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Forum topic by JADobson posted 04-04-2016 07:00 PM 525 views 0 times favorited 11 replies Add to Favorites Watch
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JADobson

682 posts in 1578 days


04-04-2016 07:00 PM

Topic tags/keywords: bow saw failure buck saw cherry hickory design

Hi everyone,
So I built this bow saw last week. I was really happy with how it turned out.

then I tried to use it. It was working alright until crack:


(I didn’t take a picture with actual crack so this will have to do)

I’ve since filled the crack with glue and clamped it. It seems to be holding the tension well.

I’m wondering what you might think happened though. I have three theories:

1) The crack started in exposed endgrain on the bend of the arm. Is my design poor in that it creates a weakness there?

2) The wood came out of a scrap bin but I believe it is cherry. Is it just not strong enough. I’ve built a smaller scale saw out of hickory and haven’t had any problems (this is a 20” blade)

3) Overtensioned. – While possible, I don’t think this is the answer. This was my first time using the saw so I started with little tension and kept tightening it until the blade stopped flexing.

Any thoughts you might have would be appreciated. Thanks.

-- James


11 replies so far

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JADobson

682 posts in 1578 days


#1 posted 04-04-2016 07:00 PM

Sorry for the bad pictures. The only camera I have is on a really old ipod touch. This is as good as it gets.

-- James

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Ocelot

1471 posts in 2105 days


#2 posted 04-04-2016 08:52 PM

I’m certainly no expert.

That said, it’s clear that the grain ran straight through while the part you made was curved. If you could find wood with a curved grain and follow the grain with the piece you are making, it’s bound to be stronger. Or, you could make the piece straight so that you don’t end up with a short grain situation. It makes sense to me that it would crack there.

[edited to remove nonsense]

After a minute of thought, it occurs to me that what you want is to have the grain running at 90 degrees to what you have here. That is, you have used a quartersawn board where you needed a flatsawn board.

-Paul

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HokieKen

1807 posts in 605 days


#3 posted 04-04-2016 08:52 PM

I can’t really see the grain direction in the pictures, but you suggest that the crack may have originated from end grain. That makes me think the grain is running parallel to the blade? If that’s the case, I would think that might be the problem. It seems to me that the grain should be running perpendicular to the blade on the ends and parallel to the blade on the cross bar. I’ve never made a bow saw though, so I may be way off :P

After looking at the pics again, I think I misunderstood. I think you do have the grain oriented as I suggested. In that case, all I can say is, wood is a fickle mistress. Sometimes, no matter how carefully you designed it and no matter how selective you were when selecting your wood, it’s gonna break. Hopefully the glue will take care of it and you’ll have a good-working saw for years to come. Either way, congratulations on a nice looking saw.

-- Kenny, SW VA, Go Hokies!!!

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TravisH

452 posts in 1402 days


#4 posted 04-04-2016 08:57 PM

I think add 1 and 2… I think you would need a little more meat or less radius if using cherry but have little experience as have only made 1 bow saw and that was a smaller 12 inch Gramercy set up. I used hickory as was concerned about the tension issue.

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Hammerthumb

2533 posts in 1442 days


#5 posted 04-04-2016 09:16 PM

I think that hickory would have been a better wood to use for a bow saw. That being said, I think if you want to keep that design with cherry, you would need to a little thicker material.

-- Paul, Las Vegas

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JADobson

682 posts in 1578 days


#6 posted 04-04-2016 09:21 PM

Thanks for the replies everyone. Just for scale the wood is 3/4” thick and about 1 1/2” wide with rounded over edges.

To clear up any confusion about grain direction, the grain runs vertical in the arms. When I wrote that there was end grain where the crack started it is because the arm curves and exposes the end grain in an otherwise straight grained piece.

-- James

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Hammerthumb

2533 posts in 1442 days


#7 posted 04-04-2016 09:23 PM

I took another look and noticed that the blade size was a little large also for the arm thickness. Here is a bowsaw I made for another LJer in a swap.

-- Paul, Las Vegas

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Ocelot

1471 posts in 2105 days


#8 posted 04-04-2016 09:27 PM

If you used flatsawn boards, the end grain would not be exposed in the same way.

What I mean is that the layers in the wood should lay down like plys in plywood. The long grain should still run perpendicular to the blade as you have it, but the grain should run so the edge grain is visible at the ends of the saw, not at the face.

-Paul

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Hammerthumb

2533 posts in 1442 days


#9 posted 04-04-2016 09:32 PM

I understand what Ocelot is saying and it has some merit, but I think the design, as good looking as it is, presents functional issues especially with cherry, and the size of the blade.

-- Paul, Las Vegas

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Nubsnstubs

826 posts in 1197 days


#10 posted 04-04-2016 11:21 PM

Dobson, take your picture and question to an Archery site. They should have any answer you are looking for. Your picture of where the crack started is exactly what any bow maker does his/her to avoid. You can’t cross over a growth ring with suffering a loss. I know enough self bow enthusiasts to know the mistakes made in stressing wood like you did.

The size of the blade has nothing to do with the wood breaking.. You cut into a growth ring, and for that, you deserve 10 lashes….....Hope this helps. heeheheh.

I’m in edit mode. You can cross over a growth ring, but not on the face of the bow, or in this case, a bow saw. When a bow is strung, what you’re looking at is the belly of the bow if you have an arrow in it ready to shoot. The portion facing the receiving end is the face of the bow….. Jerry (in Tucson)

-- Jerry (in Tucson)

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uMinded

104 posts in 1319 days


#11 posted 04-05-2016 03:39 AM

The exact same break happened to my bow saw and I made it out of razor straight grain hickory. The problem is that the curve of the bow caused the straight grain to be bisected so instead of one long arch it was a bunch of 2” long laminations. (See my awesome picture)

On my next saw I will profile it to match grain and if it’s to out of whack maybe try steam bending it into a gentle curve.

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