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Thread: Bow saw blades
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8th June 2021, 10:54 AM #61GOLD MEMBER
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Hi Ian. This bowsaw thread has got me all inspired! Quick question to you (and others) . What do you think of the suitability of Douglas Fir (oregon) for the frame? I have some very close grained pieces rescued from a 100 x 400 beam section I had to replace recently. I would guess that is old growth timber.
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8th June 2021, 11:49 AM #62
No idea, MA! I'll have to defer to others with more experience on that one. I know it's used in building for its structural properties, and my guess is it would probably be fine, just a matter of getting the dimensions right. If I were to use a conifer wood myself, the first one that would spring to my mind would be Yew - the stuff of the famed longbow..
The traditional wood in Britain is Ash (& I think Hornbeam across the Channel), but many woods will do the job, My preferred wood is the black Wattle that grows in my yard, tough as hell and more attractive than Ash. Blackwood might be ok if you get a good piece, the acacias tend to be tough & springy, which is why so many were used as 'wattle' for building houses in early colonial days & how it got its common name. "Hickory" wattle grows down your way & should be ideal, though I've not actually tried it.
Whatever, the best way to find out is to give it a whirl. There's not that much involved in making a simple frame with straight members, making curved arms like I do is just more work & adds nothing to function (& unless you find bits with the grain matching the curves actually weakens them!). So whip something up, I reckon your intuition & experience will suggest cross-section dimensions (there are plenty of pics of old ones to be found, too, which will give you an idea of sizes & proportions). String it & put more tension on it than you reckon you'll ever need & if the frame doesn't bust, you're on your way. You'll probably over-build the first frame you make, but it's easy enough to pare it back bit by bit 'til it starts to feel light & balanced but doesn't look like flexing when you tension it.
If you do try the Douglas fir, let us know what you think - this thread is becoming a useful repository of info for local bowsaw makers...
Cheers,IW
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8th June 2021, 12:54 PM #63GOLD MEMBER
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Hickory wattle....something else to have a look at
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8th June 2021, 01:03 PM #64GOLD MEMBER
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Do you mean Acacia Melanoxylon? This is what we call Blackwood locally. And yes we have plenty. Currently burning in the fireplace as I type. I tried resawing some on my bandsaw when I first started splitting it but too much for my machine. But you have got me thinking
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8th June 2021, 01:05 PM #65
MA
Douglas Fir is quite a bit stronger than Radiata pine for example, but not as strong as Beech, which was another commonly used timber. "Springyness" is probably a desirable feature of any timber used, although coupled with strength I think it is more prevalent in hardwoods. I think I would begin with timber sections, particularly for the end pieces, that are a little larger and then pare down the timber as deemed appropriate rather as Ian suggested. Have at look at the commercial frames made by Peugeot in post #54. They were basic to put it mildly. Ian would have started with that shape before sculpting his much more aesthetically pleasing frames. Some of it could be done even after you have completed the saw, but probably not the section that holds the blade.
Regards
PaulBushmiller;
"Power tends to corrupt. Absolute power corrupts, absolutely!"
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8th June 2021, 01:11 PM #66GOLD MEMBER
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Hi Paul. The other thing about DF is that it is has a good strenth to density ratio. I cut down and dressed a piece off the beam and it was stunning.
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8th June 2021, 06:28 PM #67
Yep, A. melanoxylon. It grows right up the east coast, but the further north you go, the scraggier they get, which is the opposite of what intuition would suggest. There are dozens of other species of acacias that grow along the coastal strip that are all lumped under the generic moniker of "Black wattle" in common parlance. I mentioned hickory wattle (A. implexa) 'cos an old bloke in Benalla used to swear by it for axe handles & the like. It's also called "Lightwood", presumably because of its pale brown colour, not its density.
Several species of "black wattle" grow in our backyard, & they are easy to key out by their phyllodes ('leaves') & flowers, but I can't tell them apart by the wood, there seems to be as much variation in colour & density within species as between them so they overlap completely. They are all small scraggly trees, with trunk diameters rarely over 400mm, & don't seem to be able to make up their minds which direction they want to grow in, & keep changing their minds. They don't live long, about 20-30 years is as far as they can go, so there's usually one dying somewhere on our land or on the adjoining properties. About the longest straight pieces I get out of them is chair-leg length, but some of the curvy bits have just the right 'flow' for bowsaw arms (& a lot of it is too twisted &/or punky for anything but the fire). Some of the bits around branches can have nice figure - the wedge for this chariot plane is from a piece of rescued firewood:
Done b.jpg
Good bits of wood for small projects are all around us - I'm hopeless splitting wood for fireplaces, I often want to save more than I chuck on the pile......
:U
CheersIW
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8th June 2021, 06:28 PM #68SENIOR MEMBER
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Woo hoo! Thanks Ian,
That's great news. I can blame the tool rather than the user!
On a more serious note, how to you determine when the blade is at the correct tension?
