Re-Drying Kiln Dried Lumber

      An architectural woodwork mill gets advice on acclimating stock before machining it. December 9, 2010

Question
I use a lot of eastern white pine lumber in the common grades that by standard can be up to 19%MC. Most mills tell me they target 12% but don't guarantee it. I've been having problems for years with edge glued panels cupping and shrinking, and sometimes open joints at the ends if I don't get it glued up right away.

Most of my problems occur about a week after the lumber is processed. For instance I make a lot of 12" wide panels. You go back a few days after they've been glued and sanded, they average 11.875" in width, and some even shrink down to 11.75". 11.5" is the record so far! The top panel on the stack always cups (even in summer with 70%RH). Sometimes I can get a 1" cup. So I'm thinking is there any reason I can't sticker the bundles and let them shrink prior to secondary processing?

I have two bundles that I'm playing with right now. I have them in front of a large modine gas unit heater. Our shop right now has about 28% RH, and I figure between that the low RH, and high CFM air movement and high temp of the air, I sort of have a poor man's kiln in front of the heater.

The mills do offer a furniture grade board that is KD to 6-8" at the mill. The only problem is they are almost double the price, and they want to sell it on a random width/random length basis which I cannot do.

Forum Responses
(Sawing and Drying Forum)
From contributor L:
Some EWP mills do dry to 6-8% for certain markets. Many dry to 10-12% because they are planing the lumber into siding and paneling and that is where EWP planes best. The key thing in making glued up panels is having all the lumber at the same (+/- 1/2%) MC and it should be at 6% if being used in the north. You need to pay attention to the grain because shrinkage varies, radial vs. tangential shrinkage. Your best bet will be to have the space at 35% RH and let the temperature drop until you get that. Keep it there by manipulating the temperature. It is harder at this time of year. Lower temp raises humidity, all other factors being equal. Try to find a supplier who does furniture pine and you will get what you need. Unfortunately there are not many of those any longer but there are some.



From Gene Wengert, forum technical advisor:
Be aware that 6% MC EWP will be prone to developing long splits (called planer checks or splits or roller splits). I suggest that you target 9.0% MC with all the lumber within a narrow band around this value. Under 9%, machining defects increase (chip out, planer splits, etc.) and gluing requires a higher spread rate. Your present heater is a bit low in RH (5% EMC). Remember that it is the outside of the wood that will achieve 5% MC and it is this dry wood that the planer will be processing.

I think that 50% RH is better and then make sure the entire load sees this RH. There is also a risk that such high MC wood will not have really good stress relief; drying stress (also called casehardening) causes immediate warp when processing. It would be best to search hard for someone that dries the wood in a kiln for you to the correct MC rather than trying to do it yourself. Second best would be to get a small kiln (perhaps a DH unit) that can re-dry your wood. In any case, make sure you also store your wood at the correct MC so it does not change MC in storage.



From the original questioner:
I think I left out some important information in my first post. The wood that we bring in is already dimensioned after the mill kiln drys it to NELMA rule. It is surfaced to 3/4". Most of it is nominal 4", so it is 3.5" wide +/-.

Prior to glue up we send it through a moulder to take it down to 3" width, then we cut it to any of eight usable lengths. Then the staves are glued up the desired panel width in an L and L RF press. Panels are then sanded on a 3 head sander both sides to a nominal thickness of .700". So we take off about .025" per side when we sand.

Perhaps I used the wrong terminology in my original question. Since it seem like our wood shrinks about a week after it's taken out of the pack and processed, I'm wonder if it would hurt to take it out of the mill pack and acclimate prior to moulding to its final width and gluing into panel sets?

This isn't quite scientific, but I did sticker two packs of 325 boards each after taking the mill wrapper off. I took a measurement of width and most were right on at 3.500" +/- .01". Just three days later many of those same boards have shrunk an average of .02" in width. Some have shrunk .04" in width. I placed one bundle about 20' in front of the shop heater. The other I placed in the shop, but not in the direct path of the heater. Both packs have the same amount of shrinkage. If all my boards shrink an average of .03" in width, I am theorizing that will take care of my problem of my 12" panels shrinking .125" in width.

I am thinking that allowing the wood to air dry and lose moisture will provide a better end product then simply moulding the wood oversized to allow for shrinkage? A few mills do offer a NELMA furniture grade pine which is targeted to 6-8%MC. However as stated in my original post, it is often times more than double the price of what I'm currently using, and is sold on a random width and random length basis. I buy fixed widths and lengths for ease of manufacturing. Being a small American furniture shop I can't simply double the cost of my raw materials. I certainly can't double my retail prices to make up for it.

Thank you for your insights. Regardless I will follow-up with a post in a short while to let all interested readers know if our panels are behaving better after allowing the wood to acclimate and air dry prior to processing.



From Gene Wengert, forum technical advisor:
Indeed, within the limits already mentioned (especially note: 9.0% (maybe even 8.0% MC) MC and very uniform from piece to piece), further dry the wood before milling and gluing. Incidentally, wood shrinks for only one reason - it is losing moisture. To stop shrinkage, get the moisture content at the equilibrium value that the wood will experience in your shop and in use by the ultimate customer.

If you were drying your own wood initially, it would cost you roughly $1.00 to $1.50 for each % MC loss per MBF. To charge double for going from 19% to 7% MC is excessive (unless the grade is also changed or there are other quality issues addressed, such as whiter color).



From contributor G:
We used to do about the exact same thing as you (we used 1x6) and experienced the same problems in the winter. We solved our problem by crosscutting to length and letting the stock acclimate for a few days before machining and gluing. We had very few problems after that.



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