Blade problems with Band Saw Circle Jig
Howdy all,
I am trying to make a bandsaw jig to cut circles. it’s a simple setup just some plywood about 18″ square, with a runner glued underneath to slide in the “miter gauge slot”. I have a nail punched in through the bottom with the point just protruding through the top of the plywood. this allows me to pivot the stock on the nail. The pivoting works fine, but I keep getting the blade running out of whack. so the end result looks like a section from a cone, with the bottom of the circle bigger than the top of the circle. I have tried a 3/4″ blade to a 3/16″ in blade, and I am trying to cut 5 inch circles. I don’t know what I am doing wrong, I have adjusted the guides several times to hold the blade as tight as I can, but it still seems to wander. I have aligned the center point of the nail with the front edge of the teeth. I am using delta blades and a delta 14″ saw.
This is really not making sense to me, it seems like it should be easy to do, a simple jig and a simple process. there must be something simple that I am missing.
Some help would be appreciated.
Thanks,
Mike
Replies
Do you have the material supported on both sides of the sawblade or just the circle side? If the blade is tight, the only other reason I can think of for a tapered cut is the material is tilting, or the table on the saw is not perpendicular to the blade.
Jeff
With a narrow blade you shouldn't have a problem cutting that radius. But something is applying pressure to the side of the blade causing it to flex as much as it can. My guess - move the pivot point up or back just a bit. It needs to be in correct relation to the cutting angle of the blade, and not necessarily what "looks" like 90°.
David Ring
http://www.touchwood.co.il/?id=1&lang=e
David
The pivot on mine is aligned with the back of the gullet, having said that, I cant imagine how that geomtery would cause the taper you describe.
Dave.
Success!!I checked the table and made sure its square to the blade. I ensured that the pivot point is aligned to the gullet of the blade and the blade guides are right up next to the work and it's working now! So I don't know which one of those things I was doing wrong (maybe all of them to some degree)Thanks for your help!
Meilcowski,
I had the same issue with the blade binding about 3/4 way thru the cut(demelude table). I decided I'd never be able to adequately control the jig and it's relationship to the blade without major figuring and giggling. I decided that using a template and post setup would be more reliable.
Dave, It's beyond my geometry skills. I believe that the basic problem is that a straight blade needs to run in an arc within its own kerf, and that one of the walls of the cut is applying sideways pressure to the blade. Speaking from experience I've seen that when setting up the pivot point in relation to the blade, a small movement front or back really influences the cut, perhaps because the relevant tangent is the line the blade wishes to move in, not what looks like "straight". Dunno, but playing with it has solved the problem for me in the past. All this is assuming that the blade is narrow enough to pull of the job in the first place. A 1" wide blade will obviously not make a 5" diameter circle.David Ring
http://www.touchwood.co.il/?id=1&lang=e
Meilcowski,
Is your blade sharp? Is your upper guide close to your work?
Paul
Meil, If the finished piece is the 'disc',(and not the 'hole in the square',)why not cut the disc roughly by hand on the band saw about 1/8" wider than needed?
Then ,attack the pivot cut to the finish dimension? Also, by arranging the cutting angle at the pivot point to describe a 'tangent' to the circle so you are just 'paring' the cut by glancing across the blade's teeth.
A blade with a wider 'set' will reduce pinching. Another trick is to round off the 'heel' of the blade by holding a medium grit honing stone in a rocking motion while the blade is running.
Just round off the back of the blade this way and DON'T let the stone touch the teeth. Steinmetz.
.Sounds like the table may be out of square to the blade.
mike
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