Regards,
Adam
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8th June 2021, 07:03 PM #69
Trial & error, Adam. That's not meant to be as flippant as it sounds. It's pretty hard to over-tension the blade, think of the tension you can put on a bandsaw blade, if you have one. If you don't then small saws are usually tensioned to the point where if you "pluck" the blade it will sound a note around middle C. You will never get that out of a bowsaw, of course, no practical frame would stand it, I'm just making the point that the blade can take a lot more tension than you can generate with a bit of cord & a toggle. So the idea is to get it as tight as you can without breaking the frame or the cord. I have broken a few arms in my time, deliberately, just to see what they are capable of. It takes a ridiculous amount of tension to break a reasonably well-made frame, you are unlikely to ever take it to that extreme unless your frame is a bit too flimsy.
Technique does matter a bit - it took me a while to get used to a bowsaw when I first started using one. I made a twisted wire "cord" for my first saw, with one end wrapped around a long 3/16" bolt that went through the upper arm & tightened with a wing nut. Back then it was the only way I could get enough tension on the blade, can't remember what sort of cord I tried, but I do remember it was pretty useless, so I turned to the bolt & wire instead. Eventually, I found some cord that had the requisite properties & was able to tension the blade adequately. Bowsaws don't like being forced, no saw does, of course, but with bowsaws, forcing it can buckle the blade slightly which will cause some binding & make it very hard to make a smooth, straight cut. You have to train yourself to keep the damn thing square to the cut at all times, but especially when turning tight curves.
Hang in there, it'll all come together for you shortly, every tool has a learning curve....
Cheers,IW
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9th June 2021, 12:51 PM #70
Traditional timber names can be confusing.
Down here, Acacia melanoxylon grows to quite large trees and is a much prized timber, always referred to as Tasmanian Blackwood or just blackwood.
But confusingly, in the colonial era, the bushmen called Acacia melanoxylon lightwood, not on account of its colour, but because the green logs floated. This was much easier to extract from the bush than eucalypts that sank.
Originally Posted by IanW
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24th June 2021, 03:28 PM #71SENIOR MEMBER
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As an update:
Cranking up the tension on the thread as much as i thought was safe dramatically improved performance. I'm more reasonable comfortable at sawing close to the time and the kerf is much tidier.
I've not tried using the Venetian cord yet that ian kindly sent down, largely as i didn't want to jinx the saw now is going well! I will definitely give it a try soon. The kite thread though can take quite a tension, much more than i believed.
All four table legs are now cut, just working on adding the "wings" (no idea of their proper name) and rounding the leg.
First semi complete leg is shown below and I'm happy with my efforts. Need to refine the curves but it's a good learning experience
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24th June 2021, 03:57 PM #72
Been watching, but was not sure if you were worried about breaking the cord or the wooden arms!
If your trials with Venetian blind cord are successful, but you think that you need even more strength then you might consider one of the "exotic" cords used on racing yachts - spectra, dyneema, vectran, kevlar, etc.
For example, 3mm spectra has about double the breaking strength of 3mm polyester venetian blind cord and much less stretch.
Donaghys Superspeed (Spectraspeed) Rope, $1.65 | Whitworths Marine
But it is extremely difficult to cut without a hot knife.
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24th June 2021, 05:52 PM #73
Hmm, I suspected blade tension might have been an issue, Adam...
If the cord you're using is getting you sufficient tension, then no real need to change it, the other cord will keep. Or, you might get carried away like certain silly people I know & make yourself several sizes of bowsaw.
I just find having to have multiple strands makes it more awkward when I'm setting the blade through a hole for a cutout, which I do quite often. While I'm holding frame & blade & trying to reassemble the saw, the loops of cord like to slip off the toggle & make things awkward for someone with only two hands.
Graeme, I will definitely give some of that 'spectraspeed' a go. It only costs a few $$ per metre & a metre would almost do two 10" saws, but they seem to have a flat rate for postage, which puts the price of a couple of metres of 4mm cord to a silly amount, & that offends my stingy nature. But there's a Whitworths store in Woolloongabba, so I'll just pop in next time I go by that way & pick up a couple of metres. I'm thinking the 4mm should be good for my larger saw, but I'll report back when I get hold of some...
Cheers,IW
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24th June 2021, 09:16 PM #74
Hi Ian
I did not intend to give an adv for Whitworths or Spectraspeed - any yacht chandlery should have a range of cords - so just go to whichever is convenient to you.
The "exotics fibres" are much stronger than polyesters and other "conventional" fibres, but they are harder to use. Most of them have an outer braid layer of polyester to protect the magic "exotic" fibres in the middle. You might find:
- extremely difficult to cut with a sharp knife or a hacksaw. Really need to use a "hot knife" to melt through the exotics,
- cords are stiffer and springier,
- knots can be prone to untying themselves.
You pay a price for the strength!
Because of the extra strength, I would use either the same diameter as your venetian cord, or one size smaller. There is no real reason to go thicker.
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24th June 2021, 10:10 PM #75SENIOR MEMBER
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I was concerned about both.
Bow saw mk1 snapped an arm in half which kind of kicked off this thread. It had slender arms plus was made from meranti or merbau from bunnings which was very light.
Mk2 is both meatier and uses a denser timber, sheoak. When tightening the string became quite resistant and i was guessing i was approaching its limit .
Thankfully i was wrong and I've been able to see the big improvements the extra tension gave.
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