Primitive Archer
Main Discussion Area => Bows => Topic started by: stuckinthemud on October 10, 2018, 12:27:33 pm
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If an admin wants to move this post then that's fine with me.
Here is my yew and sinew crossbow prod on my 'Heath Robinson' windlass with some cheap braided nylon rope. I have shown fully unwound and fully drawn, I have to get it to draw to 9" but it's so strong it's stretching the rope rather than drawing any further . I am re-lacing the windlass in heavy duty paracord tomorrow. I have rasped 2mm off the belly and have reached the edge of the sinew; there is now a sharp edge where the belly meets the sinew as it wraps over the sides, is it ok to round this edge over as this will start to cut into the sinew ? Any and all tillering advice is most welcome.
(https://stuckinthemudsite.files.wordpress.com/2018/10/20181009_102115.jpg)
(https://stuckinthemudsite.files.wordpress.com/2018/10/20181009_102421.jpg)
I realise there's very little to go on, I'll post more as I go on
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Lol...Sorry I don't have anything to add I just wanted to laugh at your title.
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yeh, made me smile too, figured a short bow might get more views than an angry bow
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I've just been through this with the Xbow I've built... nightmare to string. The bracing/stringer/cocking string needs to be made of some decent non stretch material. I used an old crossbow string in the end.
Paracord is stretchy isn't it? (as it's designed to absorb shock). I'd look for some decent hemp cord/rope, I've used that for warbow stringers, a bit of initial stretch then it settles down.
Del
PS, I just looked at cheaprope.co.uk they have some interesting looking sash cord, unfortunately, no breaking strain figures >:( ::)
Toolstation do 10m of sash cord for less than 4 quid, and as it's made to run over a pulley it should be good.
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Hi Del, I've been following your crossbow adventure with much interest. Got to agree, its all a huge learning curve; I've already destroyed an old linen stringer - 14 strands of 18/4 managed to bend it a 1/2". I'm hoping the paracord will stretch initially and then stabilise but if it doesn't I will have to source something with a bit more oomph. The differential windlass has been a revelation though - one hand to wind back a bow that powerful with zero effort!
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Paracord is working ok at the moment. Strikes me the left limb is a tad stiffer than the right. Right limb might have a hinge developing?
(https://stuckinthemudsite.files.wordpress.com/2018/10/20181011_122844.jpg)
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Hi, interesting post going here. I'm not into crossbows, but was wondering. Could you use plastic coated braided wire for tillering?
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Would have thought that as long as it could travel through the pulley and wrap around the winch it would be fine.
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If I remember correctly, paracord stays stretchy for quite a while. Don't remember the mil specs, but the 'chutes had a set number of jumps before the gore (shroud) lines were replaced. Maybe a big bunch of strands of B50 or fast flight would work, like 50 or so! But I don't know anything about crossbow technology, haven't even shot one!
Hawkdancer
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Just a thought but if you could get a hold of some mule tape electricians use it to pull wires long distances the stuff is like cloth cable you could tow with it, its brutally strong and no stretch ,I use it for fire house forms , the size in the pic is 5/8 but they make it down to 1/8 its tuff tuff stuff that doesent stretch or break ! It would easely string any bow.
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I'l keep an eye out for it, one of my friends works as a power-cable design engineer, he might know someone who knows someone.... My next angry bow is a full repro of a medieval gothic composite bow, should be 500# or so, and I will definitely be needing heavy duty winding eqt. for that one, tho its currently little more than a plastic bag full of small pieces of horn - shaping the horn, grooving and gluing it together is my project for this winter. >:D
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This stuff will be your ticket it holds that pressure form under full pressure it broke the 3/4 hard wood dowels replacing with metal great for pulling dear out to it's amazing stuff !
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Biggest problem I'm now finding is that as the cord travels across the drum it pulls the centre of effort sideways. Still, any comments as to the tiller?
(https://stuckinthemudsite.files.wordpress.com/2018/10/20181013_203339-e1539459686178.jpg)
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You can get the mule tape on amazon.
