Author Topic: Microwaving - blank or rough turn first?  (Read 3071 times)

Offline Dancie

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Microwaving - blank or rough turn first?
« on: August 25, 2016, 04:13:22 PM »
I’m a newbie, but I do wonder how many years I’m allowed to use that phrase. :-\

What I should say is that I’m an enthusiastic amateur who doesn’t turn as often as I’d like.

I know there have been previous posts about microwaving wood and I did a search on this site but couldn’t find what I was looking for.

I have an end sealed, bark on, Birch log about 7” long and about 7” across, it has a 25% moisture content.

I intend to turn an end grain hollow form with about ¼” thickness this coming weekend.

I thought that I might try reducing the moister content by microwaving.

My questions are:

1.   Should I rough turn the wood to 1” thickness and then microwave it or should I microwave the blank before I start?

2.   If I microwave, I guess I should remove the end wax first, right? And should I also remove the bark?

Any help suggestion opinions etc would be appreciated.

Dancie.
Mistakes are proof that at least you're trying

Offline fuzzyturns

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Re: Microwaving - blank or rough turn first?
« Reply #1 on: August 25, 2016, 05:14:58 PM »
You would always rough turn the form first and then do the microwaving.
You need to understand how wood works: basically you have a bundle of straws. When the wood is green, they are full of water, both as in filled with water as well as they are saturated with water. When you dry out wood, you first lose the water that fills the straws. Once that is gone, you lose the water that saturates the material. During both steps you end up with straws that shrink in diameter, which causes shrinking of the timber. If the timber is still quite solid, then the loss of straw diameter on the outside of the piece isn't so bad, because they can move inwards, and all that happens is you lose some of the diameter. However, at the core of your piece, that doesn't work and the straws start to come apart from each other, hence why cracks generally are radial in nature and start close to the pith.
Now, once you have rough turned your hollow form, all the straws can move. Instead of cracking/checking/splitting, you simply end up with some warping. If you can manage to mount your hollow form right on the pith in both headstock and tailstock, and keep the walls to even thickness (which must include the base!), and the wood was evenly grown (i.e. the pith is roughly in the center), you may get lucky and end up with very little distortion.

I do have to warn you, though: depending on the type of wood you have, this works more or less well. Some time ago I turned 4 end grain bowls from plum, and they all developed huge cracks, in some cases about 1" wide (on a 9" bowl diameter). I didn't mind, because I can always cut them up, put some other wood in the gap, and glue them back together. YMMV.

Offline Dancie

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Re: Microwaving - blank or rough turn first?
« Reply #2 on: August 26, 2016, 07:54:27 AM »
Fuzzy,

Many thanks for your reply, its much appreciated.

I felt that I would have to rough turn first before microwaving that seemed to make sense and I'm sure I read it somewhere, but I just wanted to see if anyone had any alternative ideas.

You've been very helpful.

All the best.

Dancie.
Mistakes are proof that at least you're trying

Offline Lazurus

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Re: Microwaving - blank or rough turn first?
« Reply #3 on: August 26, 2016, 10:53:45 AM »
I would suggest that if you are aiming for a thin wall section you may as well finish the job and let it dry naturally if you dry then remount there will be a lot of distortion to turn away. Also turning green wood is much nicer than dry blanks.
Living and working on the Norfolk Broads

Offline Dancie

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Re: Microwaving - blank or rough turn first?
« Reply #4 on: August 26, 2016, 02:01:00 PM »
Hello Lazurus,
 
Many thanks for your help.

I’ve turned thin walled hollow forms from green wood and left them to dry naturally in the past with some success.

Basically I bought a microwave in the January sales with the intention of using it solely for wood and this weekend was going to be the first time I’ve been able to get round to it.

As this is all new to me I was wondering if anyone had any do’s and don’ts.

All the best,

Dancie.
Mistakes are proof that at least you're trying

Offline Mark Sanger

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Re: Microwaving - blank or rough turn first?
« Reply #5 on: August 30, 2016, 09:05:08 AM »
Try this link loads of vids on YouTube, https://www.youtube.com/watch?v=Cv-EljgMqTE

Offline Dancie

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Re: Microwaving - blank or rough turn first?
« Reply #6 on: August 30, 2016, 11:32:27 AM »
Thanks Mark.
Mistakes are proof that at least you're trying