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4 jaw chuck axial allignment

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Tony Pratt 118/08/2019 09:58:27
885 forum posts
2 photos
Posted by duncan webster on 17/08/2019 20:44:37:

use a small length of engagement, set true near the chuck, then move the dti to the far end and set it true by tapping it with a lead weight, then go back to the chuck end, repeat till you get fed up

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.

Tony

Michael Gilligan18/08/2019 10:22:37
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13801 forum posts
599 photos

Martin,

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 ?

MichaelG.

Martin of Wick18/08/2019 10:28:48
91 forum posts
4 photos

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 Rimmer19/08/2019 08:28:09
397 forum posts
18 photos

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 Lousick19/08/2019 10:28:48
1150 forum posts
492 photos

Martin,

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.

.Paul.

Ian P19/08/2019 10:48:31
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2144 forum posts
89 photos
Posted by Pete Rimmer on 19/08/2019 08:28:09:

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.

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!sad

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.

Ian P

Martin of Wick19/08/2019 14:09:28
91 forum posts
4 photos

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.

  • short jaw surface looks reasonably normal and parallel to the square - caveat as it is only a short length and quite heavily serrated. - so no problem with one side of jaws
  • Long jaw surface - two are quite severely and obviously not parallel - around 20 thou out! the other 2 are also not parallel but to a lesser extent. In addition, by varying amounts, the serrated teeth do not form an even surface when viewed against a straight edge.
  • the two worst jaws are roughly parallel to each other but not to the chuck body
  • the other two effectively create a bell mouth.

'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.


I will attempt a regrind, but first I will need to make some sort of height adjustable fixture to hold a rotary tool in the toolpost - no slotted table on the CML otherwise I could use a vertical slide.

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 Tucker19/08/2019 18:44:22
179 forum posts

Hi Martin,

The vendor should replace it as "not fit for purpose".

Peter.

John Pace19/08/2019 22:18:25
150 forum posts
149 photos

Posted by Martin of Wick 19/08/2019 14:09:28

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.

Martin

The jaw needs to be restrained towards the front edge so when the chuck
is tightened up the jaws will grip along the length.If you trap the jaws at the rear
there is the possibility that when the jaws grip the front edge will not be so secure
effectively slightly bellmouthed.
The photo here uses a washer and a piece of wire to apply some tension to
the front of the jaw ,this is all that is all that is needed to do this on this 80 mm
chuck .It would be fairly easy to make a more substantial clip from more
solid material if required.
The washer is cut to clear the jaw edge ,the other jaws are retracted clear
the chuck key is used to oscillate the jaw to and fro past the grinding wheel.

4 jaw grind.jpg


John

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