Uke backs

[vandenboom]

Hi guys - not much activity in the shed over the past year for various life reasons but just finished 2 ukes that I started 15 months ago. Concert & Tenor. I used Hand Lima plans. The backs have an arch lengthwise and started out flat side to side. Backs and back braces were qtr sawn. I was surprised to notice today that the backs have sunk inwards 2-3 mm width wise across lower bout. Does that sound to be expected?
thanks. Frank.

[kiwigeo]

Frank, what has humidity been doing in your shop?

[Allen]

It's a humidity related issue Frank. Braced up at a higher RH than it is now in your shed. Conversely if you braced at a low RH and then got very humid the back would have got a dome in it.

It's always best to build with a bit of a arch in them. That way when the humidity drops the plate will just flatten out, and not end up sunk like you are seeing now.

[vandenboom]

Thanks Gents.
Should have been aware of that. Assembled last Mar-Apr at approx 60-65% RH
Around 40-45% in the house over past week.
Will introduce a bit of lateral arch next time.
Frank

[Mark McLean]

If you can't keep your workshop humidity controlled all of the time, at least ensure that you do the two critical steps - bracing, and the closing of the box - at a time when the humidity is lower. 40-50% is probably the ideal for Melbourne and Sydney. And it needs to be stable at around that number for a few days before you do the job so that the plates and braces have equilibrated their moisture content.