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These instructions, and the attached file address the issue of what causes the blade on your saw to mis-track and/or pop off:
What Causes the Blade to Jumo Off and How I Fixed It By John Vreede
I went through John Pitkins 'manual' (attached) and, while I found stuff out of whack by his description, none that I corrected seemed to make a substantial difference. I replaced the top axle block guides (turned them over in fact) to reduce clearance , as they were sloppy as I thought any play here would be bad, but when the blade is under tension, I now can't see that it would make any difference.
My bandwheels were not in the same plane L&R (Zero Twist section of JP's manual). I didn't fancy shimming the guides so I provided a means to tilt the top axle sideways with 2 grubscrews left and right above the axle tightening bolt, like the central grubscrew below it. The lower central grubscrew tilts the top wheel in and out at the top, but because the top of the axle-carrier is square, the wheel won't move left or right (which was where my wheels were out of alignment). My solution meant remaking the axle carrier block about 1/2" longer at the top, to have something for the grubscrews to bear on. If you've got twist in the alignment of your wheels like mine, I can send a photo or drawing of what I did. I'm pleased I did it but it didn't stop the blade popping off
JP's instruction go to great length to get the wheels in alignment and then after the blade in properly tensioned, they're all out of alignment again, and you tweak the top wheel alignment with that lower grubscrew and shim the wheel in or out to make the blade ride where it should. What matters is the alignment of the wheels with the blade tensioned up. Although his alignment process didn't stop my blade from jumping off, it's such a well-reasoned and informative document that I'd do what he says first, and if that doesn't help then maybe try what I did below.
Here where my experience differs from JP's and this stopped my blade jumping off (and may be what you need): the roller blade guides above and below the cut were too far forward on my machine, so that, when the blade travels off the bandwheel, the rear roller (ball bearing) pushes the blade forward (in the direction of the toothed side of the blade) with respect to the flange on the rim of the bandwheel. I figured that if the guides were too far forward, even though the blade might track properly on the bandwheels when running not in a cut, with the weight of the sawframe on it in the cut, the back guide rollers were acting as pivot points - press the middle of the blade between the rollers and that part of the blade out-side of the rollers would pivot away - away from the flange of the bandwheels and the blade then rides off the wheel in the cut and pops off.
To adjust properly, I loosened the guides so they could move backwards, in relation to the teeth of the blade, until the roller behind the blade wasn't touching the back of the blade at all (i.e. roller stopped turning). Do that with the blade running and tensioned up and aligned per JP's instruction. I then moved the guide forward so that the back roller just touched the blade. That way the blade is running from the flange on the top wheel to the flange on the bottom wheel in a direct line and all the guides are doing is twisting the blade so it drops square through the cut.
Loosening the guide fixing bolt on my RF machine allows the guide to move every-which-away, so I left it partially tightened and adjusted the guide angle with a 10" Crescent and tapped if forward or back with a machinists hammer, re-tightened and checked again for squareness in the cut. JP says use 2"x2" dressed all sides timber for this but I suspect rectangular plastic drain pipe would work even better since only the top and bottom edges matter just don't let it drop to quickly through the cut as you could overload the gullets of the teeth).
What Causes the Blade to Jumo Off and How I Fixed It By John Vreede
I went through John Pitkins 'manual' (attached) and, while I found stuff out of whack by his description, none that I corrected seemed to make a substantial difference. I replaced the top axle block guides (turned them over in fact) to reduce clearance , as they were sloppy as I thought any play here would be bad, but when the blade is under tension, I now can't see that it would make any difference.
My bandwheels were not in the same plane L&R (Zero Twist section of JP's manual). I didn't fancy shimming the guides so I provided a means to tilt the top axle sideways with 2 grubscrews left and right above the axle tightening bolt, like the central grubscrew below it. The lower central grubscrew tilts the top wheel in and out at the top, but because the top of the axle-carrier is square, the wheel won't move left or right (which was where my wheels were out of alignment). My solution meant remaking the axle carrier block about 1/2" longer at the top, to have something for the grubscrews to bear on. If you've got twist in the alignment of your wheels like mine, I can send a photo or drawing of what I did. I'm pleased I did it but it didn't stop the blade popping off
JP's instruction go to great length to get the wheels in alignment and then after the blade in properly tensioned, they're all out of alignment again, and you tweak the top wheel alignment with that lower grubscrew and shim the wheel in or out to make the blade ride where it should. What matters is the alignment of the wheels with the blade tensioned up. Although his alignment process didn't stop my blade from jumping off, it's such a well-reasoned and informative document that I'd do what he says first, and if that doesn't help then maybe try what I did below.
Here where my experience differs from JP's and this stopped my blade jumping off (and may be what you need): the roller blade guides above and below the cut were too far forward on my machine, so that, when the blade travels off the bandwheel, the rear roller (ball bearing) pushes the blade forward (in the direction of the toothed side of the blade) with respect to the flange on the rim of the bandwheel. I figured that if the guides were too far forward, even though the blade might track properly on the bandwheels when running not in a cut, with the weight of the sawframe on it in the cut, the back guide rollers were acting as pivot points - press the middle of the blade between the rollers and that part of the blade out-side of the rollers would pivot away - away from the flange of the bandwheels and the blade then rides off the wheel in the cut and pops off.
To adjust properly, I loosened the guides so they could move backwards, in relation to the teeth of the blade, until the roller behind the blade wasn't touching the back of the blade at all (i.e. roller stopped turning). Do that with the blade running and tensioned up and aligned per JP's instruction. I then moved the guide forward so that the back roller just touched the blade. That way the blade is running from the flange on the top wheel to the flange on the bottom wheel in a direct line and all the guides are doing is twisting the blade so it drops square through the cut.
Loosening the guide fixing bolt on my RF machine allows the guide to move every-which-away, so I left it partially tightened and adjusted the guide angle with a 10" Crescent and tapped if forward or back with a machinists hammer, re-tightened and checked again for squareness in the cut. JP says use 2"x2" dressed all sides timber for this but I suspect rectangular plastic drain pipe would work even better since only the top and bottom edges matter just don't let it drop to quickly through the cut as you could overload the gullets of the teeth).
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