Sweet Gum movement

[Wally Crawford]

I will soon start the rough turning on some Sweet Gum I scored. I understand that it moves a lot when it dries, so my question is: Do I need to leave material thicker than the normal 10% to allow for the anticipated warpage?

 

[Richard Coers]

I'm pretty confident you can find its rate of shrinkage on the internet. I would not call that 10% guess as a normal allowance. You have to take size of the object and final wall thickness into consideration every time.

 

[Jon Rista]

With gum, I would err on the side of more movement is more likely than less. I had a bunch of gum woods from early on when I started turning (2020), and over the years some of them moved a very significant amount, and entire blanks were extremely warped. They went from perfectly round to HIGHLY oval/oblong, for the blanks that survived longer than the first couple years (I received all of them fully waxed.) I think I still have two left, and both are quite oval compared to the perfect circles they originally were.

A couple of the original pieces I turned were given to family members, and once turned the warping was probably less significant than the blanks, however, they did still move quite a lot and I think some checking occurred on one of them. Neither of the originals sit flat anymore, and are also no longer round. FWIW, they weren't even freshly cut when I received them, they had been drying in their wax coats for a bit already.

This stuff can really move. So, either, err on the side of caution, or....let em dry for a while first, get a chunk of that movement out of the way, then turn.

 

[John K Jordan]

I will soon start the rough turning on some Sweet Gum I scored. I understand that it moves a lot when it dries, so my question is: Do I need to leave material thicker than the normal 10% to allow for the anticipated warpage?

Can find info on shrinkage and warping on the Wood Database. The entry for Sweetgum:

Sweetgum | The Wood Database (Hardwood)

www.wood-database.com

The nominal shrinkage/warping numbers are Radial: 5.3%, Tangential: 10.2%, Volumetric: 15.8%, T/R Ratio: 1.9
Maybe compare those to other species you have experience with.

The T/R (Tangential/Radial) ratio can be useful describe the warping. T/R for Red Oak is well over 2. Cherry is 1.9.

Their article on Dimensional Shrinkage explains:

Dimensional Shrinkage | The Wood Database

www.wood-database.com

But how the wood moves and warps as it dries also depends on other factors - I think the biggest for bowls is grain orientation (commonly straight face orientation), proximity to the pith, figure, wood from a straight trunk vs a leaning limb, etc.

The best practical resource I know of for all this is R. Bruce Hoadley's book "Understanding Wood".

JKJ

 

[Marc Banka]

I haven’t experienced major movement from the Sweetgum I’ve worked. I aquired quite a lot from a neighbor who had to take their large old tree out. It had broken his sewer line. Some, I turned immediately. Other logs sat under tarps,,tucked in for a few months. The wood that sat under the tarps was, I think, the most beautiful. It had generally darkened. The ends of the logs had turned black, with just a bit of slime. Scared me just a little. The wood inside though was simply spectacular. Rich golds, greys, blues and reds. Really gorgeous stuff.

I abide by the 10% rule for roughing, and cut almost all of that away. It hadn’t moved much at all. Red oak is the only wood in where the 10% wasn’t enough. A few pieces couldn’t be cut round without the inside becoming larger than the outside! Others, I had no creative reshaping. Only one profile possible.

Enjoy the Sweetgum experience! It’s great stuff to work with.