Modification to Soft Jaws

[Ian PParticipant @ianp]

I am making an underwater housing to contain a gyroscopic stabiliser for a video camera. The outer tube is a 200mm long slice of 9″ OD x 1/4″ wall ali tube. In order to machine the tube ends square I intended to grip it with the external jaws of my 160mm chuck (the jaws have ‘just enough’ scroll engagement) but I had forgotten that once the tube was in place the pinion key holes would be inaccessible!

Fortunately the used ebay chuck came with two sets of soft jaws (which I have never needed up to now) so I drilled and tapped two M6 holes of the outer ends of the jaws and have bolted on some ali jaw extensions that now allow the tube to be quite stably gripped. Eagle eyed readers might wonder what purpose the semi circular notches serve? I do too, basically I thought that they would allow chuck key clearance to give just a little more jaw length!

 

Ian P

[Peter Cook 6Participant @petercook6]

I suspect that the jaws are not intended to be used as you have, but to replace/fit on top of the steel jaws. In that case the circular notches would allow you to tighten the Jaws on a washer or small disk to keep them tight while you machine the faces to suit the thing you want to grip.

[Ian PParticipant @ianp]

No, the notches are due to a brain f*rt!

In the picture the ali soft jaw extensions are mounted the opposite way round to what I had intended as the holes are not quite symmetrical so the grip length is slightly longer.

I intend to machine the jaws in situ so their OD is close to the tube ID but so far have not worked out how to preload them.

 

Ian P

[Nigel Graham 2Participant @nigelgraham2]

I assume this is posed, before installing a plug for tailstock support? Even with the extensions that is not a very secure mounting.

To pre-load the jaws for rounding their outside surfaces, tighten them onto a billet of appropriate diameter.

[JasonBModerator @jasonb]

How would you keep a plug in place and be able to machine the end?

As it is being used to hold the work internally you don’t want to preload it by tightening, if it were needed then open up the jaws against a ring so the force is in the direction they will grip. But As it is just for end facing I doubt a bit of eccentric running will matter, tube is likely to distort anyway.

[Nigel Graham 2]

On Nigel Graham 2 Said:

To pre-load the jaws for rounding their outside surfaces, tighten them onto a billet of appropriate diameter.

As above, that puts the pre-load force in the opposite direction to which it is needed.

Drill and ream the face of the jaws for three dowel pins. Expand the jaws onto the ID of a bearing race and turn the aluminium. Choose dowel pin PCD to suit bearing ID making sure at the same time aluminium ones are in correct position.

A rotating tailstock chuck with similar outside jaws could be used to support the free end. The advantage of that is if the entire chuck body is inside the pipe, the shank on which the chuck is mounted is much smaller diameter, giving a lot more space for tool access.

To put a centre plug in the end of the tube, it would be possible to make an internal version of a cat head – a solid bar with three projecting bolts that can be secured in the end of the piece and then a centre drilled into it insitu.

Edit: the dowel pins also can be used to grip the outside of a bearing race if you need to modify the inside gripping surface of the soft jaws.

[Martin ConnellyParticipant @martinconnelly55370]

I think, on the basis of the ali jaws being so long and extending in front of and behind the scroll, that in this case it does not matter if something is gripped in the centre of the chuck or not. The scroll force on the jaws will force the freshly skimmed ali bars to align with the surface anyway and, as Jason pointed out, eccentricity is not an issue for facing off only. If there is intention to cut threads on the outside of the tube ends then that might require restraint for the jaws being expanded outwards but then that requires that the wall thickness is the same all the way round as well.

Martin C

[Ian PParticipant @ianp]

Only just got to my PC so apologies for not replying sooner.

Picture is posed but it was just meant to show what the job looked like. My intention was to drill and tap the ends of each jaw and fasten an MDF disk slightly larger than the tube bore. The disk would serve three purposes:-

1, To allow the jaws to be preloaded outward

2, To give me something to measure (diameter)

3, To give support whilst machining, after turning the MDF to be a snug fit in the tube.

I ran the lathe with the tube posed on the only loosely tightened jaws and was surprised how concentric and true it ran even though I knew one jaw was half a millimeter ‘high’ although that was with chuck running fast enough to throw the jaws outward against the scroll.

I only need to square up the tube ends and I will do this very gingerly with a polished carbide insert. I also need to put a generous radius on the inside edge to ease entry of the O-ring fitted into the end cap, I will do this with a hand deburring tool (Noga) at slow speed.

The 6082T6 ali tube is listed as 9″ and 1/4″ wall. I have not checked the 9″ but the wall thickness is actually 6.5mm which is great from my PoV as it gives more room for the M3 that will be retaining one end cap semi-permanently.

Ian P

[Author]

Posts