Circular Arcs With a Bandsaw

[ajayre]

I have a 1/4" blade in my bandsaw and I want to cut a board into curved segments with a radius of 5.5".

Where do I put the pivot point of the board in relation to the blade? So I know it has to be 5.5" away, but should it be lined up with:

- front of the teeth
- teeth gullets
- middle of blade (1/8")
- doesn't matter

Thanks!

 

[dickm]

Can't see that the difference between those positions on a 1/4" blade would be detectable, except for some fraction of the kerf width between teeth and the body of the blade. Probably the best thing is to do some trial cuts.

 

[WellsWood]

Front of teeth.

 

[Phil Pascoe]

From the front of the teeth set towards the pivot. As Dick said, it won't make much difference - if accuracy were that important you'd probably be looking to finish them on a sander or something anyway.

 

[CHJ]

+1 for front edge of the teeth, if you don't the blade will try to cut a different arc.

Forward of the teeth will see the blade trying to spiral inward to cut a smaller circle.
Behind the leading edge of the teeth the blade will try to spiral outwards.

Small errors may not be noticeable if blade has wide kerf.

 

[PAC1]

Just use trial and error with a few bits of scrap. You will soon find out what works and what does not.

 

[ajayre]

Thanks all!

 

[Steve Maskery]

Whoa!

The position of the pivot WRT the cut is CRITICAL, and the difference of one or two mm fore and aft can make the difference of cutting true and cutting spiral. At the risk if banging on about my videos, I demonstrate this very well with a circle-cutting jig, and just how much difference the pivot point makes. It has to be spot on. Perfect. If not, you will get circular drift and end up with a spiral. And given that the radius is so tight anyway, the precision of the pivot is even more critical.

 

[flh801978]

Steve is 5.5 "considered a tight radius?

 

[Steve Maskery]

I would say so. If it were a dining table you have more leeway. You definitely need a practice run or two. The tiniest error can throw you out significantly.

On my demonstration I deliberately move the pivot pin out of line. I forget by how much but it is not very far. The consequences are dramatic: blade stalling, sparks from the guides, very spiral cut, total disaster. And I'm pretty sure the curve was >5,5" radius. I'm not certain, I'd have to watch it, not done that for a long while.

Precision is everything.

 

[ajayre]

I lined up the pivot with the front of the teeth and it cuts the arcs perfectly. Thanks all!