Mandrel diameters?

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Mandrel diameters?

Postby ericgoodson » Mon Nov 25, 2013 3:18 pm

I am about to try turning a larger bowl, say 10-12", and will make a larger mandrel for those larger bowls. But what diameter should that mandrel have?
Has anyone worked out a ratio of optimal mandrel dimensions to bowl width?
How does, "1 inch of mandrel diameter for every 3-4 inches of bowl" sound?
Thanks for your time.
"Use it up, wear it out, make it do, or do without."
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Re: Mandrel diameters?

Postby gavin » Tue Nov 26, 2013 8:55 pm

ericgoodson wrote:
Has anyone worked out a ratio of optimal mandrel dimensions to bowl width?

Dunno.

ericgoodson wrote:
How does, "1 inch of mandrel diameter for every 3-4 inches of bowl" sound?

Sounds ok. But you please try it; report back. First mandrel you make is a pain. After you make 4 you get better. I find the important thing is the tang profile.Work out a tang shape you like and produce a number of them. I like a flat-head screw driver shape on a tang. Decide how many tangs you like per mandrel and what length you like and what dimensions. Ideally you need a forge and anvil to beat them out. I use coach screws approx 50 mm long and 4 mm dia ( I think they are lag screws in US). I'll cut of the hexagonal head and beat the tang out from the remaining stock after discarding the head.
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Re: Mandrel diameters?

Postby ericgoodson » Tue Nov 26, 2013 9:40 pm

Thanks Gavin.
I have made one mandrel, about 2" in diameter, and used large nails as my tangs. Drove four in, cut off the heads, and filed to a flat-head screwdriver profile, like you suggest. It works just fine.
I like your notion of using screws instead of nails. That would keep the tang from being driven further into the mandrel (and less into the bowl) with successive uses. 4mm diameter for the tangs seems big. Does it have any tendency to split your bowl blank when driven in? How many tangs pr. mandrel do you use?

On a related note, I am thinking that I should really make a bunch of mandrels. A few months ago I did not have time to finish a particular bowl, so I buried it in shavings and left it. Well, I could not get back to it for weeks, and by then the nearly finished bowl had dried quite a bit. Since the mandrel was still attached, I decided to try to finish it off. The dry wood was certainly harder to work than wetter wood, but the finish was excellent. It has become one of my favorite bowls. So now I am thinking about turning bowls green to a nearly finished state, letting them slowly dry with the mandrel still attached, and then giving them one final pass once dry.
To do that type of production, I need a few mandrels.
Anyway, I will report back when I make the 3" and let you know how it turns out (no pun intended.)
E
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Re: Mandrel diameters?

Postby TonyH » Wed Nov 27, 2013 12:25 pm

I recently watched Robin Wood's video of him turning a large nest of bowls. I noted that for the first bowl, the mandrel was so large that the bowl was only turning slightly more than one complete revolution - obviously this is the biggest you can go, somewhere about 4" I imagine for normal length legs ! He changed the mandrel for the smaller bowls.
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Re: Mandrel diameters?

Postby gavin » Wed Nov 27, 2013 7:15 pm

ericgoodson wrote: 4mm diameter for the tangs seems big. Does it have any tendency to split your bowl blank when driven in? How many tangs pr. mandrel do you use?

Actually, the diameter would be more like 6 or 5 mm.
It can split blank when driven in - so do make a few half-log test pieces before you commit to driving mandrel to a roughed out blank. I think the orientation of the tang to the grain can alter the tendency to split - but I cannot vigourously assert this.
I use 2 , 3 or 4 tangs- again, you must suck and see. The more tangs the more grip BUT the more likely to get a split in the blank.
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Re: Mandrel diameters?

Postby anobium » Fri Nov 29, 2013 8:38 pm

In May this year I attended a bowl turning course run by Mace Bryant. His mandrels were about 65 mm diameter with the end turned down to about 25mm increasing to about 30mm and were attached to the workpiece by being driven into a 25mm hole, so were simply a friction fit. I am working with a mandrel about 100mm long by 65 mm diameter and it is attached to the workpiece with 3 woodscrews. Using a Kreg jig I drilled the holes so I screw directly onto the workpiece; seems to work fine.
Anyone know Mace? His website http://www.bodgersbarn.co.uk no longer functions.
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