VCB203 – “Before It Happens: Master Every ‘Pre’ &
‘Fore’ Word Like a Pro
Ripping - Cutting along the grain
Cross-cutting - cutting across the grain
What are the advantages and disadvantages of the different saw types? - - Circular saw: Fast,
robust. large kerf
- Band saw: fast, small kerf, flexible sawing pattern. 1 (2) cut per pass.
- Frame saw: multiple cuts per pass. Slow, inflexible, high maintenance.
- Chipper: fast, precise cut, no kerf. High capital cost.
- Cant/not boards: Somewhat flexible sawing pattern, best for small diameter logs.
- Chain saw: mobile, low capital demand. slow, large kerf.
What saw blade characteristics determines the feeding speed, depth of the cut? - Clearance
angle in combination with blade speed determines feeding, gullet area/pitch determines in
combination with the bite the depth of cut. Deeper cuts also need thicker saw blades.
Discuss the compromises in the geometry of saw blades - Large clearance angles increases
feeding speed, reduces sharpness angle, large sharpness increases energy demand, standing
times and tooth strength, large rake angle reduces sharpness (sum usually 90 degrees). Larger
gullet area increases depth of cut but increases likelihood of blade fracture. Thicker blades/set
increase stability/reduce friction but increase kerf (reduce conversion).
How does the kerf effect the economy of a sawmill? - The wood in the kerf is transformed into
sawdust and therefore not recovered as sawn product. The conversion rate decreases.
Therefore, fewer deep cuts are made with smaller saws having a smaller kerf. Example: a large
circular saw can have a kerf of 10mm, cutting 25mm boards implies a 30% loss of a log volume
(10mm / (10 + 25 mm))
Why does sawing accuracy effect the economy of a sawmill? - How is sawing accuracy achieved?
The less variation (wandering/vibration) of the saw blade the smother the surface and the less
planing is required after sawing. A perfect cut would reduce the conversion loss to the loss due
to kerf. Otherwise, additional planing allowances are required. Sawing accuracy is achieved by
thick enough saw blades, pre-tensioning and guides as well as accurate log
positioning.
,Explain pre-tensioning of saw blades? - The edge is set under tension by rolling/hammering the
centre. Increased temperature and deformation due to centrifugal forces are counteracted and
the saw becomes more stable.
What are the advantages of debarking? - Debarking increases standing times of saw blades, high
quality/value bark-free chips, better assessment of log shape (yield optimisation). Bark as by-
product less fouling of sawmilling machinery.
Discuss log debarking techniques - - Ring-debarker: problems with stringy bark. requires round
logs, max diameter restriction, fast.
- Rosserhead debarker: slow, damage to wood (lower yield), but can accommodate large and
poorly shaped logs by removal of butt/branch swell.
Hydraulic debarkers: Work on logs with poor form, trade-off between water and electricity
usage, water filtration required, wet bark has low calorific value - not used.
Why and by which criteria are logs sorted before sawmilling? - To optimise production, logs are
sorted by size, species, grade. This allows the sawmill to cut the demanded product (e.g.
species), to saw batches of similar logs with an optimised sawing pattern, to increase
productivity of down-stream operations by feeding appropriate logs, in case of multiple
headrigs to feed the appropriate logs.
What is the difference of nominal size and target size. What accounts for their difference? - The
nominal size is the size of the dry product. The product dimensions are smaller than what needs
to be cut from the wet logs, because of shrinkage (~3%), planing allowance to get defined
dimensions and a smooth surface and sawing variations (to have no undersized pieces as there
is a normal distribution of piece sizes around the set target size).
What are the typical conversion rate in sawmills? How can they be optimised? - ~50% in
sawmills cutting commodity products, 25% in sawmills cutting valuable high grade heartwood
(furniture). Conversions rates increase with smaller kerf, smaller sawing variation (machinery),
larger logs, higher quality logs (shape, defects), more flexible sawing patterns/more products,
optimised logs placement in the saw, scanning technology.
