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View Full Version : Trying to 'true' a 3 jaw chuck



aviatakl
09-24-2010, 05:28 AM
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
09-24-2010, 06:10 AM
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
09-24-2010, 06:31 AM
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
09-24-2010, 06:45 AM
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
09-24-2010, 07:19 AM
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
09-24-2010, 07:57 AM
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
09-24-2010, 08:09 AM
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
09-24-2010, 08:19 AM
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
09-24-2010, 08:26 PM
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
09-24-2010, 09:08 PM
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:

http://img.photobucket.com/albums/0803/jstanley/chuckgrind.jpg

aboard_epsilon
09-24-2010, 09:17 PM
ive got a chuck ..that someones made a right mess of trying to grind the jaws ..


what are they made of.... steel?..can i add weld to them and start again ..i know they would have been hardend ..but they are so bad ..i have nothing to loose .

all the best.markj

Dr Stan
09-24-2010, 10:16 PM
ive got a chuck ..that someones made a right mess of trying to grind the jaws ..


what are they made of.... steel?..can i add weld to them and start again ..i know they would have been hardend ..but they are so bad ..i have nothing to loose .

all the best.markj

Don't weld them, you'll just made a bad situation worse. As previously pointed out you need to apply a load to the jaws. One way is to clamp down on a hardened disk (hardened flat washer?) with the master jaws then machine the jaws. You may want to try an insert for hard turning to rough them in and then finish with a grinder. You would at least reduce the amount of grinding dust on your lathe.

If it is a quality brand of chuck you may want to try finding replacement jaws and possibly also a scroll.

philbur
09-24-2010, 10:21 PM
Where do you buy your chucks Nick.

Phil:)


A good chuck will be off at about 1/100 mm.

Nick

Richard-TX
09-24-2010, 11:11 PM
If it were me I would buy a new one and grind the old one then keep the old one as a spare.

winchman
09-24-2010, 11:52 PM
"A good chuck will be off at about 1/100 mm."

Yeah, 0.00039" runout on a 3-jaw is REALLY, REALLY good.

Arcane
09-25-2010, 02:53 AM
"A good chuck will be off at about 1/100 mm."

1/100 mm is .00003937 of an inch.



Obviously your definition of "good" differs from mine...:D

darryl
09-25-2010, 03:56 AM
1/100 mm- ? Are you kidding? It's been a long time since chucks were carved by hand with a file- get a decent chuck.














:)

Timleech
09-25-2010, 04:12 AM
"A good chuck will be off at about 1/100 mm."

1/100 mm is .00003937 of an inch.



Obviously your definition of "good" differs from mine...:D


Not quite that small ;), too many noughts.

One to 1.5 thou seems to be a pretty good figure for a 3-jaw.

Tim

Timleech
09-25-2010, 04:15 AM
ive got a chuck ..that someones made a right mess of trying to grind the jaws ..


what are they made of.... steel?..can i add weld to them and start again ..i know they would have been hardend ..but they are so bad ..i have nothing to loose .

all the best.markj

I've got an old 3-jaw where someone has welded blocks onto the jaws, to use as soft jaws. Quite a feasible idea, but this particular chuck has so much slop everywhere that even freshly turned soft jaws are not very consistent.

Tim

MuellerNick
09-25-2010, 04:18 AM
Where do you buy your chucks Nick.

It is not the place, but the brand. :)
These chucks cost money. Sorted by sticker shock:. Röhm, Schunk, Kitawara(SP?) There are other good brands.
But one mishap when parting off and the precision might be gone.

The 250 mm Röhm chuck for my TL-1 costs 2500 EUR.


Nick

Bguns
09-25-2010, 04:25 AM
I don' t have enough money to by good crap (ultra precision) 3 jaw, will stick to OK 4 jaws for fine work...

Almost nothing I work on, is concentric to exterior..(factory belt sander polished barrels)

CNC work, in one chucking, is another matter......

MuellerNick
09-25-2010, 04:43 AM
I don' t have enough money to by good crap (ultra precision) 3 jaw ...

That's right. The basic principle is, that jaw chucks are not precise.
But there are alternatives:
Collets (depends on the quality of the collets and the adaptor)
Soft jaws (but they are of no help if the chuck doesn't repeat well)
4 jaw chucks (time consuming)
Work between centers.

If one method gives good enough results, it's the one to go with. The limits can be stretched quite with better quality.

It simply depends on the job. Often enough, the worst chuck is perfect.

Speed, precision, low price. Pick two of them.


Nick

winchman
09-25-2010, 06:50 AM
They must convert mm to inches differently in Saskatoon, Saskatchewan.

aboard_epsilon
09-25-2010, 07:13 AM
Don't weld them, you'll just made a bad situation worse. As previously pointed out you need to apply a load to the jaws. One way is to clamp down on a hardened disk (hardened flat washer?) with the master jaws then machine the jaws. You may want to try an insert for hard turning to rough them in and then finish with a grinder. You would at least reduce the amount of grinding dust on your lathe.

If it is a quality brand of chuck you may want to try finding replacement jaws and possibly also a scroll.

There aint no ORIGINAL HARDEND jaws in our country available cheap ..the only guys who have the cross reference lists of what fits what chuck ..are complete rip off merchants ..and you would end up paying £100 for a set of jaws for a half worn out chuck

you cant get any badder, when the jaws dont come to a point any more ..theres a two inch radius on them...with what were once points with say a quarter radius...now have hollows about half inch across and are bellmouthed to the back ..

like Tim said ..think that is a good solution ..

