Black walnut staircase shrinkage

[stumpy]

I am sawing some walnut for a customers staircase.  It's a rustic staircase with 2"x12" treads and risers It will be kiln dried.  How much shrinkage should I expect?  Would I end up with 1-7/8" by 11-7/8" ?

[beenthere]

Stumpy
Just follow the formulas Swampdonkey provided in that other thread.  ;D ;D

Sawing the logs, right?

Kiln dried to 8% mc ?

Figuring 12" green could end up around 11 3/8" dry to 8%

The thickness would be 1-7/8 as you figured.

[SwampDonkey]

 :D :D :D It's really not that ominous considering you only need two of them formulas. It's just easier for someone else to do the math.  ;)

Ditto to beenthere's shrinkage.

[stumpy]

I'm sorry, I missed something.  Where are the formulas?

[SwampDonkey]

Sorry stumpy, I'll type the relevant ones in here so you don't go bonkers with all the math jargon

=====================================================

for black walnut G_g = 0.51, V_sh = 12.8% to oven dry condition, T_sh = 7.8% tangential shrinkage. The Wood Handbook has these figures from USDA site, Table 3-5 lists volumetric and tangential shrinkages from green to oven dry condition, Table 4-3a lists specific gravity green and 12%..

thus FSP = V_sh% = 12.8% = 25% no shrinkage at this MC
                G_g       0.51

G_g is what we call specific gravity in green condition


So,

→8%_MC = 7.8% * (25% -8%) = 5.3%
  T_sh       25%

Be careful, use the MC% you will be reducing the lumber to in the kiln. Here we used 8%.

And applying to your dimensions.... 2" x 12"

2" - (2" x 0.053) = 1.89 " (~1 and 7/8") thick, rounding down

12" - (12" x 0.053) = 11.36" ( ~ 7 and 5/16") wide, rounding down

oh and yes factor in your planing after kilned, so probably closer to 13" wide by the time an 1/8th comes off each edge. Hope it doesn't cup when it comes to planing the flat sides, but factor planing, otherwise a good 3/8" thicker.

But it was discussed in here.

[stumpy]

Thanks SwampDonkey and Beenthere 8)  And speaking for all of us Mathematically challenged folks, another big Thanks 8)

[SwampDonkey]

Keep in mind this is empirical data (averages) from many wood samples. You can sample your own wood and find your own values, but I doubt it's worth the effort other than to satisfy or enhance your understanding. You'll probably find that you loose around 1/8" for each 2" width/thickness after kilning to 6-8%. Always good to do the math to familiarize your self with the wood species and shrinkage you would expect when you kiln to what ever moisture content your client expects.

I edited my previous post by inserting "%" by the numbers in the T_sh % formula so you can see they are calculated values and not constants. In the dimensional change calculations those decimal fractions are T_sh% @ 8% MC/100.