Editor’s Note: Mr. Peter Sporring, who heads up the technical side of the O’Hara Saw Mill restoration and operation, has contributed the content for this section. His insight and familiarity with the minute technical details of the mill provide us with the opportunity for greater appreciation of this historic treasure.
When the Mill is in operation for public viewing, it is often impossible and sometimes confusing for visitors to comprehend everything that is going on in and around the the mill. This page should help clarify it all to some extent.
1. Starting the Water Wheel
A large header tank, located on top of the dam, is fitted with a gate valve to control the amount of water is flowing into the tank and the sluiceway (Fig 1: 1). At the discharge end of the sluice way, there is a spillway (Fig 1: 2) that diverts the water flow until the water wheel is to be powered. To start the water wheel, the sawyer pulls a lever at his station inside the mill and, via cable and pulleys (Fig 1: 4), the control gate (Fig 1: 3) is opened and water flows into the buckets in the water wheel (Fig 1: 5) causing it to spin.
Fig 1: Starting the Water Wheel
2. Starting the Saw Mill
Mounted on the water wheel shaft on the inside of the building is a large flat pulley (Fig 2: 1). A loose fitting 6” wide belt (Fig 2: 2) runs between it and a smaller pulley (Fig 2: 4) on the saw’s main shaft. To put the saw mill into action, the sawyer operates a lever to release a cable attached to a counter weight on the idler pulley (Fig 2: 3). This tightens the drive belt and the main shaft begins to turn, rotating the pitman wheel (Fig 2: 7). The bottom end of the connecting rod (Fig 2: 5) pivots on the pitman crank shaft, and its top end is connected to the bottom of the saw frame causing it and its blade to travel in its up down motion. The pitman wheel also has a large counter weight on one side and this offsets the weight of the heavy saw frame. Hence, we have fluid power first converted to rotary motion and then to the linear motion required at the saw blade.
Fig 2: Starting the Saw Mill
3. Feeding the Saw: The Log Carriage Feed
The carriage link pole (Fig 3: 2) which is an extension of the upper saw frame (Fig 3: 1) is always in motion while the saw is running. It is linked to a rocking beam (Fig 3: 3) which in turn is linked to the top of a center pivoted arm, the bottom of which holds the feed finger and its detent (Fig 3: 4).
To start the log moving, the sawyer lifts a lever that lowers the feed finger onto the toothed rim of the large ring gear (Fig 3: 5). As the finger rocks forward it engages a tooth and forces the wheel to turn a fraction. Just before it returns to pick up another tooth, the detent drops into a tooth of its own thus preventing the wheel from rotating backward. On the big horizontal shaft on which the feed wheel is attached, there are two 8” ring gears that mesh with racks (linear gears) (Fig 3: 6) on the bottoms of the log carriage. For each upward stroke of the saw frame, the log carriage (Fig 3: 7) is advanced toward the blade by about one half inch. The saw blade cuts only on the down stroke.
So in a simple but ingenious way the vertical motion of the saw frame is converted to the horizontal motion of the log carriage. There is safety feature intrinsic to this arrangement in that the log can not move toward the saw blade unless the saw blade is in motion on its down stroke.
To stop the feed, the sawyer drops the lever again, which lifts the feed finger and detent just off the toothed rim.
Fig 3: Feeding the Saw: The Log Carriage Feed
4. The Log Carriage Return
Once the log has been sawn through and the board removed the next step is to return the carriage to its home position so the log can be positioned for its next cut. The sawyer lifts a lever to raise the feed fingers (Fig 4: 1) off the feed wheel and the saw is shut down to stop the blade. Another lever is operated that lifts a gate (Fig 4: 2) and water enters a small horizontal tub wheel causing the large vertical shaft (Fig 4: 4) to rotate. At the top of this shaft is a beveled crown gear (Fig 4: 5) that, when the Sawyer pulls yet another lever, meshes with the large beveled ring gear on the feed wheel. The horizontal feed wheel shaft now rotates in reverse and the log carriage is pulled back to its home position through the rack and pinion gears. This is considerably faster than the feed rate.
Originally, this rush of water through the tub wheel also flushed accumulated sawdust out through an opening in the basement foundation to be carried away by the creek. This, of course, is no longer permissible in the interests of environmental stewardship.
Current restoration efforts dictate that the tub wheel be removed. The vertical shaft powering the log carriage return will be connected to the main waterwheel via a jack shaft and clutch.
Fig 4: The Log Carriage Return
