Cutting circle on bandsaw - what went wrong?

[Alan Lightstone]

I'm completing a large round table top. As I have a new, much beefier 20" Felder bandsaw now, I thought it might be easy to cut the top into a circle on my bandsaw instead of with a router/circle jig.

Thankfully, I tested this approach on 1/2" plywood instead of my finished piece.

First I tried with my 1" bandsaw blade I usually use for resawing wood. It was having a very hard time with the curve (I quickly tried a 16" diameter circle - probably too small a curve for the 1" blade, but I though with this kind of jig that didn't matter). So I abandoned that, and put a 1/2" blade in the saw. I tensioned it to 25K psi, and gave it a go.

Here's the result:
IMG_3640.jpg

Totally scalloped cut, along the entire circle.

What's the cause of this? I would think 25K psi would clearly be tensioned enough, and the Carter Circle jig rotated the board smoothly.

I don't remember ever seeing this happen before, although I don't use that jig much. I would think if this is how it usually goes, the jig is useless.

[Matt Day]

My guess would be too big a blade. Guessing the scalloping is from the blade going in and out of the cut because it can’t cut the radius. Just my guess. I’d cut a rough circle on the BS then use a router jig.

[Jared Sankovich]

What is the diameter of the part you cut?

[Edwin Santos]

Would you post a photo of your circle cutting jig and set-up? I have a theory on what caused the scalloping, but would like to see the conditions to be sure. And yes, knowing the diameter of your circle would be good too.

I do not think the scalloping has anything to do with insufficient tension.
Edwin

[Kevin Jenness]

Could be that the pivot point is off. Get your fence aligned to the blade drift, then your circle should be centered on a line square to the fence running through the front edge of the blade.

[Alan Lightstone]

Here's some pictures of the Carter circle cut jig. I have modified it, screwing it to a plywood base, that attaches to the bandsaw table with magnets. I actually also clamped it this time, as the board was very large.

The circle I cut was 16" diameter, with a 1/2" blade. Not sure the tpi on it.
IMG_3646 (2).jpg
IMG_3647 (2).jpg
IMG_3645 (2).jpg

[Kevin Jenness]

Where is the pivot point on the jig?

[Jared Sankovich]

I'm more confused after seeing the jig.

It doesn't looked like you could line up the pivot with the blade

20200516_103551.jpg

[Kevin Jenness]

Ok, I see that the pivot point is at the upright stud location and slides in to the stop at which point you spin the blank. It looks as though the pivot is aligned with the blade teeth when slid in. Is the jig aligned with the blade's actual cutting path, that is, if you rip a thick piece on a line square to the jig member with the scale do you get a clean cut without drift? If that alignment is skewed it might cause the problem you see, as with resawing with a misaligned fence.

Other than that, I got nothing. A 1"blade should cut a 7" radius circle. 1/2"plywood shouldn't tax a sharp, well-tensioned blade.

[Edwin Santos]

I do not believe the pivot is aligned properly with blade's teeth. You would need to loosen the bolts and slide the blue bar either toward the outfeed or inside side of the table until the pivot point is in line with the teeth. If the pivot is the part with the bolt stud sticking up out of it, then it is way way too far forward. But perhaps you've just moved it there out of the way for the photo or something. See that box with the dotted line on the jig? I think the center of your pivot should be at or near the dotted line on the left side of that box.

There is a simple way to check this. Align the pivot to the cutting edge of the blade. Try cutting a circle from a piece of scrap and stop the bandsaw partway into the cut so you can carefully observe whether the blade is centered in the kerf. If the blade is rubbing (as I suspect) the keeper circle side of the cut, the pivot needs to move to the outfeed side. If it is rubbing the waste side of the cut, you would move the pivot toward the infeed side. Again, play with it on a piece of scrap and then you identify whether an adjustment as I'm describing is necessary.

What is causing the scalloping is that the mis-aligned pivot is trying to force the blade to one side or the other, and the blade guides are forcing it back, over and over again.

[Alan Lightstone]

I do not believe the pivot is aligned properly with blade's teeth. You would need to loosen the bolts and slide the blue bar either toward the outfeed or inside side of the table until the pivot point is in line with the teeth. If the pivot is the part with the bolt stud sticking up out of it, then it is way way too far forward. But perhaps you've just moved it there out of the way for the photo or something. See that box with the dotted line on the jig? I think the center of your pivot should be at or near the dotted line on the left side of that box.

There is a simple way to check this. Align the pivot to the cutting edge of the blade. Try cutting a circle from a piece of scrap and stop the bandsaw partway into the cut so you can carefully observe whether the blade is centered in the kerf. If the blade is rubbing (as I suspect) the keeper circle side of the cut, the pivot needs to move to the outfeed side. If it is rubbing the waste side of the cut, you would move the pivot toward the infeed side. Again, play with it on a piece of scrap and then you identify whether an adjustment as I'm describing is necessary.

What is causing the scalloping is that the mis-aligned pivot is trying to force the blade to one side or the other, and the blade guides are forcing it back, over and over again.

The pivot is way forward in the picture. That’s where you start the cut and move the board/pivot point into the blade.

I’ll try your suggestion and see what happens with the cut.

[Alex Zeller]

If it was me I would cut some wood the same thickness of the height of the blue part of the jig to support the work piece and put it in front of the blade. It may not be your problem but it's not going to hurt. I have a homemade jig like this I use for cutting turning blanks and the more support the better. Once you move away from plywood you are going to be dealing with the grain of the wood changing directions as you spin the piece. Having more support will help with your technique since it'll be easier to hold the piece flat. I use a 1/2" blade to cut much tighter diameter circles than that so unless it's a band blade it shouldn't be your problem.

[Edwin Santos]

The pivot is way forward in the picture. That’s where you start the cut and move the board/pivot point into the blade.

I’ll try your suggestion and see what happens with the cut.

Got it, so the question will be whether the stop point of the pivot is the issue. Please report back on what you find out.
It might also be interesting to check the scalloped circle to see if it is true.

[Lee Schierer]

The trick to cutting circles on a band saw is to get the front edge of the teeth exactly aligned with the center of rotation. Once you accomplish that cutting circles is much easier and precise. Your 1/2" blade should work just fine assuming that it is sharp on both sides.

The problem I see with the Carter jig is that there is no check that the arm is perpendicular to the saw cut line. If the arm is not perpendicular to the cut path, the center of the circle drifts off of the optimum setting as you get closer or further from the blade. What happens if your pivot point is ahead of the teeth, the blade will try cutting your circle under size. Likewise if the pivot is behind the teeth the blade will attempt to cut the circle larger than the radius and may bind as it tries to move further away from the pivot.

With your jig mounted on plywood it is essential that you align the blade with the miter slot. You might want to check out this recent thread on bandsaw set up.

The jig that I have was from Kreg and it uses a piece of plywood and has a miter gauge bar that fits in the miter slot, which insures that the center of rotation is always perpendicular and it also has a adjustable stop so that when you slide the work piece into the cut that the center aligns perfectly with the sweet spot for cutting circles. Different width blades need different stop points.

[Bruce Wrenn]

Your blade might have been too wide for the created kerf. For circle cutting, I prefer a 1/4", 6TPI blade. Remember that you can always cut circle slightly over sized, then using router and trammel bring it to finished size.