Sawmill Analysis

Preliminary analysis based on requirements at Sawmill - Design Rationale. As more details of the rest of the village system is developed, this will need to be updated in an iterative fashion.

Production Requirement

Modern sawmills are highly productive devices, so assume 1250 person Village size, the largest size for analysis. Allow 15 year construction period, followed by maintenance and remodeling. Thus 6.67% of construction lumber is needed per year. Allow 1% replacement lumber per year for maintenance and 2% for remodeling, thus 3% per year after construction. Given those numbers, it seems reasonable to build two units with 3.33% annual capacity each. The second unit is either recycled after 15 years or sold off.

Standard wood frame home construction consumes about 6 board-feet(bf) (1x12x12 inch unit volume of wood) per square foot of floor area. Allow an additional 6 board feet for other wood used such as workshops, fences, and furniture. USA average dwelling size is 750 square feet per person. Therefore 750x12 = 9000 board feet of finished lumber/person is needed for construction. Allow 20% defects and drying loss and 10% waste during construction, thus 70% net used lumber. So initial cutting need is 9000/70% = 12,850 bf/person.

1250 people x 3.33% annual construction = 41.67 cutting need per year. 41.67 x 12,850 bf = 535,400 bf/year/machine. Assume 240 working days per year. Thus daily production would need to be 535,400/240 = 2230 bf/day/machine. Experience shows about half the time is consumed in log setup, board stacking, sawdust removal, and blade maintenance. Thus you have a daily productive cutting time of 4 hours per day. The Wood Mizer commercial bandsaw mill indicates their LT40SH model can reach the required 2230/4 = 560 bf/hour production rate.

An average crew of 2 is required to operate this size sawmill. During construction, when 2 units are required, that means 4 working people are needed out of 568 available working people in the 1250 total person village, or 0.7% of total village labor. Some additional labor will be needed to cut trees into logs, dry the wet lumber in a kiln, and dispose of offcuts (the parts of a log not turned into lumber) and sawdust.

This result is highly dependent on the assumptions made. Buildings can be primarily made from other materials than wood. Techniques like Roundwood Construction use whole log segments without cutting. So the requirement for the sawmill will depend on how the rest of the Village is designed.

Sawmill Specifications

Log Capacity - Freshly cut wood is called "green", and contains a lot of water. The densest green hardwoods have a density about 64 pounds per cubic foot. The Wood Mizer product page noted above shows a log capacity of 36 inch diameter x 21 ft long. This produces net lumber up to 20 ft long, allowing for some losses at the ends due to uneven cuts and splits. Thus the maximum log weight is 148.4 cubic feet x 64 lb/ft^3 = 9,500 lb. The lightest woods are at least half the density when green. If logs these weights are considered too heavy to handle, then a smaller dimensional capacity can be set for the sawmill. In that case, large logs would be quartered in the forest using a chainsaw or hydraulic splitter to reduce them to a size that fits the mill. Also, you can intentionally cut smaller dense hardwood logs than lightweight species logs to stay within whatever weight limit you need to work with.

The log weight will determine how you transport them, how they get loaded into the sawmill, and how sturdy the sawmill bed and hold-downs (what keeps the log from moving while cutting) need to be.

Depth of Cut - Some wood species will dry straighter if done in "flitch" sections. This is a slice through the entire log. Individual boards are then cut from the flitch after drying. If that is not required, then the depth of cut needs to be at least 1/2 of the largest lumber dimension you expect to need. You can get by with 1/2 the dimension by cutting from both sides, and cutting away the round edges of the log first.

Power - The Wood Mizer product page shows engine power of 25-50 HP for the LT40SH bandsaw. A circular saw will require higher power, since the blade makes a wider cut. The required power is proportional to how much kerf (the cut away portion) is cut per second. A smaller engine will still cut a large log, just slower.

Design Options

Circular vs Band Saw

A band saw blade is kept straight by tension, while a circular blade is kept rigid by it's diameter to thickness ratio. Therefore circular blades are generally 2-3 times thicker. This requires more power to operate and results in more kerf loss (sawdust created by the cut). Circular blades are limited to less than half their diameter depth of cut. Band saw depth of cut is determined by the full diameter of the wheels the metal band rides on. The perpendicular two wheel design proposed for the circular sawmill requires two drive motors, while a bandsaw requires one. So overall the bandsaw appears a better design.

A comparison of relative labor and maintenance costs shows bandsaws are lower, which is why most commercial sawmills have converted to them.

Stationary vs Portable

The alternatives are to bring the logs to a stationary sawmill or bring the mill to the logs. In the first case we have to move 1 unit of log weight to the mill, and about 0.4 units of offcuts and sawdust back to the forest for recycling. In the second case we only have to transport 0.6 units of cut lumber out of the forest. Since the above analysis shows the mill will be sized for a large village, numerous forest locations and destinations for the lumber are likely. For a stationary mill the average distance is set by the entire radius of forest used by the village. For a portable mill the distance can be lower, down to zero if the trees are cut, and then a building put up on the same location

Moving heavy logs tends to compress the forest soil and damage it. Cut lumber can be transported in smaller loads and so cause less damage. Although bad weather would interfere with using a portable mill in the field, it can still be used under a shed if you need to run it at times like that. On the whole, portable seems a better option. There are numerous commercial and home-made portable band sawmills that demonstrate it is a practical alternative. The home-made designs are pretty close to what OSE design would be, in the sense of simple to build. Here is an example: Garage Built Bandsawmill

Portable mills require terrain within the leveling capability of the stabilizer feet. A moderate slope is useful to load logs in the absence of mechanized loaders. Any sawmill requires a power source. Portable mills can use whatever source powers the transportation vehicle, such as the OSE Power Cube.

Productivity

The value of cut green lumber over raw logs is about $0.20/bf higher. At a daily production of 2230bf/day, that means an added value of $446 per day. With a crew of two working 8 hours, that is about $28 per hour, meeting the GVCS general productivity goal.