Dear Reader:
We would like to provide what could be considered a complete engineering design for the O’Hara Sawmill on this page. We welcome any and all contributions to this page – especially if you happen to be a sawmill engineer. Please e-mail the Webmaster if you have anything to offer.
Energy & Power Calculations
Question: Given an average year, how much hydraulic energy flows through the mill pond dam?
Answer: Not having the benefit of a proper water flow meter, I have done it the simple way during the last two spring thaws. Knowing the depth and width of water going over the dam spillway, I timed a small float from the time it crested the spillway until it reached the stream bed. Using a specific formula I came up with an average of 65 cubic meters/minute during spring runoff. I would estimate that the average over the year, given drought conditions and rainy seasons, would be in the area of 10 cubic meters/minute. Spring conditions therefore was when most of the sawing was done with the mill running 24 hours a day in three working shifts. The original mill flapper wheel and later the metal turbine both required amost 50% of full spring flow to run the Mill.
Question: Torques and stresses on shafts and other components:
Answer: This depended much on the size of logs that were being sawn at the time. The larger and heavier the log, the more strain was put on all the wooden moving parts, so maintainance and lubrication times were probably more frequent. Downtime for blade sharpening was kept to a minumum by making sure logs were kept fairly free of dirt.
Question: There would, presumably, much more hydraulic energy available in Spring than in other seasons. Was the saw mill designed to accommodate these variations or was there more than enough energy available regardless of season?
Answer: No. Spring runoff was the predominant sawing season but it’s a known fact that, as water flows decreased into late spring, James O’Hara would send a couple of men upstream to "Let out" one or more of the several beaver dams, which could extend operations for as much as another three weeks.
Materials Specification
Question: How were construction materials specified? was it based primarily on what was available in 1850, or economics or engineering calculations or a combination?
Answer: It is most likely that when the O’Haras ordered the components for the saw mill, they would have received only the metal castings, shafts, fittings and a bag of nuts and bolts along with a set of working drawings. It would have been up to them to prepare the right location for the buildings’ stone foundation, hand hue all the needed timber framing to right dimensions and assemble it all to quite accurate tolerances. Some fairly fine carpentry skills were required (and all with hand tools then) as many of the wooden linkages are very close fitting mortise and tenon joints. I can attest to this as am in the process of replacing some of these lovely old fittings. I am also assuming that many of the wooden shafts and other parts were made on site in the carpentry shop by James’ carpenter brother-in-law. Economics very likely entered into the building of the mill as the area was fast being settled and there was a growing demand for sawn timbers and dimension lumber for barns and new homes.
Question: What Lubricants were specified?
Answer: Lubrication instructions, still visible pencilled on a few beams around the mill, called simply for the application of #50 gear oil to all moving parts on at least a daily basis. It’s what we still use.
Economics
- How important were engineering economics in the design and construction of the O’Hara Saw Mill?
- Around 1865, lumber was worth about $12Ref/1000 board feet delivered to Great Lakes ships.
Operations
Question: What technical skill level was required to maintain the mill?
Answer: Probably much the same as today. Whoever was in charge of lubrication was also required to keep a sharp eye out for possible wear and tear on all of the wooden moving components. Unless there was an actual breakdown, full maintenance and replacement of suspect parts would have been done during the summer while the mill was shut down.
Question: What skill was required to operate the mill?
Answer: The Sawyer was probably the supreme being while the mill was in operation. It was up to him to decide how each log was positioned on the bunks and rotated to yield the most usable lumber. He was also in control of all of the various poles and levers that determined water flow, feed rate and carriage return; kind of like a conductor of an orchestra ! Having built over 300 modern sawmills and sawn a couple million feet of lumber. I figure I am maybe half way through the learning curve of mastering this old gem and all its quirks.
