Cutting a radius with a Chainsaw Mill.....

[andy]

....Part of a Radius anyway.

I looked at a Job yesterday that has a small wooden foot bridge.
It got me thinking... I have a Cedar Log that I could cut the curved Bridge deck out of one piece.

Has anyone created a curved jig to run their chainsaw mill against?
Can a Chainsaw go any direction other than dead flat straight?

The radius on this is bridge top is relatively slight. I would cut the jigs out of plywood on the bandsaw.
Iirc the bottom is drawn to be flat but I could make a separate jig to cut it in a parallel curve to the top.
It seems like I should be able to get the chainsaw mill to follow the curve but,.... maybe I'm asking for huge trouble.
I have done tons of laminating curves for stairs and other architectural details and structures.
Chainsaw is pretty new to me but it seems to follow the rules of most other wood working tools.
The bridge deck is 24" wide so bandsawing is not going to work.

Anyone get near something like this before?

 

[Brian72]

In my experience, it's tough to be real precise with a saw. I usually rough it down with the saw and finish with a drawknife, chisels, grinder etc. My buddy just got a wheel for his angle grinder. Basically a saw chain converted to a grinding wheel. It works really well for shaping and smoothing.

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[BobL]

For a CS to cut a curve the B&C has to fit within the kerf which means only large radius bends are possible.
But the main real not to do this is the curve will tilt the chain over both on the cutting side and on the other side of the bar and the net result will be preferential wear of the chain and bar on one side. If you do this even for a short while the chain will always want to do that and pretty soon you will always be cutting curves to the point where the B&C jams in the cut.

I have seen this many times when friends/colleague/idiots who should never be let near a saw, bring me their saws to say it gets jammed in the cut. Most of teh time the bar groove is so sloppy the chain has tilted over to one side and one it cuts on that side the problem compounds as the B&C wears more and more until it jams in the cut. The most WTF??? one was the guy who sharpened the cutters on one side way more than the other and that ended up completely ruining his B&C.

When carving with a CS its common to cut curves but really ir involved just making a series of short straight cuts with a small pointed carving bar and then cutting/hogging away the unwanted waste around those cuts.

 

[Brian72]

For a CS to cut a curve the B&C has to fit within the kerf which means only large radius bends are possible.
But the main real not to do this is the curve will tilt the chain over both on the cutting side and on the other side of the bar and the net result will be preferential wear of the chain and bar on one side. If you do this even for a short while the chain will always want to do that and pretty soon you will always be cutting curves to the point where the B&C jams in the cut.

I have seen this many times when friends/colleague/idiots who should never be let near a saw, bring me their saws to say it gets jammed in the cut. Most of teh time the bar groove is so sloppy the chain has tilted over to one side and one it cuts on that side the problem compounds as the B&C wears more and more until it jams in the cut. The most WTF??? one was the guy who sharpened the cutters on one side way more than the other and that ended up completely ruining his B&C.

When carving with a CS its common to cut curves but really ir involved just making a series of short straight cuts with a small pointed carving bar and then cutting/hogging away the unwanted waste around those cuts.
View attachment 651431 View attachment 651434

Nice!

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[andy]

I should have given more detail... It looks like about a 20' radius so quite large (6" rise over 9' run).
It's a pretty slight curve. I understand it will come at a cost to the bar and chain.
Omelets... broken eggs,.. and all. This client won't care if it costs them a chainsaw (or 2).
Also understand that there will be plenty of clean up on the surface.

Is it even doable? I'm sure I can get an even easier radius put into the plan.

It would be pretty trick in the end but maybe it's just not going to work.

 

[Brian72]

I should have given more detail... It looks like about a 20' radius so quite large (6" rise over 9' run).
It's a pretty slight curve. I understand it will come at a cost to the bar and chain.
Omelets... broken eggs,.. and all. This client won't care if it costs them a chainsaw (or 2).
Also understand that there will be plenty of clean up on the surface.

Is it even doable? I'm sure I can get an even easier radius put into the plan.

It would be pretty trick in the end but maybe it's just not going to work.

I don't see why it wouldn't work. That's not a drastic curve. Probably have to make more straight cuts and then go back and smooth it out. As long as you don't twist the bar, I don't see any damage issues.

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[BobL]

6" rise over 9" is nothing but see if you can get a narrow width bar.
I'd get a thinnish guide board and tek screw it to the log with different thickness risers underneath it to generate the curve.

 

[buzz sawyer]

Were you planning on cutting relief cuts down to the curve - like you do when bandsawing?

 

[andy]

Thanks for the replies everyone.

I guess first is to try and see how the mill will ride on a curved section...
Sometime next week I'll re-essmble the long compass arm we have here and cut a couple plywood test sections (forms).
I'm seeing the top of the log would be flattened first to make the "bottom" of the bridge.
Then the jig would be mounted as a Concave curve to cut the bottom of the log for the bridge top.
The limiting factor will be the Chain saw Mill post length.

When I mentioned to the architect we might be able to take that section out of one log rather than stringers and plank she lit up.
We'll see.

Quick squiggle sketch.

This guy is free-handing a curve on a beam. Cutting a drawn on line. Big difference between a 6" beam and a 27"wide piece of log.

 

[andy]

Were you planning on cutting relief cuts down to the curve - like you do when bandsawing?

Maybe but probably not. I'm sort of stingy with materials and don't like to cut things until I know what purpose they serve.
Depending on the piece and how it turns out, The fall may turn into a couple handrail bits or trim.
otoh ...This is at the idea stage. Relief cuts could be useful.

 

[Timberframed]

It will definitely work with minimal strain. Go for it.

 

[BobL]

I'd think about cutting it the other way otherwise your mill may jam working on the inside of a curve radius.
It's a bit like using a flat spokeshave to shave a curve, it will shave an outside but not an inside curve.
Cut a flat top then Tek screw ply through suitable shaped blocks/supports direct onto the log - this should easily generate the curve
Then mill a slab off to remove the screw holes in the flat top.

 

[andy]

Well.... this project is dead for now.
The Architect just sent a revised plan with a stone bridge per the owners request.
Thank you all for the input and ideas.

 

[Tin-knocker]

Well.... this project is dead for now.
The Architect just sent a revised plan with a stone bridge per the owners request.
Thank you all for the input and ideas.

Yeah but for the sake of research and science and whatnot...

 

[Albert Beerstein]

I sometimes cut a curve without meaning to, until it all jambs up....

 

[Marine5068]

I should have given more detail... It looks like about a 20' radius so quite large (6" rise over 9' run).
It's a pretty slight curve. I understand it will come at a cost to the bar and chain.
Omelets... broken eggs,.. and all. This client won't care if it costs them a chainsaw (or 2).
Also understand that there will be plenty of clean up on the surface.

Is it even doable? I'm sure I can get an even easier radius put into the plan.

It would be pretty trick in the end but maybe it's just not going to work.

Layout the radius you want to achieve, then mark straight lines along it and cut away straight and finish up by nipping away the points and using a grinder with chain wheel or draw knife.