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Club Member question.

10 July, 2010

NOTE this is no longer an active question, but is left here for interests sake. Thanks to all those who responded with suggestions!

 

Club member Derek Luke has a wood turning challenge with which he would like assistance. Maybe some of our more experienced members can offer some advice.

I have made a cone which I hope to mount on a spiral silver strip if I can finish it.
Measures  Ht. 155 m.m.   width 160 mm
Now I want to hollow it to make it into a bowl
The shape seems difficult to make a wooden holder
How can I reverse mount this shape.
Specific details would be an advantage as I have not tried this before.
Maybe someone on the club help board can help

Regards
Derek LUKE



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3 Comments leave one →
  1. Jon Gibbs permalink
    13 July, 2010 9:37 am

    Hi Derek – Interesting problem.

    I don’t know what others think but I think I’d resort to some deep wooden chuckjaws (like these http://www.davidreedsmith.com/Articles/WoodenJaws/wooden_jaws.htm) and turn a shaped recess for your cone in them. They’ll need to be very deep jaws (nearly 6″) so that the point doesn’t foul on the chuck body and so you may have to be careful not to put too much stress on your chuck if you tighten them down too tightly. I then think that you’ll need to shape the insides of them to suit your cone and have an inward facing lip (a bit like a dovetail) on the front edge to stop the cone being forced out when you tighten the jaws down.

    Alternatively, and if you haven’t got a chuck, then you could try shaping a conical hole into a sacrificial block on a faceplate and applying hot-melt glue like a weld where the cone meets the front lip of the conical hole. I think it would need to be quite a deep block (maybe 4″ or so) but you could laminate a block up from 3/4″ MDF disks. Provided you take light hollowing cuts you should be ok – but you will need to keep an eye on the glue joint. If it starts to move use a heat gun to resoften the hot-melt welds. When you’ve finished the cone, use the heat gun to remove the cone from the recess. If you need to remove any hot-melt glue residue from your cone at the end then cellulose thinners will work a treat.

    I am tempted to say what the village idiot said when asked for directions – I wouldn’t try to go there from here 😉 With 20:20 hindsight (the best sort), I think I’d have left a spigot on the pointy end of your cone to allow you to hollow it out and then reverse turned it like a conventional bowl in wood-jaws to remove the spigot and get the pointy shape.

    I hope this helps
    Jon

  2. 25 July, 2010 9:56 am

    Dave Springett showed an interesting method of chucking odd shaped items when he demonstrated at HWA – he turned cup chucks and then used expanding foam to form the receiving shape inside the cup chuck, using the actual items wrapped in cling film as his forms.
    Workholding relies on a screw-on exterior rim holding ring (which would hold on your front edges) while the hollowing commences, and at the end of the hollowing, the tailstock could be brought up with a soft cone shape on it to hold the work in place in the foam chuck while you turn away what you need to at the edges to make it perfect.

    If thickness is important, a slot/slots could be cut down into the jig and through the foam – allowing access to the cone with thickness callipers. The integrity of the wall strength of the jig (against centrefugal force) could be maintained by the screw on rim being fastened into each independant section of the jig, plus when you remove the ring to finish the edges, you could use those plastic pull-fasteners or a large jubilee clip or two to hold everything up tight while you turn.

    If I were to do that project, that is the way I would try first. It is a method which hold the work in from all sides at all times.

    Good luck with it, and I hope we get to see the finished project at one of the HWA meetings!

  3. Jon Gibbs permalink
    26 July, 2010 10:14 am

    Incidentally I am re-reading Dave Springett’s book Woodturning Wizardry today and have another couple of thoughts prompted by Jeans response….

    You could use something like a donut chuck with a short cone at the headstock end to hold the pointy end concentric and then have a ring with a recess turned on the back to hold the rim (as Jean suggests) and some long carriage bolts between the two parts so that the cone can be trapped in-between. I think if you look here you’ll get the idea… http://azwoodturners.org/DoughnutChuck.pdf

    You could then hollow through the ring on the front.

    Some precautions though….
    Have the nuts and any protruding threaded section at the headstock end and try to have the bolt heads flush with the front ring!
    Feel the thickness only with the lathe stationary – not through the rotating bolts!

    Dave S says that you can use a microwave (he says 20 seconds on high) to melt hot-melt glue rather than using a heat gun which means you can use something other than a surface weld for holding it into a chuck.

    Happy turning
    Jon

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