centering the chuck

hiya all,

it's me again.

the help so far has been awesome but I have notice a slight issue with the lathe.

the chuck seems slightly off centre. I used one of those finger measuring scale to see if I could get it centered. I can't seem to get it any better than 0.09mm which strangely enough is noticeable.

so I used a centre finder in chuck and measeure via that but I am still struggling to get it as good. I check the face plate on to which the chuck is mounted and that is fine.

any tips advice would be greatfull.

cheers

Becky

Hi Becky,

Is it a three jaw self centring chuck? If it is then check the manufacturers specification. It is rare for three jaw chucks to be perfectly accurate and they can be off centre by 0.1mm. Often the lack of concentricity varies with the diameter of the chucked item since it depends on the accuracy of the scroll.

The way round the problem is to machine the part without removing it from the chuck. If all the machining is carried out in this way then all surfaces should be concentric. The other ways to get round the problem is to use a four jaw chuck and a dial indicator to set the job up concentrically, or to use collets to hold the job.

Mike

thank you mike,

I noticed the problem when I had to machine a little more and it went somewhat wrong.

it is a 3 jaw chuck self centering.

am glad to here that it's not me being a biff. think that 0.09 is as close as I get it.

Hi Becky

If you look in the recent thread " Installing a new lathe chuck" Chris Heapy was exploring ways of improving the accuracy of a three jaw chuck which you may find useful in you search for precision!

Cheers

Norman

Support the workpiece with the tailstock whenever possible, this always improves stiffness on a hobby machine.

A half decent tailstock live centre is an essential asset for a hobbylathe IMO

Becky,

If you want to improve the alignment, clock the job and find the jaw that gives the highest reading on the clock, undo the chuck and slip a peice of paper between that jaw and the job. Clock again and see the result, it might mean having to shim others but you will get it better than 0.09mm.

Another trick when holding in a 3-jaw is to tighten loosely but allow to rotate, rotate the job and gently push the job in and out of the jaws....if the job is out of round you will feel it either get tight or loose, also if there is any dirt it will get rubbed off.

Might sound a bit fiddly but is cheaper than buying a new chuck.......

Trevor

Hi Rebekah

A three jaw chuck has a large disk inside with a spiral milled into the face that engages the back of the jaws. The internal plate sits in an accurately machined and possibly ground recess. The fit is not an interference fit, there is some play, there has to be otherwise the mechanism would jam. This play is one of the reasons that when place a nice precision ground bar in the chuck it often does not run true even if the chuck body is correctly centred. The internal plate has a gear machined into the rim, That is driven by the three bevel pinions driven by the chuck key.

Something worth trying is to try tightening the chuck using one pinion hole only, measuring the run out on the job then trying the same from the other key holes. You may fined one hole centres the work more accurately. This is caused by the pinion gear tending to force the scroll plate sideways the previously mentioned play allows this.

While I hesitate to say this and I can hear the screams of horror from the forum already you can sometimes (Lightly) tap the jaws with a plastic hammer to take up any slack in the scroll. while at the same time with the other hand you increase the pressure on the chuck key until it is tightened. This works well on older chucks. on bigger lathes. I would not do it on a small lathe in case I damaged the bearings. Before doing this you need to know which direction is off centre.

One thing I do do fairly often is to disassemble the chuck and give it a good clean in solvent,

Fine swarf often welds itself to the scroll and sometimes the Jaw threads (on the back) It happens quite easily due to the high pressures involved, This can really affect the accuracy of your chuck.

Most chucks have a plate on the back affixed with screws, it may be hidden by a backplate that attaches onto your spindle. Some chucks break in the middle, the screws are on the back. Note the position of the back plate or plates They should be reattached in the same position You will probably find small alignment marks If not make your own with a fine prick punch.   

Getting the scroll plate out can be a bit problematic. It can easily jamb if you allow it to turn sideways, take care and try to keep it level as you withdraw it or reinsert it. If it does jamb use a piece of wood to tap it square through the chuck jaw slots. I use grease on the bevel gears,they are effectively in a separate compartment from the scroll face, for that face I just lightly oil, grease there will just help any swarf that gets in gum up and gall.

Between strip down cleans I remove the Jaws and while rotating the scroll with a chuck key blow into each slot with compressed air. Protect your eyes with goggles, you will be surprised what comes out. Also check the scroll for attached swarf.

