|Tony Pratt 1||18/08/2019 09:58:27|
|916 forum posts|
I'm afraid that is the way to do it, I use copper packing also between jaws & work piece, it's a real pain but in the real world chucks don't always perform as you would like them to.
|Michael Gilligan||18/08/2019 10:22:37|
14150 forum posts
I may be wrong, but I don't think you have told us the size of the chuck
All the discussion has therefore been made on the basis of our assumptions.
... Could you please confirm ?
|Martin of Wick||18/08/2019 10:28:48|
|93 forum posts|
These are all really useful tips that I will keep in mind for the future, but for any of them to succeed, I think you need to have some degree of parallelism in the jaws to start with!
Using a bump centre to attempt to address the issue was quite entertaining. With the jaws set semi tight the topslide and toolpost would flex and twist in the most alarming manner. Slackening off the jaws to allow the work piece to move would get the axis OK at the bump, but everywhere else on the bar would be out of true.
I am certainly not an uber-perfectionist (slipshod and cavalier would be a better description), so could have lived with 4 or 5 thou axial drift over 4 inches, but not 30 thou!
I had been using the 4 jaw in an attempt to be a little more rigorous in my approach and working on short pieces, all seemed to be going OK . The issues only became apparent when working on the spindle for the lathe handwheel, a longer piece that needed internal and external concentricity on re-chucking to drill, thread from both ends etc.
Same problems showed up when mounting 3 inches of 45mm steel bar to skim a piece for the clamp fitting - and even setting the working end to true, the chuck end would precess and it would go out of centre with the lightest skim.
Thing is, until you examine the jaws in detail, the chuck looks quite good - the jaws are a good fit in the slots and the movement is smooth. It is just that the jaw faces have not been ground properly. If the chuck had been second hand, I would have attributed the problem to someone having attempted a bit of casual, freehand jaw grinding for personal amusement. That is how bad the clamping surface of the jaws are. Strange as the 3 jaw chuck from the same manufacturer was outstandingly good.
I shall give the vendors a call on Monday to see if they can help.
|Pete Rimmer||19/08/2019 08:28:09|
|446 forum posts|
What are you using as a gauge? Simply touching two opposing jaws together and observing the light gap should tell you if they are parallel or not.
|Paul Lousick||19/08/2019 10:28:48|
|1199 forum posts|
It is very easy to fix your chuck. As you have previously said " Checking against a backlit straight edge was utterly disheartening - some jaws were convex, some hollowed, others had uneven teeth (serrated jaws). A new chuck!!"
If its new, take it back for a replacement or a refund.
|Ian P||19/08/2019 10:48:31|
2220 forum posts
Two jaws might pass that test and be parallel to a 1 micron in 10 meters, but they may not be parallel to the lathe axis!
Apart from using the toolpost grinder method to true up the jaws, consider doing each jaw individually with a diamond file.
I have dramatically improved two different SC three jar chuck using little more than a diamond file and a bright light source. Since your chuck has independent jaws it would only be a few minutes work to touch up each jaw tip,.
Start off by checking each jaw (out of the chuck) with a set square, that will show any gross error which a diamond file will quickly correct. Once all the jaws are somewhere near they can be checked in the clamped position with a suitable diameter piece of ground test bar or even a length of silver steel.
Its a bit fiddly but an additional check you can do is to wrap a single layer of aluminium cooking foil around the test bar than tighten the jaws on it. You can then see if the whole length of the jaw tip is applying equal pressure.
|Martin of Wick||19/08/2019 14:09:28|
|93 forum posts|
This is a 4in chuck that I hadn't used for precision work until now because the 3 jaw that came with the minilathe was so good! On that basis I wasn't expecting problems. Come the day I needed an accurate set up there was a wailing and a gnashing of teeth.....Having not ever experienced problems with a 4 jaw chuck before, until someone pointed out it might be an issue with the chuck I had assumed it was some fundamental operator error!
Eventually checking the jaw mating surfaces alignment with the chuck body when tightened on a short round of silver steel, using a square off the back of the chuck shows the following.
'Go figure' as they say....
It would appear that the long surface of the jaws were not ground originally parallel and there has been some bodged attempt to correct the problem at the factory (possibly explaining why the clamping surfaces are much wider than expected).
The result is it is almost impossible to get an acceptable axial setting for longer work using the jaws with the long edge holding the work.
In other respects, the chuck is not to bad, useable with the jaws reversed, the axial surface of the jaws is true and the jaw fit in the slots good. with smooth movement.
The approach I would use based on a posting above is to lightly clamp up a washer in the gap between the rearmost two teeth. with the chuck stationary and spindle locked, grind all the forward teeth of the jaw to something resembling an even height and hopefully parallel to the lathe axis, repeat for other jaws.
Then go back in with the grinder to grind down the tooth that was behind the washer to below the level of the newly ground jaw surface so it will not impact clamping geometry. This may mean that the minimum stock size that is chuckable increases from 6mm to about 8mm, but I don't think that is going to be a particular issue.
I would like to think the vendor might offer to replace it, but it has been over 18 months since purchased - moral here is if you get some new stuff give it a good teardown and test the day you get it!
|Peter Tucker||19/08/2019 18:44:22|
|181 forum posts|
The vendor should replace it as "not fit for purpose".
|John Pace||19/08/2019 22:18:25|
|156 forum posts|
Posted by Martin of Wick 19/08/2019 14:09:28
The approach I would use based on a posting above is to
The jaw needs to be restrained towards the front edge so when the chuck
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