Sawmilling operations are complex and yields can be optimised. What decisions by sawmillers
impact on the conversion rate? - - Orientation of the log (taper/live sawing)
- Opening face
- sawing pattern
- Log rotation (sweep parallel to saw cut).
, Split taper - Sawing parallel to the pith.
- Eual width of central cant on both its faces
- Reasonable conversion of tapered long logs (short side boards, good conversion of central
cant)
Full taper - Sawing parallel to the cambium
- Central cant asymmetric
- higher conversion with short logs (recover extra piece from the side)
- Full-taper sawing on all four faces (turning log) maximizes recover from the outside (high
quality)
What factors influence the design of a sawmill? - - wood resource: species (HW/SW), quality
(old growth, plantations), log size, log supply (sustainable amount)
- Markets: domestic/export, potential products, product diversification
- Location: log transport costs (proximity to resource), Infrastructure for product dispatch,
proximity to market
- Business environment: available capital (interest rates, subsidies), legal system, work
force.
How would a change in the log resource, i.e. a reduction of SED impact on sawmill
performance? - Negatively: smaller logs have lower (geometrical) conversion rates. Additionally
younger trees (fast-growing) have more low-quality corewood.
Comment on the economy of scales in sawmills - Indirect costs are spread over more product
and therefore total costs per product unit are smaller at larger scale. Direct costs related to
production are only part of the costs incurring - additionally, direct costs can also drop if bulk
purchases (e.g. electricity) are possible with size. However, log haulage costs increase with mill
size and diminish/counter these savings.
Characterise the saw milling sector in NZ - Sawmills in NZ are of smaller size. A consolidation
process (fewer bigger mills) is in progress, largely by brown-field expansions/upgrades of
existing mills and the closure of smaller mills.
What is an efficient sawmill layout for small diameter logs from slow-growing softwood trees?
Why is that no suitable for fast-growing softwood stems (e.g. radiata pine)? - Chipper/canter
lines. This technology recovers the core fo the logs as timber and the outside as chips. Logs from
fast-growing young trees contain mainly corewood of low quality, which is recovered in full and
‘Fore’ Word Like a Pro
Ripping - Cutting along the grain
Cross-cutting - cutting across the grain
What are the advantages and disadvantages of the different saw types? - - Circular saw: Fast,
robust. large kerf
- Band saw: fast, small kerf, flexible sawing pattern. 1 (2) cut per pass.
- Frame saw: multiple cuts per pass. Slow, inflexible, high maintenance.
- Chipper: fast, precise cut, no kerf. High capital cost.
- Cant/not boards: Somewhat flexible sawing pattern, best for small diameter logs.
- Chain saw: mobile, low capital demand. slow, large kerf.
What saw blade characteristics determines the feeding speed, depth of the cut? - Clearance
angle in combination with blade speed determines feeding, gullet area/pitch determines in
combination with the bite the depth of cut. Deeper cuts also need thicker saw blades.
Discuss the compromises in the geometry of saw blades - Large clearance angles increases
feeding speed, reduces sharpness angle, large sharpness increases energy demand, standing
times and tooth strength, large rake angle reduces sharpness (sum usually 90 degrees). Larger
gullet area increases depth of cut but increases likelihood of blade fracture. Thicker blades/set
increase stability/reduce friction but increase kerf (reduce conversion).
How does the kerf effect the economy of a sawmill? - The wood in the kerf is transformed into
sawdust and therefore not recovered as sawn product. The conversion rate decreases.
Therefore, fewer deep cuts are made with smaller saws having a smaller kerf. Example: a large
circular saw can have a kerf of 10mm, cutting 25mm boards implies a 30% loss of a log volume
(10mm / (10 + 25 mm))
Why does sawing accuracy effect the economy of a sawmill? - How is sawing accuracy achieved?
The less variation (wandering/vibration) of the saw blade the smother the surface and the less
planing is required after sawing. A perfect cut would reduce the conversion loss to the loss due
to kerf. Otherwise, additional planing allowances are required. Sawing accuracy is achieved by
thick enough saw blades, pre-tensioning and guides as well as accurate log
positioning.
