I'm part of a group that runs a demonstration circular sawmill in Bracebridge , Ont.
None of us are experienced sawyers, and need every bit of help we can get.
We understand that speed of the blade is important as is keeping the saw teeth as sharp as we can.
The only means of measuring blade speed was a mechanical tachometer on the end of the saw shaft , no, not at the blade:-) , but it was really quite impractical for the sawyer to see what was going on. About the only thing we could go on, was the sound of the blade and the sound of the speed of the engine.
Our blade is hammered for 500 rpm.
I knew that we needed something better, so I built an electronic tachometer that uses 12 small magnets inside the rim of a big pulley on the saw blade shaft, and a pick up coil that senses each magnet as it zips by. From that I got a frequency and used a chip that converts frequency to a voltage. I then, set up the voltage output of the chip so that the bottom of the scale on my meter was about 380 rpm and the top was about 580 rpm ( blade speed) .
We can now see with good resolution when our blade starts to slow down or speed up. The response of the meter is pretty well instantaneous.
I set up the meter so that it was in the line of vision of the sawyer, so he didn't have to look away to see what the blade ( and mill) were doing.
After a while most of the guys got the hang of when to back off on the feed rate to prevent slow down, and when to ease up to prevent over-speeding ( i.e. at the end of a cut) . You get the "feel" of the blade and mill when it's doing a good job and you can tell when it has slowed too much.
I calibrated the tach, with a revolution counter used on steam engines and counted exactly 250 revs in 30 seconds - bang on.
I kinda look at it as "training wheels" on a bike - after a while you don't need them any more.
I had thought about a digital read out, but decided that an old fashioned needle in a meter ( like a gas guage) was a better approach.
We're still learning but I can say the guys now have more confidence in the machine.
The mill is a Waterous, patented 1868 , with a 44 in blade and is run by a Case Model D stationary power unit. We mostly saw softwood ( pine, hemlock) .
I could go on and on , but that's enough for now.
miro
I can say that all the guys liked the tach and the idea that they have a reliable means of "learning: the mill.
I made a saw tack by taking parts off an old combine, works pretty good
Miro, what do you use for a power plant.?? I use a simple hand held Stewart Warner tack on the end of the arbor and adjust the diesel governor. I only need to check it once in a wile. If your using steam or various power plants I can see the need for a tack, what you skillfully built should work, it doesn't have to be perfect. Frank C.
We had six tacs in the saw cab two for the block chippers and one for each of the four bandmills (a slow turning block chipper likes to climb the log and come off the base and that gets ugly real quick like) a dull saw will slow down more then the rest and show on the 4 saw deviation monitors as well.
A great tool to have for the sawyer. I was realy surprized the lt-70 did not come standard with a tac located right beside the saw deviation gauge :o
I have a digital tach with a pickup that rides on the mandrel that I started working on but never got it to work right. My attention then turned to all of the issues I have had with bearings and bent mandrel so I have not worked on it anymore. Once I get the mill up and running, I will turn my attention back to the tach.
Dave C
WE have a 46 in saw that was hammered fo 550 but it still ragged [wobbled ] until I speeded it up to 600 . After that it never touched the guide pins . You should be able to tell from the sound of the saw when you get it right . Gearbox
Old thread, but.
Here is a vw diesel homemade tach. Adapt as needed.
http://www.4crawler.com/Diesel/CheapTricks/Tachometer/index.shtml
eddif