truing a 3 jaw chuck

I have a small lathe - Made in China! - with an 80 mm self centering 3
jaw chuck. It is about 0.003" out of true. Has anyone tried truing up
chuck? I had a look at the US 7 x 10 web site and someone there
describes a method, but it is not all that clear, or at least to me it
is not. Any ideas?
Eddie Price
Reply to
eddie price
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Hit it with a mallet!
It depends on what is out it could be the scroll, it could be the mounting of the chuck or it could be the jaws.
Does the body run out? How is it mounted, screwed or bolted?
If work holding is out then for what diameter.
If always out in the same for any diameter then it may be a jaw which could be ground.
If it out various amounts for different diameters then could be the scroll.
3 thou out is not a lot to be out on a basic three jaw chuck.
Adrian
others may disagree.
Reply to
Adrian Hodgson
if you have a dremmel type tool , set it up in the tool post , lightl
tighten the jaws against small pieces of wood between the sides of th jaws , then lightly grind them out
-- willowkevi ----------------------------------------------------------------------- willowkevin's Profile:
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Reply to
willowkevin
Just use shim... It's accuracy will change over time 8-)
Joules
Reply to
Joules Beech
Sounds about right for a 3 Jaw - use a 4 jaw for closer work.
Steve
Reply to
Steve W
Here's my idea.
Before you go all crazy with a grinder of any sort, do some troubleshooting.
You will need a dial indicator with a stand, and a couple different size objects that are known to be accurately round.
The shanks of drills, dowel pins, bearing races, ground drill rod. Gotta be round. Straight is good too.
Set yourself up with the dial indicator and a sharpie marker, and work through a series of your round objects, inserting them in the chuck. Using the dial indicator, find the highest point, and mark it on the chuck with the sharpie.
Object of this is to see if the high point is consistant (as it would be if the chuck was not installed on the backplate properly, or one jaw was out of whack) or if the high point moves around (result of worn or just out of whack scroll)
If the high point is constantly in the same spot, the cheap fix is to loosen the mounting screws or bolts a bit and tap the high point with a soft mallet. With a bit of work this way you can dial in the chuck a lot closer than the 3 thou you have. Once done, check it with the other diameter objects. If the register on the backplate (the part of the backplate that fits into the chuck) is a tight fit, it is possible to take a light cut off the register to allow a bit of movement, then tighten the bolts back down.
If the chuck does not have an integral thread mount, this is a better bet than grinding jaws, as you can redo this when you must have best accuracy for a particular part size.
Grinding the jaws on a chuck that has a worn scroll is just moving the high spot around, except that the jaws will be truer at the exact diameter they were when they were ground. Not much of a gain.
Cheers Trevor Jones
Reply to
Trevor Jones
Trueing up by jury rigged regrinding is certainly possible but before you commit yourself try a couple of simple checks.
1 Check the runout of the chuck body. If it's more than 0.001 TIR a few gentle thumps with slackened fixings will usually improve matters
2 There is always a very small residual clearance between the scroll and its centre bearing. Each of the three tightening pinions takes up this clearance in a different direction. Check the runout resulting from using each of the three pinions - mark the best pinion for future use.
3 Don't chase perfection. Different diameter workpieces will have slightly different runouts.With run of the mill three jaw chucks it's difficult to stay much below 0.002" runout, If you need better than this use collets or use soft jaw inserts that can be trued to fit individual workpieces.
Jim
Reply to
pentagrid
Thanks Trevor.
I started off by checking the headstock shaft with the backplate off. This was okay. I then added the backplate which has a recess for the shaft, which is a good fit. This was okay. I then added the chuck, which again had a recess for the backplate. I found the best position, which resulted in a eccentricity of about 0.0005", and marked the chuck and backplate for future alignment. I then mounted a 10mm drill shank in the chuck, and there now seemed to be an optimum position. I can't imagine that the drill shank is out of true, although I suppose it could be. More likely the chuck jaws tighten in slightly different positions each time I slacken off and retighten. Anyway the best position now gives me eccentricty of about 0.001" So I think I will leave well alone.
My next problem is to reduce the chatter when parting off. Are roller bearings the answer. A recent article in ME reports on replacing the bearing in a Clarke 300M with much improved performance. Has anyone else tried this? I will need to find a suitable bearing.
Eddie Price
Reply to
eddie price
May be nothing new: One of the chuck's key holes is marked with a "0". This is normaly the one you should use for highest accuracy. At least it was so when it left the factory.
Nick
Reply to
Nick Müller
Below is an extract from a letter sent to ME last year. Hope this helps.
Archie
Like Harry Barrett, (ME 4238 7th January 2005) I have been having chattering problems with my Chester Conquest mini-lathe. This was possibly due to me getting a parting tool jammed under the work a while back. It was at the stage where a 15 mm steel bar could not be faced when only 10mm from the chuck. I phoned Chester for some advice and was told to try and tighten up the C nuts on the end of the spindle to take up the play. This did not make any difference and the reason for this was found out later. I asked about the feasibility of fitting taper roller bearings but was told that they were slightly larger than the originals at 17.25mm compared to 16mm and that fitting angular contact bearings would be easier as they are the same size. The gentleman from Chester ( I didn't get his name) also gave me the confidence to tackle the job myself.
In stripping down the headstock, the biggest problem I found was removing the woodruff key from the spindle. This was eventually removed by gripping the key in a vice and levering it out. I fitted the two new 7206 bearings sourced from RS Components and after reassembling the headstock, could not tighten up the C nuts enough to take up the play in the bearings. Even though I did not have the proper C spanner and was using slip joint pliers something was obviously wrong and I had to strip it down again. The distance between the inner faces of the bearings on my lathe is 60mm give or take a bit. The spindle gear and spacers which keep the gear centered between the two bearings added up to 62mm. No wonder that my initial attempts to take up the play were unsuccessful. The spacers are made from plastic so I decided to sand off about 1.5mm from each which would allow the axial tension required by the bearings. After reassembly, it was a joy to be able to cut metal without chattering. Things were going so well that I attempted a parting operation again. Another jammed tool and more play on the spindle. This time, a quick tightening up of the C nuts and was working again.
I don't know if the lathe alignment is seriously affected by replacing the bearings but for me, it is much more important to be able to cut without the dreaded chatter. If my skills ever reach the stage where accurate alignment is important, I will look at it then and maybe even mount the lathe on a solid surface. Do I hear shouts from the more experienced that my lathe will be wrecked first?
I do not know whether the taper roller bearings or angular contact bearings are superior in a lathe headstock but did notice that Myford use a plain bearing. A search for angular contact bearings on the internet found a reference to the fact that a Hardinge HLV which is described as a super-precision toolroom lathe is fitted with them.
Reply to
Archie

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