ALL THE BEST.MARKJ

PixMan
09-25-2010, 07:55 AM
I've gotten new jaws for old chucks from these guys in the past. Also a great source for soft jaws if you're too busy to make them yourself.

http://www.huronmachine.com/

Of course I don't know how or if it's economical to buy from Huron if you live outside the US, but might still be a bargain.

aviatakl
09-25-2010, 08:42 PM
Gentlemen et al

Thank you for your suggestions and tips.
OK 1. I realized after two passes of the grinder, that I had to hold the jaws tight.
2. I did this with a known steel ring and set it on the side of the jaws for holding work on the internal bore.
3. I then set the the known jaw to be 'long' at centre across the lathe bed and proceeded to grind off what I felt was enough material to bring the jaw to correct centre.

After two or three passes of the grinder, I would then dismount all the tooling and set the Dial Gauge and roll the cu
chuck by hand and observe the run out.
I had the run-out down to .1 mm after 3 sets of passes. After this, I would then machine a 30mm round bar for 60mm of travel, turn it around in the chuck and turn the same dia on the other end. I found that there was still a bit of run out to the order of .06mm with micrometer.
I set the grinder up once again as I noticed that I had ground just one side of the jaw face. I fixed this problem by rotating the chuck 5º either side of centre, thus giving me a sort of 3 facet finish to the jaw.
Repeated the testing process once again and had a near zero run out. +/- .02mm.
To answer one other suggestion you made. Yes I did check the mounting stub, the backing plate and the chuck body, all had from zero to +/- .002mm.

I then set the jaws to fully tight to the centre. I checked the 'offending jaw' for run out to the internal grip and sure enough, it had just under .23mm run out. I set the tool grinder up once again and ground the jaw down to size and then did a full turn of the chuck and corrected the other two jaws to be all the same .

I am now very happy with the chuck, turning out 2 special 3/8NPT to 3/8 hose tails in Bronze for a custom built 'dripless shaft seal' for a mates motor launch.

FYI. the chuck cost $130.00NZD, it is listed at $450.00NZD new in the box.

Thank you all for your input and help.

Jim O'Donnell (HSM 40 years and counting)
Auckland
New Zealand

vpt
11-16-2010, 10:08 PM
My jaws had a slight taper to them so I ground them today using ball bearings to load the jaws.

http://img258.imageshack.us/img258/8301/jawgrind002.jpg

jugs
11-17-2010, 05:58 AM
ive got a chuck ..that someones made a right mess of trying to grind the jaws ..


what are they made of.... steel?..can i add weld to them and start again ..i know they would have been hardend ..but they are so bad ..i have nothing to loose .

all the best.markj

Yes you can - it gives you a soft jaw chuck that you bore to size (use the tip in post 10) for each job (it only takes a few mins), that is the only way you can get a truly accurate hold in a 3 jaw, infact it's more accurate than a collet, not bad from a scrap chuck :D .
When the soft jaws are used up, just weld some more in.

john
:)

aboard_epsilon
11-17-2010, 06:55 AM
Yes you can - it gives you a soft jaw chuck that you bore to size (use the tip in post 10) for each job (it only takes a few mins), that is the only way you can get a truly accurate hold in a 3 jaw, infact it's more accurate than a collet, not bad from a scrap chuck :D .
When the soft jaws are used up, just weld some more in.

john
:)

well then ..i could weld stainless onto them and then the would be semi hard jaws .

all the best.markj

gwilson
11-17-2010, 11:09 AM
O.K.,Nick. Pick Speed,and Precision. They DON'T match up.

jugs
11-17-2010, 01:43 PM
O.K.,Nick. Pick Speed,and Precision. They DON'T match up.

Strange statement :confused: many industries do both all the time.

john
:)

TriHonu
11-17-2010, 03:54 PM
It is not the place, but the brand. :)
These chucks cost money. Sorted by sticker shock:. Röhm, Schunk, Kitawara(SP?) There are other good brands. But one mishap when parting off and the precision might be gone.

The 250 mm Röhm chuck for my TL-1 costs 2500 EUR.

Nick


I just downloaded the Rohm Operating Instructions.pdf (http://www.rohm-products.com/uploads/tx_userproducts/RN_398_neutral_06.pdf) (2.16 meg).

Page 15 states the 250 mm (9.84 inch) chuck with Class I precision is rated at 0.06 mm (.00236 inch) TIR.

The Class II chuck is rated 0.075 mm (0.00295 inches) TIR.

The 200 mm (7.87 inches) and smaller Class I chucks are rated at 0.04 mm (0.00157 inch) TIR and the Class II is rated at 0.075 mm (0.00295 inches) TIR.

The only way I see you could get down to 0.01 mm (0.0004 inch) TIR is with an adjust true version.

Considering an Albrecht Drill chuck is rated for 0.003 inch TIR. Getting less TIR on a much larger lathe chuck is obviously going to be much more difficult and expensive.

MotorradMike
11-17-2010, 04:10 PM
O.K.,Nick. Pick Speed,and Precision. They DON'T match up.

Nick said "Speed, precision, low price. Pick two of them."

In Ontario we say "Fast, Cheap, Good; pick two".


If you pick fast and good, it won't be cheap.
Pick any two and you don't get the third.

squirrel
11-17-2010, 07:18 PM
Grinding the jaws is futile, the non linearity of the scroll is the problem combined with unequal wear on the jaw teeth. Yes, you can grind the jaws but it will only be true in the exact position it was ground at. If you need precision go to a 4 jaw chuck and dial it in. A collet chuck is better than than a 3 jaw IF you have good collets. Lyndex and Hardinge have a pretty tight run out spec. All the eBay el cheapo and discount tool place collets are not very good, some one sent us a 16C sample and it was so bad I could acutally see the run out!!!