New carver with a question

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New carver with a question

Postby dervishcarving » Sun Aug 12, 2012 7:14 pm

Hi all. I started using our bodging-club's pole-lathes to make bowls recently and am completely hooked.4/5 have come out quite well and seem to be drying without cracking. Mainly I am carvign bowls of 4 - 6inch diameter but am slowly workign up to larger ones. Today i tried a 10-inch that was going well untill i had a dig-in that turned into a hole. Its close to the rim so i can trim the bowl down to make a shallow bowl/plate.
Anyway, I have a question. I use cord not leather straps, usually with 2 turns round the mandrel and i find that large bowls (12 inch diameter, 3 inch thick) have so much momentum that when i reach teh bottom of the down-stroke on the foot-pole the cord stops pulling but the bowl will keep turning. Of course i just wait until it stops and then let the cord back up (we use Bungee cords). However, this made me think. I am loosing a lot of energy here and i wonder if its possible to do somethign that will allow these larger bowls to keep turnign in the same direction? I have heard ideas such as a piece of leather loosely wrapped around the mandrel, under the cord. when the cord is pulled downwards teh tension would tighten it around the mandrel and the bowl will start to spin. release teh tension and it would loosen and be 'sprung' back up whilst the bowl continues to turn, then you push down again. I have no idea if this would work, any opinions out there?

On a similar note, those of you that carve the larger bowls (Robin in particular i have seen in videos carving some very large lumps of wood), do you use cord or something else?
Thanks to all of you who have shared advice and videos, they help poor beginners like me get startd
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Re: New carver with a question

Postby Davie Crockett » Sun Aug 12, 2012 8:47 pm

Hi Dervishcarving and welcome to the forum.....

David Mann made a one way mandrel for the Bodgers Ball a couple of years ago http://www.theslidymandrel.co.uk

There are also ideas in this forum using a bike sprocket/chain/bungee mechanism. (use the search function to find threads).
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Re: New carver with a question

Postby gavin » Mon Aug 13, 2012 9:34 am

For earlier comments and Robin Wood's take on this refer here.

I submit that if your blank is spinning very strongly at the end of your stroke it is really too heavy for a beginning turner - you don't need such a lot of kilos spinning round to start with. And for an experienced turner it is probably too heavy - you only have about 500- 700 watts at your disposal, and generating that all day is like hauling sledges over the Arctic or competing in Tour de France. You can do it, but you need to be supremely fit and you'll eat about 6,000 to 8,000 calories per day. Far better to work at a sustainable rate and use a blank max 8 inches diameter to start with and say 3 inches deep.

You certainly can come up with a ratchet, one-way drive system. You can embed bike sprockets and other things. But would you rather be turning wood in a primitive and accessible way that anyone COULD do should they apply themselves, or would you rather invent something that will work, but will be beyond the ability of most to access, much less execute?

If you did wish to be engineering-creative I would applaud your effort and be interested to see it. If you wish to be a engineering or technical sort of turner, then you'll probably be more interested in powered woodturning.

I reckon the value of what we do is that it is really democratic: anyone can do it. You don't need a lot of cash, much less access to engineering skills. So your ratchet idea is probably too advanced for this forum. And I may be wrong: you could well invent some new scheme which is jaw-dropping simple and gorgeous-effective. For the moment, I would concentrate on volume production: try and make 6 nearly the same. Then worry about ratchets - if it then seems appropriate.

I have read ( but never tried ) that seal-fur has a ratchet effect. I have seen it laid in the soles of cross-country skis where it stops you slipping backwards. Perhaps you can buy some strips of seal skin and incorporate that in your drive strap and report back as to whether this will work as a clutch.
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Re: New carver with a question

Postby dervishcarving » Mon Aug 13, 2012 12:47 pm

Hi and thanks for the replies, The beauty of a forum is that you can ask questions or pose ideas and get feedback from many brains, thus you benefit from the experience of others :) The purpose of this question was simply to see what ideas people out there had. I dont see me making many large bowls ever and certain;y not yet. I have managed 6 bowls so far and the first one was a disaster. the next 5 have been ok but I need experience and practice. I tried a large one simply because i had a piece of wood that would make one and i thought 'why nto have a go'. now i know why not :shock:
Ok.. so the Slidy Mandrel is amazing, that really is teh work of a gifted mind. a simple solution that only requires a lot of skill with the chisel and you are set! Unfortunately i dont have that kind of small-scale skill (my main practical skill seems to be chainsaw carving which isn't really appropriate for that kind of work :D ). I might give the fur or skin idea a try sometime, one of the other bodgers in Stonehaven had seen something along those lines but couldn't remember the details, hence the thread...
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Re: New carver with a question

Postby Davie Crockett » Mon Aug 13, 2012 1:49 pm

Here's something I knocked up in about 1/2 an hour. (I had made the mandrel previously)....... (Picture heavy)
Image
The basic "4 gear" Mandrel.

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A second hand rear sprocket (£2.50) from http://www.bricycles.co.uk/

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Rear view of sprocket.

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The Mandrel forced into the splined end of the sprocket (Mandrel turned .5mm oversize and drifted in). Note the end piece is recessed to accept the end of the mandrel when it's driven in.

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The finished article.

