Using a Shrinkage Calculator

[Jan Bianchi]

I have been trying to use The Shrinkulator http://www.woodbin.com/calcs/shrinkulator.htm to calculate the length of each slotted screw hole I need to attach my madrone table top to its base and to keep it flat. (Thanks Kent Bathurst)

My problem with using the calculator is how do I know what either the initial or final moisture content will be? I don't know what the initial moisture content after air drying was, but I know it is 10% now according to my moisture content meter. How would I know that the final will be?

If I use the relative humidity values instead of moisture content, then do I use lowest possible humidity and highest possible humidity in my Seattle home (I don't have a humidifier in my house). My humidity monitor reads 63% right now in a house that is currently heated to 63.4--sunny outside today--surprise. But my monitor reads a low of 54 and a high of 89 degree humidity and 50 and 73 temperature. (this is in a heated basement which isn't heated in the summer,)

I tried reading the Hoadley book on wood movement, but it doesn't really explain, at least not to me, what values you use to calculate this.

In case a description of the table would be helpful: the table top is 42 inches wide and 72 inches long with the grain running along the length. It is an extension table so the top is cut into two leaves. The middle end of each leaf will be attached to three runners, one on each side and one in the middle, that slide open lengthwise inside a structure between the aprons. (The structure will also hold an extension leaf that folds underneath the table top in a butterfly fashion). The leaf will be attached to each runner with at least three screws. A cleat three inches from the other end of each leaf is a concession to madrone's tendancy to move and intended to keep the end of the table flat. It is not connected to the base. The cleat will have a minimum of five screws in it. Thus I understand the slots will be a different size on the cleat than on the runners.

[Jan Bianchi]

I gues it's really an expansion/contraction calculator, not a moisture calculator.

[Kent A Bathurst]

Jan - I can tell you how I deal with this, but be aware that there are smarter folks than me here, and also that [as noted in my post to your initial thread] I take a base-line-pragmatic approach to this particular construction detail [which means, they won't have me do an article in a magazine :p].

I use the shrinkulator. I go to a historical weather chart for the planned home of the item, and look at hi-low humidity ranges and combine those with my home numbers, take the two extremes from the two locations, and put those numbers in the shrinkulator. This is a bit overkill, since the extremes of the humidity levels aren't around long enough for the piece to fully acclimate/respond. I make the slot based on the max expansion from low to high, and center the screw, without usually worrying what "today" is, unless that would peg me near one end of the range.

OTOH - for the wood[s] I tend to use, a change in the rel humidity input to the shrinkulator doesn't really move the wood a whole lot more - I wouldn't know madrone if a piece fell on me, tho, so YMMV.

Last - figure out how much wood each slot will be responsible for. You don't need a slot in the center [assuming you want equal expansion on both sides - in some cases, I want all the expansion to go one direction only]. As you get farther from the fixed-location-centerline, each slot will have to accomodate more movement. In your example, if the outboard runners were at 18" from the centerline, then they have to accomodate the movement of 18" of wood surface.

Again - and this is just me - my approach to this issue is "don't let it fail, don't over-analyze, and don't worry what it looks like if someone gets on their back and crawls under the table with a flashlight to look". Anybody does that and calls me about it is looking at the last piece they'll ever get from me :D

[Jan Bianchi]

Thanks for responding Kent, I was hoping you'd see this.

It does seem like this approach would handle the worst the weather could throw at me, so I'll give it a try. I would not be pleased either with people crawling on on the floor. I'm just hopeful of getting this to actually stay flat, open and close, and be the beautiful sunset wood it is. When I'm done I'll post pictures.

[Kent A Bathurst]

....... I would not be pleased either with people crawling on on the floor......

I don't care if they do that - more power to 'em, if they are that bored - as long as they don't call me with that detail as their one gripe on a free gift. :D

I only make stuff for me, family and friends - and one family member doesn't get anything anyway because she has been griping non-stop since she was 2 [internally, I refer to her as "my brother's little sister" :p]

Again - I make no claims to being an expert resource on this topic [a bit surprised one of the smarter guys haven't thrown down on this topic to correct me, which I certainly don't mind], I was just passing along to you how I personally address the issue. No problems so far. The FWW guys would be aghast at mine, but to me they're overdriving their headlights.

And - one last comment - my personal favorite is the metal Z-clips, or their wood block equivalents - but that approach won't work for your slides/runners.

Good luck, Jan !!!!!!