AWGB Woodturning Forum
General Category => General Discussion => Topic started by: Lazurus on November 05, 2015, 08:18:34 AM
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On my last lathe I had a natty little gadget that allowed me to turn perfect spheres, now on the VB36 I am going to have to use "skill". Any advice as I wish to make some balls for a local pub skittle game?
Steve over to you for advice........
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was that a self made ,bed mounted ,gizzmo youhad on you last lathe.
if so how about remaking it on a 45mm column to fit in vb tool post.
Dave Willcocks
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Yes it was home made, however the local guy who did the metal work / welding has since died and now stuck for a decent engineer, but I think that one sat on the bed bars as opposed to the tool post to give a vertically central pivot point. Not really sure of the geometry to give perfect spheres from the tool post?
All suggestions gratefully received.
Stuart
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you can see the one i have here,
http://www.awgb.co.uk/awgbforum/index.php/topic,2885.0.html
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the geometery is quite simple ,and can be seen in the linked pictures. the pivot point is aligned to the axis of the machine ,the cutting edge is then set to centre height ,the radiusyou wish to cut is a function of how far you move the cutting edge toward the perpemdicular axis of the pivot point.
so as in the photos the 2 axis's of the pivot and cutter height have to be at right angles to one another to coincise with the centre of the sphere you are making.
there for the whole mechanical setup could be rotated around the machine longtitudinal axis and mounted off of the tool post (as there is no bed to speak of on the VB) this would have the cutter describing an arc lenghtways between centre vertically over the top of the sphere.
i can visualise it ,not sure i've explained that clearly enough.
Dave Willcocks
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Perhaps for a toolpost mounted version it would be easier if the axis of rotation of the cutting assembly was horizontal instead of vertical? That way you don't have to centre the toolpost under the blank, you just set the cutter's axis at spindle height.
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hi Paul
thats what i was trying to get at without using the vertical and horizontal wording.
the gizzmo i have fits in toolpost and the cuutter describes a horizontal arc over the top of the sphere along the long axis of the machine.
the guy that makes them is on ebay mine will do up to 100mm dia sphere he currently makes the gizmo to do up to 150mm diameter.
Dave Willcocks
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Paul: you still need to have the axis aligned, otherwise you'll never get to the "poles" of the sphere. Although I can see the benefit of using the banjo with an existing clamping device, I think it'll be a faff to set it up every time, as the banjo moves freely in both directions (laterally as well as longitudinally). It's probably easier to make a new stand for the old gizmo, from plywood, with a central protrusion that fits snugly between the runners of the lathe bed.
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before i had my jig i looked at all that i could find for sale and was depressed by the lack of options in this area. the ones for sale on ebay are very well made but inadequate for turning my skittle balls as they can be up to 6" in diameter and i also thought that the one on ebay looked a bit feeble on the construction side.
the one i ended up designing was done so to overcome any possibility of breaking it as if i did so whilst turning it would not only be dangerous but i would have to disappoint my customers.
mine is a simple design but it does the job and is only 1/16" out of spherical once finished.
for doing variations on a sphere my design is useless but it won't move around like a banjo can and will be in the same place every single time.
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This was made by me originally for my graduate lath with a 1" stem then adapted for my VB inserting it in a 40mm stem, this
enables the Beniston adaptor to be locked on the 40mm stem, the 1" stem left loose .... if you start off with 40mm you will
have to put it in the main tool post & leave loose .... the pillars are 3 1/8 enabling a 6" ball to be turned it can then be slid
in & out for smaller sizes . It does take some setting up in center to traverse right round the front for a complete ball
Regards Tony
(http://i986.photobucket.com/albums/ae345/dadothebold/IMG_68181_zpswd4qtnfx.jpg) (http://s986.photobucket.com/user/dadothebold/media/IMG_68181_zpswd4qtnfx.jpg.html)
(http://i986.photobucket.com/albums/ae345/dadothebold/IMG_68171_zpsrznz7sqs.jpg) (http://s986.photobucket.com/user/dadothebold/media/IMG_68171_zpsrznz7sqs.jpg.html)
(http://i986.photobucket.com/albums/ae345/dadothebold/IMG_68161_zpshnnfbb8m.jpg) (http://s986.photobucket.com/user/dadothebold/media/IMG_68161_zpshnnfbb8m.jpg.html)
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In a recent demo we were shown a good way to deal with the spigots produced on parting off. He had two jigs with short pieces of 2" dia plastic pipe glued on. These were slightly chamfered on the inside of the bore. So with one on the headstock and the other held by a running centre the ball was sandwiched between them at 90 degrees to the knobs. These now easily removed. The final sanded finish was done by slacking the tail stock and randomly resetting the ball.
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Yes, that works, but on soft woods it leaves marks. I have found it better to turn two pieces of wood with an approximately spherical concave shape, then cover them with double-sided tape and stick some rubber mat on. Much friendlier on soft woods, and just as functional. Best made out of moderately soft wood, e.g. sycamore or poplar.
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www.paulhowardwoodturner.co.uk
Had some correspondence with the guy above he has and can make a jig specifically for the VB36 - however like most things its quite expensive.
Stuart
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I'm a little late on this one, but I have never used a jig for this, just turn to a template, then use wooden cups to hold the ball. This might explain it better:
https://www.instagram.com/p/-MWIhugSSk/?taken-by=richard_findley
HTH
Richard
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Thanks Richard, very accurate for free hand. There is hope for me yet. ;)
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how are your balls looking stu? ;D
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Been put on "hold" due to a few Christmas requests. Still on my to-do list though........