a new 3 jaw chuck that was removed from a new machine due to a slight 'run 'out'.
OK I figured that I could true the chuck by grinding the jaws.
The run out is in the order of 0.18mm. It was 0.26 when I stated.
Is grinding the jaws the way to correct this? I have a flexi drive dremel type tool and it does a good job, positioned on centre in the tool post.
Any advise is welcome.

Jim O'Donnell
New Zealand

[MrDan]

To the collective.

I have an 'old' CZ300C lathe made in the ROC. The 3 jaw chuck is worn out, I have owned this lathe for 12 years and it has done a fair bit of work.
I have acquired a new 3 jaw chuck that was removed from a new machine due to a slight 'run 'out'.
OK I figured that I could true the chuck by grinding the jaws.
The run out is in the order of 0.18mm. It was 0.26 when I stated.
Is grinding the jaws the way to correct this? I have a flexi drive dremel type tool and it does a good job, positioned on centre in the tool post.
Any advise is welcome.

Jim O'Donnell
New Zealand

You'll get more learned, detailed answers here but you're on the right path. I'm assuming the flexi drive can be rigid mounted? Key word Flexi here.

Make sure to cover your bed ways to keep all the grinding dust off of everything.

[EVguru]

You need to load the jaws to grind them. With soft jaws the usual thing is to bolt a ring to the jaw tops. With hard jaws, you need to have a ring behind the jaws that the jaw teeth can tighten onto.

Is it the jaws that are off, or the chuck register?

Is the chuck body running out?

Are the jaws parallel?

I got a free (new) chuck that was off in all sorts of ways and got it much closer by tightenin the jaws onto a bar held in a collet chuck so I could re-machine the chuck back and register.

[Black_Moons]

Also note if the scroll is worn, Grinding it to 0 runout at one diamiter will make it still have runout at all other diamiters.

Options if scroll wear is suspected: Mod the chuck to 'set true' type.
Or convert the existing jaws into a 2 part jaw setup that accepts soft (aluminum) jaws, Then you can recut them to the proper size to hold a part when you need to hold a odd shaped or fragile or perfect alignment needed part.

[Forrest Addy]

All the above is good advice.

Would I be out of place to suggest you use a 4 jaw chuck for work requiring accurate centering?

[Evan]

As Forrest suggests, the three jaw is for machining that requires relative concentricity only, or none at all. The four jaw is for when absolute concentricity must be maintained. Those aren't the only applications of course but that is how concentricity shakes out.

[MuellerNick]

A good chuck will be off at about 1/100 mm.
But that has some prerequisites:
Your spindle nose must be running dead true. Below 0.005 mm.
If your spindle nose has a short taper, it must fit perfectly. If it is a cylinder, you won't get very happy.
The chuck has to be mounted with always the same relative orientation to the spindle. Find the best position and mark it (0, or center punch).
You always have to lock the chuck with the key inserted where the "0" is stamped in the chuck.
If the jaws were replaced, you'll have to grind them. The procedure has already been described.


Nick

[strokersix]

You may wish to check runout at different diameters, which has been implied above. If runout is the same at all diameters then grinding the jaws should help. If it isn't then grinding the jaws probably won't help.

[darryl]

Such things are best checked out in a proper order. As suggested, make sure the spindle is running true with low runout. Register and shoulder surfaces must run true. The chuck surfaces that contact should be checked for damage and the chuck should mount reliably every time. Then chuck some scrap and carefully true it. Note which hole you used to tighten the jaws, or the sequence you used. Make sure that when you true up the scrap piece, the tool doesn't keep cutting as you make successive spring passes. Good so far? Now leave the piece in the chuck, remove and remount the chuck. Check the piece for runout. Do this a couple or three times to be sure there isn't a variable in the chuck mounting.

Still good? Now mark the scrap piece at a particular jaw so you can remove and remount it the same way. With care you'll be able to feel how it was oriented to the jaws when you sneak up on tightening it again. Each jaw will imprint a very slight flat which you might be able to feel by slightly rotating the piece at the point where the jaws are just starting to get tight. So remove the piece, crank the jaws open a bit more just to move the scroll around, then remount and check the runout again. Do this a few times to see if you can get some indications. If you can't remount the scrap piece and have it remain within a couple thou, there's some work to be done inside the chuck, if that can be helped at all. If you can stay pretty close during the few rechuckings, then and only then should you grind the jaws. You might want to do this test on a few different diameters before committing to the grinding job. The information you get from this testing will tell you (us) what to do next.

If you get to the point where the grinding is the appropriate step, then follow the methods of jaw control to get the job done properly.

[J Tiers]

Old, used chucks usually get "bell-mouthed", either from jaw abuse, or from wear of the slots they move in. At that point, the stock begins to flop around and develop flats and out-of-round when turned. You may even suffer from crashes and dig-ins due to the stock flopping outwards suddenly.

You ABSOLUTELY CAN fix that by grinding.....

Now, DO NOT LISTEN to folks who decry grinding as worthless etc..... It fixes bellmouth VERY nicely.

It even can help with true-ness, but as mentioned, you ought not to trust a 3 jaw for that in the first place. I assume you are smart enough to know that and you just want to clean up the grip. That will work fine.

The best loading system is a Rich Carlstedt idea..... drill some holes in the jaws, put in pins, and close on a ring to load the jaws. use a carbide drill , even a masonry type if you can get a small one around 0.125"/3mm.

if you don't do that, or if you try rubber bands, loading the BACK of the jaws, or other fairly worthless substitutes, you won't get as good results. DAMHIKT

Example: