wall thickness

[diamind]

Hi there,

I'd like to turn a wooden jar/ cyclinder, 200mm in diameter, top and bottom, and height of about 200mm, then hollow it out, basically it will look like your typical cookie jar... the question is what kind of wall thickness should be left to keep the integrity of the shape??

https://encrypted-tbn3.gstatic.com/images?q=tbn:ANd9GcRy8XCSoHoeGro1fwerbe_8T-YE9kysSvjDKA_gxCvDQrQxAIg7ig

[The Hun]

Tough question

Many variables - type of timber, age etc, your equipment ie will you do it free hand or use any attachment

I suspect the biggest limitation will be your skill and experience - certainly has been in my case as I sought to go thinner and thinner

Best wishes

Peter

[Pat]

Depends on how brave and how the piece is going to be used. If a display piece around the 5mm range, to be used everyday by kids 10 - 15mm, as it will be dropped:;

[Skew ChiDAMN!!]

A 200d x 200h cylinder is relatively difficult to hollow, given basic skills & tooling, especially if you want the bottom 'square' to the sides. It's a tight space to squeeze the average tool-rest into, so catches and chatter can cause some... "interesting" effects down deep inside. DAMHIKT. :rolleyes:

I'd suggest that you hollow it out the same way you would as a bowl; ie. start with a rounded, concave bottom and slowly deepen it, bringing the top section of the walls square to a constant thickness. Once you get to the depth you want inside, then you can start working on squaring off the bottom... but I suggest that you still take off nice, smooth curves so that you can stop at almost any time should you get to a stage where it's either awkward or near-impossible to leave a nice finish off the tool. (If you get to a depth where you find it difficult to cut without chatter or catches, it'll be twice as awkward to try and sand the mongrels out if you keep going! :doh:)

IMHO Pat's pretty much spot on as to the wall dimensions: 5mm(ish) if it's gonna be a show-off piece, double that if it'll be a daily user.

[ian thorn]

After the hollowing to get the bottom square use the Soren Berger tool you don't get catches with it

Ian

[diamind]

ok great thanks for all the replys that will give me some good guidelines. I have another question tho if i were to first ruff this out of green wood, what would my ideal dimensions be of the outer diameter and inner diameter of the ruff out? the allowances for warping etc.. I will be playing it safe and keep the finished wall thickness at 15mm.

Cheers

[Pat]

Two minds, 15mm finish thickness in wet timber I would at least double the wall thickness. You still run the risk of the timber cracking, warping and carry on.

If it was me, I would turn round, turn the spigot, use a forstner bit to drill 170mm deep, then hollow out in stages to get the 30mm wall thickness, grab all the shavings, stuff them back in the vessel, seal the outside and leave in a cool dry place for a year or two. I would do a few so if any are unturnable, you can dispose of them.

[diamind]

Two minds, 15mm finish thickness in wet timber I would at least double the wall thickness. You still run the risk of the timber cracking, warping and carry on.

If it was me, I would turn round, turn the spigot, use a forstner bit to drill 170mm deep, then hollow out in stages to get the 30mm wall thickness, grab all the shavings, stuff them back in the vessel, seal the outside and leave in a cool dry place for a year or two. I would do a few so if any are unturnable, you can dispose of them.

Ok so this is the bit where I originally had trouble with, with the wall thickness of 30mm for the ruff, if i want a finished outer diameter of 200mm, what would i ruff it out the outer diameter too? around 210mm? 205mm? Because if i go right to 200mm with 30mm wall thickness and if the outside warps then I cant turn it down and still have a finished diameter of 200mm. You know what I mean. maybe go 7.5mm either side out inner and outer finished diameter?

[Pat]

I'd go to 210mm external diameter. Unless you are a patternmaker, no one will notice a few extra mm.

This allows for surface cracks/crazing and retruing the blank.

Make sure you locate the spigot onto the correct jaws. I mark around No 1 jaws in texta and when I re-chuck locate the spigot against this jaw and tighten the 3 other jaws onto the spigot. Remember to well support tailstock end with a cone shape live centre. Yes you have to turn an auxiliary (http://www.youtube.com/watch?v=acqZRDdkmkk) cone to fit the opening, unless you find a pipe center the correct size in an engineering shop ($190 - $300 MT4).