Degrade Related Issues in Hem Fir

[AliciaS]

What planing degrade related issues will occur when my dimension hem fir is getting over dried?...I'm interested in the answer to this at planer speeds of 1200 ft/min.  I assume most of these losses through the planer would be related to excessive skip. The target moisture content range for the hem fir is 15%, and there's a concern about over drying of 2% or more moisture content.  At approximate current market values what kind of drying related dollar losses per mbf can we expect at any particular level of over drying?

[TimothyD]

Hi Neissa,
Of course that will depend on some other factors as well and not just on the average %mc that the batch is overdried to.  Another factor would be how tight your distribution is, can you tell me what sort of variability you are talking about, in terms of the standard deviation?  Mike Milota did a study some years back where, if my memory is correct, there was about a 1% value loss for every 1%MC a batch was overdried to and the percent loss increased the more down the scale you went.  I believe his paper also went into other input variables related to that degrade.

Tim

[AliciaS]

Hi, Tim.

A typical batch (production run) would be 10,000 pieces; looking at 2.5 to 3.0 standard deviation.

Thanks!
Neissa

[PhilM]

Hi Neissa,
All types of warp (crook, bow, twist, and cup) will increase the lower the moisture content, though proper stacking and stickering techniques can help minimize warp.  Severe crook and twist, and to some degree bow, will have an impact on the planer's productivity as these pieces may jam the planer.  And the excessive warp may exceed that allowed by a particular grade and thus cause the piece to be downgraded.  Overdrying will increase the amount of cup, and this may result in planer or roller splits at the planer.  Overdrying will also lengthen pre-existing end splits and can also increase torn grain.

I could not find the Milota publication that Tim referred to, but I think the 1% value loss for every 1% MC overdried is a reasonable estimate.  So if your Hem-Fir sells at $300 per MBF and you know you have overdried this material by 2% MC, the estimate of degrade cost would be $6 per MBF.

To quantify this in your lumber, you would grade the lumber using your grading rules and give two grades: the first grade is the grade of the lumber in its dried condition; the second is the grade the lumber would have been if the drying defects were not present.  A tally of the volume and grade loss and cause of the degrade would be kept.  You would need the MC of each piece so you would know the average MC of the test bundle.  This would allow you to begin to make an assessment of the cost of drying degrade due to overdrying.

[AliciaS]

Thanks.