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Can't really see enough bend to comment on tiller... there's nothing obviously wrong tho'.
maybe a more conventional single roller windlass with a spreader bar so it's pulling the rope back from 2 points , say 6" apart?
Dunno if you can see what I mean below, the \\\/// is how the rope is on the roller. ___ is the spreader bar
___
| |
\\\\ /////
Del
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I like the 'nothing obviously wrong', I'm being excessively cautious this time. I understand the 'drawing,' a spreader bar is a really great idea, I think I'll fit one on this set up and see what happens. Cheers
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not far off brace, tho still on a long string
(https://stuckinthemudsite.files.wordpress.com/2018/10/20181014_124949-e1539519466373.jpg)
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Brace height, still on a long string (hanging down 1 inch). How far past brace do you guys go before you switch to a short string?
(https://stuckinthemudsite.files.wordpress.com/2018/10/20181015_175752-e1539625554117.jpg)
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That doesn't look quite up to brace yet... but to answer the question... just enough to get it comfortably braced.
BTW. Right limb look much thicker, but I'm assuming it's a trick of the light/camera angle tc?
Del
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It is a little thicker, by about 1mm at the point where there is some natural reflex as it seems to be a weak point, I am a bit worried I've reduced the left outer a bit too much though. A short straight edge seems to show a tiny fraction more bend just in from the horn insert.
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Had a really good squint at it across the rim of my coffee mug and I think the right limb is marginally stiffer than the left so I'll rasp a little off in the morning and see how it looks. I was going for a 3" brace height and a 6" power stroke, the lath is 27" ntn. Not really too concerned regarding draw weight but it's way strong at the moment, though I have no idea what its draw weight is, not even a ball park figure, any creative ideas how to measure draw weight of somewhere in the region of 150Lb?
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Right, now I'm ticked off with myself and I do not know what to do.
I got the bow to bend evenly and with no set (Yay! first time I managed that) and to draw to 3" (brace height, but I haven't braced it yet) - no pictures because just as I was about to take one, the bow pulled the winch apart. I put the bow over a bar and pulled my full weight down on it - I weigh about 170# and the bow bent about 2". I would like something in the region of 150 at 9" so I figure the bow is about 50# over weight; that's why I'm ticked.
I guess my options are:
1. live with it and build a cranequin
2. reduce width to reduce weight - but the sinew runs right over the sides and down to the belly - does that matter?
3. reduce thickness to improve flexibility
4. reduce width and thickness
so, what would you do? Are there any other options? Opinions please
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I'd remove wood from the belly if necessary.
IMO you really need to get it braced.
I'd suggest making some decent way of fixing it to the wall and use a pulley set up like on my tiller tree. A 220# fishing scale is relative ly cheap.
I've pulled 150# on my set up and I dare say it would take 200#
For actual use in the field a goats foot type lever would do it, as per pictured (bottom right) in this blog post, the bow in the pic is 275# if I recall.
https://bowyersdiary.blogspot.com/2017/06/crossbow-progress.html (https://bowyersdiary.blogspot.com/2017/06/crossbow-progress.html)
This video gives a detailed explanation of the tiller rig and its pulleys to give 2:1 leverage
https://www.youtube.com/watch?v=YGNYlCQk9mU&t=66s (https://www.youtube.com/watch?v=YGNYlCQk9mU&t=66s)
Del
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Thanks Del, that goat's foot might be a game-changer for me.
Found an image taken from not long before I stopped tillering, its bending a 1/2 inch more than this now and the slightly thick section on the right of centre now matches the section on the left. Tillering gizmo shows even and constant bend from the ends of the horn inserts to the fades.
(https://stuckinthemudsite.files.wordpress.com/2018/10/20181017_174334-e1540131186378.jpg)
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5. Make a stronger winch??
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I'll rebuild the winch in better materials with short drums for tillering but I am also looking forward to shooting it where I had hoped to draw by hand, hence a target weight of 150# but a goats foot or wippe will be ok
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I've just weighed my crossbow (108#). I did a video showing the set up.
Not the best video but it shows how I did it.
https://youtu.be/1JVTiy5BKXk (https://youtu.be/1JVTiy5BKXk)
Del