by Harry Hopcroft, Tide Mill Institute
When the circular saw blade on display at this summer’s Georgetown Historical Society exhibit was closely examined, it was found to have an extra plate mounted in the center, and on only one side. The extra piece is severely rusted in place now, but was originally held on by a series of short machine screws installed through holes from the back side of the blade. It is about a third to half the diameter of the main part of the blade, and thicker at the spindle than at its edges. The main blade itself is about 1/8 inch thick, and the added piece is ½ inch thick at the spindle, then tapers to almost knife edge at the outside. The main blade also has very little “kerf”, which is the measure of the width of the cut. The kerf is created by bending the cutting teeth outward in an alternating pattern from one side to the other. That allows the blade kerf to be a tiny bit wider than the thickness of the blade, and helps prevent the saw from binding.
The sort of obvious answer was that the extra piece was there to stiffen the blade, but it also appeared that it would severely limit the depth of the cut available with this blade, so we wondered why it was added. We knew the blade came from the site of the Oliver Grist and Shingle mill on the west end of the dam in Georgetown Center, so presumed it had something to do with cutting shingles, not logs.
When we put the question to the mailing list at the Tide Mill Institute, confirmation came quickly from two sources. The first was Rob Grassi, who worked as a millwright at the Hanford Mills Museum for more than 14 years. He writes, “We operated (on water power) an old Chase Shingle Mill, utilizing a 36 inch blade similar to the one pictured. Yes, it did have a stiffener plate bolted to the main sawblade only on the outer side. The inner side had machine screws through the saw plate threaded into the stiffener plate. It had 2 shear pins and it was held all in place on the saw with a large nut over the threaded mandrel at the center of the sawblade. Yes, you are correct, these are particular to shingle saws. The stiffener plate allows the blade to be thin thus giving a narrower curf and less waste of material.”
John Morse, of Bath, whose family ran saw mills at Winnegance for decades, writes” “Your questions probably have to do with the thickness of the blade. The hub is a sort of stiffener for the blade since it is probably thin enough to heat up and get limber during use. The hub helps to prevent this. Since this is a presumed shingle saw it was probably used as a table saw. Its maximum cut being 10”. … The reason for a thin blade is to reduce the effort involved in pushing the wood through the saw manually. Also to make a smoother cut and to not waste the stock by making excess sawdust.”
Ron Klodenski, at TMI, then found, on the website of something called Gas Engine Magazine (gasenginemagazine.com) of Sept 1, 1978, the following notation (actual author not listed):
Shingle mills still exist which are known to have been built in the 1850’s and 1860’s, but I doubt if any were made earlier than that, because the circular saw did not become popular until the 1830’s, and even then it was very crude and inefficient. Almost all the blades were made the same as the sawmill blade. They were of a very thin gauge, and were attached to a tapered plate which was about half the diameter of the blade. This plate came to practically a knife edge around the outside, and its center varied from a half-inch to several inches. This plate performed three very important functions.
First, it strengthened the thin blade, and helped to keep it sawing with the grain and from sawing around knots. Second, it acted as a flywheel, keeping up the momentum of the saw as it cut through the block, allowing faster feeds to be used, and to be run with less horsepower. Third, as the shingle was sawn, the plate separated or spread it from the block and saw, preventing any binding or pinching which may otherwise occur.
The blades for shingle mills were usually about nine gauge and ranged from 24′ to 42′ in diameter.
For reference, this exactly describes our blade, which is itself about 9 gauge (with 9 gauge being 5/32 inches, and 11 gauge being 1/8 inch, but our tape measure not being marked well enough to distinguish between them on a rusted edge). The single carat in the numbers above refer to the blade width in inches, not feet. Similar plates are common today for stiffening very thin blades favored by artists and professional woodworkers for finer cuts and less waste, but today’s are square edged, mounted on both sides of the blade, and definitely limit the depth of cut available.
When cutting shingles, a length of wood is cut into blocks 16 inches long, then mounted on a small carriage affair, and held there by metal pins, or “dogs”. For each shingle, the block is then cut lengthwise, whether the block is mounted horizontally or vertically. A clever arrangement of rotating shafts and wedges tips the block slightly forward for one slice, then back for the next, in order to achieve the characteristic wedge shape of a shingle. The edges are then cleaned up in a separate operation. The thinness of the blade allows it to turn much faster, and the extra piece keeps it from wobbling and wandering as it heats up from all the friction.
Shingle and heading machine from a 1902 catalog of circular saw mills offered by Chase Turbine Manufacturing Co. (Courtesy of vintagemachinery.org.)
Very good research, and a good explanation!
I agree with Dennis: It was a very good article, with good research and good explanation.
This is a really nicely done explanation. So the shingle as it was cut was actually wedged outward a bit by rubbing along the tapered hub plate? Interesting way to make a splitter and more than compensate for any pinching as the saw cut. I don’t know if cedar is prone to warp or residual stresses, though I have seen it with wind and certainly other woods might have any of those. But that also would explain why the sawblade would be heating up even more than any sawblade in use.
On a side note, I don’t think you mean ‘single carat,’ as a carat (karat) is the measure of a diamond grade, though you were probably thinking ‘caret’ but that is the wedge-shaped insert character (^). But — and here I put on my super-pedant hat — the straight quote used for feet and inches are single and double ‘primes’. Fun fact of the day!