,Explain pre-tensioning of saw blades? - The edge is set under tension by rolling/hammering the
centre. Increased temperature and deformation due to centrifugal forces are counteracted and
the saw becomes more stable.
What are the advantages of debarking? - Debarking increases standing times of saw blades, high
quality/value bark-free chips, better assessment of log shape (yield optimisation). Bark as by-
product less fouling of sawmilling machinery.
Discuss log debarking techniques - - Ring-debarker: problems with stringy bark. requires round
logs, max diameter restriction, fast.
- Rosserhead debarker: slow, damage to wood (lower yield), but can accommodate large and
poorly shaped logs by removal of butt/branch swell.
Hydraulic debarkers: Work on logs with poor form, trade-off between water and electricity
usage, water filtration required, wet bark has low calorific value - not used.
Why and by which criteria are logs sorted before sawmilling? - To optimise production, logs are
sorted by size, species, grade. This allows the sawmill to cut the demanded product (e.g.
species), to saw batches of similar logs with an optimised sawing pattern, to increase
productivity of down-stream operations by feeding appropriate logs, in case of multiple
headrigs to feed the appropriate logs.
What is the difference of nominal size and target size. What accounts for their difference? - The
nominal size is the size of the dry product. The product dimensions are smaller than what needs
to be cut from the wet logs, because of shrinkage (~3%), planing allowance to get defined
dimensions and a smooth surface and sawing variations (to have no undersized pieces as there
is a normal distribution of piece sizes around the set target size).
What are the typical conversion rate in sawmills? How can they be optimised? - ~50% in
sawmills cutting commodity products, 25% in sawmills cutting valuable high grade heartwood
(furniture). Conversions rates increase with smaller kerf, smaller sawing variation (machinery),
larger logs, higher quality logs (shape, defects), more flexible sawing patterns/more products,
optimised logs placement in the saw, scanning technology.
Sawmilling operations are complex and yields can be optimised. What decisions by sawmillers
impact on the conversion rate? - - Orientation of the log (taper/live sawing)
- Opening face
- sawing pattern
- Log rotation (sweep parallel to saw cut).
, Split taper - Sawing parallel to the pith.
- Eual width of central cant on both its faces
- Reasonable conversion of tapered long logs (short side boards, good conversion of central
cant)
Full taper - Sawing parallel to the cambium
- Central cant asymmetric
- higher conversion with short logs (recover extra piece from the side)
- Full-taper sawing on all four faces (turning log) maximizes recover from the outside (high
quality)
What factors influence the design of a sawmill? - - wood resource: species (HW/SW), quality
(old growth, plantations), log size, log supply (sustainable amount)
- Markets: domestic/export, potential products, product diversification
- Location: log transport costs (proximity to resource), Infrastructure for product dispatch,
proximity to market
- Business environment: available capital (interest rates, subsidies), legal system, work
force.
How would a change in the log resource, i.e. a reduction of SED impact on sawmill
performance? - Negatively: smaller logs have lower (geometrical) conversion rates. Additionally
younger trees (fast-growing) have more low-quality corewood.
Comment on the economy of scales in sawmills - Indirect costs are spread over more product
and therefore total costs per product unit are smaller at larger scale. Direct costs related to
production are only part of the costs incurring - additionally, direct costs can also drop if bulk
purchases (e.g. electricity) are possible with size. However, log haulage costs increase with mill
size and diminish/counter these savings.
Characterise the saw milling sector in NZ - Sawmills in NZ are of smaller size. A consolidation
process (fewer bigger mills) is in progress, largely by brown-field expansions/upgrades of
existing mills and the closure of smaller mills.
What is an efficient sawmill layout for small diameter logs from slow-growing softwood trees?
Why is that no suitable for fast-growing softwood stems (e.g. radiata pine)? - Chipper/canter
lines. This technology recovers the core fo the logs as timber and the outside as chips. Logs from
fast-growing young trees contain mainly corewood of low quality, which is recovered in full and
