Log Size and the Effect

on Lumber Production

April 10, 2019

Key Factors in Optimal Lumber Production

Uptime Log Size (diameter)

What is the PERFECT Log Size?

What is the PERFECT Log Size?

• There is no right answer

• It depends on the application and what is being produced

What is the PERFECT Log Size?

• There is no right answer

• Even within the same application/product line

What Affect Does Log Size Have?

• Production Volume/Yield

• Production Efficiency

• Fiber Recovery

• Lumber Quality

Production Volume/Yield

• Larger diameter logs have more

geometric volume and therefore

more board footage

• Volume increases exponentially

with diameter

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

0 5 10 15 20 25 30

Cub

ic V

olu

me

(ft3

)

Log Diameter (inches)

8' Log Cubic Volume by Diameter (ft3)

Production Efficiency

• Larger, high volume logs don’t always equate to more efficiency

• Larger logs often take longer to cut

• Too many large diameter logs can cause bottlenecks at downstream machine centers

• Too many small diameter logs can cause upstream machine centers to not keep up

Production Equipment Limitations

• Logs can be too big for equipment…

Production Equipment Limitations

• Logs can be too small for

equipment…

Production Efficiency

• Many mills have multiple Primary

Break Down Machine Centers

• This requires a mix of large and

small diameter logs

Fiber Recovery

• Recovery is the measure of the

amount of lumber that is cut out

of a log

• Small diameter logs generally have a

higher Scribner gross recovery

• Why is this?

Fiber Recovery

• Review of Gross Scribner Scale

• Gross Scribner Scale is based on the geometry and size of a log

• It is independent of any log defects, natural or man-made.

Fiber Recovery

• Why do smaller logs have higher recovery?

• The short answer: Scribner log rule overestimates volumes on smaller logs

• Two most significant influence on changes in recovery over the years are

technology and log size

• Because LRF and CLR use a consistent measure of log input (i.e., cubic feet)

they overcome the limitations of LO by eliminating the measurement

distortions of the SLR

Fiber Recovery

• Net Scale subtracts any type of defect, both natural and man-made

• Larger logs commonly have more natural defects which affects net recovery

• This includes:

• Checks

• Rot/Decay

• Burls

• Fire Scar

• Insect Damage

• Oversize Knots

Natural Defects

• Checks and Pitch Seams

• This includes heart checks and frost

checks

• Tends to occur in older,

larger trees

Natural Defects

• Rot/Decay

• This includes heart rot, butt rot and

conk rot

• Tends to occur in

older, larger trees

Natural Defects

• Fire Scars

• Oversize Knots

• Insect Damage

Operational Decision Making

• Decisions of sawmill personnel significantly affect lumber recovery

• The opening face (first sawline is critical) to determining the recovery from a log

• A shift of 2/10” on an 8” log can result in a 25% change in lumber recovery

• The affect of this is most significant on logs 12” and less

Lumber Quality

• Defects in logs often translate to defects in lumber

• Boards from larger diameter logs typically have more:

• Shake/split

• Rot/decay

• Larger knots.

• Smaller logs typically yield more boards with wane

Lumber Quality - Large Logs

• Shake, split, rot, decay and large knots all lower the grade of the lumber

Lumber Quality – Small Logs

• Small logs yield more boards with wane

• The surface area to volume ratio is higher

• SA = 2πrL; V = πr2L

• SA / V = 2/r

What is the PERFECT Size Log?

• There are pros and cons with each size log

• Productivity, efficiency, fiber recovery and lumber quality are all affected

• Talk with Foresters or Log Buyers to know what size log is best suited for each application

• This helps to yield optimal value for mill and land owner