Image
Another view.
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Re: New carver with a question

Postby Davie Crockett » Mon Aug 13, 2012 1:58 pm

I should add that if you don't want to use chain drive. The sprocket cogs are removable, it would be a simple matter to turn a wooden sleeve to replace them.
IMG_3484.JPG
Sprocket removed and collar turned to fit.
IMG_3484.JPG (163.42 KiB) Viewed 13068 times
Attachments
IMG_3488.JPG
Collar in place (Secured with 4 screws).
IMG_3488.JPG (114.42 KiB) Viewed 13068 times
Last edited by Davie Crockett on Tue Mar 04, 2014 6:50 pm, edited 1 time in total.
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Re: New carver with a question

Postby dervishcarving » Mon Aug 13, 2012 3:09 pm

wow, thats a very neat/simple solution there. I am trying to see how one uses it as it stands... The (on the picture) short end forced into the rear of the cogs i assume (if it were longer) would accept the drive-cord and the impetus. the other half of the mandrel would take on the impetus when suppliled but would allow the moment (angular momentum) of the work-piece to continue rotation whilst the cord is returning ? I dont use a chain drive so i would need to make the right hand side longer to accept the drive cord.

or have i gotten it backwards?
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Re: New carver with a question

Postby Davie Crockett » Mon Aug 13, 2012 3:52 pm

Not quite dave, the Mandrel itself acts like a solid piece. it's the cogs which drive and freewheel. I'm figuring out how to utilise the chain drive on my bow lathe.

I thought bow string link to chain, round cog and back up, chain back to cord, over a pulley and down to the foot plate.

It may well be possible to take the chain directly to the footplate and only having 1/3 contact with the cog, I'll have a play and report back.

EDIT: There is another option....to mount the bow underneath with the chain coming up and over to the footplate, that eliminates the pulley and subsequent cord wear. A Bungee is also an option in this scenario.

You could remove the cogs and turn a hollow spindle to replace them so that cord/strap drive can be used. (You'd probably need to epoxy this into position).
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Re: New carver with a question

Postby Baggy » Mon Aug 13, 2012 6:23 pm

dervishcarving wrote:Hi all. I started using our bodging-club's pole-lathes to make bowls recently and am completely hooked.4/5 have come out quite well and seem to be drying without cracking.

Hiya

Glad to see that you found your way here :-)

Mark AKA baggy
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Re: New carver with a question

Postby gavin » Mon Aug 13, 2012 6:33 pm

Davy Crocket: I like your simple drive. This aint complicated at all - I thought you'd need to be an advanced engineer. More pix pls of the mandrel in action - can you youtube it?
If you had a one-sprocket freewheel, you could inset that into an external wooden sleeve and run your drive-strap or cord round that.
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Re: New carver with a question

Postby dervishcarving » Mon Aug 13, 2012 8:40 pm

yep, got here eventually. Love this simple mandrel-modification but cant help thinking it would be nicer if a drive cord/strap could be used. So, Davy, how to overcome that (you being patently better at this than us poor mortals) :D
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Re: New carver with a question

Postby Davie Crockett » Mon Aug 13, 2012 9:32 pm

If you look at the second picture, there's a threaded collar which holds the cogs in place. Remove this and the cogs leaving an inner casing around the ratchet. if you turn a tube of this dimension to cover it, you could epoxy it in place and use a normal belt/cord drive.
Last edited by Davie Crockett on Mon Sep 10, 2012 11:26 am, edited 1 time in total.
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Re: New carver with a question

Postby Davie Crockett » Mon Sep 10, 2012 11:14 am

OK, I've now removed the cogs and inserted the hub into an oak sleeve. It strikes me that using a greenwood sleeve in the round (think shrink pot) would also work very well.
Images here:
https://www.dropbox.com/sh/tx21czsri3cnevp/qpXnlWLGcE


Image
I've yet to try it in action (no bowl blanks), but I'll video my attempt when I do and post a link here.

I should add that the Sleeve is slightly concave in the cylindrical profile so that the drive belt doesnt wander and that the hub is inserted so that it is as near the centre of the sleeve as possible to minimise the leverage on the bearings from an offset hub.
Last edited by Davie Crockett on Tue Sep 11, 2012 8:13 am, edited 1 time in total.
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Re: New carver with a question

Postby robin wood » Mon Sep 10, 2012 2:49 pm

When anyone with half a brain sees a pole lathe they start to work out how to get continual rotation thinking it will make it work much better. I spent a while on it at one stage, in fact I got as far as buying some nice slick one way bearings to insert into the end of a mandrel. Problem is when you turn your mandrel round to do the inside you need it to slip the other way, it should be possible, cheap ratchet socket spanners and screwdrivers manage it with a little lever to flick to ratchet either way or lock. I reckon 10" is at the small end of where a ratchet would be of benefit, below that the lost time in the reverse stroke is no more significant than the lost time in the reverse stroke of a hammer, or the inefficiency of walking, fancy having to lift one leg and put it down every time, how wasteful. In practice most continual rotation devices add friction, complexity and inefficiency to the point that they are not as good as the simple solution. From the description of your first attempts I suspect your mandrel is too small diameter for your 10" bowl.
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Re: New carver with a question

Postby dervishcarving » Tue Sep 18, 2012 6:46 pm

I think you have some very good points there Robin. I only had one mandrel which i tried to use for all bowl-sizes. I have learnt that this is not a good plan and am now in the process of making new mandrels but unfortunately (or fortunately) i have been given a few Chainsaw-carving commissions which has pulled me away from teh bowls for a few weeks. I hope to get back to it over the next few weeks and see how it goes. I will still try the ratchet system to try on larger (10inch +) but for the smaller ones i will stick to the simple method with a range of mandrel sizes.
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