Spiral bowl

[Ray153]

Some avid watchers of the Woodwroking channel may have seen the American Woodshop where he turned what he termed a "spiral bowl". I am tempted to try the same idea but cannot recall the finer points of how he made the blank.
He started off with a large diameter blank approximately 25 mm thick, then marked out a spiral using a pencil on a string from the centre. The spiral was then cut out on the bandsaw at an angle leaving a flat base about 150-200 mm across. The effect looked like a very large wooden mosquito coil that he then glued up while pulling the outer parts of the coil downwards to make a height of 400+ mm.
I am unclear on the width between the cuts, which I imagine has a fairly critical bearing on the degree to which the bandsaw cuts are made. I seem to recall that he tilted the table to around 15 degrees but can't remember exactly.
Can the more mathematically competent help me with the maths on this one. Without fingers, I can't really get much past 10 so you might understand my dilemma. Or if anyone made notes during the show, they would greatly assist as well.

[Skew ChiDAMN!!]

Some avid watchers of the Woodwroking channel may have seen the American Woodshop where he turned what he termed a "spiral bowl".

Sounds interesting... I'm sorry I missed it.


I am unclear on the width between the cuts, which I imagine has a fairly critical bearing on the degree to which the bandsaw cuts are made. I seem to recall that he tilted the table to around 15 degrees but can't remember exactly.

The width between the cuts has no effect on the angle of the cut, but the narrower the distance, the "steeper" the sides of the bowl will be when pulled up.

What does affect the angle of the cut is the thickness of the blank. When lifted into position, the bottom edge of the raised coil doesn't want to come up any higher than about a 1/3 to 1/2 the thickness of the blank, to leave you with enough material to turn. The thicker the blank, the lower the blade angle you can get away with. :)

A bit of maths to help work it out? here's a diagram...

A = the width of the bandsaw's kerf. B = how far over you want the top of the cut to be compared to the bottom... which ideally would be around 2 or 3 times A. C = the thickness of the blank, while X is the angle you need to make it all work.

So, make a test cut and measure the blade kerf to determine A. (eg 2mm) Triple it to get B (6mm in this example) and measure the thickness of the blank to get C. Let's say that's 30mm.

Using basic trigonometry, TanX = C/A = 30/6 = 5, so X = 78°... or 12° off square. (Subtracting from 90°)

That help any? :rolleyes: Really, I wouldn't worry about the maths... this is the sort of thing best done by hands-on experimenting. :)

[BernieP]

G'Day Ray153

I think this bloke is talking about the same bowl http://www.forums.woodnet.net/ubbthreads/showflat.php?Cat=&Number=2653140&Main=2653140

Cheers
Bernie

[joe greiner]

[(I downloaded it a few days ago, but couldn't remember where)]
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AHA!!!!!!!!!!!!!

http://www.woodworkforums.ubeaut.com.au/showthread.php?t=32197&highlight=relief

This contraption should work for the spiral bowl too.

No matter how good your maths, the wood and the saw will always have a different opinion. This lets you know what they want.

Joe

[Skew ChiDAMN!!]

True, but I'd really like to see someone make such a device on a bandsaw. :D 'Tis hard enough on a scrollsaw!

[joe greiner]

1. Increase the scale to match your saw's capabilities.
2. Can't remember where, but a while back, I found a website for some folks making compound-cut 3-D puzzles. They use a 1/8-in blade for very tight curves (on the order of about 10mm diameter, I think). YMMV
[3. The jig is general purpose. Your "experiment" is one-off. Same-o same-o]

Joe

[dazzler]

Hi

I watched this last nite and it was very clever.

He set the bandsaw table at a 4deg angle to give the taper on the sides.

Didnt give a close up of the finished product though.

cheers


dazzzler

[joe greiner]

4 degrees sounds about right, but as Skew and I have said, it's best to make some test cuts or a relief gauge. I don't recall seeing this done with a bandsaw. Yes with a scroll saw. Note that with a bandsaw, you have to either back the workpiece all the way around to the beginning or cut the blade ($$$). With a scrollsaw, you either use a pinless blade and extract it directly, or with a pinned blade just cut it because they're cheap enough to toss. (Although you could pre-drill an exit hole.)

Joe

[Hardenfast]

I didn't know there was a whole channel devoted to wood working - that would be great. Is this on Foxtel?

[jchappo]

The Woodworking Channel (http://www.thewoodworkingchannel.com/)is an internet resource running streaming video of woodworking programs 24/7 - and its free:D
(http://www.thewoodworkingchannel.com/)

[OGYT]

If you're on dial-up... forgeddaboutit. Takes forever and a day to view anything.

[joe greiner]

I've been wondering why this thread started in the Woodturning forum. Finally figured it out. My last post (yesterday, unless International Date Line has goobered the date) mentioned awkwardness of releasing the blade. If the spiral layout is for future glue-up, you can of course just cut across the strips to reach outside, and glue those gaps as well.

I'd been thinking of a collapsible spiral bowl, like shown at the bottom of this page: http://users.adelphia.net/~plemieux/wood.htm

I've bought a couple bowls like this at garage sales, but haven't made one myself, just made some test cuts. Pics are from the link. [Only the initial gap needs to be glued.]

Joe