Cheers
John

Edited By John McNamara on 07/06/2013 15:02:53

(5) Use an ER collet chuck instead of the 3-jaw

Hi Becky, many good advices have been said above. I especially can endorse the trick to lightly tap the jaws or the workpiece with a plastic hammer. I have a small handle-less type residing on the lathe for this.

For what I will write now, I'm sure I will be nailed to the wall by others...
If you are really desperate, remove the chuck from its backplate and turn down the register one or 2 tenths of a mm. Remount the chuck afterwards. Then, if you loosen the screws which attach the chuck to the backplate (just a tiny bit), you can bring your finger thingy scale to good use and tapping the chuck and workpiece true. Don't forget to give these screws a bit of torsion again when you are happy with the runout.

Greetings, Hansrudolf

My three jaw chuck is on a back plate, I eased off the mounting bolts, just a fraction(still quite firm against the back plate), with a bar in the chuck, and a DTI in the tool post, found the high side and gave the chuck a tap with a wooden mallet. The back plate is spigoted into the back of the chuck, but there was enough movement, and I moved the chuck nearly .001", then tighten the bolts and recheck. Ian S C

Lots of advice.

I will have a look and see which one would work the best for me.

Thanks guys, i really appreciate the help you give.

Becky

Hi Becky

Lots of decent advice above but, in my view, its "get you out of jail" stuff. Handy when you have no other options but far too much faff for general use.

Best, and correct, solution is to get a four jaw chuck and use the indicator to set things up as accurate as you need. Once in practice you get decently quick at the mount up and indicate business. The old boy who claims he can set stuff in a four jaw as fast as you can use a three is exaggerating but not vastly so.

Various tricks to speed things up. For example if doing several parts the same size only release two jaws at 90° to make the change. Once you have the knack in intial setting and changing things will go back to rather better than a thou (0.02 mm). Another time save is to have a quick mount set-up for the indicator. My old Heavy 10 had a flip down chuck guard pivoting off a rod above and behind the chuck running parallel to the bed. I made up some simple bracketry to mount a Verdict indicator and fine adjuster rod from a magnetic base kit on the spare part of the chuck guard rod. Four four jaw work I'd leave it all mounted and just pivot it out of the way when turning.

Realistically its unreasonable to re-chuck work in a 3 jaw and get it to run true whatever you do. My best 3 jaw is a Pratt Bernard precision one bought new (ahem) "several" years ago when I picked up a very nice Heavy 10 lathe. £ 500 for the lathe £ 550 for the chuck as I recall matters and significantly less than 0.5 thou error last time I measured. Even that isn't good enough to reliably re-chuck work without faffing.

For small jobs consider a master-slave system where you make a suitable carrier to go in the chuck and put the job in that. Theoretically you get perfect results if you make the carrier and do the job with it still in the chuck. In practice its pretty acceptable to mark the carrier so it can be aligned with the correct jaw to go back in the right place when re-mounting. Maybe a little twist ajustment using the indicator for reference if you want things to be just so.

Clive

Hi MikeW, Can you enlighten me on your suggestion of chucking bushes please. Use chucking bushes for accurate work and especially for second operation work . There are several types - split , thin wall and threaded are most common . Regards, Bear..

Becky, some decent quality 3-jaw chucks came with a mark against one of the key sockets indicating which should be used last when tightening up to get best accuracy. My Chinees lathe came with a chuck marked like that, though I suspect it is just cosmetic .

Mark or no mark, it does no harm to use all three sockets in turn at the end of tightening up, just as you would with a keyed drill chuck..

As has been said, any self centring chuck will exhibit runout unless you are very lucky, and even then you may find that though it might hold one diameter with tolerable accuracy, other diameters will be held out of true.

Andy

Hi Rebekah,

If I understand what you are trying to do correctly.

The problem is not yours alone in that any work done in a 3jaw chuck is never going to go back in the right place again so as already has been noted try a 4 jaw or do all the machining at one sitting.

Or you could do as I have done get a collet chuck and then round items can be put back at anytime.

Peter

Edited By peter walton on 09/06/2013 13:13:34

Or..... get some soft jaws for your chuck and bore them on the machine

Mark